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

To kill, cheetahs use agility and acceleration not top speed

Researchers have used gadget-laden collars to record cheetahs’ movements in the wild. They found that cheetahs succeed not because it is the fastest animal on land, but because of its incredible acceleration and unmatched turning speeds.

Most of what we know about cheetahs in the wild is based on direct observation, or through videos from remote cameras. This limits our understanding of cheetahs to open habitats and daytime. Alan Wilson at the University of London’s Royal Veterinary College wanted to study cheetahs better.

Over the past ten years, Wilson and his team have been perfecting devices to study the locomotion of animals. For cheetahs, they assembled a collar that carries a GPS to record location data, an accelerometer to measure speed, a gyroscope to understand angular motion, and a magnetometer to make location data more accurate, which it does by measuring tiny changes in Earth’s magnetic field. The data were transmitted back to the researchers in real time through radio.

“The key development,” Wilson said, “was to pack all that in a low-power device”. The collar relies only on solar cells for recharging, but carries a battery in case of failure.

After tracking 367 runs by five cheetahs in the wild, Wilson found many surprising results.

First, the top speed of most cheetah hunts is on average half the “record speed”. That record speed is 102 km per hour, and was noted in 1965 (though not published until 1997), by a veterinary surgeon in Kenya.

The average length of a cheetah’s hunt was about 180 meters. Instead, on average, cheetahs covered about six kilometers every day. With only two hunts made every three days, high speed runs make for only a tiny fraction of a cheetah’s daily routine.

Second, he found that cheetahs can successfully hunt in all terrains, not just open fields. The run data were overlaid on Google Earth to visualise the landscape the cheetahs were operating in. This showed that only 20% of chases in open fields were successful, compared to 31% in dense cover. Wilson thinks that dense cover, such as trees, might give cheetahs vantage points that open fields cannot.

Third, cheetahs can decelerate faster than they can accelerate, much as sports cars with powerful engines need beefed-up brakes. While both these processes require different sets of muscles and depend on different conditions, the rates of acceleration and deceleration beat those of any other land-dwelling animal. Based on the recorded data, Wilson calculates that the muscle power output of cheetahs is about four times that of Usain Bolt, three times that of polo horses, and nearly double that of greyhounds.

The top speed of a cheetah hunt had no correlation to the successful outcome of the hunt. Instead, Wilson found that success depended more on how fast the cheetah could slow down, rather than on how fast it could speed up. It is this last phase of a hunt that was critical for success, where the cheetah slows down. When these two observations are put together, Wilson thinks that it seems cheetahs don’t abandon hunts early to save energy or reduce risk of injury.

Finally, cheetahs are not built to be able to turn at their highest speed. In an artificial setting, which astronauts and fighter pilots are put into for training, the force felt by a cheetah trying to turn around at top speed could knock it unconscious. Instead they use their ability to slow down and their ridged footpads and claws to grip the ground well enough to turn quickly.

The results of Wilson’s work are published in the journal Nature today. Craig McGowan at the University of Idaho, an expert in understanding animal locomotion who was not involved in this, was impressed by Wilson’s work. “This research has been able to collect a huge amount of data from animals behaving naturally in their environment. No other dataset of this kind exists,” he said.

Roger Kram at the University of Colorado, Boulder, another biomechanics expert who was not involved in the study, said, “The technology used is absolutely fantastic. Most people studying biomechanics of running do so in labs. I’d like to see this technology applied to prey, such as impala and Thomson’s gazelle.”

Wilson is keen to see the technology used widely. “My aim is not to commercialise this. We’ve revealed all the technology and methods in our paper,” he said. His team has already started using it on lions and wild dogs.The Conversation

First published on The Conversation.

Image credit: photosbyflick

This story made it to the front page of Reddit and Digg, receiving over 130,000 views in two days.

Animal Behaviour: The benefits of schooling

Nearly four-fifths of the 28,000 known species of fish swim in schools, harmoniously aligning their movements with others around them. Besides reducing drag for those not in the front of the pack, coming together makes it harder for a predator to single out just one prey; a mass change of direction by the entire school might act to confuse the attacker further.

That, at least, is the theory. The rub is that testing it requires manipulating the behaviour of real fish—trickier even than herding cats. Now, though, Christos Ioannou, from Bristol University, may have found a way around it. As the researchers report in Science, he and his colleagues have developed a video game for piscine predator to play.

They put their gamer, a hungry bluegill sunfish, into a tank and projected computer-generated prey on one of its walls. Each digital fish in the 16-strong school was programmed to maintain their speed and to move away to avoid collision if they get too close to each other. But each was also endowed with a mind of its own: some ignored what their neighbours did while others followed their every move.

It turns out that the real sunfish is indeed more likely to go after the lonely virtual minnows than the more gregarious ones. It seems, then, that there really is strength in numbers, though it be some time before Dr Ioannou manages to coax his bluegill into disclosing precisely why it prefers the loners.

Also published on economist.com.

Reference: Ioannou CC, Guttal V, & Couzin ID (2012). Predatory Fish Select for Coordinated Collective Motion in Virtual Prey. Science PMID: 22903520

Free image from stock.xchng.