National Geographic

Why do killer whales go through menopause?

Here’s yet another reason why humans are weird: menopause. During our 40s, women permanently lose the ability to have children, but continue to live for decades. In doing this, we are virtually alone in the animal kingdom. From a cold evolutionary point of view, why would an animal continue to live past the point when it could pass on its genes to the next generation? Or put it another way: why don’t we keep on making babies till we die? Why does our reproductive lifespan cut out early?

One of the most popular explanations, first proposed in the 1966, involves helpful grandmothers. Even if older women are infertile, they can still ensure that their genes cascade through future generations by caring for their children, and helping to raise their grandchildren.* There’s evidence to support this “grandmother hypothesis” in humans: It seems that mothers can indeed boost their number of grandchildren by stepping out of the reproductive rat-race as soon as their daughters join it, becoming helpers rather than competitors.

Now, Emma Foster from the University of Exeter has found similar evidence among one of the only other animals that shows menopause: the killer whale.

Killer whales, or orcas, become infertile during their 30s or 40s, but they can live well into their 90s. Individuals stay within the pod they were born in, which gives older mothers plenty of chances to help their children and grandchildren. The same is true for humans and pilot whales – the only other species known to have a long menopause. The question is: Does this actually matter?

There’s no better place to get an answer than the Pacific North-West. Since the early 1970s, when Mike Bigg discovered that individuals could be identified from photos of their fins, scientists have conducted a thorough census of all the whales swimming off Washington state and British Columbia. Led by Ken Balcomb, they have recorded the lives of 589 individuals (and the deaths of around half of them). They have even deduced the whales’ family ties.

By tapping into this rich vein of data, Foster found that a mother’s presence does help her offspring survive, even if they are full-grown adults. If sons are 30 or younger at the time of their mother’s death, they are 3 times more likely to die themselves in the next year. If they are older than 30 when mum dies, they are 8 times more likely to die.

It’s clear that mothers who had been through menopause were just as useful to have around, and probably more so, than those who are still fertile. On average, a 30+ male is 8 times more likely to die in the next year if his mother passes away, but his odds actually go up by 14 times if mum had gone through menopause. This confirms that mothers are helping their sons well into adulthood, since older orcas actually benefit from mum’s presence more than young ones. Perhaps she helps them to hunt, or maybe she watches their backs during fights with rivals.

“It would be great if we knew more about orca social behaviour, in particular just what benefits mothers are conferring,” says Michael Cant from the University of Exeter, who has studied killer whale menopause himself. “But working on [these animals] is just enormously challenging, and this is very rare and hard-won data. Given the constraints…  this is a provocative and stimulating result.”

Foster also found that killer whale daughters don’t depend on their mothers in the same way. If mum died, younger daughters were fine, and older ones were just 3 times more likely to die in the next year. You can see this in the graph below: the death of a mother has a far smaller impact upon her daughters (red lines) than upon her sons (blue lines).

This gender difference is exactly what you’d predict. Put yourself in a mother orca’s perspective. Orcas mate between groups, so your son’s offspring will be raised in another pod but your daughter’s offspring will stay in the home pod and compete for resources. For as many grand-calves as possible, but as little direct competition as possible, it’s better for you to help your sons with their parenting than your daughters. “It is exciting to see this result, which would otherwise be rather unexpected and puzzling,” says Cant.  “If we find out more about menopause in cetaceans, it may help us to better understand the forces that shaped our own unusual life history.”

*****

*There are some other explanations that we can rule out. It’s not that old age is a symptom of modern society, and humans never used to live beyond menopause. Hunter-gatherers go through menopause and are surprisingly long-lived. Among the Ache of Paraguay or the Hadza of Tanzania, around half of women survive to 45, and continue living into their late 60s.

Other scientists point to the fact that we’re born with a full set of eggs that constantly depletes, but this disappearing act doubles in speed at the age of 40. Chimpanzees, monkeys and mice, which also have the same egg starter kits, don’t go through a similar acceleration. Finally, it’s not an inevitable consequence of long life that ovaries just stop working, since long-lived mammals like elephants and blue whales can breed into their 60s and 90s.

There’s another idea that’s more compelling, suggested by Dustin Penn and Ken Smith. I covered it back in 2007: “Women retire early from child-bearing for the same reasons that athletes retire from their sports at a young age – their bodies cannot handle the strain. Childbirth is a taxing process for a woman and at some point, it becomes too risky for mother and child. Menopause is an evolutionary respite from the burdens of having children.

Reference: Foster, Franks, Mazzi, Darden, Balcomb, Ford & Croft. 2012 Adaptive Prolonged Postreproductive Life Span in Killer Whales. Science http://dx.doi.org/10.1126/science.1224198

Image: by NOAA

There are 11 Comments. Add Yours.

  1. Zach Miller
    September 13, 2012

    Hold up, minor correction: At the end of the opening section, you write that orcas are the “only other animal” to go through menopause, but in the very next paragraph, you say that pilot whales do, too. So that begs the question of what other whales go through menopause. Even if the answer is zero, the orca can’t be the only other animal (besides humans) to go through menopause if pilot whales do, too.

    Certainly an interesting story, though. I’m intrigued that orcas and humans go through menopause but stay alive for a long time for the same reasons. I wonder why this hasn’t happened in other strongly social mammals?

  2. labellaflora
    September 14, 2012

    “Orcas mate between groups, so your son’s offspring will be raised in another pod… For as many grand-calves as possible, but as little direct competition as possible, it’s better for you to help your sons with their parenting than your daughters.”

    How can she help son raise offspring if they are in another pod?

    I think it’s odd that sons would be in greater danger of dying without mother around. More research is obviously needed.

  3. Igor Galić
    September 14, 2012

    “Women retire early from child-bearing for the same reasons that athletes retire from their sports at a young age – their bodies cannot handle the strain. Childbirth is a taxing process for a woman and at some point, it becomes too risky for mother and child. — I don’t understand how this is different for elephants or blue whales…

  4. Michael Cant
    September 14, 2012

    A very good question. In 2010 Rufus Johnstone and I suggested that the answer may lie in the demographic system of humans and menopausal cetaceans. We showed that two unusual and different dispersal systems result in mothers becoming more closely related to other local group members as they age – female biased dispersal coupled with local mating (thought to characterise ancestral humans) and no dispersal with inter group mating (the killer whale/pilot whale pattern). In the whales our model predicted at mothers should direct care toward sons rather than daughters – just as Foster et al have found. By contrast male biased dispersal which is the usual mammalian pattern selects against late life helping and menopause.
    Johnstone & Cant 2010. The evolution of menopause in cetaceans and humans: the role of demography. Proc Roy Soc B 277:2219-2226

    Labellaflora Cetacean biologists have suggested for some time that mothers help their sons in aggressive inter group mating competitions – so at is one way in which they might increase their sons fitness even thought the offspring are reared elsewhere. Also in pilot whales mothers appear to wean their daughters at 6 or 7 years but continue to suckle their sons into their teens (at least this is the what it looks like from whaling records of milk in stomach contents).

  5. chris y
    September 14, 2012

    Zach @1, Ed wrote, ” one of the only other animals that shows menopause” (my emphasis), which is a neologism I hate, precisely because it gives rise to confusion like you have suffered. I don’t see what’s wrong with old fashioned “one of the few other…”, which is clearly different from “the only other…”

    But I’m not a linguistic prescriptivist, so I’ll shut up.

    It’s a fascinating article though, because it suggests that if there is a selectionist explanation for the survival of post-fertile females, it’s a distinctly different reason in Orcinus from in Homo.

  6. Elizabeth Moon
    September 14, 2012

    In considering why humans and orcas, and not elephants and blue whales, go through menopause, consider the nutritional and other physical strains placed on the pregnant and birthing female. Human pregnancy costs the pregnant woman in nutritional resources even if she has access to adequate nutrition; early in human history (and still where adequate nutrition is not ensured for women) nutrition through pregnancy was an iffy matter, depending on when and where the woman became pregnant. Because human women–unlike many other species–are fertile year around, human societies without access to contraception tend to time pregnancies close enough that full recovery is not possible, and without reference to the probability that the pregnant woman will have nutritional support throughout, and between pregnancies. In species that do not have frequent fertile periods, pregnancies cannot be spaced as closely.

    Human pregnancy and birth also places a physical load on women because of the relative size of the human fetus and specifically the size of the head. Large fetuses and multiple births begin stressing the body early (not just nutritionally) and multiple pregnancies increase the risk of permanent injury and death.

    So if healthy post-fertile women are beneficial to the survival of their genes, then those who lose their fertility before they lose their lives have an advantage even with fewer actual offspring. This would be enhanced, I would think, by being social animals and large-brained animals. Grandma might be useful to a more solitary animal…but in a group, grandma’s influence would benefit more individuals. And a smart grandma–a grandma with more skills, more experience, and a bigger toolkit with which to meet novel challenges–would be especially helpful to a group. Even those the smart grandma did not specifically aid could benefit by observing the smart grandma’s skills.

    I question whether humans and orcas are the only species to have a menopause. In livestock, some animals show an end of fertility before the end of life: cattle, horses, dogs, and cats may cease bearing young, but continue to live for longer than one or two reproductive cycles. Human management decisions can obscure the actual life history (many infertile animals are sent to slaughter, and some fertile animals are not bred) but in situations where animals have gone feral (as with horses) or are managed in a more natural situation, it’s clear that these four species at least live past fertility. Whether this is true menopause would require more investigation…is it cessation of ovulation or something else that causes the older females to cease becoming pregnant? What is the fate of infertile females if their lives are not terminated by owners? And what effect does long survival have on the fate of their offspring?

    I have anecdotal evidence on cattle from one ranch and there’s some published data on feral/free-ranging horse herds, but research on the social structure of domestic livestock is rarely done (and mostly, with horses, in feral herds, not closely managed ones.) It’s clear that social rank (herd rank, for cattle and horses) is strongly affected by the mother’s social rank. A rancher friend can track the “boss cow” effect down through generations (in one case producing a boss bull who took over the actions of “boss cow” in any herd he was moved to…deciding when to move, leading through gates, etc.) I’ve seen the “alpha mare” effect with horses through a few generations. Social animals learn from their mothers–and others in the group–which animals are dominant and how to behave toward them. For animals with a long enough lifespan to accumulate useful experience, and especially those that are nomadic, having a “wise grandma” could well confer benefits.

    As always “more research is needed…”

  7. Andrew
    September 15, 2012

    Hmmm. “Individuals stay within the pod they were born in, which gives older mothers plenty of chances to help their children and grandchildren. The same is true for humans and pilot whales” – actually, women are exogamic, and are often not in the same community as their grandchildren, since their daughters may leave.

  8. Howard Garrett
    September 15, 2012

    This study is important mainly because it describes – and publicizes – the discovery that at least some orca females live long past their reproductive years, that their offspring stay with them for life, and that their male offspring often don’t live long after she dies. The evolutionary model used to explain this unprecedented mother/son bond invokes a benefit to genetic inheritance from the practice, i.e., that orca moms help their sons get dates and procreate. The other rationales for the lifetime bonding – help with foraging or as backup during fights – are less likely, since foraging for Southern Resident orcas is pretty much a solitary effort, and orca males don’t seem to fight.
    The data could also be explained by considering recent studies showing that:
    The complex and stable vocal and behavioural cultures of sympatric groups of killer whales (Orcinus orca) appear to have no parallel outside humans and represent an independent evolution of cultural faculties. (Rendell and Whitehead, 2001).
    When the orcas’ capacity for cultural faculties is combined with studies of captive dolphins in which self-awareness has been demonstrated, a different model emerges that is not totally dependent on the selfish gene simply replicating itself, but is also driven by factors like social identity, role, tradition, and inter-individual interactions as motivating values. In other words, could these dependent male orca offspring depend on their mothers to confirm their social status, or even their identities as members of their families? Does their self-value diminish when their mothers die? That may be the kind of animal we’re talking about here.

  9. oddjob
    September 15, 2012

    “How can she help son raise offspring if they are in another pod?”

    She can’t, but if she helps her son stay alive longer then presumably he’s able to sire more children than he otherwise would.

  10. Falk Schlesinger
    September 16, 2012

    I understand that the data for this study is recorded from resident orca pods.
    Is there a difference in life expectancy / probability of survival between resident and transient orcas? And if there is a difference, how significant is it?

  11. kimw
    September 22, 2012

    Using the logic detailed above: in female-biased dispersal with local matings, or even in arranged non-local matings, the story of humans, shouldn’t the human mother/grandmother assist her daughter more than her son?

    Orca mothers assist sons more (again, according to above) because the offspring are more likely to be reared away from the mother’s group, and therefore the mother does not have to compete with grandchildren for scarce resources. The same could be said for human mothers and daughters. Remember, not too long ago, we “gave our daughters away.”

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