Scientists learn that mice nest together to confuse paternity and reduce infanticide

mice

It is a cruel world out there, particularly for young animals born into social groups where infanticide occurs. This dark side of evolution is revealed when adults – often males – kill offspring to promote their own genes being passed on, by reducing competition for resources or making females become sexually receptive more quickly.

This behaviour proves expensive for females, who have evolved strategies to avoid this fate. One strategy is to join forces with other females to physically ward off killer males. A more interesting strategy is to mate with several males, known as “polyandry”, so fathers can’t distinguish their young from others’, which means they avoid killing pups so that they don’t accidently kill their own.

Now, researchers at the University of Zurich have found a new type of infanticide counter-strategy: mothers can achieve paternity confusion even if they don’t mate with multiple males, through nesting with other females, which they call “socially mediated polyandry”. And such a strategy might be happening close to home, in the unassuming house mouse.

Yannick Auclair and his colleagues put their theory to the test on a wild population of mice living in an old agricultural building outside Zurich. They measured the genetic relationships within litters and found a complex picture of female social relationships and mating patterns. These allowed them to identify mothers nesting alone or with others and those who mated with one or more males. To examine the risk of infanticide for pups born into these different types of litters, they assessed survival until just before weaning, which is about two weeks after birth.

Direct observations of infanticide are extremely rare in natural systems. But studying an enclosed population without the presence of a predator meant that infanticide becomes the most likely cause of death for young pups. And indeed, from the corpses of pups that were recovered, most gave direct evidence – missing limbs or holes in the skull – of this harsh fate.

The results of the study were published in the journal Behavioral Ecology. The researchers found that pups born to females nesting alone and who had only one mate had the lowest survival rate (50% surviving, the rest presumably killed by males who were confident they were not the father). Meanwhile, those born to females nesting together were better off (80% surviving).

Key evidence supporting their theory was that some of these communal litters were composed of pups whose mothers had actually only mated once, but the different females had different mates. These litters had similar survival to those where paternity was mixed for individual mothers, suggesting that mothers can achieve the same survival benefits of communal nesting without mating with multiple males.

There were also a few communal litters (nine of the 90 studied) where the different females had mated with the same male and, as such, featured multiple mothers but no paternity confusion. These litters had worse survival rate (40% surviving) suggesting that – as predicted by the theory – paternity confusion is a more important driving factor of communal nesting than the physical warding off of males.

According to Elise Huchard of CNRS Montpelier: “This study presents an interesting idea, and an interesting system to test it.” Yet the data raise more questions than they answer – and additional experiments or comparative work would be insightful.

For example, it is not clear whether higher survival in litters with multiple fathers might actually reflect variation among females if, as in the case of mouse lemurs, higher-quality females have more mates. Dieter Lukas of Cambridge University concurs that the theory is interesting, but believes it is too early to assess its generality.

Infanticide occurs across diverse mammal systems – from meerkats and rabbits, to lions and gorillas – and comparative analyses could help assess how this theory fares among the many hypotheses about the evolution of infanticide.

Communal nesting may have evolved as an alternative to mating with multiple mates (which is costly when males harass females during mating or transmit disease) as a strategy to avoid infanticide through paternity confusion. “We don’t know whether other social behaviours may have evolved through similar ways,” said Auclair.

Comparative analyses will lead to new insights and future research on the nest-box population will also address such interesting questions as how females choose their nesting partners – and why some still nest alone even if this comes at a cost to offspring survival.

http://theconversation.com/whos-your-daddy-mice-nest-together-to-confuse-paternity-and-reduce-infanticide-31796

Monogamy may have evolved to keep baby-killers away

monogamy

Social monogamy – when a male and female of the species stick together for the long term, although may mate with others – is rare in mammals generally. However, it occurs in over a quarter of primate species, including humans, gibbons and many New World monkeys, such as titis.

To investigate what originally drove us to establish such pair bonds, a team led by Kit Opie of University College London and Susanne Shultz from the University of Manchester, UK, gathered data on the mating behaviour of 230 primate species. They selected behavioural traits associated with several possible evolutionary drivers of monogamy, including the risk of infanticide, the need for paternal care and the potential for guarding female mates.

Using data on the genetic relationships between the species, the team ran millions of computer simulations of the evolution of these traits to work out which came first.

All three were linked to the evolution of monogamy but only behaviours associated with infanticide actually preceded it, suggesting that this was the driver. Suckling infants are most likely to be killed by unrelated males, in order to bring the mother back into ovulation.

With pair-bonding in place, not only would a mother have a male to help protect the infant from marauding males, but there would then be the opportunity for the male to help care for it by providing extra resources. This means the infant can be weaned earlier, again reducing the chance of it being killed.

“Until recently, reconstructing how behaviour evolves has been very tricky as there are few behavioural traces in the fossil record. The statistical approach we have used allows us to bring the fossils to life and to understand the factors that have led to the evolution of monogamy in humans and other primates,” says Shultz.

Carel van Schaik, a primatologist at the University of Zurich in Switzerland says the results are solid but questions whether they can be extrapolated to humans. He says evidence suggests that humans were never really monogamous and that the monogamy we see today in many cultures is socially imposed.

Shultz counters that there is fossil evidence pointing to monogamy in australopithecines, the hominin genus from which modern humans descended.

“Although we suggest that infanticide may help explain the evolution of monogamy in humans, we do not argue that it is the only factor nor that monogamy is universal,” Shultz says. “I would suggest that where infanticide risk is high, as it would be with our ancestors, having a father provide protection and care would facilitate the evolution of the modern human extended childhood.”

Journal reference: PNAS, DOI: 10.1073/pnas.1307903110