Animals | Erin Zimmerman

February 17, 2017February 17, 2017

Forever Young

How Evolution Made Baby-faced Humans & Adorable Dogs

human_development_neoteny_body_and_head_proportions_pedomorphy_maturation_aging_growth

Who among us hasn’t looked at the big round eyes of a child or a puppy gazing up at us and wished that they’d always stay young and cute like that? You might be surprised to know that this wish has already been partially granted. Both you as an adult and your full-grown dog are examples of what’s referred to in developmental biology as paedomorphosis (“pee-doh-mor-fo-sis”), or the retention of juvenile traits into adulthood. Compared to closely related and ancestral species, both humans and dogs look a bit like overgrown children. There are a number of interesting reasons this can happen. Let’s start with dogs.

When dogs were domesticated, humans began to breed them with an eye to minimizing the aggression that naturally exists in wolves. Dogs that retained the puppy-like quality of being unaggressive and playful were preferentially bred. This caused certain other traits associated with juvenile wolves to appear, including shorter snouts, wider heads, bigger eyes, floppy ears, and tail wagging. (For anyone who’s interested in a technical explanation of how traits can be linked like this, here’s a primer on linkage disequilibrium from Discover. It’s a slightly tricky, but very interesting concept.) All of these are seen in young wolves, but disappear as the animal matures. Domesticated dogs, however, will retain these characteristics throughout their lives. What began as a mere by-product of wanting non-aggressive dogs has now been reinforced for its own sake, however. We love dogs that look cute and puppy-like, and are now breeding for that very trait, which can cause it to be carried to extremes, as in breeds such as the Cavalier King Charles spaniel, leading to breed-wide health problems.

cavalier-king-charles-spaniel-blenheim-dog-tag-id-image-nm — An undeniably cute Cavalier King Charles spaniel, bred for your enjoyment. (Via Wikimedia Commons)

Foxes, another type of wild dog, have been experimentally domesticated by scientists interested in the genetics of domestication. Here, too, as the foxes are bred over numerous generations to be friendlier and less aggressive, individuals with floppy ears and wagging tails – traits not usually seen in adult foxes – are beginning to appear.

But I mentioned this happening in humans, too, didn’t I? Well, similarly to how dogs resemble juvenile versions of their closest wild relative, humans bear a certain resemblance to juvenile chimpanzees. Like young apes, we possess flat faces with small jaws, sparse body hair, and relatively short arms. Scientists aren’t entirely sure what caused paedomorphosis in humans, but there are a couple of interesting theories. One is that, because our brains are best able to learn new skills prior to maturity (you can’t teach an old ape new tricks, I guess), delayed maturity, and the suite of traits that come with it, allowed greater learning and was therefore favoured by evolution. Another possibility has to do with the fact that juvenile traits – the same ones that make babies seem so cute and cuddly – have been shown to elicit more helping behaviour from others. So the more subtly “baby-like” a person looks, the more help and altruistic behaviour they’re likely to get from those around them. Since this kind of help can contribute to survival, it became selected for.

chimpanzee-mastiff-dog-friends-10 — You and your dog, essentially. (Via The Chive)

Of course, dogs and humans aren’t the only animals to exhibit paedomorphosis. In nature, the phenomenon is usually linked to the availability of food or other resources. Interestingly, both abundance and scarcity can be the cause. Aphids, for example, are a small insect that sucks sap out of plants as a food source. Under competitive conditions in which food is scarce, the insects possess wings and are able to travel in search of new food sources. When food is abundant, however, travel is unnecessary and wingless young are produced which grow into adulthood still resembling juveniles. Paedomorphosis is here induced by abundant food. Conversely, in some salamanders, it is brought on by a lack of food. Northwestern salamanders are typically amphibious as juveniles and terrestrial as adults, having lost their gills. In high elevations where the climate is cooler and a meal is harder to come by, many of these salamanders remain amphibious, keeping their gills throughout their lives because aquatic environments represent a greater chance for survival. In one salamander species, the axolotl (which we’ve discussed on this blog before), metamorphosis has been lost completely, leaving them fully aquatic and looking more like weird leggy fish than true salamanders.

axolotl_ganz — An axolotl living the young life. (Via Wikimedia Commons)

So paedomorphosis, this strange phenomenon of retaining juvenile traits into adulthood, can be induced by a variety of factors, but it’s a nice demonstration of the plasticity of developmental programs in living creatures. Maturation isn’t always a simple trip from point A to point B in a set amount of time. There are many, many genes at play, and if nature can tweak some of them for a better outcome, evolution will ensure that the change sticks around.

Sources

Martin & Gordon (1995) Journal of Evolutionary Biology 8:339-354
Trut (1999) American Scientist 87(2):160-169.
Trut et al. (2009). BioEssays 31:349-360.
Waller et al. (2013) PLoS ONE 8(12): e82686.
https://www.scientificamerican.com/article/being-more-infantile/

*Header image by: Ephert – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39752841

October 4, 2016

An Inconvenient Hagfish

1280px-eptatretus_stoutii

We think of scientific progress as working like building blocks constantly being added to a growing structure, but sometimes a scientific discovery can actually lead us to realize that we know less than we thought we did. Take vision, for instance. Vertebrates (animals with backbones) have complex, highly-developed “camera” eyes, which include a lens and an image-forming retina, while our invertebrate evolutionary ancestors had only eye spots, which are comparatively very simple and can only sense changes in light level.

At some point between vertebrates and their invertebrate ancestors, primitive patches of light sensitive cells which served only to alert their owners to day/night cycles and perhaps the passing of dangerous shadows, evolved into an incredibly intricate organ capable of forming clear, sharp images; distinguishing minute movements; and detecting minor shifts in light intensity.

584px-diagram_of_eye_evolution-svg — Schematic of how the vertebrate eye is hypothesized to have evolved, by Matticus78

In order for evolutionary biologists to fully understand when and how this massive leap in complexity was made, we need an intermediate stage. Intermediates usually come in the form of transitional fossils; that is, remains of organisms that are early examples of a new lineage, and don’t yet possess all of the features that would later evolve in that group. An intriguing and relatively recent example is Tiktaalik, a creature discovered on Ellesmere Island (Canada) in 2004, which appears to be an ancestor of all terrestrial vertebrates, and which possesses intermediate characteristics between fish and tetrapods (animals with four limbs, the earliest of which still lived in the water), such as wrist joints and primitive lungs. The discovery of this fossil has enabled biologists to see what key innovations allowed vertebrates to move onto land, and to precisely date when it happened.

There are also species which are referred to as “living fossils”, organisms which bear a striking resemblance to their ancient ancestors, and which are believed to have physically changed little since that time. (We’ve actually covered a number of interesting living fossils on this blog, including lungfish, Welwitschia, aardvarks, the platypus, and horseshoe crabs.) In the absence of the right fossil, or in the case of soft body parts that aren’t usually well-preserved in fossils, these species can sometimes answer important questions. While we can’t be certain that an ancient ancestor was similar in every respect to a living fossil, assuming so can be a good starting point until better (and possibly contradictory) evidence comes along.

So where does that leave us with the evolution of eyes? Well, eyes being made of soft tissue, they are rarely well preserved in the fossil record, so this was one case in which looking at a living fossil was both possible and made sense.

Hagfish, which look like a cross between a snake and an eel, sit at the base of the vertebrate family tree (although they are not quite vertebrates themselves), a sort of “proto-vertebrate.” Hagfish are considered to be a living fossil of their ancient, jawless fish ancestors, appearing remarkably similar to those examined from fossils. They also have primitive eyes. Assuming that contemporary hagfishes were representative of their ancient progenitors, this indicated that the first proto-vertebrates did not yet have complex eyes, and gave scientists an earliest possible date for the development of this feature. If proto-vertebrates didn’t have them, but all later, true vertebrates did, then complex eyes were no more than 530 million years old, corresponding to the time of the common ancestor of hagfish and vertebrates. Or so we believed.

This past summer, a new piece of research was published which upended our assumptions. A detailed electron microscope and spectral analysis of fossilized Mayomyzon (the hagfish ancestor) has indicated the presence of pigment-bearing organelles called melanosomes, which are themselves indicative of a retina. Previously, these melanosomes, which appear in the fossil as dark spots, had been interpreted as either microbes or a decay-resistant material such as cartilage.

This new finding suggests that the simple eyes of living hagfish are not a trait passed down unchanged through the ages, but the result of degeneration over time, perhaps due to their no longer being needed for survival (much like the sense of smell in primates). What’s more, science has now lost its anchor point for the beginning of vertebrate-type eyes. If an organism with pigmented cells and a retina existed 530 million years ago, then these structures must have begun to develop significantly earlier, although until a fossil is discovered that shows an intermediate stage between Mayomyzon and primitive invertebrate eyes, we can only speculate as to how much earlier.

This discovery is intriguing because it shows how new evidence can sometimes remove some of those already-placed building blocks of knowledge, and how something as apparently minor as tiny dark spots on a fossil can cause us to have to reevaluate long-held assumptions.

Sources

Gabbott et al. (2016) Proc. R. Soc. B. 283: 20161151
Lamb et al. (2007) Nature Rev. Neuroscience 8: 960-975

*The image at the top of the page is of Pacific hagfish at 150 m depth, California, Cordell Bank National Marine Sanctuary, taken and placed in the public domain by Linda Snook.

September 5, 2016

Sex & the Reign of the Red Queen

Tenniel_red_queen_with_alice

“Now, here, you see, it takes all the running you can do to keep in the same place.”

From a simple reproductive perspective, males are not a good investment. With apologies to my Y chromosome-bearing readers, let me explain. Consider for a moment a population of clones. Let’s go with lizards, since this actually occurs in lizards. So we have our population of lizard clones. They are all female, and are all able to reproduce, leading to twice the potential for creating more individuals as we see in a species that reproduces sexually, in which only 50% of the members can bear young. Males require all the same resources to survive to maturity, but cannot directly produce young. From this viewpoint alone, the population of clones should out-compete a bunch of sexually-reproducing lizards every time. Greater growth potential. What’s more, the clonal lizards can better exploit a well-adapted set of genes (a “genotype”); if one of them is well-suited to survive in its environment, they all are.

Now consider a parasite that preys upon our hypothetical lizards. The parasites themselves have different genotypes, and a given parasite genotype can attack certain host (i.e. lizard) genotypes, like keys that fit certain locks. Over time, they will evolve to be able to attack the most common host genotype, because that results in their best chance of survival. If there’s an abundance of host type A, but not much B or C, then more A-type parasites will succeed in reproducing, and over time, there will be more A-type parasites overall. This is called a selection pressure, in favour of A-type parasites. In a population of clones, however, there is only one genotype, and once the parasites have evolved to specialise in attacking it, the clones have met their match. They are all equally vulnerable.

The sexual species, however, presents a moving target. This is where males become absolutely worth the resources it takes to create and maintain their existence (See? No hard feelings). Each time a sexual species mates, its genes are shuffled and recombined in novel ways. There are both common and rare genotypes in a sexual population. The parasite population will evolve to be able to attack the most common genotype, as they do with the clones, but in this case, it will be a far smaller portion of the total host population. And as soon as that particular genotype starts to die off and become less common, a new genotype, once rare (and now highly successful due to its current resistance to parasites), will fill the vacuum and become the new ‘most common’ genotype. And so on, over generations and generations.

Both species, parasite and host, must constantly evolve simply to maintain the status quo. This is where the Red Queen hypothesis gets its name: in Wonderland, the Red Queen tells Alice, “here, you see, it takes all the running you can do to keep in the same place.” For many years, evolution was thought of as a journey with an endpoint: species would evolve until they were optimally adapted to their environment, and then stay that way until the environment changed in some fashion. If this was the case, however, we would expect that a given species would be less likely to go extinct the longer it had existed, because it would be better and better adapted over time. And yet, the evidence didn’t seem to bear this prediction out. The probability of extinction seemed to stay the same regardless of the species’ age. We now know that this is because the primary driver of evolution isn’t the environment, but competition between species. And that’s a game you can lose at any time.

1280px-Passiflora_in_Canary_Islands — Passionflower. Photo by Yone Moreno on Wikimedia Commons.

Now the parasite attacking the lizards was just a (very plausible) hypothetical scenario, but there are many interesting cases of the Red Queen at work in nature. And it’s not all subtly shifting genotypes, either; sometimes it’s a full on arms race. Behold the passionflower. In the time of the dinosaurs, passionflowers developed a mutually beneficial pollinator relationship with longwing butterflies. The flowers got pollinated, the butterflies got nectar. But then, over time, the butterflies began to lay their eggs on the vines’ leaves. Once the eggs hatched, the young would devour the leaves, leaving the plant much the worse for wear. In response, the passionflowers evolved to produce cyanide in their leaves, poisoning the butterfly larvae. The butterflies then turned the situation to their advantage by evolving the ability to not only eat the poisonous leaves, but to sequester the cyanide in their bodies and use it to themselves become poisonous to their predators, such as birds. The plants’ next strategy was to mimic the butterflies’ eggs. Longwing butterflies will not lay their eggs on a leaf which is already holding eggs, so the passionflowers evolved nectar glands of the same size and shape as a butterfly egg. After aeons of this back and forth, the butterflies are currently laying their eggs on the tendrils of the passionflower vines rather than the leaves, and we might expect that passionflowers will next develop tendrils which appear to have butterfly eggs on them. These sorts of endless, millennia-spanning arms races are common in nature. Check out my article on cuckoos for a much more murderous example.

Egg-like glands at the base of the passionflower leaf (the white dots on my index finger).

Had the passionflowers in this example been a clonal species, they wouldn’t likely have stood a chance. Innovations such as higher-than-average levels of cyanide or slightly more bulbous nectar glands upon which defences can be built come from uncommon genotypes. Uncommon genotypes produced by the shuffling of genes that occurs in every generation in sexual species.

And that, kids, is why sex is such as fantastic innovation. (Right?) Every time an illness goes through your workplace, and everybody seems to get it but you, you’ve probably got the Red Queen (and your uncommon genotype) to thank.

Sources

Brockhurst et al. (2014) Proc. R. Soc. B 281: 20141382.
Lively (2010) Journal of Heredity 101 (supple.): S13-S20 [See this paper for a very interesting full explanation of this links between the Red Queen hypothesis and the story by Lewis Carroll.]
Vanderplank, John. “Passion Flowers, 2nd Ed.” Cambridge: MIT Press, 1996.

*The illustration at the top of the page is by Sir John Tenniel for Lewis Carroll’s “Through the Looking Glass,” and is now in the public domain.

August 15, 2016

The Cost of Colour

Sobo_1906_324 Try to imagine a colour you’ve never seen. Or a scent you’ve never smelled. Try to picture the mental image produced when a bat uses echolocation, or a dolphin uses electrolocation. It’s nearly impossible to do without referring to a previous experience, or one of our other senses. We tend to tacitly assume that what we perceive of the world is more or less all there is to perceive. It would be closer to the truth to say that what we perceive is what we need to perceive. Humans don’t require the extraordinary sense of smell that wild dogs do in order to get by in the world. But it wasn’t always this way.

Scent molecules are picked up and recognized in our noses by olfactory receptors. Each type of receptor recognizes a few related types of molecules, and each type of receptor is written into our DNA as an olfactory receptor (OR) gene. In mammals, OR genes make up the largest gene family in our genome. There are over a thousand of them. Sadly for us, over 60% of these genes have deteriorated to the point of being nonfunctional. Why? In what must be a hard piece of news for X-Men fans, extra evolutionary features tend not to hang around unless they’re actively helping us to survive longer and breed more. If a gene can develop a fault that makes it useless without causing its host a major competitive disadvantage, it’ll eventually do so, and an incredible number of these broken genes – called “pseudogenes” – have built up and continue to sit in our genome. This isn’t specific to humans; cows, dogs, rats, and mice all have about 20% of their OR genes nonfunctional. But that still works out to a difference of hundreds of different types of scents that we can’t detect. Even compared to our closest relatives, the apes and old world monkeys, we have twice as many OR pseudogenes, and are accumulating random mutations (the cause of pseudogenes) at a rate four times faster than they are. This is all quite logical, of course; humans have evolved in such a way that being able to smell prey or potential mates from a distance just isn’t key to our survival.

Phylo tree image — From: Gilad et al. (2004) PLoS Biology 2(1): 0120

What’s more interesting is that when scientists looked at the OR genes of apes and old world monkeys (OWMs), they found elevated rates of deterioration there, too… about 32%, compared to only 17% in our next closest group of relatives, the new world monkeys (NWMs). So what happened between the divergence of one group of primates and the next that made an acute sense of smell so much less crucial? The answer came with the one exception among the NWMs. The howler monkey, unlike the rest of its cohort, had a degree of OR gene deterioration similar to the apes and OWMs. The two groups had one other thing in common: full trichromatic vision. Nearly all other placental mammals, including the NWMs, are dichromats, or in common parlance, are colourblind. Using molecular methods that look at rates of change in genes over time to determine when a particular shift happened, scientists determined that in both instances of full colour vision evolving, the OR genes began to deteriorate at about the same time. It was an evolutionary trade-off; once our vision improved, our sense of smell lost its crucial role in survival and slowly faded away. In apes and monkeys, this deterioration process seems to have come to a halt – at a certain point, what remains is still necessary for survival – but in humans, it is ongoing. We know this because of the high number of OR genes for which some individuals carry functional copies, and some carry broken copies. This variability in a population, called polymorphism, amounts to a snapshot of genes in the process of decay, since the broken copies are not, presumably, causing premature death or an inability to breed amongst their carriers. So as we continue to pay the evolutionary price for the dazzling array of colours we are able to perceive in the world, our distant descendants may live in an even poorer scentscape than our current, relatively impoverished one. There may be scents we enjoy today that will be as unimaginable to them as the feel of a magnetic field is to us.

As a quick final point, it turns out humans aren’t the only animal group to have undergone a widescale loss of OR genes. Just as full colour vision made those genes unnecessary for us, so moving into the ocean made them unnecessary for marine mammals. In an even more severe deterioration than that seen in humans, some whales and porpoises have nearly 80% OR pseudogenes. As you may already know, whales, dolphins, and other marine mammals evolved from land-dwelling, or terrestrial mammals (want to know more about it? Read my post here). Using methods similar to those mentioned above for primates, researchers found that at about the same time they were adapting anew to life in the ocean, their scent repertoire was beginning to crumble. And since anatomical studies show that the actual physical structures used to perceive scent, such as the olfactory bulb in the brain, are becoming vestigial in whales, it’s likely the loss isn’t finished yet. Interestingly, the researchers behind this study also looked at a couple of semi-marine animals, the sea lion and the sea turtle, which spend part of their time on land, and found that they have a sense of smell comparable to fully terrestrial animals, with no increased gene loss.

The widescale and ongoing loss of the sense of smell in certain animals, particularly ourselves, is a nice illustration of an evolutionary principle which can be summarized as “use it or lose it”, or more accurately, “need it or lose it.” We tend to think of evolution as allowing us to accrue abilities and features that are useful to us. But unless they’re keeping us and our offspring alive, they’re not going to stick around in the long term. Which makes you wonder, with humans’ incredible success in survival and proliferation on this planet, which relies overwhelmingly on our cognitive, rather than physical abilities, what other senses or abilities could we eventually lose?

Sources

*The image at the top of the page comes from Sobotta’s Atlas and Text-book of Human Anatomy (1906 edition), now in the public domain.

August 17, 2013

Movin’ On Up: Hermit Crabs & the World’s Only Beachfront Social Housing

Common Name: The Hermit Crab

A.K.A.: Superfamily Paguroidea

Vital Stats:

There are around 1100 species of hermit crabs in 120 genera
Range in size from only a few millimetres to half a foot in length
Some larger species can live for up to 70 years
Most species are aquatic, although there are some tropical terrestrial species

Found: Generally throughout the temperate and tropical oceans, in both shallow and deep areas (I was unable to find more specific data on this.)

It Does What?!

If there’s one thing nature loves, it’s symmetry. Sometimes radial symmetry, as we see in starfish or sea anemones; sometimes bilateral symmetry, as in mammals and insects, which have a right half and a left half. External asymmetry is extremely rare in living organisms, and when it does occur, it is generally in a minor form, such as a bird species with beaks bent to the side, or a type of flower with oddly distributed stamens. One of the very few groups with entire bodies that lack symmetry are the gastropods; specifically, the snails. They develop helical shells with asymmetrical bodies to match.

caption (Via: Wikimedia Commons) — The shells also hide how ridiculous they look naked.
(Via: Wikimedia Commons)

But this post isn’t about snails. It’s one thing to evolve an unusual asymmetrical bodyplan to go with your asymmetrical home. It’s another to evolve an asymmetrical bodyplan to go with somebody else’s home. Which brings us to the hermit crab. When snails die in ways that leave behind perfectly good shells on the beach, these guys literally queue up for the chance to move in. Hermit crabs are part of the decapod order of crustaceans, as crabs are, but are not in fact true crabs, and unlike most other crustaceans, they lack any kind of hard, calcified plating on their abdomens (think shrimp shells). Instead, they have a soft, spirally curved lower body that fits perfectly into a snail shell, with muscles that allow them to clasp onto the interior of the shell. Paleobiologists have found that hermit crabs have been living in found shells for over 150 million years, and that they made the move to snail shells when their original shell-producer, the ammonite, went extinct. Living in shells has strongly restricted their morphological evolution, meaning the crabs of aeons ago look pretty similar to the crabs of today, because their housing situation doesn’t allow a lot of change.

caption (Via: Telegraph.co.uk) — Housing shortages hurt everyone.
(Via: Telegraph.co.uk)

Back to those line-ups I mentioned. Unoccupied snail shells are a limited resource, and an unarmoured crustacean is an easy lunch, so of course a lot of fighting goes on over them; crabs will actually gang up on an individual with a higher quality shell and just yank the poor bugger out. But it actually gets much more complex than that… these little pseudo-crabs aren’t as dim and thuggish as you might think. You see, as a hermit crab grows over the course of its life, it needs a series of progressively larger shells in which to live. A crab stuck in an undersized shell is stunted in its growth and is much more vulnerable to predation, since it can’t fully withdraw into its armour. The easiest way to find your next home? Locate a slightly larger hermit crab about to trade up and grab its shell afterward. This is how the crabs form what are called “vacancy chains.” A series of individuals will line themselves up in order of size (I’ve seen groups of schoolchildren unable to perform this task), waiting for hours sometimes, and as the largest crab moves to its new shell, each successive crab will enter the newly vacated one. Brilliant… new homes for everybody, and no one gets hurt. In fact, if a given crab chances upon a new shell that it judges to be too large for its current size, it will actually wait next to the shell for a larger crab to come along and a vacancy chain to form. That’s pretty impressive reasoning for a brain smaller than a pea.

[Fun Fact: Larger aquatic hermit crabs sometimes form symbiotic relationships with sea anemones; the anemone lives on the crab’s shell, protecting its host from predators with its deadly sting, while the crab shares its food with the gelatinous bodyguard.]

Today in Words You Didn’t Think Existed:
carcinisation / car·si·nə·ˈzā·shən / n.
a process by which an organism evolves from a non-crablike form into a crablike form.

That’s right, glossophiles, thanks to a British zoologist, we actually have a specific word for turning into a crab. English rules.

Says Who?

Angel (2000) J. of Experimental Marine Biology and Ecology 243: 169-184
Cunningham et al. (1992) Nature 355: 539-542
Fotheringham (1976) J. of Experimental Marine Biology and Ecology 23(3): 299-305
Rotjan et al. (2010) Behavioral Ecology 21(3): 639-646
Tricarico & Gherardi (2006) Behav. Ecol. Sociobiol. 60: 492-500

Say hello to my little friend. (Via: dailykos.com) — “Say hello to my little friend.”
(Via: dailykos.com)

May 19, 2013

Back to the Deep, Part 2

Common Name: Whales, Dolphins, and Porpoises

A.K.A.: Order Cetacea

Vital Stats:

While the lifespan of most whale species is unknown, evidence indicates bowhead whales can reach ages of around 200 years.
Sexual maturity in whales occurs at around 7-10 years of age.

Found: Throughout the world’s oceans, save the very northernmost regions

It Does What?!

Last time, we looked at how whales evolved from a deer-like creature the size of a housecat into the aquatic behemoths they are today. This week, we’ll cover a couple of the odds and ends of whale weirdness.

One important thing to understand about evolution – particularly in cases of a major habitat shift, as we see in whales – is that it’s not an orderly or “well-thought out” process. A good analogy is to think of an old building that’s being renovated and rewired. New additions may be built onto old structures and new wiring overlaid on old plans, creating a product very different, and often much less efficient, from what would have been created were a new building made from scratch. Because you have to work with what’s already there.

Whale respiration is an excellent example of this point. When the ancestors of modern cetaceans took to the water, developing gills and breathing like fish wasn’t an option, because the machinery wasn’t intact… they were already much too far down the evolutionary path of a terrestrial mammal. What they could develop were more efficient lungs and greater control over how they used them. For humans, breathing is an unconscious and largely involuntary act – it just happens, whether we think about it or not, and we can’t choose to stop doing it for very long. Even if you were to hold your breath until you passed out, you’d just start breathing again the moment you lost control (take note, parents of tantrum-prone toddlers). For whales on the other hand, respiration has become voluntary; they breathe because they choose to do so. Life underwater and the need to hunt without distraction made this ability more valuable than the safety of an involuntary mechanism.

caption (Via:) — Whale-snoring.
(Via: The Telegraph)

There is, of course, a major drawback. For whales, control over respiration came at the price of ever being able to fully fall asleep. If a cetacean were to sleep as we do, it would stop breathing and drown. As a result, they’ve evolved the ability to sleep with one brain hemisphere at a time. So while one hemisphere rests, the other is awake, one eye is open, and the whale is in motion, surfacing periodically. In fact, they appear not to experience REM sleep at all, meaning that these creatures gained their mastery of the oceans, very literally, at the cost of their dreams.

Another interesting problem during whale evolution was that of temperature regulation. Anybody who’s been swimming knows that even relatively warm water can start to give you the chills after a while, especially if you’re not expending a lot of energy. This is because water is an excellent conductor of heat, and will constantly draw warmth away from the skin’s surface. Now once you get into the sunless depths of the ocean, to say nothing of the polar oceans that many whales live in, things get very chilly very fast. To counteract this, whales have developed a thick, insulating layer of fat that holds in the heat and keeps their core body temperature from plummeting. Easy peasy, right?

But what happens when the whale expends a lot of energy, say, on an intense feeding session, and builds up too much heat? Ever shovel snow in a heavy winter coat? After a few minutes, you’re ready to tear the coat off because you’re sweating so much. Not so easy when the coat’s under your skin. Well, researchers have recently discovered what they think may be the answer to this problem.

caption (Via: National Geographic) — That’s 144 inches, in case you were wondering.
(By: Craig George, Via: National Geographic)

In the bowhead whale, which lives exclusively in frigid arctic and sub-arctic waters (and therefore has a great deal of insulation), biologists found a mysterious, twelve foot long organ positioned along the roof of the mouth, made out of what is, essentially, the same tissue found inside penises. That is to say, spongy tissue filled with a lot of blood vessels which can expand as it fills with blood. So how does a giant mouth-penis help a whale cool off? It’s quite clever, really. The brain being the major point of concern for overheating, the organ, called the corpus cavernosum maxillaris, lies directly beneath it. Hot blood is pumped into the organ, filling the spongy tissue, as the whale opens its mouth, letting in a great volume of icy water which surrounds the engorged tissue, quickly drawing off much of the heat. The cetacean equivalent of a cold shower. This cooled blood then drains from the organ and lowers the temperature around the brain.

And if this extra-penis-as-thermoregulator wasn’t cool enough, it seems to have a secondary function as well. The organ is also packed with sensitive nerve endings (naturally…), which the researchers believe the whales may use to determine the prey density in a given area (bowheads are filter feeders), helping them to decide whether to remain in a location and feed, or move on in search of better pickings.

Fun Facts:

The ability to “sleep” with one eye open was likely also highly valuable to the much smaller ancestors of the cetaceans, for whom predation was a bigger problem.
Whales have fleshy nasal plugs with which they can plug their blowholes while diving.
Oceanic dolphins have the highest relative brain size among extant cetaceans.

Says Who?

Ford et al. (2013) The Anatomical Record 296: 701-708
Gatesy & O’Leary (2001) Trends in Ecology and Evolution 16(10): 562-570
Gatesy et al. (2013) Molecular Phylogenetics and Evolution 66: 479-506
Lyamin et al. (2008) Neuroscience and Behavioral Reviews 32: 1451-1484
Uhen (2010) Annual Review of Earth and Planetary Sciences 38: 189-219
Zimmer (2013) The Loom, March 4th.

April 21, 2013

Back to the Deep, Part 1

Common Name: Whales, Dolphins, and Porpoises

A.K.A.: Order Cetacea of Class Mammalia

Vital Stats:

Consists of 88 living species
Order is divided into Odontoceti, the toothed whales (73 sp.), and Mysticeti, the baleen whales (15 sp.)
Odontoceti includes both dolphins and porpoises
The largest whale, a blue whale, can grow up to 30m (98’) in length and weigh as much as 20 elephants

Found: Throughout the world’s oceans, save the very northernmost regions

It Does What?!

caption (By: Nobu Tamura, via: Wikimedia Commons) — The twenty pound vermin that went on to rule the oceans.
(By: Nobu Tamura, via: Wikimedia Commons)

Picture it: the time is just over 50 million years before the present – the early Eocene – the climate is much warmer than today, undergoing a period of rapid global warming… it is the Age of Mammals. On the shores of the tropical Tethys Sea, in what would eventually become India, a small, deer-like animal, not much larger than a housecat, wades into the water and dives briefly to retrieve a fish before returning to dry land. This has become a successful strategy for its species, avoiding competition from other mammals by eating marine life. Well-fed, the creatures reproduce rapidly, creating competition amongst themselves. Those individuals with greater lung capacity and better swimming ability catch more food, outcompeting those who don’t. Over great stretches of time, characteristics enabling speed and skill under water become more important than those enabling life on land, and selection tilts in favour of a longer, more lithe body, smaller hindlimbs, stronger forelimbs for paddling, and less fur.

caption (Via: AccessScience) — Swimming: great for a slim figure.
(Via: AccessScience)

Millions of years pass as our small hunter’s descendants eventually lose the ability to ever return to land. They have no fur now… it isn’t useful for retaining heat beneath the waves. Fat is, though, and this begins to accumulate in thicker layers under their bare skin. Their front legs are nearly inflexible at the joints, trading range of movement for strength and widening into precision rudders to control direction as they swim. In concert, the tail becomes more muscular and widens into flukes at the tip, propelling them forward powerfully with each stroke. Their back legs – unneeded – atrophy, gradually losing both size and bone structure, until the foot is completely gone. A small stub lingers for a time before the last vestigial bones simply remain inside the smooth body wall, evidence of a distant terrestrial past. The nasal opening has migrated to the top of the head for ease in surface breathing. Ten million years have passed since the scene on the shore, and we now have our first fully aquatic whale.

Of course, much still had to happen before we arrived at the whales of today. In the time since aquatic mammals first arose, a major division took place within the Cetacea. One group, the toothed whales, or Odontoceti, continued to hunt and eat fish and large marine fauna, including squid and even other whales. To aid in finding their prey, these whales developed echolocation, the use of projected sound to create an image of the surrounding area, thereby becoming the loudest mammal, with vocalisations of more than 180 decibels (a jackhammer tops out at about 120dB). The large bulge we see on the forehead of dolphins and other toothed cetaceans is an organ called a ‘melon’ (because they couldn’t think of anything more science-y sounding just then), which is thought to help direct and focus these sounds.

Being a top-level predator isn’t very energetically efficient, though, and there isn’t always enough prey to go around. So at some point, one group of whales began to move toward a different strategy. The origins of the Mysticeti, the baleen whales, are still a bit unclear, but these animals switched from hunting large fauna to eating colossal numbers of tiny sea creatures such as krill. In order to do this, the whales lost their teeth and developed baleen in their place. Baleen is essentially a fine-toothed comb that filters small animals from the water as it passes. The whale takes a giant mouthful of water and pushes it out against the combs until only food remains. While this may seem less efficient than just grabbing a big fish and eating it, filter feeding is what allowed the largest whales to evolve to their present size. The blue whale, Balaenoptera musculus, is believed to be the largest animal which has ever existed on Earth, and it got that way eating mostly shrimp the size of your thumbnail. Amazing, isn’t it?

Now that we’ve covered how they got that way, tune in next time for part two, where we’ll explore the many weird and wonderful aspects of life as a modern whale.

Fun Facts:

Baleen whales still have teeth during the embryonic stage of their development, much as human fetuses briefly develop tails.
Toothed whales do not chew their food; it is eaten whole or torn into large pieces and swallowed. This may be related to the fact that, unlike most mammals, they have only one set of teeth.

Says Who?

Gatesy & O’Leary (2001) Trends in Ecology and Evolution 16(10): 562-570
Gatesy et al. (2013) Molecular Phylogenetics and Evolution 66: 479-506
Lyamin et al. (2008) Neuroscience and Behavioral Reviews 32: 1451-1484
Uhen (2010) Annual Review of Earth and Planetary Sciences 38: 189-219

March 14, 2013

Cuckoos: Outsourcing Childcare, Hogging the Bed

Common Name: Parasitic Cuckoos

A.K.A.: Subfamily Cuculinae (Family Cuculidae)

Vital Stats:

Range in length from 15-63cm (6-25”) and weigh between 17g (0.6oz.) and 630g (1.4lbs.)
The majority of cuckoos are not parasites, but around 60sp. are (about 56 in the Old World, and 3 in the New World)
Babies of brood parasites are initially coloured so as to resemble the young of the host species

Found: The cuckoo family is present throughout the temperate and tropical world, with the exceptions of southwest South America and regions of North Africa and the Middle East. Parasitic cuckoos occupy a subset of this range, principally in the Old World.

It Does What?!

Parenting is tough… less sleep, less free time, all those all those hungry mouths to feed. What’s a busy mother to do? You know you need to perpetuate the species, but who has the time? Impressively, cuckoos have come up with the same answer that many humans have: outsourcing! Involuntary outsourcing, in this case.

One of these things is not like the others.(Via: Timothy H. Parker) — One of these things is not like the others.
(Via: Timothy H. Parker)

Once a female cuckoo has mated and is ready to lay the eggs, rather than build a nest and slog her way through childcare, she waits for another female with freshly laid eggs to take off for some food and just lays her egg there, spreading her clutch across several nests. In theory, when the duped female returns, she’ll just settle in and care for the new egg along with her own. Cuckoo eggs have a shorter incubation period than that of their host, so the foreign egg usually hatches first, at which point the baby cuckoo just gives the other eggs (or chicks, if the timing didn’t quite work out) a good shove, and enjoys having both a nest and a doting mother to itself. The cuckoo chick will tend to grow faster than its host species, so it keeps its adoptive parent busy with constant begging for food, having eliminated the competition.

But this wouldn’t be a fun evolutionary arms race if the host species just took it on the chin. Birds plagued by cuckoo eggs have worked out several ways to try to cope with the problem. First off, and not surprisingly, they’ve developed a burning hatred of cuckoos. Adult cuckoos seen in the area of the hosts’ nests will immediately be mobbed and run off by a group of angry mothers. The cuckoos, however, have learned to use this to their advantage by having the male of a pair tease and lure the angry mob away while the female lays her eggs in peace. Advantage: cuckoos.

And this, kids, is how you deal with those annoying younger siblings.(Via: M. Bán, PLoS ONE) — And this, kids, is how you deal with those annoying younger siblings.
(By: M. Bán, PLoS ONE)

A second strategy used by the parasitised birds is to learn to recognise foreign eggs and pre-emptively toss them out of the nest. Cuckoos responded to this in two ways. First, they slowly evolved eggs to match those of their host bird in colour and size (or, in the case of covered nests, very dark eggs which aren’t easily seen at all). Bird species with higher levels of egg rejection just end up with cuckoo eggs which look more and more similar to their own. Second, if a host does reject the foreign egg, the cuckoo who laid it will sometimes come and just destroy the entire nest, killing anything left inside it in an act of motherly vengeance. Advantage: cuckoos.

A third strategy, developed by the Superb Fairy Wren (not to be confused with the equally floridly named Splendid Fairy Wren) is a bit more clever. As soon as the host mother lays her eggs, she begins to sing to them in a very specific pattern. Now, in this case, the cuckoo egg will hatch around the same time as her own eggs, but was deposited there several days later than her own. This means that her own chicks have been sitting there, unborn, learning her song for a longer period of time than the cuckoo has. Once the eggs are hatched, only her own chicks will be able to properly replicate her calls. Can’t sing the song? No food for you. And if, prior to starving to death, the parasite chick does manage to push her chicks out of the nest, the mother will fail to hear the proper response at all and know to simply abandon the nest entirely. Advantage: Fairy Wren. Superb indeed.

Shrikes: don't try to outsmart a bird that kills mammals for sport.(Via: Arkive.org) — Shrikes… don’t try to outsmart a bird that kills mammals for sport.
(Via: Arkive.org)

There is at least one known case of a former host species throwing off the yoke of cuckoo parasitism entirely. The red-backed shrike, aside from being particularly murderously aggressive toward adult cuckoos (and many other things), became very good at identifying cuckoo eggs, very quickly. So quickly, in fact, that researchers believe the cuckoos simply didn’t have time to adapt. In laboratory experiments, the shrikes correctly identified and rejected 93.3% of all cuckoo eggs placed in their nests. Pretty good pattern recognition for a brain the size of a pea. While cuckoo-red shrike parasitism has been known historically for some time, it hasn’t been seen in nature for the last 30-40 years.

Shrikes for the win.

Fun Facts:

Even typically non-parasitic cuckoos will sometimes lay their eggs in the nests of their own or other species, but will still help to feed the chicks (parental guilt, perhaps?).
The eggshells of parasitic cuckoos are unusually thick, helping prevent them from cracking as their mother drops them from above into the host nest.
Striped cuckoos, not content to just shove their adoptive siblings out of the nest, actually peck them to death with their beaks.
A few birds deal with homicidal cuckoo chicks by building steep-sided nests, making it difficult for any chick to be pushed out (and raising them as one big, happy family, I guess).

Says Who?

Colombelli-Négrel et al. (2012) Current Biology 22: 2155-2160
Feeney et al. (2012) Animal Behaviour 84: 3-12
Lovaszi & Moskat (2004) Behaviour 141(2): 245-262
Spottiswoode & Stevens (2012) American Naturalist 179(5): 633-648
Wang & Kimball (2012) Journal of Ornithology 153: 825-831

March 1, 2013

The Devil You Know, the Devil You Don’t

Common Name: The Tasmanian Devil

A.K.A.: Sarcophilus harrisii (Family Dasyuridae)

Vital Stats:

Latin name translates to “Harris’s Meat Lover” after naturalist George Harris
Weigh 6-13kg (13-29lbs.), around the size of a small dog
Largest carnivorous marsupials in the world after the extinction of the thylacine in 1936
Live up to five years in the wild; fully grown at two years of age

Found: On the Australian island-state of Tasmania

It Does What?!

Spins around in circles and chases talking rabbits, if the cartoons are to be believed. But Tasmanian devils have suffered from some bad press over the years. While they’re often portrayed as incurably vicious, dangerous creatures, this isn’t really the whole truth. Yes, they can scream like a person getting dismembered. And yes, they’re good little hunters that can take down prey larger than themselves, partly thanks to having the strongest bite per unit body mass of any living mammal. (Crunching through large bones is not a tall order for a Tasmanian devil.) But they just as often scavenge carrion killed by other causes, frequently in the form of roadkill. They don’t tend to attack humans, either (unless that human happens to be dead already). Faced with live humans, devils will usually just hold still and hope you don’t see them, sometimes trembling nervously as they do so. Doesn’t exactly strike fear into your heart, does it?

In fact, more than anything, devils deserve a bit of sympathy (just ask the ‘Stones)… life is tough for them right from the word ‘go.’ You see, Tasmanian devils are marsupial, meaning the young are born very under-developed and must crawl from the birth canal into their mother’s pouch to find a nipple to latch onto while they finish baking. The problem here is, devils give birth to between twenty and thirty babies, but possess only four nipples, which aren’t shared. In fact, they’re effectively stuck in the infant’s mouth from the time they latch on, preventing them from falling out of the mother’s pouch. So as newborn babies, fresh from the womb, they already have as much as an 87% chance of immediate death. That is some harsh selection right there. Somewhat tellingly, the babies can’t open their eyes until three months after their birth, yet come out of the womb with a full (if small) set of claws. You can see where evolution’s priorities were here.

But it doesn’t get much easier for the four that win the nipple race. Tasmanian devils are already working with a rather restricted range, having been hunted to local extinction on mainland Australia around 3000 years ago (probably by dingoes, which aren’t found in Tasmania). Nevertheless, they were doing pretty well in keeping their numbers up and had a healthy population until the mid-90s, when disaster struck.

Because the entire Tasmanian population of devils was originally based on only a few individuals, they’ve experienced a ‘Founder Effect,’ which basically means that the genetic diversity from one animal to the next is quite low. In terms of disease, they’re all susceptible to the same things. So when a form of transmissible cancer known as Devil Facial Tumour Disease (DFTD) suddenly popped up in 1996, it spread like wildfire from one devil to the next, mostly via their tendency to bite one another during sex and mealtimes.

An infected devil quickly develops tumours on its face and inside its mouth. This eventually makes it difficult to eat, leading to starvation within a year of contracting the disease. DFTD is estimated to have already killed up to 50% of all devils, rushing them from a healthy population to an endangered species in record time. While the government has taken the step of building up a healthy, captive population which will be isolated from the disease, in the long term, this will have the effect of reducing the species genetic diversity even further. As a small glimmer of hope, researchers are now reported to have found a few individuals with at least partial immunity to the disease, and hope to try to build a cure based on their physiology.

Fun Facts:

Tasmanian devils store fat reserves in their tails… a fat-tailed devil is a healthy devil.
See the white spots on the devil pictured above? All bite marks. Each scar leaves a patch of white fur. The natural white streak on the devil’s thick-skinned chest is thought to draw attacks away from more sensitive areas.
Unlike most other marsupials, the devil’s pouch opens to the rear of her body rather than the front (like a kangaroo), making it impossible for her to interact with her babies while they’re nursing there.
Devils tend to try to eat whatever’s available when they’re hungry. The following have been found in their droppings: steel wool pot scrapers, tea towels, parts of leather shoes, blue jeans, plastic fragments, dog collars (minus the unfortunate dog that had been in it), and echidna spines.
The only other known form of non-viral, transmissible cancer is a type of venereal disease that occurs in dogs.

Says Who?

Attard et al. (2011) Journal of Zoology 285: 292-300
Coghlan (2012) “’Immortal’ Tasmanian devil brings vaccine hope” New Scientist, 17 February
Grzelewski (2002) Smithsonian 68: February
Hamede et al. (2013) Journal of Animal Ecology 82: 182-190
Hesterman et al. (2008) Journal of Zoology 275: 130-138
Marshall (2011) “Tasmanian devils were sitting ducks for deadly cancer” New Scientist, 27 June

February 21, 2013

Axolotls in Never Never Land

Common Name: Axolotls

A.K.A.: Ambystoma mexicanum

Vital Stats:

Grow to a length of 15-45cm (6-18”)
Can live up to 15 years
Have no eyelids
Usually black or brown in colour, but mutation occasionally produces pink skin
Eat insects, worms, and small aquatic animals
Commonly kept as pets and, in parts of Mexico, food

Found: In the Xochimilco lake system, near Mexico City

It Does What?!

Axolotls are the Lost Boys of the amphibian world… they never grow up. These bizarre little salamanders are found only in a single lake system near Mexico City and, if the city’s pollution gets much worse, may soon not be found there, either.

First, a little background on salamanders in general. These amphibious, lizard-like creatures begin life in a larval stage. While adult salamanders have lungs and spend much of their time out of the water, larvae have only gills and are completely aquatic. They commonly undergo a metamorphosis in which the gills are lost and the body changes shape, thinning out and losing its ‘tadpole with legs’ appearance. Many salamanders have displayed the ability to occasionally forego metamorphosis, remaining in their larval stage for life. This phenomenon of looking like a juvenile even during adulthood is called “neoteny.”

What makes axolotls special is that they’re what’s called “obligate neotenes,” meaning they simply never go through metamorphosis… every adult axolotl looks like the larval stage of other salamander species. At some point in their evolution, it became either more beneficial or downright necessary for them to remain aquatic. Biologists have speculated that this is because their smaller larval form requires less food, and because the lakes where they live are low in iodine, an element required for their transformation.

Interestingly, while axolotls almost never go through metamorphosis in the wild, in a certain percentage of them, the genetic instructions for doing so seem to still be intact. If you have a larval axolotl and you want an adult form, you can either give it an injection of iodine, or, for the more deranged among you, gradually deprive it of its pool of water. Either method of forced metamorphosis has a high mortality rate and, at best, causes a hugely decreased lifespan, but it does show they haven’t entirely lost that capacity.

caption(From:) — The future of multi-tasking.
(From: McCusker & Gardiner (2011) Gerontology 57: 565)

An eternally youthful appearance isn’t even the axolotls’ only superpower. The creatures also possess a Wolverine-like ability to heal themselves. Not only can they – and other salamanders – regrow lost limbs, they can actually regenerate parts of vital organs, including sections of the brain, spinal cord, and, in one study, up to 50% of the heart ventricle. Axolotls can also receive organ transplants from other individuals without rejection or problems with lack of function in the new tissue. Obviously, these traits have made them of intense interest to a certain species which doesn’t regrow limbs, hearts, or spinal cords. Researchers hope that by studying the genetic and biochemical basis of these heightened healing abilities, they can create their own army of X-Men help amputees and victims of spinal cord injuries. But this research is still in its early stages. In the meantime, it would probably be in our best interests not to drive them to extinction.

Fun Facts:

Axolotls have tiny vestigial teeth, which in other salamanders only grow during metamorphosis.
Sometimes, an axolotl with a heavily damaged limb will both repair the old limb and regrow a new one, ending up with an extra leg (see above).
Forced metamorphosis can be only half-successful, producing adult forms with juvenile characteristics, such as a thickened neck.
Obligate neotenes like axolotls end up with a lot of extra “junk” DNA [biologists: via duplications of the pseudogenes created when their life history changed], which has actually resulted in their having larger cells than other salamanders.
It’s hard not to look crazy when you have no eyelids.
(Via: Aquadisiac News)

Says Who?

Chernoff (1996) International Journal of Developmental Biology 40: 823-831
Martin & Gordon (1995) Journal of Evolutionary Biology 8: 339-354
Neff et al. (1996) International Journal of Developmental Biology 40: 719-725
Rosenkilde & Ussing (1996) International Journal of Developmental Biology 40: 665-673