With a body length of just 27 centimeters and a maximum weight of 300 grams, the Northern giant mouse lemur is indeed one of the larger representatives of mouse lemur species in Madagascar, but it is not a true giant. Scientists from the Oxford Brookes University, the Bristol Zoo and Peter Kappeler of the German Primate Center have now discovered what is really big on the small, nocturnal primates: These are their testicles. In relation to their body size the giant mouse lemurs have testes that have on average 15.5 cubic centimeters and thus are the largest among all primates species. If humans had testicles of an equivalent size to the lemur, they would be as large as two grapefruits.
The Northern giant mouse lemur is not known yet a very long time. Researchers of the German Primate Center have discovered and described the lemur species in Madagascar only in 2005. Therefore, little was known about mating system, seasonal mating and reproduction times of the animals until now.
During the field study on Madagascar, the researchers examined twelve Northern giant mouse lemurs. Five males were collared with radio-transmitters and caught in May and again in July. The researchers found that the testicles of the males were very large in relation to their body size. In addition, they could not detect any differences in testicle size between May and July. Furthermore, some of the females were in met-estrus, although they were not caught in the originally presumed mating period.
The results showed that the Northern giant mouse lemurs mate not seasonal like most of the other lemur species, but mating can take place year-round. In addition, the giant lemurs have a polygynandrous mating system where most males mate with several females. There is usually a high competition for fertilization in such mating systems, which is counteracted by a high male sperm production. In adaption to aseasonal reproduction and polygynandrous mating system, males of the Northern giant mouse lemur developed those large testicles.
Rode-Margono, E.J., Nekaris, K.A.-I., Kappeler, P.M. and Schwitzer, C. (2015): The largest relative testis size among primates and aseasonal reproduction in a nocturnal lemur, Mirza zaza. American Journal of Physical Anthropology, DOI: 10.1002/ajpa.22773