While female killer whales usually spend their old age surrounded by a growing family, the female spotted hyena has fewer and fewer annoying relatives around her as she gets older. Because whether there are many relatives in the group in which a mammal lives changes with age, depending on sex and species. Does that also shape the behavior?
A team led by Samuel Ellis from the University of Exeter recently tried to predict these so-called kinship dynamics using a theoretical model. The scientists looked at seven mammal species living in groups: Banded mongooses, chimpanzees, European badgers, killer whales, rhesus monkeys, spotted hyenas and yellow baboons. The results were published in the journal Nature ecology & evolution released.
How the relationships develop in the group depends on the way of life of the species. Female chimpanzees are more likely to leave their original group and only surround themselves with more and more relatives as they get older. With rhesus monkeys or spotted hyenas it is the other way around, it is more the males who look for a new group and only have their own family there over time.
It may be worth supporting the group for older female killer whales
Banded mongooses and killer whales, on the other hand, have both males and females staying in their groups but looking for outside mates to reproduce. In both species, the relatedness of females in the group increases with age, as more distant relatives are progressively replaced by the direct offspring of the females, while the males’ offspring tend to grow up in other groups. The model calculated by the researchers was able to predict the real data well in most cases.
Such changes can affect the behavior of group members towards each other. It can actually be evolutionarily worthwhile for older females to no longer reproduce themselves and instead to support the group – provided that this then consists largely of close family members, as is often the case with killer whales, for example. This could be one reason for the development of menopause.
In fact, many species show age- and sex-dependent differences in whether the animals help or harm each other, for example when foraging or fighting. Older female killer whales, who usually have many family members around, are more likely to share their food than younger ones. At the same time, it is known that degrees of kinship can play a major role in cohesion and status. For example, male spotted hyenas are supported by their relatives in conflict situations within the group.
Thus, the development of kinship of individuals with the group over their lifetime may play a role in how age and sex affect the incentives for selfless or aggressive behavior in different species – and thus help explain why grandmothers, for example, often behave very differently as young male individuals.