Despite Two New Studies on Motives for Monogamy, the Debate Continues

The golden lion tamarin, a one-pound primate that lives in Brazil, is a stunningly monogamous creature. A male will typically pair with a female and they will stay close for the rest of their lives, mating only with each other and then working together to care for their young.

To biologists, this deeply monogamous way of life — found in 9 percent of mammal species — is puzzling. A seemingly better evolutionary strategy for male mammals would be to spend their time looking for other females with which to mate.

“Monogamy is a problem,” said Dieter Lukas of the University of Cambridge in a telephone news conference on Monday. “Why should the male keep to one female?”

The evolution of monogamy has inspired many different ideas. “These hypotheses have been suggested for the past 40 years, and there’s been no resolution of the debate,” said Kit Opie of the University College London in an interview.

On Monday, Dr. Opie and Dr. Lukas each published a large-scale study of monogamy that they hoped would resolve the debate. But they ended up coming to opposing conclusions, which means the debate over monogamy continues.

Dr. Lukas, co-author of a paper in the journal Science with Tim Clutton-Brock of Cambridge, looked at 2,545 species of mammals, tracing their mating evolution from their common ancestor some 170 million years ago.

The scientists found that mammals shifted from solitary living to monogamy 61 times over their evolution. They then searched for any factors that these mammals had in common. They concluded that monogamy evolves when females become hostile with one another and live in ranges that do not overlap. When females live this way, they set up so much distance between one another that a single male cannot prevent other males from mating with them. Staying close to one female became a better strategy. Once males began doing so, they sometimes evolved to provide care to their offspring as well.

For his study, Dr. Opie and his colleagues examined 230 primate species, because monogamy is especially high in that group. They came down in favor of a different hypothesis: the threat of infanticide drove the evolution of monogamy.

“What we found was a very neat pathway for primates to evolve monogamy,” Dr. Opie said.

In many species of mammals, males will sometimes kill the young offspring of other males. Scientists have proposed that they do so because nursing females do not ovulate. By killing a female’s offspring, a male then gains the chance to have offspring of his own with her.

In The Proceedings of the National Academy of Sciences, Dr. Opie and his colleagues argue that in many primate species, males responded to the threat of infanticide by sticking with females after they gave birth.

Dr. Opie offered possible explanations for why his team and Dr. Lukas’s came to different conclusions. It is possible that the forces driving the evolution of monogamy in primates are different than in other mammals. Dr. Opie also noted that he and his colleagues had used a more powerful type of statistics, known as Bayesian probability, to reconstruct the evolution of monogamy.

“They don’t use the latest methods, which is a bit of a pity,” Dr. Opie said.

But Jacobus Boomsma of the University of Copenhagen, who was not involved in either study, found Dr. Lukas’s paper to be superior. “It makes perfect sense to me,” he said.

Sergey Gavrilets, an evolutionary biologist at the University of Tennessee, also favored Dr. Lukas’s conclusions. But he also noted that neither study tested all of scientists’ proposed explanations, like monogamy’s benefits in lowering the risk of sexually transmitted diseases or the possibility that females chose to mate with males who repeatedly brought them food.

“It is still unknown how these other scenarios fare,” Dr. Gavrilets said.