When it comes to mating, males rarely cooperate. In most species males compete for females, through roaring, head-banging, or even eye-length comparison.
However, this is not the case for Geoffroy’s tamarin. In these tamarins, the females mate with all the males in her group that are not related to her. This is called polyandry (the opposite being polygyny). These males then cooperate to raise her two offspring: they rarely fight with each other, they take it in turns to carry the young, and are just generally nice guys. This is a very unusual system, as the males cannot be sure that the young that they are raising is their own (females on the other hand can always be sure a child is theirs: a baby coming out of you it’s a bit of a give-away).
In terms of evolution, it generally does not make sense to rear offspring that are not your own (as doing this will cause your own genes to die out). So why would male tamarins do this?
A recent study looked into just this. The researcher looking into this question predicted that the males in the group of tamarins who were all with the same female would be fairly closely related to each other. If you’re trying to get your genes into the next generation (as every animal is), and if you cannot have your own offspring, then sometimes raising a sibling’s is as good an option. After all, siblings do share half of your DNA. The researcher also predicted that more than one male would actually father the offspring so that, for each male, there would be a greater chance that he would be the father.
The scientist found the answer to these questions by catching tamarins and taking small bits of their hair for DNA sampling, as well as observing their behaviour in the wild. He found that, sure enough, the males were closely related to each other. In fact, they were as closely related as brothers or father and son. This backs up other studies done in captivity which have found that polyandry only occurs when the males in the group are closely related.
Surprisingly, whilst in a few groups the DNA analysis showed that there was more than one father, in most groups only one male fathered the offspring. However, across different breeding events this was not the case, and the father varied more often. Even within a single brood of two fraternal twins, it was found that there were two different fathers (yes, this can happen in humans too, it’s called superfecundation and is more common that you might think!)
In very few species will the males cooperate this much when it comes to breeding. The findings of this study have helped us understand more about how this uncommon occurrence in nature can come about.
Reference: Díaz-Muñoz, S. L. (2011). Paternity and relatedness in a polyandrous nonhuman primate: testing adaptive hypotheses of male reproductive cooperation. Animal behaviour, 82(3), 563-571.