Evolution and the Brown Mouse LemurPosted: November 21, 2012
The rainforests of Madagascar highlight, with great clarity, the power the physical environment exerts on evolution. As a study abroad student in the fall of 2006, I was researching the sleep habits of the brown mouse lemur in Ranomafana National Park, a protected tract of land in the high rain-forested mountains of Madagascar’s east coast.
During the day, I bushwhacked through this dense rainforest, attempting to locate two or three of these nocturnal mouse lemurs, who had been fixed with tracking collars, as they slept. In the evening, I waited for the lemurs to wake up so that I could record the size and consistency of their sleeping groups.
One day, as the sun was setting on the bamboo, ferns, and mossy trees of the forest, I watched as multiple lemurs suddenly emerged and attempted to rouse the female lemur I was tracking from her sleep. These lemurs, all male, were attempting to mate with my study subject.
Female brown mouse lemurs, and indeed many species of female lemurs in Madagascar, are only receptive to mating for a very short period of time each year. To make the most of this short mating season, the male lemurs, deathly focused on a single goal, spend the winter months growing testicles that end up being a quarter of their entire body mass. It is no question, given the males’ months of stored hormonal energy, that there would be a significant interest in my study subject that day.
More and more male lemurs arrived, seemingly out of nowhere, each attempting to gain access to the tiny nest housing the lemur I was observing. They approached, leaping from tree to tree, from all sides, at first standing their own ground. But then, as two lemurs attempted to make it into the female lemur’s nest, a frenzy ensued. Trees shock violently as several brawls broke out. There was a fierce stand off, and the lemurs, fighting their way closer to my study subject, were literally falling out of the sky to the ground where I was sitting. Eventually one lemur managed to force his way in, besting the competition, and as my study subject subsequently fled to the darkness of the jungle, the excitement died off.
Years of research conducted by primatologists have shed some light on why such a violent ritual might have developed in the first place. Each species of lemur mates during a different narrow window of time throughout the year. This means that each species gives birth to their young at different times as well. The result is that nearly all species of Ranomafana lemurs, each with differing cycles of fertility as well as varying gestation times, nevertheless each wean their young at the same time of the year.
This time coincides seems to coincide with the maximum abundance of feeding resources for the young, and it allows their parents to be unencumbered by their offspring as the harsh winter arrives. Being weaned at this time allows the young lemurs the highest odds of survival when first given the opportunity to feed on their own, and allows their parents to be more prepared when resources become scarce.
It has taken millions of years for this system to evolve, but the physical environment has been the ultimate force behind it. Food availability, ultimately controlled by rainfall patterns, themselves controlled by monsoonal circulation patterns in the Indian Ocean, itself controlled by the axis of our planet as it travels around the sun, dictates when each individual lemur in the Ranomafana rainforest will have a chance to mate. It is a vivid example of the interplay between the physical and biological worlds.