Why Are Some Mice (and People) Monogamous? A Study Points to Genes

Dieter Lukas, an evolutionary biologist at the University of Cambridge who was not involved in the research, hailed the study as a sophisticated tour de force, saying that uncovering these links “is like designing a tool to follow individual threads through a large colorful tapestry.”

The findings may one day help scientists make sense of how human couples bond and care for their children. Mammals share many of the genes governing the production of hormones and neurotransmitters in the brain.

Variations in how they function may explain why most species are promiscuous, why a few are monogamous — and why some, like humans, are somewhere in between.

“We can go from the bottom up and build our knowledge base, and then ask questions about human biology,” said Gene E. Robinson, a biologist at the University of Illinois who was not involved in the new work.

In the vast majority of mammal species, males mate with as many females as possible and offer no help with raising offspring. Oldfield mice belong to the about 5 percent of species in which a male forms a long-term bond with a single female and offers paternal care.

Hopi E. Hoekstra, an evolutionary biologist at Harvard University and senior author of the new study, first became aware of oldfield mice as she was digging through dusty old reports from naturalists.

Their closest living relatives, deer mice, dwell in forests instead of open spaces, she learned, and they live promiscuously instead of monogamously. In the wild, deer mice and oldfield mice never interbreed.

But the two species will do so in laboratory experiments if a single male and female are placed in a cage together. Their offspring are healthy and fertile.

Dr. Hoekstra realized that it might be possible to compare parenting behaviors in the two species, and then see how their hybrid offspring behaved. The differences could lead the scientists to the genes influencing these behaviors in each species.

Andrés Bendesky, a postdoctoral researcher at Harvard, began the research by observing as each mouse species raised its young in the lab. He tracked a number of parenting behaviors, from building a nest for pups to licking them.

Deer mice put in less parenting, but it was possible that the cause was not genetic. Perhaps they were neglected by their own mouse parents, and the behavior was learned. To compare nature and nurture, Dr. Bendesky and his colleagues moved deer mice pups into oldfield nests — and vice versa.

The swap, they found, had no effect on how the mice behaved when they grew up: Oldfield parents were still solicitous, deer mice much less so.

Dr. Bendesky and Dr. Hoekstra concluded that the differences in the mice’s behavior must be anchored in their DNA. So the researchers carried out a large-scale breeding experiment.

They paired five mice from each species, producing 30 hybrids, which then yielded 769 pups…

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