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Behavior Brief

A round-up of recent discoveries in behavior research

By | August 10, 2012

image: Behavior Brief FLICKR CREATIVE COMMONS, MANU.1421

The benefits of cannibalism

Spiders that cannibalize potential mates have bigger broods, according to new research published last month (July 27) in Animal Behaviour. Females of some spider species eat males without copulating with them first, but exactly why has been a mystery. To find out, researchers at the University of Pittsburgh captured local funnel-web spiders (Agelenopsis pennsylvanica) and tracked the fitness of aggressive females, who tend to eat more males, versus more passive females.

A popular hypothesis is that the choice to eat a male or not is due to females gauging whether they would be more useful as mates or meals, but no link was found between aggressiveness and female or male body size. Instead, it was found that when females ate males, more hatchlings emerged from thicker eggs sacs. Hatchlings have to tear their way out of the egg sacs, and while cannibalistic females were no more likely to produce more eggs or offspring, the offspring appeared more capable of ripping out of the naturally thicker egg sacs.

"Either how good the offspring are at developing or how adept they are at ripping out of the egg case appears to be the mechanism by which they most receive a benefit," co-author Jonathan Pruitt told BBC News. "So it appears for the energetic investment for the amount of calories that they (spider mothers) put into these egg cases, cannibals get more bang for their buck."

Elephants’ quiet calls

Most of the sounds elephants make we can't hear. Between the trumpeting calls, elephants communicate in low frequency rumbles that travel over long distances of up to 10 kilometers. While the low-frequency calls have been known about since the 1980s, it wasn't clear until now how exactly the elephants made them.

Animal sounds are usually created either by the passage of air through the vocal folds, as happens when humans speak, or by neuronal signals causing the larynx to contract in a constant rhythm, as when a cat purrs. Finding out which mechanism elephants use has been difficult. “You cannot just walk up to an African elephant and stick an endoscope in his mouth and ask him to say 'Ahh,'" lead author Christian Herbst of the University of Vienna told ScienceNOW.

So instead, the researchers isolated the larynx of a recently deceased African elephant  and hooked it up to an artificial lung to test f the passage of air was enough to produce the sound. The movement of air indeed vibrated the vocal folds, and the recorded sounds from the experiment fell well with the range of elephant calls, according to a database of wild elephant recordings. The authors say the results, published August 3 in Science, may reveal how a variety of other animals make noise.

http://www.youtube.com/watch?v=o32E2LAzYBA

Every man for himself

While "women and children first" is a noble call that rings out on sinking ships in movies, an analysis of real maritime disasters reveals the opposite is nearly always true. Swedish researchers looked back at the records of 100 major ship disasters from the past three centuries involving more than 15,000 passengers, and found that the survival rate for men was approximately twice that of women. Even the idea that the captain goes down with his ship was trashed, with 61 percent of the crew typically surviving, compared to just 37 percent of male passengers.

The authors of the analysis, published last month (July 10) in Proceedings of the National Academy of Sciences, are actually economists, interested in how far people will go to protect their own interests. The sinking of the Titanic is a rare example in shipwrecks where there was a clear survival advantage to women, but it goes against the observation of self-interest many economists observe in day-to-day activities. In researching the overall trend, the authors show that the Titanic is a remarkable exception to the general rule that every man is out for himself.

Two for one meal

Of all the flies crawling on the ceiling of a cowshed, bats prefer to snag mating pairs. What makes a copulating couple more at-risk is not their movement, but the male's attempts to woo the female with a series of clicking sounds.

Bats hunt most often by echolocation, sending out sounds and waiting to hear what echoes back, giving a picture of the landscape. But in a dark cowshed, flies lining the walls and ceiling are concealed between the myriad other bumps and grooves, and echolocation is not much use. So the bats have to rely on other cues, and the male fly's mating call is a dead giveaway.

Researchers watching Natterer's bats in a cowshed in Germany reported in Current Biology (July 23) that the mammals attacked 26 percent of the mating flies, catching them 60 percent of the time. To confirm it was the clicking sounds that attracted the bats, the researchers played mating calls through a set of speakers, and watched as the bats attacked the speakers.

http://www.youtube.com/watch?v=Me0eARgq6MU

Sloths lazy at sex, unbalanced

Sloths are famously sedentary, and a new census of a Hoffman's two-toed sloth population in Costa Rica revels their slow pace may prevent them from gaining all the advantages of promiscuity, according to the authors of the study, published last month (July 12) in Animal Behaviour. The sloths appear not to be monogamous by nature, but only around a third of male sloths fathered offspring with more than one female, despite each sloth roaming a home territory containing an average of three females. Furthermore, males showed only limited interest in defending their territory from other males and amassing a harem of females.

But they do fight. "They're not necessarily just puppy dogs—they will battle and we do find males with considerable facial scars," co-author Zach Peery told BBC News. “So although there is overlap [between territories], and although it's a somewhat more flexible mating system... there is structure to it.”

Credit: WIKIMEDIA COMMONS, CHRISTIAN MELFUHRER

Recent research has also discovered a surprising diversity in the sloth's apparatus for controlling balance. Structures in the inner ear are responsible for keeping us upright while moving about, but the dimensions of this structure in three-toed sloths varies on nearly twice as much as other mammals. The authors of the study, published August 1 in Proceedings of the Royal Society B, suggest this may be due to the fact that sloths move so slowly and infrequently. Not having to move swiftly and with precision means the sloths likely don't need a perfect balancing system.

 

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