Cannibal Crickets Can Control a Crowd: How Eating Your Friends Aids Collective Motion

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On the wide, open plains of the American West, it’s more than the buffalo and the antelope that roam. Mormon crickets (Anabrus simplex) also sweep across the land in huge migratory swarms that can stretch six miles long and three miles wide. The crickets (a misnomer, they’re actually flightless katydids) can march up to a mile and a quarter a day in these groups, devouring every scrap of vegetation in their path and devastating agriculture in areas they pass through.

It sounds like a Biblical plague*, but consider the poor crickets. While the swarming behavior isn’t completely understood, entomologists think that it’s partly a strategy to avoid being eaten. Observations and experiments have shown that crickets that become separated from the group are easy prey and a big, cohesive group minimizes the risk of predation for any individual cricket.

Not that life in the swarm is any easier. In addition to consuming any and all plants they come across, the crickets often eat each other. One reason for this should be obvious, says a new study, “huge, concentrated numbers of crickets require huge, concentrated amounts of food. If the landscape doesn’t provide it, a fellow cricket will.”

A not so obvious side effect of this crickety cannibalism is that it might be helpful, even necessary, in keeping the swarm moving as a unit. A swarm of insects is simply the sum of its parts. The group’s movement, coordination, cohesiveness and persistence are the simple decisions and interactions of millions of individuals scaled up to the population level, and some of those decisions and interactions happen to involve one insect eating another. Another swarming insect, the desert locust (Schistocerca gregaria), tends to cannibalize traveling companions that have stopped moving or can’t keep pace with the group, and the threat of cannibalism influences their marching behavior. Individuals keep moving and maintain proper direction and pace to keep from becoming lunch for the guy behind them, and this helps maintain coherent swarm motion. [Read more]

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From my cold, dead paws: Sneaky kidnappings and daring rescues among baboons

ResearchBlogging.orgFor baboons, running away from home is something a boy is expected to do. Most baboon species rely on young males leaving the social group they’re born into and starting or joining another group to disperse genes and ensure diversity. In one species, though, the hamadryas baboon (Papio hamadryas) of northeast Africa, genetic evidence suggests that it’s the females who are the genetic movers and shakers. How that could be was, for the longest time, a real head-scratcher. The most basic hamdryas social group is made up of one male and a harem of 2-11 females. No one was sure how these females could leave, since the males can be a little clingy, keeping their females close through aggressive, and sometimes violent, herding.

In the late 1960s, biologists suggested that females might move to other groups not by leaving on their own, but when they’re abducted by other males. Researchers had, without seeing the abductions directly, found evidence that females in groups they were studying had been taken and then retrieved by the male from their original group within a matter of days. In the four decades since, though, no one had actually seen an abduction happen in the wild (probably with good reason, since observing and recording abductions in detail would require long-term, simultaneous observations of multiple baboon groups).

That changed in a few years ago, when Mathew Pines from the Filoha Hamadryas Project in Addis Ababa, Ethiopia and Larissa Swedell from Queens College in New York were the first people to witness not just one, but three, attempted baboon-nappings and rescues live and in the fur. The pair recently described these abductions in the journal Primates. [Read more]

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How is a mantis shrimp like a punching bag? The way it takes a hit.

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Mantis shrimp are, ounce for ounce, some of the most fearsome predators that you can pull out of the ocean. The marine crustaceans of the order Stomatopoda (neither shrimp nor mantids, they got the name because of their physical resemblance to both) are tiny and unassuming, but can use their front claws to attack with incredible speed and tremendous force. Stomatopods armed with “smasher” claws (there are also those armed with spearing claws) regularly crack open crabs and snails with cudgels that work on the same principal as crossbows: a spring-and-catch mechanism allows potential energy to be built up and stored and then released all at once. When all that power is unleashed, stomatopods can bludgeon prey with 45 mph strikes (the fastest known limb movement in the animal kingdom) and 340 pounds of force.

These war hammers aren’t just for hunting meals, though. Stomatopods use them on each other in territorial disputes, too. Given what these strikes can do, how have mantis shrimp not power-punched each other into extinction?

There’s two parts to the answer. One is the way they hit each other. When sparring over turf, two mantis shrimp will usually exchange a few strikes to each others’ tails as a way of sizing each other up before committing to a full-on and rumble and mutually assured destruction. The second part is where they hit each other. These ritual test blows are made to each other’s telsons, armored tail segments that are strong enough to take the punishment.

To find out just how strong telsons are and how they withstand such force, Sheila Patek from the University of Massachusetts and Jennifer Taylor, from the University of Indiana took a few shots of their own at them. They got some mantis shrimp, let them live out a few final days eating grass shrimp in luxurious plastic cup accommodations and then put them in the freezer until they were dead, but not frozen solid. Then, they superglued the stomatopods to a strip of Plexiglass and dropped stainless steel balls on them.

The pair recorded the impacts with high-speed video cameras and used the data to calculate the tails’ coefficient of restitution, a value representing the elasticity of an object. The basic principle of the measurement is that the amount of elastic energy absorbed by an object can be measured by the loss of momentum of a colliding object suffers, so the figure is expressed as a ratio of and the post- and pre-impact velocities of the striking object. Coefficients of restitution are often used to characterize and regulate products that take their fair share of blows, like automobiles, body armor, sports equipment and even fruits and vegetables.

Patek and Taylor calculated the telson’s coefficient of restitution as 0.56. This is similar to a major league baseball, which has a coefficient of restitution between 0.45 and 0.50 when hit with a bat. The telson dissipated a significant amount of energy, 69%, when it compressed during impact with the steel balls. The incredible loss of energy implies that the telson absorbs impact inelastically, like a heavy punching bag does.

Patek and Taylor also used micro-ComputedTomography (a 3-D imaging method that uses penetrating waves) scans to examine mantis shrimp exoskeletons to see if they could find anything that might explain the telson’s resilience.

They found that the stomatopods’ tails are two times thicker than normal at three ridges, called carinae, that run along the telson. While the center area of the telson crumples inward upon impact, the carinae don’t deform. This provides a balance of stiffness and compliance that helps impact resistance by both absorbing energy and resisting penetration, a strategy human engineers have co-opted for designing armor.

Reference: Taylor JR, & Patek SN (2010). Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp’s telson. The Journal of experimental biology, 213 (Pt 20), 3496-504 PMID: 20889830

Patek, S., Korff, W., & Caldwell, R. (2004). Biomechanics: Deadly strike mechanism of a mantis shrimp Nature, 428 (6985), 819-820 DOI: 10.1038/428819a

Images: Female Odontodactylus Scyllarus by Roy L. Caldwell, UC Berkeley, for the National Science Foundation. Odontodactylus Scyllarus by Flickr user prilfish, used under a Creative Commons license.

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It’s not lonely at the top, after all: dominant chimps have more parasites

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This post was chosen as an Editor's Selection for ResearchBlogging.org

It wasn’t more than a few decades ago that stress was seen merely as an unpleasant mental state or a mild irritation. Stanford neurologist Robert Sapolsky recognized early on, though, that it had real, significant impact on one’s health. In a Wired piece from last summer,  “Under Pressure,” Jonah Lehrer relates how Sapolsky first connected the dots between stress and health while studying baboons: “He noticed, for instance, that the males at the bottom of the hierarchy were thinner and more skittish. ‘They just didn’t look very healthy,’ Sapolsky says. ‘That’s when I began thinking about how damn stressful it must be to have no status. You never know when you’re going to get beat up. You never get laid. You have to work a lot harder for food.’”

The sustained social stress that social animals with low status put up with can grind down their bodies and numerous studies have shown the health gap between high and low ranking individuals several species. High ranking greenfinches clear viral infections more quickly than lower ranking ones, high ranking dairy goats have fewer gastrointestinal parasites less dominant ones and high ranking pigs have more white blood cells available to fight off Aujeszky disease than lowlier swine.

This isn’t to say its all fun and games for the animals at the top of the social hierarchy, though. Sure, dominance has its perks. Everyone wants to groom you, no one’s trying to beat you up and you can always get laid, and, therefore, have the best chance of reproductive success, but acquiring and maintaining that dominance is no easy task and often involves frequent aggression. Testosterone is considered the driving physiological factor of aggression – the hormone has a permissive effect, altering bodily functions that heighten pre-existing patterns of aggression – and is frequently associated with social dominance rank (though no study has shown solid, significant correlations among the three). Testosterone might facilitate a climb up the dominance ladder, but it is appropriately costly, and elevated testosterone levels are associated with increased metabolism, production of oxygen radicals and – mirroring the effects that stress has on low ranking animals –suppression of the immune system.

Michael Muehlenbein and David Watts from Indiana University wondered, since testosterone is immunosuppressive, and high testosterone levels are associated with a high dominance rank, if a high rank would also be associated with a higher parasite burden, a common effect of a suppressed immune system. To get a better grip on the links between dominance, testosterone, health and parasitic infection in nonhuman primates, Muehlenbein and Watts collected fecal samples and behavioral data from a community of chimpanzees in Ngogo in Kibale National Park in western Uganda. With 150 members, the Ngogo community is the largest described in the wild and contained 24 adult males and 14 adolescent males when Muehlenbein and Watts did their research.

The pair logged 1,700 hours of observational data to determine the dominance rank of 22 adult males and collected 67 fecal samples from those individuals. They hypothesized that dominance rank would be directly associated with fecal testosterone and cortisol levels (cortisol is a steroid hormone that’s produced by the adrenal gland in response to stress) and intestinal parasite burden. In order analyze the fecal samples for hormone and parasite levels, the researchers had to dehydrate the chimp droppings for two hours in a small portable oven on top of their camp stove before packing them up and shipping them stateside, an activity that likely caused the occasional cortisol spike at camp.

After analysis, Muehlenbein and Watts found that 1) fecal testosterone, but not cortisol, levels were directly associated with dominance rank, 2) both testosterone and cortisol were directly associated with intestinal parasite richness (number of unique species recovered) and 3) dominance rank was directly associated with worm-like helminth, but not single-celled protozoan, parasite richness (possibly because helminth parasites impose greater immunological costs than protozoans). As suspected, higher ranking chimps had higher testosterone levels and greater parasite burden than the lower ranking ones. Heavy is the head that wears the crown, and the gut below is isn’t doing so well, either.

Testosterone’s effect on the immune system is mainly suppressive. It inhibits antibody production and impairs natural killer cell activity. Muehlenbein and Watts think that a testosterone-suppressed system can’t muster the allergic responses needed to clear gastrointestinal infections. They also suggest that it could be the behavioral, and not the physiological, aspects of alpha male status that leads to an increased risk of parasites. A chimp’s daily travels, social network, nutritional status and certain personality factors – like sociability – all impact its susceptibility to disease to varying degrees, and all those factors vary greatly depending on where a chimp sits in the social hierarchy. High-ranking males usually have wider social circles, get around more and have more mating opportunities and, hence, more risk of acquiring directly-transmitted infections.

Reference: Muehlenbein MP, & Watts DP (2010). The costs of dominance: testosterone, cortisol and intestinal parasites in wild male chimpanzees. BioPsychoSocial medicine, 4 (1) PMID: 21143892

Image: “Alpha male chimp and older chimp jump on fig tree” by Wikimedia Commons user Caelio, used under a Creative Commons license.

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