Why some rodents have multiple biological fathers and one mother

Anthropologists have found that polyandry—the union of one woman and more than one man—is a rarity in humans. Across thousands of studied societies, just a few dozen polyandrous cultures exist, widely scattered around the world. For the most part, the guess is that cultural factors are at work. Among rodents, however, the practice is both widespread and well understood: it cuts down on infanticide. Males who have not sired with a given female will kill her newborns to prevent the spread of his rival’s genes, and to free her from the burden of raising another’s young in favour of his own.

In a classic sexual arms-race case, the practice of polyandry won out. Males cannot distinguish their own young from a rival’s, so a female that gives birth to young from more than one male will have protected them all from any individual father’s aggression, lest he threaten his own offspring.

Natural selection, then, should have weeded out monogamy in rodents. But in the house mouse, Mus musculus, that has not happened; females can choose one or many mates. In a study published in Behavioral Ecology Yannick Auclair, of the University of Zurich, and colleagues, may have figured out why.

Mr Auclair set up and then meticulously followed the progress of a mouse colony over three years. He kept count of the litters laid, and whether they were raised in solitary nests (those that consisted of only one female and her family) or communal nests (in which females shared maternal duties, irrespective of who sired their offspring). For those pups that survived into adolescence, he took tissue samples to determine paternity by genetic analysis.

The final tally noted 146 survivors and 254 deaths among the pups. Scratch marks on the bodies suggested that almost all deaths were due to infanticide. What was more interesting, however, were the survival rates between different kinds of litters. Polyandrous litters survived well in both solitary and communal nests, but monandrous ones survived significantly more in communal nests than in solitary ones. The reason, the team concludes, is “socially mediated polyandry”. A nest full of pups from many mothers and fathers was as safe as a nest with one mother and many fathers’ pups. Females derived the protective benefits of polyandry without actually having to expend the effort to carry it out.

The authors suggest that socially mediated polyandry might apply to many more species that engage in communal care of the young, including a small percentage of mammals such as rodents. That makes it a rich seam for investigation by evolutionary biologists. The explanation for a smattering of polyandry in humans, however, remains a matter of guesswork for anthropologists.

First published on economist.com. Image by noadi. CC-BY-NC-ND.

Slo-mo mojo

FLIES live shorter lives than elephants. Of that there is no doubt. But from a fly’s point of view, does its life actually seem that much shorter? This, in essence, was the question asked by Kevin Healy of Trinity College, Dublin, in a paper just published in Animal Behaviour. His answer is, possibly not.

How animals perceive time: Slo-mo mojoThe Economist, 21 September 2013.

Image credit: The Economist

The backtrackers

“Viewed as a geometric figure, the ant’s path is irregular, complex, and hard to describe,” wrote Herbert Simon, an American psychologist. But, he added, this is really down to a “complexity in the surface of the beach, not the complexity in the ant”. Or is it?

Ants have the animal kingdom’s biggest brains, relative to their bodies. Brains account for up to 15% of an ant’s total mass in some species (humans weigh in at a meagre 2%). This goes some way to explaining their uncanny knack for finding their way back home from foraging forays. But entomologists have, like Simon, long believed that this apparently complex behaviour is the result of sticking to a handful of simple rules. Ants keep track of distance (for example with an internal pedometer) and direction (based on the position of the sun or scent, of pheromones, for example). If they lose it, they switch to a second startegy and move in a spiral around a centre they think is the nest. If the nest is not found in the first one, then the ant increases the radius and tries another.

Now Antoine Wystrach, of the University of Sussex, proposes that there is more to ants than mindless adherence to simple instincts. As he and his colleagues report in the Proceedings of the Royal Society, at least one species of ant appears to display some hallmarks of intelligence: the ability to integrate different strategies based on experience.

Dr Wystrach captured Melophorus bagoti ants just before they reached their nests after a foraging trip and shunted them into straight tubes which led to random spots about 50 metres away. On exiting the tube, the ants invariably turned around and headed straight back in the direction of the nest. They could not be following the pheromone trail, as that was enclosed in the tube. Nor were they using visual cues: they appeared to backtrack just as well with with their eyes closed. (Cruel as it may sound, the researchers used an opaque paint to cover ants’ eyes.) In other words, the ants appeared to have some sort of internal compass.

Oddly, ants do not always use this device. When Dr Wystrach put ants into the tube when they were farther than two metres away from their nest, however, they used the two basic methods to find home. But when he repeated the procedure, but dropped the ants close to the nest for a few seconds before setting them down the tube, they backtracked just as they had in the first experiment. Backtracking, in other words, appeared to be triggered only when ants possessed a recent memory of their nest. Complexity in the ant, it seems, is a tad greater than Simon would have allowed.

First published on economist.com.

Image credit: Reverend Barry