Curiously, however, for these primates, it’s sex-specific. Males showed no such correlation between group-based aggression towards outsiders and intragroup bonding, but the females did. This implies that there must be different evolutionary benefits for females in this regard.
One line of thought is that the male with the most to benefit from an aggressive encounter – more resources, more females to mate with – is the most dominant one. Regardless of the effort they put in in the fight, the lesser males will not be paid attention to by the alpha male, who doesn’t gain from socially bonding with his comrades.
On the other hand, females on the winning side will benefit more evenly, and by socially bonding with each other after a victory, they are more likely to share the spoils of war via the dominant male.
In most societies around the world, this “Us against Them” mechanism is seen more evenly between males and females, but interestingly, there is still normally a dominant figure, male or female, leading the charge. The current US Republican presidential candidate is a perfect example of this in action – his hateful, xenophobic, misogynistic and homophobic rhetoric is galvanizing his followers to rally against anyone that isn't on his side.
"Us versus Them" in action. Gina Santa Maria/Shutterstock