Wallflower or daredevil: Personality of male primates changes throughout life

Adult males are willing to take high risks, which they might pay for with their life, in order to gain reproductive advantages. Because young males are rather unsuccessful in mating, they tend not take the same risks, which keeps them alive for when they have better odds for successful mating.  Mature males, in their prime, indeed do father more young and some individuals father more young than others. It is therefore worth taking chances with some daring adventures to improve their reproductive success. Females, on the other hand, keep their personality traits throughout their entire life which makes sense, because every year they have about the same chance of having offspring.

It is still a mystery why individuals of the same species living in the same area often differ fundamentally in their personality. If only the one who is best adapted to his habitat made it, the most advantageous trait would survive while others would decline. But it is not like that and the reason might be that at certain times of the life cycle different behavior is optimal.

Melanie Dammhahn studied gray mouse lemurs, small nocturnal primates, in the forest of Madagascar. She wanted to know whether there are differences in the temperament of the animals. Therefore, she caught about one hundred of these tiny animals, which weigh only about 60 grams, and observed their behavior in new environmental settings. Are they bold and investigate the toy car or the plastic duck? Or are they rather shy? How does their behavior change while growing older?

It turned out that younger males were considerably more cautious than older ones while among females there were no differences between the age groups. The answer to this might be found in the reproductive behavior. At an age of ten months mouse lemurs reach sexual maturity. But females almost exclusively mate with older males, as these are bigger and competitively stronger driving away younger competitors. For younger males there is no advantage to risk their lives through venturous actions. "They have much to lose and little to gain as it is unlikely that they might reproduce" said Dammhahn. Thus, it seems reasonable to take care of oneself until one is old and big enough to beat competitors. For older males things are different. They have little to lose and much to gain for recklessness pays off, even if one's life is at stake.

"Wallflower or daredevil is a question of balancing between today and tomorrow," summarizes Dammhahn.