Female killer whales apparently live for about 60 years after the end of their reproductive life (totally about 90-95 yrs). Why do they stop reproducing in the first place? That is a central question in this new study (1). In many mammalian groups, once a male reaches reproductive age, they mate within the group and then leave. In the case of apes, the females leave instead of the males. But Killer whales have unique kinship dynamics or group architecture. Here neither male nor female killer whale babies (brothers and sisters) leave their group after reaching reproductive age and about 2-3 generations of them continue to live together led by the original mom whale. The males however mate outside of their group.
In this study they looked at two groups of whales (n= 200) and find that the relatedness between males and a female in a group increases up until she reached reproductive age (as predicted by a previous study). But after that, it declines steadily because the males, although they live in the same group, die around 30 years of age! The relatedness between females remains constant throughout their life span as they are born in the group and do not leave it and also as mentioned above, live twice as long as the males after they stop reproducing. Based on the unique group dynamics of whales compared to mammals and apes, the authors make predictions about why menopause might occur in females. According to this, if females were to increase their reproductive capacity (fecundity) with age, then they would be competing with the younger females in the group for food (foraging and sharing of salmon), contributing to conflicts over collective movement and not able to effectively assist their male progeny in successfully mating outside of their group. Very interestingly and seen in many human cultures around the world, the mom likes to share food with her male progeny preferentially as she wants to ensure his reproductive success. So with increase in fecundity, the female will also be required to invest more of her energies in competitive efforts and this will indirectly affect the well-being of the entire group for the above mentioned reasons.
They test this prediction by observing the intergenerational conflict in the two groups of 525 killer whale calves from Washington state, US and British Columbia, Canada. They define intergenerational conflict as when mom whale gives birth to a calf within 2 years either side of the birth of a grand-offspring. Pregnant mothers require more food and resources during gestation and lactation and so their definition of intergenerational conflict is supposed to represent these periods of highest conflict for food and resources. As predicted, they found that when mom and daughter are in intergeneration conflict, the mom’s calf (sibling of the daughter) has about 1.7 fold lesser chances of survival compared to her grand offspring. They explain that this could be primarily due to food and resource sharing in which the younger mothers might be at an advantage in competing than the older mother who has to share not only with the new born but also with her male offspring from before if any.
Among the calves that are born in intergenerational conflict, they found that the first born calf has a slightly higher chance of mortality than the later born calves. These second born calves take advantage of having two lactating females around while that benefit is unavailable to the first born calf in intergenerational conflict. Another study previously showed that grandmother killer whales provide leadership in times of food scarcity and also salmon foraging knowledge to their offspring and these might be additional benefits that the group gains (2). However, the authors of this study believe that this knowledge-sharing benefit alone has been insufficient to explain why other mammals like elephants that do the same thing continue to reproduce till the end of their lives. Based on their findings, they suggest that given their unique kinship dynamics, and as a result the intergenerational conflicts that arise in food and resource sharing, killer whales could have been selected to experience menopause.
I cannot imagine how exhausting and difficult such a prolonged study following hundreds of whales would have been but the results are interesting simply because killer whales are among the very few (3 including humans) animals that undergo menopause and live long lives after. Why were we selected to go through with this? This study presents a glimpse of one possible reason ‘why’ in killer whales but leaves us wanting to explore more, especially in humans.