The “Ooooooohhhh!” a human being cries out when they stub their toe might sound a pretty similar to the “Ooooooohhhh!” they cry out at the end of their mating ritual, but they two calls are different. An important part of human-to-human communication is our ability to extract information from context-specific calls and integrate it with other information we already have to make sense of what we’re hearing. It’s how we know, if we’re standing in one room and the TV is on in another, the difference between the scream of a serial killer’s victim in a slasher movie and the scream of a hero going into battle in an action blockbuster. We might not know what kind of movie is on in there, but we can at least identify which end of a blade the screamer might be on.

Katie Slocombe, a lecturer at the University of York’s psychology department, has spent her career tracing the evolution of different aspects of human language. More often than not, she finds herself starting with pants, grunts, hoots and hollers of chimpanzees. Many people find this surprising, Slocombe has said, but they shouldn’t. Finding an evolutionary explanation for any part of human language is difficult. Unlike, say, wrist bones, spoken language hasn’t left any fossil remains behind for us to study. Genetic evidence from our hominid ancestors suggests that we evolved our capacity for complex spoken language in a very short window of time, so it’s likely that the cognitive abilities underlying language emerged farther back in the primate lineage. Hence it makes perfect sense to look to other living primates, apes and monkeys, for clues to language’s origins. [Read more]

One if by land, and two if by sea/And I on the opposite shore will be/Ready to ride and spread the alarm/Through every Middlesex village and farm/For the country folk to be up and to arm.

On April 18, 1775, Paul Revere told three Boston patriots to hang two lanterns in the steeple of the city’s Old North Church. A militia waiting across the Charles River in Charlestown kept an eye out for these signal lanterns and were prepared act appropriately as soon as they saw one or both of the lights stab out at the darkness. The meaning of the two lanterns has been memorized by countless American schoolchildren in the century and a half since Longfellow published “Paul Revere’s Ride.” One lantern told the militia that the British Army would march over Boston Neck and the Great Bridge, and two meant that that the Redcoats would take boats across the river to land near Phips farm.

Many, if not most, birds and mammals that live in groups have their own signals and alarms that alert members of the group to predators and other dangers. An alarm call can mean the difference between life and death for animals who didn’t detect the threat on their own and younger animals who are especially vulnerable to predation. Belding’s ground squirrels take a cue from Revere and use two different alarm calls to warn of two types of danger. Whistle alarm calls signal aerial predators and trill alarm calls signal terrestrial ones. A squirrel needs to react differently to each type of call and to each type of predator. Listeners respond to whistles by entering a burrow or another hiding spot, and adopt a “posting” stance on their hind legs in response to trills.

When young Belding’s squirrels first emerge from their burrows when they’re a month old, they don’t respond appropriately to the two different calls and if they respond at all, they typically just freeze. They pick up on the appropriate behavioral responses very quickly, though, often within five days of coming above ground. Watching the responses of mom, dad and the other squirrels could teach a youngster what they need to know pretty quickly, but Jill Mateo, from the Department of Comparative Human Development at the University of Chicago, wondered if there was also a physiological factor. For many species, the sight, sound or even odor of a predator spurs physiological changes that make individuals better prepared to track predators and the responses of other animals, hide and be still, defend themselves or run/fly/swim like hell. Maybe a squirrel’s body reacts differently to a whistle than it does to a trill – to two lanterns than it does to one, if you will – and helps prime the squirrel for one response or another.

For the first five days after they come aboveground, juvenile ground squirrels show a higher level of cortisol (a steroid hormone released in response to stress) than during the days before emergence or the weeks after. To see if the hormone had some role in ground squirrels learning appropriate anti-predator behavior, Mateo tested how the levels of the hormone changed in response to different alarm and non-alarm calls. She caught pregnant female squirrels at a few sites near the Sierra Nevada Aquatic Research Laboratory (SNARL) at Mammoth Lakes, CA and brought them back to the lab so they could give birth and rear their young. Around the time the babies would normally leave the burrow, Mateo placed them, in pairs, in a large, dark wooden box once per day and played either a recording of ground squirrel whistle alarms, trill alarms, squeals young squirrels use during play or a silent control.

Every time a squirrel heard a recording, Mateo took a blood sample from it. These tests continued until she had one blood sample for each of the four recordings from a squirrel or until the squirrel turned 35 days old (in some cases, she was not able to get complete samples from a squirrel before it reached the age limit or did not have a large enough sample to analyze). After two rounds of tests in 2006 and 2008, Mateo had partial samples from 32 squirrels and complete samples from 17 of those.

Mateo analyzed the samples and, using a squirrel’s cortisol concentration following the silent stimulus as its baseline, looked at the hormone’s percent change in response to the alarm calls and play noises. Because multiple squirrels from several different litters were tested, Mateo averaged the cortisol responses to each recording for each litter.

For all litters, cortisol concentrations were higher following playback of trill alarm calls than after the other recordings. The change in cortisol levels compared to the baseline was only significant in response to the playback of the trill alarm calls. The whistle alarms did not increase cortisol concentrations, but earlier research by Mateo showed that they do elicit bradycardia, a slower than normal heart rate.

So the squirrels do have different physiological responses to the two alarm calls. What relationship do these changes inside the body have with behavior, though, and what do they have to do with air versus ground attacks? Mateo hypothesizes that cortisol might not increase in response to whistles because attacks by avian predators often only last a few seconds and most birds don’t make repeated attacks if their first one is unsuccessful; the attack would be over before circulating cortisol increased. Bradycardia, however, is associated in young squirrels with decreased motor activity and enhanced information processing. If the heart slows in response to whistles, the squirrels can stay still and pay attention in case it needs to make a break for a hiding spot.

On the other hand, the terrestrial predators that squirrels respond to with trills usually spend a significant amount of time either moving around squirrel burrows or waiting near one to attempt an ambush. Increased cortisol makes glucose available as fuel to the squirrels’ bodies for sustained vigilance in posting stances and, if needed, multiple escape attempts.

Both of these physiological reactions increase arousal and attention in a variety of species, so both might also just aid young squirrels in noticing and paying attention to the responses that nearby adults have to the alarm calls, making for a faster association between the alarms and their appropriate responses.

References: Mateo JM (2010). Alarm calls elicit predator-specific physiological responses. Biology letters, 6 (5), 623-5 PMID: 20236965

Mateo JM (1996). Early auditory experience and the ontogeny of alarm-call discrimination in Belding’s ground squirrels (Spermophilus beldingi). Journal of comparative psychology (Washington, D.C. : 1983), 110 (2), 115-24 PMID: 8681525

Image: “Belding’s Ground Squirrel in the Sierra Nevada Mountains, California, USA” by Justin.Johnsen via Wikimedia Commons. Used under a Creative Commons Attribution 3.0 license.

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.

It’s a love story as old as time itself: boy Asian corn borer moth (Ostrinia furnacalis) meets girl Asian corn borer moth; girl secretes sex pheromones; boy goes through his courtship ritual, a little song-and-dance routine where he rubs his wings against his thorax to produce a soft, whispering sound. It’s a sweet little love song that, it turns out, hides a vicious lie.

In 2009, this romantic tale played out in a lab in Japan so that Ryo Nakano, from the RIKEN Brain Science Institute’s Laboratory for Biolinguistics, could observe the males’ seduction technique. They knew that the buggy love song increased the moths’ mating success somehow, but its function and effect weren’t clear. Nakano and colleagues ran a series of experiments where the ability either to make or detect the mating sounds were removed so they could figure out the secrets of the song.

Nakano and his team silenced several male moths by surgically removing the sound-producing scales located on their forewings and punctured the tympanic membranes of several female moths so that they couldn’t hear males’ calls. Next they observed how deafened versus intact females reacted to male advances, and how frequently mute versus intact males convinced females to get it on with them.

Intact female-intact male, intact female-silent male and intact males-deafened female pairs were let loose in a flight tunnel to do their thing and the courtship behaviors of the males – exposing their genitalia while fanning their wings, raising the wings into position for producing courtship songs, assuming the position for attempted copulation, etc. – were observed until the females either accepted or rejected their advances. That last part is where things get tricky. For each attempt at sex, the male has to bend his abdomen based on the position of the female in order to get a grip on her genitalia with his genital claspers. It’s a difficult little maneuver, and like human males’ “yawn and put the arm around her” move, even the slightest movement of the female can screw it all up.

Females who could hear the males’ songs readily accepted them as partners and pairs of intact moths got busy 98% of the time. In all, 94 of the 96 intact males succeeded in mating with intact females, though only 15% succeeded on the first attempt and the rest kept trying until they either succeeded or, in the case of two unlucky guys, were interrupted by the escape of the female. When either partner was acoustically impaired – deaf or mute – though, the rates of sexual success fell dramatically. The females eventually accepted only 63% of the muted males after several mating attempts. The other 37% couldn’t get things right, even after repeated attempts, before the females wriggled away (results for the deafened female and intact male pairs were similar). The team also observed that, if the initial mating attempt failed, the males who could make noise pumped up the volume of their song during subsequent attempts and that the sound level strongly affected their acceptance by females.

Nakano and his colleagues noted that neither the intact nor muted males gave up easily. The interruption of the males’ mating attempts was almost always the fault of the female trying to get away from her suitor. The variation in the number of mating attempts and mating success among intact and impaired pairs didn’t seem to be a problem of tenacity, but female behavior and Nakano wondered if that’s where the males’ courting sounds worked their magic.

When the team analyzed the female response to courtship calls, they found that the females froze in their tracks during the serenade, making it easier for the males to mount them. Nakano proposed a “predator recognition” hypothesis that assumed the females recognized the males’ song as the sound of predatory bats. Staying still, of course, is a great way to small insects to “drop off” a bat’s sonar and not be recognized as prey, and bat calls do elicit freezing responses in many insects, including moths. Faking a bat call, then, is a great way for a male moth to cause a freeze response and get the girl to lie still long enough for him to score.

To test this whether this sonic date rape drug actually worked, Nakano and colleagues examined, in a separate study, whether female moths of the species Spodoptera litura, which exhibit mating behaviors similar to O. furnacalis, could discriminate between male songs and bat calls. They couldn’t, and simulated bat calls played over a loudspeaker while muted males attempted to mate with them had the same effect as the courtship song of intact males: they stopped moving and the males had a much easier time of “boring their corn.”

References: Nakano, R., Takanashi, T., Skals, N., Surlykke, A., & Ishikawa, Y. (2010). Ultrasonic courtship songs of male Asian corn borer moths assist copulation attempts by making the females motionless Physiological Entomology, 35 (1), 76-81 DOI: 10.1111/j.1365-3032.2009.00712.x

Nakano R, Takanashi T, Skals N, Surlykke A, & Ishikawa Y (2010). To females of a noctuid moth, male courtship songs are nothing more than bat echolocation calls. Biology letters, 6 (5), 582-4 PMID: 20219743

Image: Ostrinia furnacalis, courtesy of the Hannam University Natural History Museum

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.

There are some 1,400 described species of scorpion in the world, and while only 25 of those have proven they can take down a human being with their venom, many more of them can easily injure and kill smaller creatures. Given that, you’d expect scorpions to be important predators in desert food webs, but you might not expect them to be equally important as prey.

Yes, despite the pincers and the stinger and the venom, plenty of animals – among them, centipedes, tarantulas, lizards, owls,shrews and bats – regularly chow down on scorpions. Hemprich’s long-eared bat (Otonycteris hemprichii, at right), found in deserts in northern Africa, the Middle East and south-central Asia, considers scorpions a major food group, along with beetles, centipedes and spiders. In Israel’s Negev Desert, the proportions of these groups in the bat’s diet changes throughout the year, with scorpion fragments found in only 10% of bat droppings in the early spring and in a whopping 70% in the late summer.

That’s a whole lot of scorpions they’re eating, and while some of those are certainly weakly toxic Large-clawed Scorpions, the bats also prey on the Palestine yellow scorpion (Leiurus quinquestriatus, below). Everything you need to know about L. quinquestriatus can be summed up in its nickname, the deathstalker. These scorpions are considered some of the most dangerous in the world and possess a highly toxic venom that contains a grab bag of neurotoxins. Their sting can cause extreme pain, fever, convulsions, paralysis and death (via heart or respiratory failure), even in humans. These scorpions are also “sit-and-wait predators” that hunt by remaining quiet and still and lashing out at unsuspecting prey that wanders too close.

They certainly sound appetizing, but how do the bats deal with prey that present such a challenge to both detecting and disabling them? Carmi Korine, of Ben-Gurion University of the Negev in Midreshet Ben-Gurion, Israel, and colleagues found that Hemprich’s long-eared bat is what’s known as a gleaner, which means it takes prey from surfaces and not right out of the air, as many bats do. Korine and his team hypothesized that, like gleaning Pallid bats that prey on scorpions in North America, Hemprich’s bat might rely on passive gleaning to find its meals, simply listening for the prey to make noise instead of actively using echolocation to detect it based on its echo signature. The researchers figured that, if that was the way the bats hunted, then they would select prey based on 1) body size (bigger scorpions being easier to detect), and 2) toxicity (less venomous scorpions posing less risk of injury).

The team captured eight O. hemprichii individuals, set them up in a room with a scorpion buffet that included both living and dead Large-clawed Scorpions (Scorpio maurus palmatus), Israeli common scorpions (Buthus occitanus israelis) and, of course, deathstalkers.

When the bats took off to search for food, they spent a few minutes circling the room and dropped down directly onto a scorpion once they noticed it. They only went after the live scorpions, ignoring the dead ones and even walking right over motionless live ones if they missed on their initial divebomb, confirming that they glean passively and rely on prey noises.

Once they landed on a scorpion, the bats immediately started biting the scorpions’ heads. The scorpions did not take this lying down and fought back, stinging the bats on the head and face and, in one case, under the eyelid. The bats made no observable attempts to either avoid or disable the stingers and once they had killed the scorpions they often ate the whole thing, including the stinger and poison gland.

This is how the bats hunted all the scorpions. Contrary to the researchers’ predictions, the bats showed no preference among scorpion species based on either on size or toxicity, diving on their prey immediately after detection without any further inspection and going only on limited acoustic information. In 49% of the test sessions the bats actually went after the more poisonous of the available species (and in 24 direct comparisons, the deathstalker was chosen 50% of the time).

The researchers did not have an explanation for the bats’ indifference to the danger their meals pose, but speculated that either scorpions aren’t able to pierce the bat’s skin, or that the bats have at least a partial tolerance to the venom. Given that the bats regularly ate the stingers and venom glands, it seems more likely that their just tough enough to handle what the deathstalkers can dish out.

For other takes on this study, see Michael Marshall’s post at Zoologger and Zen Faulke’s post at Neuro Dojo .

Reference: Holderied M, Korine C, & Moritz T (2010). Hemprich’s long-eared bat (Otonycteris hemprichii) as a predator of scorpions: whispering echolocation, passive gleaning and prey selection. Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology PMID: 21086132

Images: “Otonycteris hemprichii” by Wikimedia Commons user Charlotte Roemer, used under a Creative Commons license.

“Deathstalker near Tzehelim, Israel” by Wikimedia Commons user Yair Goldstof, used under a Creative Commons license

Ed Yong recently reposted his fantastic 2008 post on assassin bug camouflage to keep us entertained while he’s away. I covered the same paper on an old incarnation of my blog, and can’t resist joining in on the reposting fun. Kevin Zelnio of Deep Sea News also has a post about it.

Remember that scene in Silence of the Lambs where Hannibal Lecter kills one of his guards, cuts the poor bastard’s face off and then wears it as a mask so he can escape in an ambulance? This great (if only for the appearance of Chris Isaak as a SWAT team captain) movie moment, it turns out, is something of a case of art imitating life.

While disguise and camouflage have a long history in the animal world (stick bugs, chameleons, decorator crabs, etc.), the assassin bug Acanthaspis petax, takes things to a Lecter-esque extreme.

The order of insects called Hemiptera is comprised of some 80,000 species, collectively known as true bugs. The order’s defining characteristic the arrangement of the bugs’ mouthparts: the mandibles and maxillae have evolved into a sheathed proboscis capable of piercing tissue and sucking out liquids. Most hemipterans use their proboscises to suck sap from plants, but the assassin bug prefers to stab them into other insects (usually ants) and inject their prey with paralysis-inducing saliva and digestive enzymes in order to break down and suck up bodily fluids.

A few other types of insect do the same thing. So what? Well, Acanthaspis petax one ups its brethren and sticks the corpse of its meal to its back, which secretes fine, sticky threads. There the corpse sits with others like it, forming a coat of bodies that earlier research suggested might protect the bug from predators.

Robert Jackson and Simon Pollard from the University of Canterbury tested this theory by matching the assassin bugs against jumping spiders in a no-holds-barred insect cage match. Three species of jumping spider which – all stalking, vision-guided predators that wouldn’t be able to detect the assassin bug by smell  - were placed in glass cages with either with naked, unmasked, assassin bugs or bugs bug wearing the bodies of its last few meals.

All three species of spider went after the uncovered bugs about ten times more than the covered ones (even if the bugs were actually dead and preserved decoys the authors used to control variables associated with using live bugs, like motion, behavior, size etc.).

Jackson and Pollard suggest that the cloak of corpses (or skin coat, if you will) successfully deters predators because the bodies break up the assassin bug’s form into something the spiders don’t recognize. They see a mound distinct from the background, but they dont recognize it as prey.

The remaining question is: why ants? Assassin bugs feed on a variety of other insects they encounter, but their camouflage is consistently composed mostly of ant corpses. Jackson and Pollard suggest the possibility that the spiders avoided the cloaked bugs because ants are formidable prey, using chemical defenses and having a nasty tendency to swarm. The assassin bugs, then, might be using ants in particular as disguises because of their tough guy reputation.

Reference: Jackson, R., & Pollard, S. (2007). Bugs with backpacks deter vision-guided predation by jumping spiders Journal of Zoology, 273 (4), 358-363 DOI: 10.1111/j.1469-7998.2007.00335.x

The segregation of habitat between native and invasive species often comes down to a competition between their physiological and behavioral abilities. This is especially true in habitats prone to frequent change; as both indigenous and invasive species respond to environmental variations in a habitat, it’s the difference in their responses that can determine their success or failure.

In South Africa, the indigenous mussel Perna perna (below, left) seems to have the odds stacked against it. Its coastal ecosystem is under heavy fire from invasive species, it’s subjected to variable, extreme environmental conditions in its intertidal home and its behavioral repertoire is more than a little limited. What’s a mussel to do? Really, the only thing it can do: open and close its shell (“gaping”). Turns out that this simple behavior has a strong influence on the outcome of the mussels’ turf war.

The Mediterranean mussel Mytilus galloprovincialis (below, right) is one of the world’s most widespread marine invasive species andcan be found all over the northern and southern hemispheres’ temperate zones. Having found its way to South Africa in the late 1970s, it slowly branched out along the entire west coast and has now spread along 800-900 km of the south coast, too. There, it shows partial habitat segregation with the P. perna in the lower eulittoral zone, or mussel zone, where P. perna typically dominates the lower zone and M. galloprovincialis dominates the higher mussel zone, with some overlap.

The bivalves are regularly covered and uncovered by the changing tide and endurea steady rhythm of wet and dry conditions. When the outgoing tide leaves them high and dry, the mussels have two choices. They can keep their valves closed, which minimizes water loss, but requires them to use anaerobic metabolism (a way for an organism to produce usable energy in the form of ATP without the involvement of oxygen; it’s basically respiration without oxygen). Alternately, they can open and close their valves, which maintains a more efficient aerobic metabolism (energy creation that uses oxygen), but opens them up to (no pun intended) to water loss and the risk of drying out.

Mussels of each species and from each zone were exposed to air at two different temperatures by Katy Nicastro, Gerardo Zardi (CCMAR, CIMAR-Laboratorio Associado at Universidade do Algarve in Portugal), Christopher McQuaid (Department of Zoology & Entomology at Rhodes University in South Africa), Linda Stephens, Gregory Blatch (Department of Biochemistry, Microbiology & Biotechnology at Rhodes University) and Sarah Radlof (Department of Statistics at Rhodes University) in three experiments conducted to observe gaping behavior, water loss and mortality due to dessication. The two species took very different approaches to air exposure. M. galloprovincialis did not show gaping behavior at either temperature, while P. perna showed gaping at both temperatures, with an increased number of gaping individuals and of number of gapes per hour at the higher temperature. Consequently, water loss rates were higher for P. perna than for M. galloprovincialis (average loss of 21% and 4% of total body water, respectively) and while water loss was greater for both species at the higher temperature, P. perna’s water loss rate was much steeper when the temperature was increased. P. perna likewise had higher mortality rates in the desiccation experiment than M. galloprovincialis, but the invasive mussels did show a greater production of stress proteins related to anoxic stress.

Gaping, as simple as it seems, has a profound effect on the segregation of habitat between the native and invasive mussels. While gaping may relegate P. perna to the lower area of the mussel zone, it doesn’t exactly get stuck with a raw deal. It’s greater attachment strength allows it to withstand greater hydrodynamic stress than the invasive mussels that might venture into the zone. P. perna initially aids the survival of M. galloprovincialis in the lower zone by providing protection against waves, but eventually excludes it competitively in the long run and takes the lower zone all for itself. Meanwhile, keeping their traps shut condemns the invasive M. galloprovincialis to more stress and a less efficient metabolism (the end products of which can be toxic or lethal if left to accumulate), but minimizes water loss and allows it to make itself at home in the upper mussel zone, where gaping P. perna can’t survive or compete with it. Territory gets divvied up and both invaders and natives find a niche for themselves based on the simple act of opening up, or not.

Reference: Nicastro KR, Zardi GI, McQuaid CD, Stephens L, Radloff S, & Blatch GL (2010). The role of gaping behaviour in habitat partitioning between coexisting intertidal mussels. BMC ecology, 10 PMID: 20624310

Images: Mytilus galloprovincialis with Symplegma reptans living on it, by Flikr user Jay Vavra. Perna perna from Collection Georges Declercq, via the World Register of Marine Species. Both used under a Creative Commons license.

Game theory models based on repeated interactions between two individuals have often been the framework for understanding cooperative interactions in humans, but these models rarely apply in nature. Non-human animals, after all, rarely find themselves in situations like the “prisoner’s dilemma.”

Instead, partner choice and competition are emerging as the framework for understanding cooperation in the natural world. Some mutualisms (biological interactions between organisms where each individual derives a fitness benefit) can be described as “biological markets,” where organisms exchange goods or services. These markets and the animals that participate in them share some similarities with humans and our markets: animals preferentially interact with partners that provide the highest-quality goods or services; animals sometimes cheat each other; competition is often a good thing, and threatening to take your business elsewhere can lead to more cooperative behavior from your partner.

In many cleaner mutualisms among fish, cleaner fish occupy cleaner “stations” where they remove parasites from cooperating client fish. Buyer beware, though, because clients often have to wait for service from a cleaner and when it’s finally their turn, they may be cheated by cleaners that feed on tissue or mucous instead of parasites. Clients don’t have many options for ensuring good service. They can’t demand their mucous back or complain to management. What they can do is go get cleaned somewhere else.

Thomas C. Adam, a graduate student at the Department of Ecology, Evolution, and Marine Biology at the University of California, Santa Barbara, investigated cleaner-client interactions involving the territorial butterflyfish Chaetodon ornatissimus . In the Maharepa lagoon on the north shore of Moorea, French Polynesia, C. ornatissimus (at left) is the preferred client of bluestreak cleaner wrasse (at right), but has the option of partnering with several other species of cleaners common to the area. Snorkelers mapped the territorial boundaries of C. ornatissimus and conducted hour-long observations of their interactions with their cleaners (in total, individual fish in 32 territories were observed for 43 hours).

The results of the study indicate that not only do bluestreak cleaner wrasse compete for access to their butterflyfish clients (the amount of time cleaners had access to clients was negatively associated with the number of cleaner stations in a territory and individual butterflyfish with access to multiple cleaner stations did, indeed, shop around and were less likely to return to a cleaner station for their next cleaning than individuals with access to just one cleaner station), but the ability of butterflyfish to take their business elsewhere got them higher-quality service from cleaners. To wit, (1) the observed clients were never ignored by cleaners (at left) when they had more than one cleaner station in their territory (in contrast, five of 11 fish with a single cleaner station in their territory were observed being ignored), (2) while there was no evidence that clients with access to multiple cleaner stations were cheated less frequently than clients without access, the clients with their choice of partners were less likely have interactions terminated early by cleaners and were inspected for significantly longer during each cleaning session.
See? The free market does work sometimes.

Reference: Adam, T. (2010). Competition encourages cooperation: client fish receive higher-quality service when cleaner fish compete Animal Behaviour, 79 (6), 1183-1189 DOI: 10.1016/j.anbehav.2010.02.023

I recently took part in what social scientists call a “vacancy chain” (a social structure through which vacancies in discrete, reusable, and limited resources propagate through a population) and all I needed was a moving truck, a few helpful relatives, a case of beer and a few pizzas. You see, when my girlfriend and I moved into a new house in May, we filled a vacancy left by the previous tenants. When we moved, someone moved into our old apartment and filled the vacancy we left. Their apartment, in turn, was filled by someone else, and their apartment was moved in to by someone else and so on and so forth. Somewhere (further up the chain than me), a vacancy was created and propagated down the socioeconomic order through a series of interdependent events and resulted in many individuals acquiring new, sometimes better (we have a patio, but no central air, so the jury is still out), resources and benefiting from them.

Hermit crabs, for whom really nice shells to call home are a scarce commodity, have evolved their own sorts of vacancy chains as way for optimizing shell acquisition and occupancy. While these shell vacancy chains have been described (and shown to provide aggregate benefits that are distributed across many participants) for several hermit crab species in previous research, not much was known about the behaviorial and ecological factors that lead to and influence them.

Cue the arrival of Randi Rotjan, Jeffrey R. Chabot and Sara M. Lewis (from the New England Aquarium in Boston, the Pfizer Research Technology Center in Cambridge, MA and the Department of Biology at Tufts University, respectively) at Carrie Bow Cay, a ¾-acre island located near the Belizean barrier reef that is home to Eighty-four palm trees and 1,084 purple-clawed hermit crabs of the terrestrial species Coenobita clypeatus.While the biologists were there study parrotfish, bad weather made the water too rough for diving, so they used their time to better understand shell vacancy chains. The researchers marked 20 locations around the island, set out a single vacant shell at dusk at each one and monitored them. Over the course of 24 hours they observed a total of 16 vacancy chains of two different types, asynchronous and synchronous.

An asynchronous chain occurs when one crab moves into a new, empty shell and abandons its old one to be found by another crab, which abandons its own for another crab to find, etc. With this type of chain, shell switching is sequential and the crabs experience little to no interference or competition. They have the opportunity to investigate any vacant shells they find and can directly compare their current shell with a new shell by switching back and forth between the two. The down side is that individual crabs aren’t very likely to just stumble upon a vacant shell that meets their specific size and quality requirements. It’s like if I told you that you could wander around your town, go into any unoccupied houses you wanted, check them out and pick your dream home, but you’d have to find the one with two bedrooms, a dishwasher and a fireplace on your own by chance, without the aid of Craigslist.

Synchronous shell vacancy chains are more social and much more interesting. They start off with “waiters,” crabs that hang around a shell that’s too big for them, and wait for a bigger crab to come along so that if the big crab moves in to the vacant shell, the waiter can grab their more appropriately-sized hand-me-down shell (the researchers note that the decision to wait, and how long to wait, based on previous experience, provides some evidence that the crabs are smarter than we thought). The chains that the researchers observed began with one to 20 waiters who spent anywhere from a few minutes to an hour-plus loitering around empty shells. As a crowd gathers, the crabs queue up by size, from largest to smallest, and once largest crab switches into the vacant shell, each crab climbs into a new shell as it’s vacated by the slightly larger crab ahead of it, quickly shuffling vacancies (literally) down the chain. In both chain types, the fun stops when the last shell vacated is so low in quality (too small or damaged) that all the crabs reject it.

A Synchronous Chain in Action

In addition to the waiting that kicks off synchronous chains, the researchers observed other unique shell acquisition behaviors that the crabs only exhibited in social contexts and appeared to be associated with the vacancy chains. At almost half of the observed locations, when the waiting crabs were all too small for they vacant shell they had gathered around, some would “piggyback,” or form lines with each crab grasping the shell of another crab from behind and frequently moving in and out of the line to jockey for a better positions. The researchers hypothesize that piggybacking may be help establish a dominance hierarchy among the waiting crabs and/or allow them to investigate some of the shells they might be able to move into. Theses piggyback lines often transformed into queues upon the arrival of crabs that were appropriately sized for the vacant shell.

At some of the locations, multiple queues formed when there were many similarly sized waiters, and the crabs in these queues appeared to engage in a “tug-of-war” for control of the vacant shell. The smallest crabs, positioned at the end of each queue, frequently switched back and forth between the lines in a possible attempt to stake its place in the winning line.

So what sets these theatrics off in the first place? Population density seems to be a key factor determining the length and type of vacancy chains. Using modeling software, the researchers created a simulated habitat space and a population of crabs of varying sizes. Rules for shell switches that realistically reflected hermit crab behavior were established and, after a while, a vacant shell appropriately sized for the largest crab in the population was placed the center of the habitat and the simulation was continued. During 100 model runs were at each combination of 2 parameters: population density (8 levels, from 10 to 900 crabs) and maximum waiting times for the waiters (2 levels), vacancy chain lengths increased along with population density at the highest population density, almost half of the shell switches that occurred were part of synchronous vacancy chains. How word about an available shell gets out among the crabs in the first place is still unknown, though. The researchers plan to address the question in a future study and speculate that the waiters may use aural or chemical signals to draw attention to the vacancy.

Reference: Rotjan, R., Chabot, J., & Lewis, S. (2010). Social context of shell acquisition in Coenobita clypeatus hermit crabs Behavioral Ecology, 21 (3), 639-646 DOI: 10.1093/beheco/arq027

Image: “Caribbean hermit crab (coenobita clypeatus)” by ZooFari, via Wikimedia.