Which came first, the rattle or the rattling?

Whether or not you’ve seen one in person, you’re probably familiar with the way a rattlesnake uses its rattle. When cornered, a rattler takes a ready-to-strike posture, stiffens its muscles, and quickly vibrates its tail. Of course on the end of that tail is a rattle, or as the authors describe it in a recent paper published in The American Naturalist, “interlocking segments of keratin that fit loosely within one another.” (Keratin is a protein that also makes up your hair, nails, outer layer of skin…and a snake’s scales.) The distinctive rattling sound that’s made when these segments rapidly vibrate against one another causes many animals (including humans) to instinctively freeze and slowly back away hoping to avoid a nasty bite. It’s a very effective warning signal.

western diamondback

A western diamondback rattlesnake (Crotalus atrox) in Arizona, rattling its tail to scare away a perceived threat – presumably the photographer.
Photo from Allf et al. (2016)

Warning Signals

It turns out the behavior of tail rattling is much more common than the rattle itself. Rattlesnakes belong to the Viparidae family of snakes, all of which have long hollow fangs that inject proteolytic (protein-degrading) venom into their prey victims or some other animal stupid enough to mess with a rattlesnake. The rattle on the tip of a rattler’s tail helps it send a clear warning to stay away to avoid those fangs. A second family of snakes called Colubridae contains about two-thirds of all snake species, and they’re generally harmless to anything that’s too big to strangle. This is why you’ll find many kinds of small colubrids—kingsnakes, corn snakes, garter snakes, etc.—in any pet store that happens to be into the whole reptilian thing.

I owned a California Kingsnake (for 21 years, she passed away this past April) and I had seen her rapidly vibrate her tail a few times when she seemed to feel threatened. As someone who studies marine invertebrates, I’m not much of an expert on the biology of snakes. But I did know that kingsnakes don’t have rattles on their tails like rattlesnakes*, so I was a bit puzzled when I saw my kingsnake “rattle” her tail. It made me wonder whether the behavior had evolved before rattles themselves, but my wondering pretty much stopped there and I went on with my own non-snake research.


The late Irma (1995–2016), my California kingsnake (Lampropeltis getula californiae).

Now researchers at University of North Carolina have done much more than just wonder about tail-vibrating behavior in non-venomous snakes. They subjected the idea to some hypothesis testing, showing that the action of a snake “rattling” its tail did indeed evolve before the rattle itself.

Reconstructing the Past

To figure this out, they took a phylogenetic tree (like a family tree, but representing the evolution of a bunch of different species) of snakes from a previous study and mapped the behavioral trait onto the tree. This statistical method, also known as ancestral character state reconstruction, uses some fancy math to calculate the likelihood that a given trait existed in the common ancestors of living species.

So let’s take a look at the phylogenetic tree in Figure 2 of Allf et al. (2016)1. First of all, each name on the right represents a different species of snake, and the circle next to the name shows whether that species vibrates its tail (white) or does not (black). Notice that all but three species included in the tree exhibit the behavior of tail vibration; even vipers that don’t have rattles and the non-venomous colubrids. Only the group of rattlesnakes has a rattle on its tail. At first glance it seems obvious that the behavior is older than the morphology, but it’s important to test these things statistically… 


Figure 2. Phylogenetic tree of snakes species in the families Colubridae and Viperidae, showing how tail-rattling behavior has evolved.
As shown in Allf et al. (2016)

Now follow the lines (branches) next to each species, from right to left. Every time two branches come together you’ve reached a node, which represents the common ancestor of those two species. A pie chart on each node shows the probability of that ancestor possessing the trait of tail vibration. You should quickly notice that almost every ancestor has a completely white pie (a single black line in it as opposed to a black wedge) indicating a nearly 100% chance that that ancestor vibrated its tail. These likelihood values provide statistical support for what already seemed obvious, that the behavior of tail vibration evolved long before the physical rattle of rattlesnakes.

But there’s more to this study than simply confirming that rattling evolved before rattles. Seeing how a particular morphological trait functions often makes perfect sense at first glance: wings are great for flying, eyes great for seeing, teeth great for biting, etc. But how do these highly specialized structures arise in the first place? In other words, how does evolutionary novelty come about?

Evolution is NOT Random

There is a common misconception that evolution is completely random, causing many people to be skeptical of how such a wide variety of well-functioning organisms could have evolved on Earth without some sort of divine intervention. You wouldn’t expect to build a computer by randomly throwing circuit boards and wires at each other, right? So how could evolution produce such a huge variety of organisms so perfectly suited to their environments, with no predetermined design or direction? It’s because the view that evolution is 100% random is wrong, there’s more to it than that.

Endless forms

It’s difficult for some people to imagine how such a variety of plant, animal, and microbial forms of life have evolved. Behavior-facilitated adaptation is just one way that evolutionary novelty can come about.
Illustration by Ainsley Seago (www.trufflebeetles.com)

We do know that genetic variation—a crucial component of evolution—arises completely at random (for the most part) by the process of mutation. Most mutations are random mistakes made by cells as they make copies of their DNA before dividing, and the vast majority of mutations have effects that are usually either negative or neutral. It’s very rare for a random mutation to actually improve the survival or reproductive abilities of the organism who gets it.

But another critical piece of the evolutionary process is natural selection, which is anything but random. Natural selection is extremely effective at teasing out even the most subtle advantages among individuals. This means that if a trait has any influence (no matter how small) on increasing the survival and reproductive chances of some individuals relative to others, then natural selection is said to favor that trait. This causes a beneficial mutation to become more and more common each generation, until it belongs to every member of the species. Natural selection is so good at doing this that it can favor changes that are imperceptible to even the most observant scientists with the most expensive instruments.

Can Environment Induce New Adaptations?

Despite the extreme non-randomness of natural selection, it can still only work with what it’s given. In other words, while the environment is incredibly effective at causing species to adapt over time by favoring individuals with the best traits, it can do nothing to make those traits appear in the first place. Or can it?

Well, the rattlesnake study says…sorta maybe. This is not to say the environment directly induces beneficial DNA mutations in an organism (except epigenetics now tells us that it sort of can), but it does appear that tail-rattling behavior provided the perfect backdrop for evolution of the rattle itself. The authors suggest that this might have happened in one of two ways.

The first hypothesis is that tail-rattling behavior as a warning signal simply provided a situation that allowed a rattle to evolve. Let’s say genetic variation among individuals who all rattled their tail meant that some snakes produced a little more keratin on the tips of their tails than others. This could cause these individuals to make a bit more noise when they vibrated their tails, making them a bit better at startling predators. These individuals would survive longer and leave more offspring, some of which would produce even a bit more keratin tissue on the tail than their parents. Continued selection for increased keratin could eventually lead to the fully formed rattle as it exists today. If tail vibration hadn’t existed first, then selection never would have favored individuals who happened to grow a bit more tail keratin.

Alternatively (and more remarkably IMO), rattling behavior may have actively facilitated rapid evolution of the rattle. Like, actually caused the rattle to form. In this scenario, tail vibrations is hypothesized to have caused a sort of callus to form on the tip of the tail. Snakes that vibrated their tails more rapidly and for a longer duration would have developed larger calluses. If certain mutations made some snakes more susceptible to callus formation than others, and if these individuals were also better at deterring predators, then selection would favor callus formation associated with tail vibration.

This would mean that the environment helped cause the formation of the rattle as opposed to simply favoring random mutations that gradually made the rattle bigger. It may sound like a subtle distinction, but in evolutionary biology it’s huge.

While the researchers are not yet able to say which scenario is more likely, more results are consistent with the latter hypothesis. When they mapped two aspects of tail vibration onto the snake phylogeny, they found that duration was longest and intensity was highest in rattlesnakes and their close ancestors:

Allf 2016 7

From Allf et al. (2016)

But if tail calluses only developed when snakes behaved a certain way (i.e. rattled their tails), then we wouldn’t expect them to pass those calluses on to their offspring. That would be similar to your kids inheriting the big muscles you earn by hitting the gym every day. This type of evolution where an individual acquires a trait during its lifetime and passes it to offspring is called Lamarckian inheritance, and it’s something evolutionary biologists disproved decades ago (although read more on epigenetic Lamarckism if you really want to fall down a rabbit hole).


A classic example of Lamarckism is the giraffe acquiring a longer neck during its lifetime and passing on that trait to its offspring. We now know evolution doesn’t work this way (mostly).

Of course reality is rarely as simple as we’d like it to be. While natural selection may not have directly induced mutations that grow rattles, it could have provided a strong advantage to snakes that rattled their way to callused tails. And if this selection pressure remained strong for a long period of time, the stage was set to favor any mutation that may have caused even a tiny bit of extra tail keratin formation, independent of the behavior itself. We’d call this genetic assimilation, where a trait that originally forms in response to environmental stimulus eventually becomes hard-wired by the genome. In other words, formation of rattles (the noun) became programmed into the development of rattlesnakes even though their ancestors had to work hard for it, by rattling (the verb).

It may seem counterintuitive that the use of a tool could evolve before the tool itself, but future studies may show that’s it far more common in nature than we now realize. While a resurgence in neo-Lamarckism isn’t likely, it is possible that behavior can be an effective precursor, and even facilitator, of the evolution of novel structures.

*The “king” in kingsnake references the fact they often eat other snakes, including rattlesnakes – they’re resistant to rattlesnake venom (which is obviously badass).

1Bradley C. Allf, Paul A. P. Durst, and David W. Pfennig,“Behavioral Plasticity and the Origins of Novelty: The Evolution of the Rattlesnake Rattle,” The American Naturalist (Volume & page numbers not yet available) DOI: 10.1086/688017


Ants can medicate themselves when they’re sick

Whether swallowing Excedrin® for a headache or sipping a cocktail to take the edge off, you’ve likely self-medicated more than once in your life. Similar to written language and advanced tool use, ingesting an otherwise harmful substance for the purposes of curing what ails you is a behavior usually considered to be uniquely human. But it turns out ants can self-medicate, too. A recent study showed that common black ants changed their diets when they were sick—eating more food supplemented with a medicinal substance—as compared to ants with no infection.


The common European black ant, Formica fusca, can alter its behavior to self-medicate when it’s sick.
Photo by Mathias Krumbholz via Wikimedia Commons

Do animals get high?

You may have heard stories of elephants getting drunk by eating fermented fruit or of dolphins getting high off pufferfish toxin – but as much as people might like these stories to be true, they’re actually driven more by myth than by the reality of evolution.

It shouldn’t be surprising that natural selection would not favor a tendency to get high. Being successful in a predator-prey interaction, for example, requires being substantially more alert than a stoner on their couch eating pizza rolls and watching Netflix.


Intentional inebriation may enhance the movie-watching experience for Harold and Kumar in their living room, but it’s not a particularly good evolutionary strategy.
Image copyright of Warner Bros.

But what about legitimate medicine? Disease is pervasive in the natural world. If a creature could cure itself of infection by ingesting a substance naturally available to them—and if this behavior was genetically inherited—then we could easily expect the trait to spread through a population over time (in other words, we’d expect the species to evolve the ability to self-medicate).

A study by researchers at University of Helsinki in Finland (published online for an upcoming issue of the journal Evolution) has demonstrated this sort of therapuetic self-medication in the common European black ant, Formica fusca. They found that the ants will intentionally ingest hydrogen peroxide (H2O2) in the presence of a fungal pathogen, and that the otherwise toxic substance helps them recover from the infection.

Specific criteria for self-medication

To show that this behavior is genuinely medicinal, some specific criteria needed to be met. If the ants ate a non-toxic substance that just happened to prevent infection, for example, this could potentially be eaten by all ants as part of their diet—regardless of pathogen exposure—and would not qualify as medicine. In order to be considered self-medication, the ingested substance must: (1) increase the fitness of infected ants (i.e., make it healthier), (2) have a negative effect on uninfected ants, (3) have a negative effect on one or more pathogens, and (4) be deliberately consumed by the ants.


Two metal ants holding a bottle of wine and looking happily medicated.
Image from http://www.frbiz.com, a website that sells things.

In a cleverly designed study, researchers found that their study subjects fulfilled all four of these criteria. First, they divided ants into two colonies, one of which was fed a standard honey-based food while the other colony was fed the same food, but supplemented with hydrogen peroxide (they use the more general term reactive oxygen species, or ROS, in the paper). Each of these groups was then divided into two, one exposed to the common fungal pathogen, Beauveria bassiana, and the other kept as a fungus-free control group. They tracked daily mortality rates of all four groups for 12 days and found that medicated ants survived at a higher rate than non-medicated when exposed to the fungus, but the opposite was true for the pathogen-free control groups. This satisfied criteria 1 and 2.

Bos 2015 fig1

Graph showing that ingesting H2O2 (dotted lines) increased the survival of fungal-infected ants (black arrow) while decreasing survival in healthy ants (gray arrow).
Figure 1 from Bos et al. (2015)

To demonstrate that these differential survival rates were the direct result of H2O2 killing the fungal infection, they cultured the fungus in petri dishes with six different concentrations of H2O2, including 0%. This part of the experiment showed a steep decline in the survival of the fungus as the concentration of H2O2 increased, satisfying criterion 3.

Bos 2015 fig2

Graph showing the rapid decline in the survival of the fungal pathogen BB in the presence of increasing concentration of H2O2.
Figure 2 from Bos et al. (2015)

Finally, for the most interesting part, the authors tested whether ants would deliberately put themselves into contact with H2O2 after being infected with the fungus. If they did, this would demonstrate that the ants are able to distinguish between medicinal and non-medicinal food, and that they only eat the medicine when they’re sick. You probably know where this is going: when given a choice, infected ants indeed increased their consumption of H2O2-supplemented food, while non-infected ants largely avoided it.

Bos 2015 fig3

Graph showing that, when given a choice, up to 50% of fungal-infected ants deliberately consumed food supplemented with H2O2, while no more than ~20% of non-infected ants were ever observed doing the same. The results also suggest ants adjust the amount of medicine they take – consuming more of it at 4% than at 6% concentration.
Figure 3 from Bos et al. (2015)

It was a very simple and straightforward experiment, but one that carefully demonstrated all the criterion necessary to say that these ants successfully self-medicate based on the presence of infection. While this work was done in the lab, the authors are careful to point out that ants are also likely to encounter reactive oxygen species such as H2O2 in nature, as they can be present in many ant food sources such as nectar and cadavers.

Free meds

The authors even speculate that the phenomenon may be part of a mutualistic association between aphids and ants. Aphids secrete a substance called honey dew as they feed on plants, and ants are known to feed on the honey dew. Because plants also produce H2O2 as a defense against aphid predation, infected ants could potentially use the honey dew food source as a way to adjust their intake of H2O2.

Also posted at: http://www.planetexperts.com/self-medicating-ants/

Nick Bos, Liselotte Sundström, Siiri Fuchs, and Dalial Freitak. 2015. Ants medicate to fight disease. EvolutionDOI: 10.1111/evo.12752

5 (more) Misconceptions about Evolution

I recently came across this excellent post debunking 5 common misconceptions about evolution (hat tip to Joe Hansen at It’s Okay to be Smart). It’s a great infographic, and in light of the recent debate about whether creationism is compatible with science, between young-Earth creationist Ken Ham and Bill Nye the Science Guy, I thought I’d take this opportunity to add five more.

Full disclosure, the five misconceptions I’m writing about here are indeed common, but the inspiration was not all mine. This list is actually presented in The Tangled Bank, by the prolific science writer Carl Zimmer. It’s a great book for a non-biology major evolution course (I happen to be using it for just that right now), or for anyone outside the field who would like to learn more about evolution. No paid promo here, it’s just my opinion.

One thing that clearly unites every argument denying evolution: they are nearly always some form of a straw man argument. With ever-growing mountains of overwhelming evidence to support the theories behind evolution, this is really the only type of argument available to creationists. They must make false claims about evolution that they then easily take down to give their arguments the appearance of credibility.


Any time a creationist makes an argument to discredit evolution that seems on its surface to be scientifically compelling, this is probably how they did it. (Cartoon by history professor Dr. James MacLeod)

So in addition to the 5 misconceptions listed by Molecular Life Sciences, I figured it would be useful to add 5 more to the list (with no claims about whether these fit at the top, bottom, or middle of such a list). It turns out that most denials are so weak, they can be refuted in short order. So let’s get started…

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Reef creatures at night in Mo’orea, French Polynesia

While I was grad student at UCLA, I twice had the opportunity to TA a field course in Mo’orea, a small island next to Tahiti in French Polynesia. These are some photos I snapped on a night dive last time I was there, in the spring 2012. Enjoy!

An anemone hermit crab at night.
From Wikipedia: Dardanus pedunculatus usually lives on coral reefs and in the intertidal zone, at depths of 1–27 metres (3–89 ft). It usually carries sea anemones on its shell, which it uses to protect itself from its main predator, cephalopods of the genus Octopus. The anemones are collected at night, and comprises the crab stroking and tapping the anemone until it loosens its grip on the substrate, at which point it is moved onto the gastropod shell that the hermit crab inhabits.

A parrotfish in its mucus “sleeping bag.”
Many species of parrotfish surround themselves with a mucus bubble at night. It is thought to protect them from predators by keeping their scent out of the water and possibly provide an early warning system if the mucus is broken by an approaching predator. I wasn’t able to identify this particular species, suggestions are welcome.

The chocolate tang Acanthurus pyroferus.
Tang and surgeonfish (family Acanthuridae) have a sharp scalpel-like blade on their caudal peduncle (the part right in front of the tail) that they can whip around to slice a predator.

The banded cleaner shrimp Stenopus hispidus.
Again, from Wikipedia: “Stenopus hispidus lives below the intertidal zone, at depth of up to 210 metres (690 ft),on coral reefs. It is a cleaner shrimp, and advertises to passing fish by slowly waving its long, white antennae. S. hispidus uses its three pairs of claws to remove parasites, fungi and damaged tissue from the fish.”

The urchin Diadema savignyi.
No much to say about this guy, just don’t step on one. The spines are every bit as sharp as they look.

A moray eel. I think the undulated moray Gymnothorax undulatus? 

The ass end of one of the most beautiful fishes I’ve ever seen. Some kind of gurnard?

The spotfin lionfish Pterois antennata, I think.
Also viewed from the posterior. This species has other common names, but who cares?

A Jellyfish’s Offense is a Sea Slug’s Defense

Arms races between predators and their prey have been common in the evolution of life. Gazelles run a little faster to help them escape cheetahs, then of course the cheetahs speed up to keep catching their meals. Bivalves (clams, scallops, etc.)  and snails have evolved some tough and elaborate shells to protect themselves, and in response the crabs, octopuses, and fishes that eat them have developed incredibly strong claws, drills, or teeth to crush, open, or bore into the shells. And of course toxins can be synthesized to ward off all sorts of enemies looking to take a bite out of you (the slow loris is by no means the only animal to do this, but it may be the cutest).

navanax eating

A cephalaspidean sea slug of the genus Navanax devours an aeolid nudibranch slug. Aeolids can sometimes steal stinging cells from their prey, jellyfish and their relatives, as a defense against predators, but that doesn’t seem to have worked in this case.
Image taken from http://www.uwphotographyguide.com/california-marine-life

But what if you’re just slow and squishy? In the case of nudibranch sea slugs, some of them make up for this vulnerability by eating cnidarians (jellyfishes, coral, sea anemones), stealing their stinging cells, and planting them in their own tissues, ready to fire at an unsuspecting predator. The process is called kleptocnidae, and even though it was discovered almost 100 years ago, it’s been the subject of very little study since then.

A while back, I wrote about kleptoplasty. It’s the process whereby sacoglossan sea slugs steal chloroplasts from their algal prey and use them for photosynthesis, essentially turning themselves into solar-powered animals. However, see a recent study calling into question the extent to which the slugs use photosynthesis for survival.

Regardless of exactly how important solar power may or may not be for the survival of sacoglossans, nudibranchs have arguably taken the art of stealing from their prey to a different level. Nudibranchia is the best known group of sea slugs, largely because their patterns are so colorful that they often look more like cartoon characters or art projects than real animals. But we all know beauty doesn’t necessarily make you nice, and nudibranchs could be considered masters of deception in that regard. In contrast to the herbivorous (i.e. vegan) sacoglossans, nudibranchs are voracious predators, devouring sponges, corals, bryozoans, or other sea slugs – some will even cannibalize members of their own species!

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Dear PBS and Joe Hanson, it’s okay to not be sexist

Part of what I enjoy about being in science academia is the fact that I’m generally surrounded by like-minded people. For the most part, my peers and colleagues hold progressive views about politics and society. They are environmentalists, they are proponents of social justice, they support gay rights, and they generally strive for improving diversity in the sciences. This is a generalization, of course, and there will always be individuals who are at least ignorant to one or more of these issues or at worst detrimental to the cause. Not to mention that the system itself still harbors a high degree of its sexist and racist history – the vast majority of professorships are still given to white men and we’ve got a long way to go to move beyond that. But generally speaking, especially among the younger scientists of my generation, there is at least an overall desire to promote both equality and diversity among scientists while also making attempts to bring science to a widely diverse audience.

Screen Shot 2013-11-16 at 8.51.45 AM

Einstein and Marie Curie at Thanksgiving dinner before the bobblehead father of relativity decides to sexually assault the likeness of the one of the greatest chemists of all time.

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Solar-powered sea slugs are energy-efficient thieves

Every animal needs energy to live, and they usually get it by ingesting other living things. We’ve all seen videos of predators hunting down and killing their favorite prey, or docile herbivores munching on grass, leaves, or berries. But one group of animals has evolved a strategy to harness energy that’s a bit more complex and far more clever than simply finding something and eating it. Sacoglossan sea slugs are close relatives of the more colorful and better-known group of slugs called nudibranchs.

Nudibranchs are carnivorous – they prey on sponges, cnidariansbryozoans, and even cannibalize other members of their own species. Sacoglossans, on the other hand, are more like their hippie vegan cousins. They feed on green algae using a specialized tooth-like feeding structure common to all snails and slugs called a radula. A sacoglossan sea slug uses its radula to pierce cell walls, allowing it to suck out all those sweet, sweet algal juices.

Close up view of functional chloroplasts in the tissues of Elysia crispata.
Photo courtesy of Patrick J. Krug.

Okay, so some sea slugs are vegan, but what’s so clever about that? Continue reading