Croft, D.P., R.A. Johnstone, S. Ellis, S. Nattrass, D.W. Franks, L.J.N. Brent, S. Mazzi, K.C. Balcomb, J.K.B. Ford, M.A. Cant. (2017). Reproductive Conflict and the evolution of menopause in killer whales. Current Biology. 27: 298–304. Open access article available here.
Killer whales, or orcas (Orcinus orca), are amazingly intelligent and social animals. Their complex society is rivaled only by elephants and higher primates, like humans and chimps. A family group (pod) is composed of an older female, the matriarch, and her siblings, children, and grandchildren. A newborn calf lives with its mother for its entire life, and is surrounded by its maternal relatives.
The mystery of menopause
Orcas, along with short-finned pilot whales and humans, are one of the few animals that undergo menopause, living for decades in a post-reproductive state. Why any animal would outlive its fertility is still a mystery that baffles scientists. The most common explanation is the “grandmother hypothesis,” which argues that a long post-reproductive lifespan allows older females to improve the success of her relatives (and indirectly ensure the continuation of her own genes). But skeptics of the grandmother hypothesis have long argued that the math doesn’t added up – why would she give up having children of her own, each passing on half of her genes, for the sake of her grandchildren, who only carry a quarter of her genes?
A group of researchers based in the UK and North America thought we might be missing something. To see if this was true, they took advantage of 43 years of demographic records from resident killer whale populations to find out. They modeled how kinship changed in killer whale populations over time, and tested the hypothesis that reproductive conflict was a major force in the development of menopause in killer whales.
Female killer whales have their first calf at about 15 years of age, and are reproductive into their late 30s and even their 40s. This means older females may be pregnant at the same time as their younger daughters. The co-breeding of older and younger generation females in the same pod at the same time, and limitations of resources like food (female whales need to consume ~42% more food to support lactation), sets up a reproductive conflict.
Calves born to older mothers were almost twice as likely to die than those born to younger mothers at roughly the same time. This was not due to the older mother being weaker or senile, as mother’s age alone had no effect on how likely a calf was to survive to maturity. The researchers hypothesized that this has to do with older mothers investing less than younger mothers in reproductive completion due to the unique social structure of killer whales.
Resident killer whales live with their mothers and her family for their entire lives. At birth, a calf is only distantly related to the males in her pod because her father is from a different social group. As a young female matures and begins to have calves of her own, her children and grandchildren make up a larger part of the group, and her genes are found in a greater proportion of the pod.
Since older mothers are surrounded by their children and grandchildren, they may feel pressured to share their food and other resources with the pod to a greater extent than younger mothers. This may lead to less food being allocated to their newborn calves and less parental investment overall (since the older mother’s attention is so divided). Looking at it from another perspective, perhaps younger females are able to hoard more food for their offspring, by being more competitive than older females because they are less concerned with sharing.
Interestingly, birth order was also an important determinant in calf survival. When two calves were born at roughly the same time, the firstborn calf was almost twice as likely to die. The researchers suggests this was due to the benefits of having a potential surrogate mother nearby, who can provide babysitting or nutrition. So while there was a cost to reproductive conflict, there were also positive to having multiple reproducing females in the vicinity.
On a similar note, calves were born without a reproductive conflict, such as when the grandmother had recently died, did not survive as well as even second-born calves. This emphasizes the key role that senior females play in their pods, as they rely on decades of knowledge to lead their family to reliable food resources.
The evolutionary value of menopause
Female killer whales can live into their 90s, decades after the birth of last calf. Why has this evolved? What is the point of having such an extended post-reproductive lifespan?
This study suggests that reproductive conflict between older and younger mothers becomes so intense that older mothers give up on having calves soon after the birth of their first batch of grandchildren. Post-menopausal whales then devote their time to ensuring their pod secures resources, making sure her many sons and daughters can successfully raise their own offspring. Instead of having calves late into their lifespan, older females ensure their genes are passed on by guiding the next generation to success.
Taking the intense cost of reproductive conflict together with the benefits of grandmotherly behaviour may help explain why menopause evolved in the first place. While researchers had known about the grandmother effect for decades, they had difficulty using it to justify the early timing of menopause, since the benefits of post-reproductive helping lag so far behind continuing to have children. This paper’s examination of the consequences and intensity of reproductive conflict in resident killer whales bridges some of this gap by demonstrating the cost of intergenerational reproductive conflict.