Why are some species monogamous? Two new studies conducted to answer a question that has intrigued biologists for more than a century have come up with two different answers.

One study, published in Science Magazine, reasoned that social monogamy derived almost exclusively from a "solitary individual social system," in which females lived far apart from one another. According to the study, monogamy is a mating strategy which developed over time because males were unable to defend multiple females, especially when the females lived far apart. 

The other study, published in Proceedings of the National Academy of Sciences, argued that males used monogamy as a tool to protect their offspring from male infanticide from rival males who may see offspring as a an obstacle to mate with females.

Social monogamy -- the pairing of one breeding female and one breeding male who are closely associated with one another over several breeding seasons and raise their offspring as a unit -- is uncommon in mammals. Only nine percent of mammal species examined in the Science paper were classified as monogamous.  Polygyny -- where one male mates with multiple females -- permits males to spread their genes more extensively.

According to an article in the Los Angeles Times, scientists have three leading theories in which to explain the reasons for monogamy. The first is that males play a role raising and protecting their offspring -- thus helping to ensure their survival. The second is that it monogamy evolved as a way for males to guard over one female -- since they were unable to guard multiple females if they lived far apart from one another. The third theory postulates that monogamy is used as a way to defend against infanticide.

The study, led by Dieter Lukas and coauthor Tim Clutton-Brock, at the University of Cambridge in England categorized the 2500 mammalian species for which information exists as solitary, socially monogamous or group-living. Their findings reinforced the theory that these species switch to monogamy as females developed their own exclusive territory.

Previously, it was thought that monogamy evolved as a result of selection for paternal support in raising offspring. This study, however, demonstrates that paternal care typically evolved after mates were already monogamous.

The other research team led by Kit Opie, a biological anthropologist at University College in London, came up with a different conclusion. Their study argued that reducing infanticide was the only explanation for monogamy.