1Institute of Biotechnology, University of Helsinki, 2Department of Anthropology, University of Texas at Austin
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The relative lengths of the second and fourth digits (2D:4D) are considered a marker for prenatal androgen exposure. Lower ratios are generally characteristic of males in humans and anthropoid primates, and have been correlated with a wide range of presumably androgen dependent behavioral and morphological traits within species (increased aggression, dominance and physical size) as well as between species (increased intrasexual competition, reduced pair bonding). 2D:4D sexual dimorphism is well characterized in humans and reported for several anthropoids, however the data presented here represent the first assessment of digit ratios in a strepsirrhine, the brown mouse lemur (Microcebus rufus). Due to the female dominance and overall sexual monomorphism typical of lemurs, and the comparable levels of fecal testosterone in this species’ males and females, we expected to find no evidence of dimorphism. Digital measurements of rays were taken and all potential ratio combinations analyzed for male (n=28) and female (n=21) brown mouse lemurs in a wild population in Ranomafana National Park, Madagascar. No sexual dimorphisms were found (p values between 0.38 and 0.82). However, the low 2D:4D (0.75) of these polygynous lemurs fits well with the negative correlation established between intrasexual competition and 2D:4D in anthropoid species, despite the typically strepsirrhine manual ectaxony of M. rufus and the mesaxony of the anthropoid species. Implications for comparisons of a full range of digit ratios within and between groups with mesaxonic and ectaxonic hands will be discussed.