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She has her father’s coat, and her mother’s testosterone
April 12, 2009Posted by on
In many primate species, there is sexual dimorphism in the color of the coat, or pelage. Males are one color, and females are another. Sometimes, infants of the species are born with a color that is different from both mom and dad, and go through a molting as they grow and develop into adults. It seems that this kind of coloration might be easily explained by adaptation and natural selection: Perhaps the infants need to blend into their environment more so they aren’t targets for predators.
The only problem with that scenario is that many of these species have brightly colored, highly conspicuous infants. Take, for example, the Silver Leaf Monkeys to the right. Adults are all dark gray and black, but the infants are bright orange! How could that possibly be adaptive?!
It turns out that predators from other species aren’t the only danger that a baby monkey might face. In some of these species, infanticide from high-ranking males is a distinct threat. Perhaps conspicuous natal coats serve to attract the attention of other members of the group and encourage them to protect and take care of the infant. And then perhaps the coats further serve to say to potentially infanticidal males, “Stay away, I’ve got backup!” Or perhaps these natal coats serve to confuse the paternity of the infants. The males can’t look at the infant and say, “Ah, he’s got the same black coat as I have, so he’s safe,” or “That kid looks an awful lot like neighbor Bob- I should make sure that I don’t have to invest any energy taking care of it.” These different hypotheses all have varying amounts of empirical support, but it’s safe to say that we’re still not really sure what’s going on.
But let’s go back to the sexually dimorphic species. In these species, infants can be born with a natal coat that resembles the male, the female, is intermediate between the two, or differs dramatically from both. Infants are usually all born the same color, regardless of sex. Julia Barthold and her colleagues chose to explore these relationships using an interesting species- the Redfronted Lemur. In Redfronted Lemurs, females are competitive for mates, and so female-female aggression is relatively common. Females have also been observed to commit infanticide, though it is very rare. All Redfronted lemurs are born with a natal coat resembling the adult coat of males.
Barthold et al. suggest that, since females are so aggressive toward other females in this species, the natal coats serve to protect the infant females and their mothers from aggression. However, during the four months that they were observing these animals in Madagascar, they observed very low aggression overall, and the rates of aggression didn’t change once the females began to molt into their adult coats. Their hypothesis wasn’t directly supported. But, perhaps the low aggression rates are the result of successful sexual mimicry on the part of the females? Or maybe aggression only occurs occasionally, under specific conditions which were not in effect at the time of observation. Or, perhaps the dark natal coat in females is merely a side effect of high prenatal androgen levels in the species. High prenatal androgen levels result in masculinization of the genitalia in lemurs, and a lot of their behavior is probably determined by these hormone levels. In that case, it might be that the adaptation is for female dominance and philopatry, and the result is that female natal coats are masculinized.
In the particular case of the Redfronted Lemurs, I think that the last explanation is the most convincing. Not everything is an adaptation, and the simplest explanation is often the correct one. But something strange is going on in those leaf monkeys…
Barthold J, Fichtel C, Kappeler P. 2009. What is it Going to Be? Pattern and Potential Function of Natal Coat Change in Sexually Dichromatic Redfronted Lemurs (Eulemur fulvus rufus). American Journal of Physical Anthropology 138:1-10.