Study finds lack of evidence for infanticide link to monogamy

A fuss over the evolution of males that kill babies has taken a new turn. A broad analysis of mammals says there’s no sign that the danger of infanticide drove species toward monogamy.

That new analysis, in the Nov. 14 Science, examines an evolutionary genealogical tree of 260 species, from lions to mouse lemurs. In 119 of the species, observers report males killing infants they hadn’t sired and thus freeing up the mothers for new matings.

The study tests basic ideas about the evolution of infanticide, but the most contentious result may be its conclusions about what biologists call social monogamy —animals living in pairs (even though there may be a bit of extra-pair mating now and then). In the new analysis, there’s “no evidence” that infanticidal males notably increase the chances of solitary-living ancestors switching to monogamy, says coauthor Dieter Lukas of the University of Cambridge.

Lukas still sounds a bit shocked that in a single week of 2013, he and another research group published papers that came to very different conclusions about infanticide as a possible driver for the evolution of monogamy (SN: 8/24/13, p. 5). 

Lukas, with Tim Clutton-Brock of the University of Cambridge, rejected the idea that risks of infanticide pushed females toward sticking near one male for protection. Instead the pair argued that in species that evolved monogamy, females’ need to defend food territories and space themselves widely across the landscape, could have made monogamy more practical for males trying to defend a female from other suitors.

Meanwhile, says evolutionary anthropologist Kit Opie of University College London, “We came to pretty near opposite conclusions.” Looking at genealogical trees to trace the deep history of infanticide, he and his colleagues argued that the rise of baby killers had indeed nudged along the evolution of monogamy among primates.

Now Lukas and Elise Huchard of the University of Montpellier in France have been much stricter in selecting species to examine. Looking only at the species with best documentation of infanticide (or a lack thereof) across the major groups of mammals, the researchers confirm such fundamental ideas as the strong link between ability of females to breed year-round and the rise of infanticidal males. Baby killers show up in only 28 percent of the one-season-only breeders but in 76 percent of any-season breeders.

However, species that arose from solitary ancestors weren’t any more likely to switch to living in pairs or bigger groups if male infanticide had developed, the researchers say.

But Opie isn’t changing his mind, at least for primates. Part of the chasm between results may come from varying definitions of monogamy. Lukas considers a species solitary if the animals spend most of their time alone in their territories. Yet Okie points out that animals that are solitary in this sense can still mate with multiple partners. Solitary dwarf lemurs, for instance, mostly mate with one partner, but the equally solitary sportive lemurs mate in a harem pattern. 

Also, Opie argues that primates may have special quirks in their evolutionary paths because long lactation periods help infants develop their generally big brains. So the slow timing of reproduction may mean infanticide is especially useful for males.

Sarah Hrdy at the University of California, Davis, who wrote many of the early papers on the infanticide issue, welcomes the careful selection of species in the new study. And she is inclined to agree that infanticide probably did not drive solitary females to living in company.