Killer whales have been revealed to be one of only three species whose females are known to undergo menopause—living on long after their reproductive years in order to help their offspring, particularly their sons, survive the rigors of young adulthood and later to help raise their grandchildren. It is a rare evolutionary trait, shared with humans and pilot whales, that may have much to do with the killer whales' success as a species, say British researchers at the University of Exeter and the University of York, who have conducted a 30-year study.
Like human mothers, female killer whales can bear offspring well into their 30s, but then may live on for another 50 years or so to look after their families, pass on their knowledge and skills to the younger generation, and assume a leadership role in the community. Their steadying presence is believed to dramatically improve a young male offspring's survival chances.
Males are particularly vulnerable when young; one study suggests there is a 14-fold increase in mortality in the first year after the death of their mother. The survival of female offspring seems less dependent on the presence of the mother.
"Killer whales have a very unusual social system whereby sons and daughters don't disperse from their social group but instead live with their mother her entire life," says Darren Croft, a lecturer in animal behavior at the University of Exeter, who led the study with University of York biologist Dan Franks.
How did you determine that killer whales go through menopause?
Franks: The data comes from tracking over 500 whales for more than 30 years. Looking at this we can see that female killer whales stop reproducing in their 30s or 40s, yet live into their 90s. This is a striking 50 to 60 years of their life in which they no longer give birth.
Does menopause occur among other species of whales too?
Franks: We know that short-finned pilot whales undergo menopause. Anecdotally, there are some suggestions that other species of whales may undergo menopause as well, particularly sperm whales. More data is needed, however, to establish whether these other species actually do undergo menopause.
You mention that only three mammals undergo menopause—humans, killer whales, and pilot whales. How do we know there are no others?
Croft: Biologically speaking, menopause is a bizarre concept, and very few species have a prolonged period of their life span where they no longer reproduce. There are only three species that we can be sure undergo menopause. It is certainly possible that other species may also have menopause, particularly in species that live in close-knit family groups, but this is not yet known.
Do the whales, like humans, appear to undergo any other physiological symptoms or changes, such as hot flashes, as they go through menopause, or is that impossible to know?
Croft: Unfortunately, we don't have the physiological data on the whales to know what changes females experience as they go through menopause. Given the size of the animals and the fact that they live in the open ocean, it is difficult to get such data.
What are the evolutionary advantages to having older menopausal females in a population? What specific things do older females do to enhance the survival of young males?
Franks: We know that older females increase the chances of their offspring's survival, but exactly how they do this is a topic of our current research. Our team has a few ideas about what could be going on here, and we can speculate that they may provide support during encounters with other whales and help with foraging through knowledge and leadership.
If there are clear evolutionary advantages to menopause, why haven't other creatures developed this? Why whales? What's so special about them?
Croft: For menopause to evolve, the benefits of stopping reproduction in late life have to outweigh the costs. Theory suggests that the key to understanding why menopause has evolved in the killer whales is in understanding their social structure and how a female's relatedness to her group changes as she ages. Killer whales live in close-knit family groups where both sons and daughters stay with their mother her entire life. Under these conditions, as females age, their relatedness to the group increases and there comes a point where females can benefit more from switching from having offspring themselves to helping to care for either their existing offspring or grand-offspring.
Given the perceived rarity of menopause across the animal kingdom, what would whales and humans have in common?
Croft: Theory suggests that the key to understanding why menopause has evolved in whales and humans is in understanding their social structure. In both humans and whales, as females age, their relatedness to their local group increases. This increase in relatedness means that evolution will favor females that switch from having their own offspring in late life to helping their offspring and grand-offspring survive and reproduce.