A scanning electron microscope image of an American house dust mite.
Our world is quite literally lousy with parasites. We are hosts to hundreds of them, and they are so common that in some ecosystems, the total mass of them can outweigh top predators by 20 fold. Even parasites have parasites. It’s such a good strategy that over 40% of all known species are parasitic. They steal genes from their hosts, take over other animals’ bodies, and generally screw with their hosts’ heads. But there’s one thing that we believed they couldn’t do: stop being parasites. Once the genetic machinery set the lifestyle choice in motion, there’s supposed to be no going back to living freely. Once a parasite, always a parasite.
Caffeine has been a part of human cultural heritage for more than five thousand years. From ancient teas and coffees to todays energy drink craze, you could say that as a species, we’re hooked. But we’re not the only ones — a new study published in Science today has found that pollinators get a daily buzz off caffeine, too, and it keeps them coming back for more.
“Oh, beauty is a beguiling call to death, and I’m addicted to the sweet pitch of its siren.” – Johnny Quid, RocknRolla
Female of the Emerald cockroach wasp Ampulex compressa manipulating an American cockroach, Periplaneta americana, which has been made docile by wasp venom and that will serve as food for the wasp larva. Image courtesy of Gudrun Herzner.
Glinting in shimmering shades of blue and green, the emerald cockroach wasp is surely a thing of beauty, but its shimmering exterior masks its cruel nature. The emerald cockroach wasp is one nature’s most impressive neurochemists. At its core, it is a parasite. The female wasp lays her eggs on a cockroach host, and when they hatch, the larvae eat the creature from the inside out. You’d think the cockroach would be opposed to this idea, but instead the insect patiently awaits its fate while the larvae mature. Cockroaches are much larger than even a full grown wasp, and certainly could put up a fight, but that is where the wasps’ ingenious manipulation of neurochemistry comes in. When she encounters a potential host, the female cockroach wasp first stings the cockroach in its abdomen, temporarily paralyzing its front legs and allowing the wasp to perch precisely on its head. She then stings the roach again, this time delivering venom directly into a part of the roach’s brain called the sub-esophageal ganglia. This doesn’t kill the roach. Instead, it puts the roach in a zombie-like trance. The roach is less fearful and loses the will to flee. It allows the wasp to lead it by its antennae, like a dog on a leash, to the wasp’s burrow where the roach will play the martyr for the wasp’s unborn children. Even though the roach is fully capable of locomotion during the week to month that passes from when the wasp stings the its brain until the hungry brood finish eating it alive, the zombified insect doesn’t move. Emerald cockroach wasps have elevated neural manipulation to an art form to create perfect living incubators.
But, though the roach has been rendered harmless, the wasp-to-be is threatened by other organisms. Humans aren’t the only species that have to worry about their food spoiling—so do emerald cockroach wasps. Cockroaches truly are dirty creatures, and their insides are home to a suite of bacteria that can harm the wasp’s vulnerable larvae. One of these potential threats is Serratia marcescens, a vile sort of Gram negative bacteria found in cockroach bodies. It’s the same bacteria responsible for a number of human urinary tract infections and the weird pink stains that form in our toilets and showers. In insects, its effects are much more deadly. The bacteria possess a suite of protein-degrading enzymes that cut apart fragile larval cells. The larvae aren’t entirely defenseless, though—as a new study published today in the Proceedings of the National Academy of Sciences reveals, larval wasps sterilize their food by secreting antimicrobial compounds.
For many parasitic wasps, microorganisms are a serious concern. Studies on another wasp, Microplitis croceip, found that contamination with Serratia marcescens can lead to a 25% reduction in successful parasite emergence, and even the young that do survive can be infected. When adults are exposed to the bacteria, almost 80% die. The emerald cockroach wasp must defend against this mortal enemy, or pay the ultimate price.
But how do you protect yourself against bacteria that live inside your food? Well, you do what we do to foods that house potentially harmful pathogens: you sterilize them.
Oral droplets secreted by wasp larvae, figure 1 from the paper
Gudrun Herzner and a team from the University of Regensburg in Germany noticed that larval wasps secrete droplets from their mouths that they disperse around before they feed on their cockroach meal. They suspected these secretions kill off potentially deadly bacteria, allowing the larvae to eat in peace. The researchers tested the antimicrobial activity of the oral secretions to see if they were right.
When added to bacterial cultures from the cockroach, the droplets killed off a wide variety of bacteria, including the potentially deadly Serratia marcescens. But the team wanted to know more: exactly what in the droplets killed off bacteria? So, the researchers isolated the secretions and ran them through gas chromatography–mass spectrometry to determine the nature of the substances in them. They found nine compounds previously unknown from the wasps or the cockroaches. In particular, the secretions contained a large percentage of two compounds, a kind of mellein called (R)-(-)-mellein, and micromolide, a natural product that may hold the key to treating drug-resistant tuberculosis. Both compounds showed broad-spectrum antibacterial activity, and the combination of the two was particularly effective.
As a final test, they extracted parasitized and unparasitized cockroaches and looked for these compounds. From the parasitized cockroaches, the same antimicrobial mixture could be extracted, but not from unparasitized ones. Thus, the scientists were confident that the wasp larvae produce and use these compounds to sterilize their food from the inside out.
“We found clear evidence that A. compressa larvae are capable of coping with antagonistic microbes inside their P. americana hosts by using a mixture of antimicrobials present in their oral secretion,” write the authors. While both compounds used by the larvae have been found in other animals, never before has the combination been discovered, in insects or otherwise.
The broad-spectrum nature of these antimicrobials may be key to the wasps’ success. “Food hygiene may be of vital importance, especially to the vulnerable early developmental stages of insects,” explain the authors. “The range of microbes that A. compressa larvae may encounter during their development in their hosts is unpredictable and may encompass all different kinds of microbes, such as various Gram-positive and Gram-negative bacteria, mycobacteria, viruses, yeasts, and ?lamentous fungi.” It would be vital, then, that the antimicrobial compounds produced by the wasps are able to fend off a wide variety of potential contaminants. “The secretion of a blend of antimicrobials with broad-spectrum activity seems to represent an essential frontline defense strategy.”
These beguiling wasps not only have mastered neurochemistry, they have aced microbiology to become proficient parasites. Already, this tiny wasp has given us great insights into brains through the study of its particularly effective zombification strategy. Now, it is shedding light on another field of science. This study is one of the first to suggest that larval parasites possess the ability to protect themselves against microbial pathogens, but the authors suspect many species of insects may have similar strategies. These small creatures may prove a vital new resource for natural products to fight against human diseases. Who knows what other pharmaceutical secrets are being kept by insects like the emerald cockroach wasp, and what ailments we might be able to treat with their chemical arsenal.
Citation: Herzner G., Schlecht A., Dollhofer V., Parzefall C., Harrar K., Kreuzer A., Pilsl L. & Ruther J. (2013). Larvae of the parasitoid wasp Ampulex compressa sanitize their host, the American cockroach, with a blend of antimicrobials. PNAS Early Edition, DOI: 10.1073/pnas.1213384110
There’s a lot to be said for smarts—at least we humans, with some of the biggest brains in relation to our bodies in the animal kingdom, certainly seem to think so. The size of animal brains is extravagantly well-studied, as scientists have long sought to understand why our ancestors developed such complex and energetically costly neural circuitry.
One of the most interesting evolutionary hypotheses about brain size is The Expensive Tissue Hypothesis. Back in the early 1990s, scientists were looking to explain how brain size evolves. Brains are exceedingly useful organs; more brain cells allows for more behavioral flexibility, better control of larger bodies, and, of course, intelligence. But if bigger brains were always better, every animal would have them. Thus, scientists reasoned, there must be a downside. The hypothesis suggests that while brains are great and all, their extreme energetic cost limits their size and tempers their growth. When it comes to humans, for example, though our brains are only 2% of our bodies, they take up a whopping 20% of our energy requirements. And you have to wonder: with all that energy being used by our brains, what body parts have paid the price? The hypothesis suggested our guts took the hit, but that intelligence made for more efficient foraging and hunting, thus overcoming the obstacle. This makes sense, but despite over a century of research on the evolution of brain size, there is still controversy, largely stemming from the fact that evidence for the expensive tissue hypothesis is based entirely on between species comparisons and correlations, with no empirical tests.
With beauty and brains, Poecilia reticulata help give insights into cognitive evolution
A unique study published this month in Current Biology has taken a new approach to examining this age old question. Rather than comparing species with bigger brain-to-body ratios to smaller-brained relatives, they exploited the natural variation of brain size in guppies (Poecilia reticulata). Guppies, as it turns out, aren’t as dumb as they look. They’re able to learn, and show rudimentary ability to count. Researchers from Uppsala University in Sweden were able to use their numerical abilities to test whether brain size affects intelligence in these simple fish.
Fig. 2 from the paper; Large-brained females outperform small-brained females in a numerical learning task. Depicted are the mean and standard error values for the number of times, out of eight tests, that an individual chose the correct option of either two or four objects in females and males selected for large and small brain size.
First, the team selected for larger and smaller brains from the natural variation in guppies. They successfully created smarty-pants guppies that had brains about 9% larger than their counterparts through artificial selection. Then, they put them to the test. While the males seemed to gain no benefits from possessing larger noggins, the females with bigger brains were significantly better at the task.
But what was really remarkable was the cost of these larger brains. Gut size was 20% smaller in large-brained males and 8% smaller in large-brained females. The shrunken digestive system seemed to have serious consequences reproductively, as the smarter fish produced 19% fewer offspring in their first clutch, even though they started breeding at the same age as their dumber counterparts. And, the authors noted, this was in an idealized tank setting with an plenty of food—what about in the wild, where resources are harder to come by? How much of a cost does a reduced gut have when meals aren’t guaranteed?
“Because cognitive abilities are important to facilitate behaviors such as ?nding food, avoiding predation, and obtaining a mate, individuals with increased cognitive abilities are likely to have higher reproductive success in the wild,” explain the authors. These benefits, though, don’t come cheap. “Our demonstration of a reduction in gut size and offspring number in the experimental populations selected for larger relative brain size provides compelling experimental evidence for the cost of increased brain size.”
There are still many questions to be answered. For example, the authors aren’t entirely sure why females were the only ones to show cognitive improvement with larger brains. They suggest that, perhaps, the researcher’s measure of intelligence (the numerical task presented to the guppies) may be be geared toward female behaviors. “In the guppy, females are more active and innovative while foraging,” they explain. “Because females feed more, they may thus have had more time to associate the cue with food in our experimental design.”
The clear trade-off between brains and guts, though, is an important finding. By providing empirical evidence for the physiological costs of brains, this study provides the ?rst direct support for the expensive-tissue hypothesis, and can provide us with insights into how our own big brains evolved. One of the prevailing hypothesis for our own brain growth is that the incorporation of more animal products into our diets, through hunting or cooking or however, allowed us to obtain more energy from less food, thus offsetting the cost of a reduced gut. The less food we needed to eat for the same amount of energy, the more our brains could grow even if our guts suffered for it. The debate, however, is far from over. Comparative analyses in primates don’t support a gut-brain tradeoff, and there are certainly plenty of other hypothesis as to how and why we developed our massive lobes, and what prices our bodies paid for them.
Kotrschal A., Rogell B., Bundsen A., Svensson B., Zajitschek S., Brännström I., Immler S., Maklakov A. & Kolm N. (2013). Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and Benefits of Evolving a Larger Brain, Current Biology, DOI: 10.1016/j.cub.2012.11.058
A simple white butterfly caterpillar (Pieris rapae) nibbles blissfully on a cabbage leaf, completely unaware of the complex interspecies interactions he has just set in motion. The cabbage, displeased with the damage the caterpillar is doing to its tissues, is releasing volatile compounds into the air, hoping to attract parasitoid wasps like Cotesia glomerata, which use caterpillars like the one eating through the cabbage’s precious leaves as incubators for their larvae—and succeeds. Drawn by the compounds wafting off of the damage plant, a female wasp arrives and finds the defenseless caterpillar. Using a needle-like appendage, she injects her eggs into the caterpillar’s body, and her larvae hatch and feed on the caterpillar’s internal organs one by one, carefully selecting the least important so that their meal survives as long as possible. Finally, when they are ready to pupate, the wasp larvae tunnel out, and through a chemical trick, convince their half-dead host to spin them a protective web of silk. Success, thinks the plant (if plants could think); its cry for help has stopped another hungry caterpillar in its tracks.
Lysibia nana parasitizing cocoons of Cotesia glomerata
But, as Dutch scientists have discovered, the story doesn’t end there. What goes around comes around for the C. glomerata, as there are other wasps that use them as hosts, laying eggs in the wasp larvae that grew in the caterpillar, like a parasitic Russian doll. Researchers have discovered that these hyperparasitoids (parasitoids of parasitoids) can smell the call being broadcast by the plant, too.
After all, the world is a large place. Parasites that need to find a very specific, small host benefit from having a way of finding what they need without wasting tons of energy searching. So it makes sense that Cotesia glomerata and other parasitoid wasps with caterpillar hosts are drawn to the chemical compounds emitted by damaged plants. If they’re drawn, the wasps that parasitize them should be drawn, too. So the team tested this hypothesis by collecting air from undamaged plants, plants damaged by uninfected caterpillars, and plants damaged by caterpillars already infected with parasitiod wasp larvae, then presented those scents to hyperparasitoid wasps to see if they were attracted to them.
Not only were the wasps attracted to the smell of caterpillar damage in general, “we found that they preferentially detected odours of plants damaged by infected caterpillars,” explained Dr Erik Poelman, lead author of the study published today in PLoS Biology. The wasps were nearly five times more attracted to the damage done by infected caterpillars. “We were excited by these results as they indicate that hyperparasitoids rely on a network of interactions among plant, herbivore and parasitoids to locate their host”.
But how did the wasps detected whether the caterpillars were infected? Poelman and his team wanted to find out. It’s known that infection can change the saliva contents of caterpillars, so they took the saliva from uninfected and infected caterpillars and presented those scents to the wasps, but the wasps didn’t care. So while the infection is altering the caterpillar’s saliva, the change in attractive chemicals had to be coming from the plant. They then tested the different air collections for volatile compounds, and found the ones damaged by caterpillars infected with Cotesia glomerata were only 40% similar to the ones damaged by uninfected caterpillars. Something about infection changes the saliva in a caterpillar, which in turn affects what volatile compounds a plant emits when it gets damaged by that saliva.
This complex web of interactions calls in to question the role of the plant compounds in the first place. Though they are often thought of as a ‘cry for help,’ the team noted that this may not be the case at all. “Although plant volatiles may function as a ‘‘cue’’ to parasitoids, they may not be a specific ‘‘signal’’ released by the plant (implying a selective benefit),” write the authors. “It is important to emphasize that volatile cues may provide many community members with information and thereby may not necessarily result in a fitness benefit to plants.”
These findings also call into question the use of parasitoid wasps as biocontrol for managing pests. Cotesia glomarata has been introduced and intentionally released in a number of agricultural areas to control caterpillars like Pieris rapae. Recently, some have suggested that farmers might be able to spray the volatile compounds emitted by damaged plants to attract more parasitoids, as a way of reducing pest populations without using pesticides. But the authors think that this strategy might not be so clear-cut. “Our results show that hyperparasitoids may parasitize up to 55% of the parasitoid offspring, therefore potentially playing a major role in parasitoid population dynamics,” they caution. “Overexpression of herbivore-induced plant volatiles [HIPVs] in crops or field application of synthetic parasitoid attractants may not benefit pest control in conditions where the responses of hyperparasitoids to HIPVs cause major mortality to parasitoids.”
In other words, the interactions between species are far more complex than we once thought, and we can’t assume we can predict how our manipulations will affect a community—which is generally the trouble we’ve gotten into when trying to use biocontrol mechanisms. The more we try to tinker with interspecies interactions, the more unintended consequences we seem to have.
Research: Poelman E., Bruinsma M., Zhu F., Boursault A. & et al (2012). Hyperparasitoids Use Herbivore-Induced Plant Volatiles to Locate Their Parasitoid Host., PLoS Biology, 10 (11) e1001435. DOI: 10.1371/journal.pbio.1001435.t005
Coral under siege by the seaweed Chlorodesmis fastigiata
Just below the ocean’s surface, a war is being waged. Coral reefs are under constant assault by seaweeds which seek to take control, stealing the coral’s prime sunlit location for themselves. Many of these plant invaders come equipped with deadly chemical weapons that knock down the coral’s metabolism, which might come off as an unfair fight against a seemingly unarmed foe. But corals aren’t defenseless; as a new paper in Science shows, corals have fish bodyguards at the ready to mount a defense.
Coral reefs are one of the most productive ecosystems on Earth. They’re also one of the most threatened. While managers and scientists struggle to find ways to protect these precious habitats, coral cover has decreased by 50% along the Great Barrier Reef and 80% in the Caribbean. The losses ripple up the food chain, causing declines in fisheries and ecosystem services. But not all organisms mourn coral declines—when corals struggle, seaweeds reap the benefits.
Corals and seaweeds are in a constant struggle for dominance. On healthy reefs, seaweeds are kept in check by plant-eating fishes and invertebrates which keep the algae from overtaking their coral homes. When these herbivores are lost, like when sea urchins underwent a massive die off in the Caribbean in the 1980s, the algae run rampant, reducing habitat complexity and leaving many fish homeless. Up until recently, it appeared that corals are relatively passive in this ongoing battle. But now, scientists from Georgia Tech have found that corals actively recruit defenders to fight algal invasions.
“We had been studying coral-seaweed interactions to determine which seaweeds were most damaging to which corals and the mechanisms involved,” explains study co-author and professor in the School of Biology at Georgia Tech, Mark Hay. He and his post-doc Danielle Dixson discovered that the seaweed Chlorodesmis fastigiata is particularly chemically toxic to corals, emitting lipid toxins that harm corals that they come in contact with. Yet in Fiji, where the experiment was conducted, corals seemed to be holding their own. Given the important role of herbivores, the team decided to see if the fishes living in the corals might be fighting back on behalf of their homes. So, they took Acropora nasuta colonies (an important reef-building coral) that had resident gobies and removed the fishes from some of them. They then placed algae or an algae mimic made of nylon line on the corals and watched the corals over a few days to see what happened.
The mutualistic fish Gobidon histrio in its home coral Acropora nausuta, coming out to investigate the presence of the toxic green alga Chlorodesmis fastigiata
While the fake algae (which physically covered the coral but lacked the chemical weaponry of the algae itself) had no effect, the corals where algae was transplanted were all damaged by the introduction of the competitive plant. But, the scientists noted, the corals that retained their fish residents were much better off. After three days, the amount of seaweed left on the corals was reduced by 30%, and the corals themselves suffered only 20% – 30% the damage of the corals without their fish colonies.
Assured of the important role of the fishes living in the corals, the team further investigated the interplay between the fish, coral and algae. The team introduced Chlorodesmis fastigiata to corals again, but this time they watched how the fish reacted. Within minutes, small gobies—only inches in size—emerged from their hiding places to pick at and eat the seaweed. “These little fish would come out and mow the seaweed off so it didn’t touch the coral,” said Hay.
But to really understand what was going on, the scientists took water samples from next to undamaged corals, corals damaged by algae while the algae was still present, corals damaged by algae after the algae was removed, and the algae alone away from coral. They exposed gobies to these water samples and watched how they responded. In less than 15 minutes, gobies were drawn to the water from damaged corals, but didn’t react to the chemical signature of algae by itself. “We found that the gobies were being “called to” the area damaged by the algae, and that the “call” was coming from the damaged coral, not from the seaweed.”
“This species of coral is recruiting inch-long bodyguards,” explained Hay. “This takes place very rapidly, which means it must be very important to both the coral and the fish. The coral releases a chemical and the fish respond right away.” The scientists even tested the gobies by using water from seaweed damage of a different but closely related coral species, but the fish didn’t react. “The gobies came to the rescue of their host coral but did not respond to a related coral’s chemical cues,” said Hay.
The gobies aren’t being entirely selfless. Gobies don’t just eat seaweed—they also eat mucus from the coral itself. “The fish are getting protection in a safe place to live and food from the coral,” explains Hay. “The coral gets a bodyguard in exchange for a small amount of food. It’s kind of like paying taxes in exchange for police protection.”
In addition to defending their homes, the team found that one of the little fish species gets an extra defensive boost from eating the invading seaweed. “One of the gobies was known to produce a toxic skin secretion,” explained Hay. “This goby consumed the toxic seaweed and became more toxic,” thus helping to protect it from potential predators. The other main species of goby found in the coral doesn’t have this defense, but it still fought off the attacker. “It trimmed back the seaweed from its host coral but did not consume the seaweed – it apparently just trimmed it and spit it out.”
The scientists were even able to narrow down what in the seaweed is causing the coral’s cry for help. The team took different fractions of seaweed chemicals and applied them to fake nylon mimics. Only the extract containing the known lipid chemical weapon triggered the fish defense system.
“I’m an ecologist that studies chemically-mediated interactions, but the wonderfully subtle, nuanced, and specific chemical dance being conducted here is still shocking to me,” said Hay. He noted that these findings highlight the significance of mutualistic interactions on coral reefs. “Competition among some seaweeds and corals has been important enough to drive the evolution of this wonderfully well-tuned signaling among a coral and its mutualistic fishes.” While similar mutualistic defense systems are well described in terrestrial species, this is the first time such an interaction has been shown in a marine environment.
Hay also emphasized that, at least when it comes to ecosystems, size really doesn’t matter. “Organisms need not be large or abundant to be ecologically important,” said Hay. “These tiny, inconspicuous fishes are important in slowing, or preventing, the damage that seaweeds do to corals and thus are important, but unappreciated, in stabalizing reef corals and preventing coral loss and reef decline.”
Einstein once said that the definition of insanity is doing the same thing over and over while expecting different results. Yet as scientists, we are taught to fundamentally question this assumption. We replicate and repeat with the express purpose of determining if a result is reproducible or merely the product of random chance. As social and emotional creatures, we do the same thing. We like to believe in second chances. We tell ourselves that stochastic circumstances are to blame when things don’t go the way we imagined, so when presented with the opportunity to try again, we often do. Or, at least, I do. But no matter how logical an argument I can make for do-overs, Einstein was right.
In retrospect, I feel like a fool. As I sit at the edge of my bed fumbling with my guitar, I can’t help but blame myself. Why did I choose time and again to trust a person whose actions have always betrayed it? Blinded by love, I had a slew of reasons, a variety of parameters I could change that I thought might affect the outcome. But now, with 20-20 hindsight, I cannot find any. I should have known better, I chide myself. I failed the scientist in me.
Yet still at the slightest mention of him, I flush with anger, jealousy and regret, and heart pounding, I fantasize about retaliation and justice. Evolutionary psychologists would tell me that the physiological experience of betrayal stems from the fact that humans, at our core, are a social species. Personal bonds were vital to our ancestors, and thus natural selection has reinforced emotional mechanisms that evaluate the connections we form with others. In a dangerous world, our ancestors had to know whom they could trust with their lives. Anyone who threatened the relationships we have with one another didn’t just wound pride or break hearts, they threatened our predacessors very survival. The reaction is strong and visceral: stress hormones spike, leading to twisting pain in our gut and heightened sensitivity. But at the same time, areas of our brain involved in deception detection activate. While we feel the rush of cortisol and adrenaline clouding our thinking, brain regions like the anterior insula process our physical and emotional state to make judgements of trustworthiness to inform future interactions.
If only my previous judgements had been more permanent. A friend of mine likes to say “monkeys learn,” but clearly, I didn’t the first time. Though the rest of our evolutionary lineage seems to be quick to categorize friends from foes, I could not.
What’s done is done, though, and I am left to collect the pieces of my heart that they shattered so effortlessly. While I might not have learned my lesson as quickly as I should have, I have learned it now. I know that this time is different. There will be no more replicates, no more re-runs with the hope of a different result. There are no variables I can change to get what I want. The data are clear, and it’s time to stop trying to bias them toward the end I prefer. All that is left is to document what happened, so like a good scientist, I write and record my final results.
Working on Coconut Island has many upsides, but one of my favorite is getting to see science in action. I’ve been in the lab for the past few years, watching as Dr. Chris Bird’s research on the Hawaiian limpets (known locally as opihi) has unfolded. The tale they tell is already an intriguing one, as they seem to be one of the only organisms with solid evidence to suggest sympatric speciation (the splitting of species without any physical barriers). They’re also one of the only marine species to have radiated here in Hawaii. But on the most recent expedition, something else strange about these little mollusks was confirmed: they tend to separate based on sex.
The scientists found that female opihi live higher up on the shore than male opihi. Why? Well, we don’t know yet, but Chris is determined to find out. He thinks it likely has to do with spawning, and may prove valuable information for managers of the opihi fishery. The recent discovery was even featured on the local news – alongside some fantastic visuals of the perils these scientists undergo to conduct their research:
Weight is a big issue in America. More than half of Americans are unhappy with their weight, spending 33 billion on dieting and weight loss programs and products every year. This obsession starts younger than we’d like to admit, as 80 percent of 10 year old girls will say they are on a diet. But whether you count the millions dissatisfied with their looks, the percentage trying to lose weight, or the billions wasted on pills and fad diets, the message is the same: being the ideal weight matters, and it matters a lot.
But what is the ideal weight? Doctors say somewhere between a BMI of 20 to 25. Looking at runway models, you’d think it was just this side of starving, as the stick figures that grace our catwalks have an average BMI of only 16. Ask the average man and… well, actually, that will depend on a number of things, including his mood.
A number of factors affect what weight a guy prefers a woman to be, and evolution is to blame. For a long time, scientists have believed that attractiveness is really just our way of interpreting how good a person will be as a mate, starting with genes. “Good-genes theory posits that human judgments of physical attractiveness, particularly in mating contexts, have evolved to respond in part to heritable cues associated with health,” explains Jason Weeden and John Sabini in their scientific review of the topic. As the theory goes, the better someone’s genetic makeup, the more symmetrical and ideal their body becomes.
But being a good potential mate isn’t just dictated by our DNA. Current health status, ability to provide for young, and other variable factors also play a role in how fit a person is as a potential husband or wife. A woman can have all the good genes in the world, for example, but if she’s starving, she won’t have the fat reserves to feed a child, let alone survive pregnancy. So, it makes sense that in times of hardship, men would prefer women better equipped to handle times of scarcity – and by better equipped, I mean with fat reserves.
“A primary function of adipose tissue is the storage of calories, which in turn suggests that body fat is a reliable predictor of food availability,” explain co-authors Viren Swami and Martin J. Tovée in their PLoS ONE paper released today. “In situations marked by resource uncertainty, therefore, individuals should come to idealise heavier individuals.”
But do times of hardship actually shift body size preferences? Science to date has supported this hypothesis, as hungrier and poorer men prefer larger women. But what Swami and Tovée wanted to know was whether the stress had to be related to food scarcity. What about other kinds of stress? Does stress in general shift preferences, or only hardship?
So, the team took college men and had half of them perform a stressful task unrelated to food or money which raised their cortisol levels. They then asked the stressed and unstressed men to take a look at some images of women, and rate their attractiveness. The images varied in body size, from underweight to obese. Finally, they recorded the participants own weight, height, and hunger status, as controlling variables.
The results were clear. The stressed out guys preferred a larger body size than their relaxed counterparts – but that was not all. “Men experiencing stress not only perceive a heavier female body size as maximally attractive, but also more positively perceive heavier female body sizes and have a wider range of body sizes considered physically attractive,” explain the authors.
The wider range of preference was notably one-sided. “This difference was driven by the shift in the experimental group’s upper limit of attractive female bodies,” the authors write. “While there was no significant difference in the lower end of the range, the experimental group appear to have shifted the maximum cut-off for attractive bodies at higher BMIs, which resulted in their wider attractiveness range.”
Why did the stressed-out guys prefer weightier women? Because, evolutionarily, more weight means better able to survive in tough times. “In contexts marked by prolonged stress as a result of resource deprivation, individuals may idealise larger body sizes because such body types are associated with better ability to handle environmental threat.” These results are consistent with cross-cultural studies on attractiveness, which found that ideal body size varies by socioeconomic status and resource scarcity. In other words, our evolutionary past has affected why different cultures throughout the world have very different ideals when it comes to beauty.
Nowadays, of course, the connection between body weight and ability to survive is uncoupled. Unlike our ancestors, Americans generally don’t worry about having the fat reserves to chase down their next meal. Modern medical technologies and an abundance of high calorie foods have made surviving and reproducing much easier. But, this evolutionary leftover does raise some interesting questions about modern life, too. What are the full implications of an economic depression, for example? I wonder if cutting taxes affects what size girls end up with modeling contracts, or if the association goes both ways, and girls on a diet become less picky. More research will have to determine if stressed women prefer larger men, too, or how chronic stress instead of acute stress affects attractiveness ratings.
Citation: Swami V, Tovée MJ (2012) The Impact of Psychological Stress on Men’s Judgements of Female Body Size. PLoS ONE 7(8): e42593. DOI: 10.1371/journal.pone.0042593.t001
Thursday 26th July saw the launch of SciLogs.com, a new English language science blog network. SciLogs.com, the brand-new home for Nature Network bloggers, forms part of the SciLogs international collection of blogs which already exist in German, Spanish and Dutch. To celebrate this addition to the NPG science blogging family, some of the NPG blogs are publishing posts focusing on “Beginnings”.
In the beginning, the earth was without form, and void; and darkness was upon the face of the deep, as a giant cloud of gas and dust collapsed to form our solar system. The planets were forged as the nebula spun, jolted into motion by a nearby supernova, and in the center, the most rapid compression of particles ignited to become our sun. Around 4.5 billion years ago, a molten earth began to cool. Violent collisions with comets and asteroids brought the fluid of life – water – and the clouds and oceans began to take shape. It wasn’t until a billion years later that the first life was brought forth, filling the atmosphere with oxygen.
Over the next few billion years, single-celled organisms fused and became multicellular; body plans diversified and radiated, exploding into an array of invertebrates. Yet all this abundance and life was restricted to the seas, and a vast and bountiful land sat unused. Around 530 million years ago, there is evidence that centipede-like animals began to explore the world above water. Somewhere around 430 million years ago, plants and colonized the bare earth, creating a land rich in food and resources, while fish evolved from ancestral vertebrates in the sea. It was another 30 million years before those prehistoric fish crawled out of the water and began the evolutionary lineage we sit atop today. To understand life as we know it, we have to look back at where we came from, and understand how our ancestors braved a brand new world above the waves.
It was a small step for fish, but a giant leap for animalkind. Though, looking at modern fish species, it’s not so hard to envision the slow adaptation to life out of the sea. Just the other day, I was feeding my pet scorpionfish Stumpy, and he surprised me with this slow, deliberate crawl towards his food:
A number of fish exhibit traits which are not unlike those of the first tetrapods: the four-limbed vertebrates that first braved life on land, direct descendants of ancient fish. Many of Stumpy’s relatives, including the gurnards, are known for their “walking” behaviors. Similarly, mudskippers have adapted anatomically and behaviorally to survive on land. Not only can they use their fins to skip from place to place, they can breathe through their skin like amphibians do, allowing them to survive when they leave their shallow pools. Walking catfishes have modified their respiratory system so much that they can survive days out of water. But all of these are only glimpses at how the first tetrapods began, as none of these animals has fully adapted to life on land. To understand how tetrapods achieved such a feat, we must first understand the barriers that lay between their life under the sea and the land above that awaited them.
Living in air instead of water is fraught with difficulties. Locomotion is one problem, though as evolution in a number of lineages has shown, not as big a problem as you might think. Still, while mudskippers and catfish seem to walk with ease, the same cannot be said of our ancestors. Some of the earliest tetrapods, like Ichthyostega were quite cumbersome on land, and likely spent most of their time in the comfort of water. These first tetrapods came from an ancient lineages of fishes called the Sarcopterygii or Lobe-Finned Fish, of which only a few survive today. As the name implies, these animals have meaty, paddle-like fins instead of the flimsy rays of most modern day fish species. These lobe fins, covered with flesh, were ripe for adapting into limbs.
But these early tetrapods had to develop more than a new way to walk – their entire skeletons had to change to support more weight, as water supports mass in a way that air simply doesn’t. Each vertebrae had to become stronger for support. Ribs and vertebrae changed shape and evolved for extra support and to better distribute weight. Skulls disconnected, and necks evolved to allow better mobility of the head and to absorb the shock of walking. Bones were lost and shifted, streamlining the limbs and creating the five-digit pattern that is still reflected in our own hands and feet. Joints articulated for movement, and rotated forward to allow four-legged crawling. Overall, it took a long 30 million years or so to develop a body plan fit for walking on land.
At the same time, these cumbersome wanna-be land dwellers faced another obstacle: the air itself. With gills adept at drawing oxygen from water, early tetrapods were ill-equipped to breathing air. While many think that early tetrapods transformed their gills into lungs, this actually isn’t true – instead, it was the fish’s digestive system that adapted to form lungs. The first tetrapods to leave the water breathed by swallowing air and absorbing oxygen in their gut. Over time, a special pocket formed, allowing for better gas exchange. In many fish, a similar structure – called a swim bladder – exists which allows them to adjust buoyancy in the water, and thus many have hypothesized that tetrapod lungs are co-opted swim bladders. In fact, exactly when tetrapods developed lungs is unclear. While the only surviving relatives to early tetrapods – the lungfishes – also possess lungs (if their name didn’t give that away), many fossil tetrapods don’t seem to have them, suggesting that lungfish independently evolved their ability to breathe air. What we do know is that it wasn’t until around 360 million years ago that tetrapods truly breathed like their modern descendants.
The other trouble with air is that it tends to make things dry. You may have heard the statistic that our bodies are 98% water, but, as well-evolved land organisms, we have highly evolved structures which ensure that all that water doesn’t simply evaporate. The early tetrapods needed to develop these on their own. At first, like the amphibians that would arise from them, many tetrapods likely stuck to moist habitats to avoid water loss. But eventually, to conquer dry lands and deserts, animals had to find another way to keep themselves from drying out. It’s likely that many of the early tetrapods began experimenting with ways to waterproof their skin. Even more important was the issue of dry eggs. Amphibians solve the dryness issue by laying their eggs in water, but the tetrapods which conquered land didn’t have that luxury.
The solution to land’s dry nature was to encase eggs in a number of membrane layers, in what is now known as an amniote egg. Even our own children reflect this, as human babies still grow in an amniotic sac that surrounds the fetus, even though we no longer lay eggs. This crucial adaptation allowed animals to cut ties with watery habitats, and distinguishes the major lineage of tetrapods, including reptiles, birds and mammals, from amphibians.
These crucial adaptations to tetrapod skeletons and anatomy allowed them to conquer the world above the waves. Without their evolutionary ingenuity, a diverse set of animals, including all mammals, would not be where they are today. Yet still we barely understand the ecological settings that drove these early animals out of the sea. Did dry land offer an endless bounty of food not to be passed up? Perhaps, but there is evidence that our ancestors braved the dry world very early on, even before most terrestrial plants or insects, so it’s possible earth was barren. Were they escaping competition and predation in the deep? Or was land important for some yet undetermined reason? We may never know. But as we reflect upon our beginnings, we have to give credit to the daring animals that began the diverse evolutionary lineage we are a part of. While we may never understand why they left the water, we are thankful that they did.
Working on Coconut Island has many upsides, but one of my favorite is getting to see science in action. I’ve been in the lab for the past few years, watching as Dr. Chris Bird’s 
