Killer whale behaviour shows granny knows best

Dr Sam Ellis is part of a team at the University of Exeter that has been studying the behaviour of killer whales and other cetaceans using data collected over several decades.

His research is helping us understand the role played by older females in killer whale communities once their breeding years are over.

Only killer whales, false killer whales, short-finned pilot whales, beluga whales, narwhals and humans live for long periods of time after they stop producing offspring. Otherwise, almost all animals reproduce until their death.

“There are only six living species in the world that scientists know for sure experience menopause,” Sam says.

“Even chimpanzees – which have more in common with humans genetically than whales – continue reproducing right up until the end of their life.”

Studying the behaviour of killer whales and short-finned pilot whales has helped scientists shed some light on why menopause happens to females.

The research has revealed that post-productive life is of important value to the social and communal lives of these two species.

Orca-ward encounters

Without actually having to set foot on a boat, the team at the University of Exeter work primarily with demographic data supplied by the Center for Whale Research in Washington State, which has been running a killer whale survey project since 1976.

The project has more than four decades of photographic records covering a population fluctuating between 80 and 100 individuals in the Pacific Northwest, known as southern resident killer whales.

“The behaviour that we're looking at is really rarely observed and therefore hard to find and monitor through contained periods of fieldwork,” says Sam.

“It’s also quite challenging to study whale behaviour in the wild because they spend so much time underwater and far from shore. We are really lucky to have this much data showing social relations over time and socialisation with other whale groups.”

The community of fish-eating southern resident killer whales are well-known to scientists.

Sam says, “we know most of their ages, when they were born and when they die, how often they are reproducing. We also know about their social structure because there are photographs that show which whales are spending time together.”

Why does menopause exist?

There are a number of theories as to why menopause exists. One popular theory is called the grandmother hypothesis.

Some scientists believe that there could be a biological need to work to protect the next generation’s offspring, rather than continuing to have more young that would be competing for resources.

It can make sense for female killer whales to have a longer post-productive life span if it helps their young to breed successfully and their grandchildren to survive.

Sam says, “as an older female in the pod, as life goes on you become more and more related to the children around you, so there is a strong pressure to help the individuals around you. This is what's known as inclusive fitness.”

“There is a cost to continue reproducing, and the offspring of older killer whales have been found to be statistically less likely to survive.”

“In addition to that, resources are finite so menopause could be considered a survival mechanism that helps overcome reproductive conflict.”

Menopause allows older females to take more of an interest in other young in the group who may also be related to them, further contributing to the survival of the pod as a whole.

The granny effect

Female killer whales have a much longer lifespan than male killer whales. While males rarely live to the age of 40, females regularly live to their 60s – and sometimes even 90s.

Studies of killer whale populations have shown that the presence of grandmothers is particularly important to the survival of young males.

Sam says, “in earlier studies of lifespans of killer whales it was found that when their mother dies, adult males have something like an eightfold likelihood of dying in the next couple of years. This is because they are so reliant on their mothers to show then where to find food.”

“It’s quite common to find massive males over 30 years of age still living and swimming with their mums,” adds Sam. “And in the research we are doing now, we are finding a similar trend with grandmothers.”

In times of abundance, killer whale hierarchies are usually fluid and see regular leadership exchange. But when times get tough, and salmon are scarce, the pod will always turn to the oldest female to lead.

Is this an underwater matriarchy? “Not exactly,” says Sam. “The relationships of killer whales are best described as matrilineal.”

”It’s not that the females are specifically in charge all the time. But when food is hard to find, the oldest female will lead as she is likely to be the most experienced hunter and to have survived lean years before.”

Mum’s the word

Killer whales have what is called a fission-fusion society, which means the size and composition of groups change over time. Within this society, however, families create stable subgroups.

“For most of the time adults will form small family groups,” explains Sam. “Sometimes they will join other family groups, and other times a whole population will be seen together. It’s very hierarchical.”

“A male might breed with a female from another family group, but then returns to his mother.”

Whales may not experience menopause in quite the same way as humans do – there seems to be no scientific evidence of hot flushes, for example. But ongoing research into their complex social worlds can dramatically improve our understanding of both the natural world and ourselves.