Comments on: Why do killer whales go through menopause?

By: kimw

kimw — Sun, 23 Sep 2012 01:05:16 +0000

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.”

By: Falk Schlesinger

Falk Schlesinger — Sun, 16 Sep 2012 05:17:45 +0000

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?

By: oddjob

oddjob — Sun, 16 Sep 2012 02:32:09 +0000

“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.

By: Howard Garrett

Howard Garrett — Sat, 15 Sep 2012 21:12:16 +0000

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.

By: Andrew

Andrew — Sat, 15 Sep 2012 14:43:02 +0000

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.

By: Elizabeth Moon

Elizabeth Moon — Fri, 14 Sep 2012 16:06:11 +0000

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…”

By: chris y

chris y — Fri, 14 Sep 2012 12:27:45 +0000

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.

By: Michael Cant

Michael Cant — Fri, 14 Sep 2012 10:45:34 +0000

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).

By: Igor Galić

Igor Galić — Fri, 14 Sep 2012 08:42:44 +0000

“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…

By: labellaflora

labellaflora — Fri, 14 Sep 2012 05:27:14 +0000

“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.