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Killer whales give new insight into the evolution of menopause

On March 13, 2024, the international science and technology journal Nature published a Center for Whale Research (CWR) paper titled The evolution of menopause in toothed whales. CWR Executive Director Dr. Darren Croft, one of the paper’s five authors, explains the research results in layperson’s terms.


Photographs © Copyright 2024 Center for Whale Research.

Post-reproductive Southern Resident orca mother and grandmother J16 (est. 52 years old) with newborn calf J60. Photograph by CWR’s Mark Malleson during Encounter #3 on January 3, 2024.


 

FOR MOST SPECIES, FEMALES REPRODUCE UNTIL THE END OF LIFE. However, in humans and a handful of toothed whales (including killer whales/orca), females can live for decades beyond their reproductive years—they go through menopause and have a prolonged post-reproductive lifespan. How and why this unusual life history strategy has evolved has been an evolutionary puzzle for much of the last century. Our latest research, using the long-term individual-based data collected by ORCA SURVEY, gives new insight into how and why menopause evolved—showing that humans and killer whales show striking similarities in their life history.


So why has menopause been an evolutionary puzzle? Under most natural conditions, we expect evolution to select females to maximize their reproductive success by reproducing for as long as possible (i.e., until the end of life). Evolution favors individuals with a heritable advantage over others—because they will contribute more offspring to future generations. Through this process, their young are overrepresented in the next generation, and these offspring will inherit the genes that allowed their parents to be successful (i.e., reproduce more). Therefore, we would expect evolution to favor females who maximize their own reproductive success by reproducing until the end of life.


As always, there are exceptions to the rule. In human societies, women can expect to live for decades after their reproductive years. At first sight, we may think this is because there is a mismatch between how long we can expect to live now compared to how long our ancestors lived centuries or millennia ago. The argument that menopause is an artifact of modern medical care lengthening our expected lifespan has been repeatedly proposed. However, the numbers don’t add up. If we look at historical human societies living with no modern medical care, women regularly live through menopause and can expect to live for decades beyond. This result is universal for all human populations regardless of the quality of medical services. The data is clear—menopause is deeply rooted in our evolutionary history, setting us apart from other primates where, in most cases, females reproduce until the end of life.


 

However, in humans and a handful of toothed whales (including killer whales/orca), females can live for decades beyond their reproductive years—they go through menopause and have a prolonged post-reproductive lifespan.

 

But what about killer whales (i.e., orca)? In the mid-70s, Ken Balcomb and others started long-term individual-based studies of the killer whale populations living in the coastal waters of the Pacific Northwest. As the long-term studies progressed, an incredible pattern emerged—adult females were living well beyond their reproductive years—indeed, no females were reproducing after their late 30s or early 40s. Still, they were living for decades after their last calf. As these long-term data sets matured, where the birth and deaths of every individual in the population were tracked, menopause in killer whales was firmly established. We discovered we shared something with orca that was once thought to be uniquely human—life after reproduction.


Over the last few decades, we have learned a great deal about menopause in toothed whales. And it is not just female killer whales that go through menopause. We have established that menopause evolved at least four times independently in toothed whales and occurs in five species (short-finned pilot whales, false killer whales, killer whales, narwhals and beluga whales). In contrast, in terrestrial mammal species, menopause is restricted to humans. However, it does occur in animal populations living in captivity, where individuals are protected from predation, starvation, and disease, supporting them to live beyond their natural lifespan, with many living beyond their reproductive years. This pattern is an artifact of captivity. From an evolutionary perspective, it does not require an explanation.


Graphical results from the Center for Whale Research’s Nature-published paper: The evolution of menopause in toothed whales, illustrating (a) that menopause evolved four times independently in toothed whales and occurs in five species (short-finned pilot whales, false killer whales, killer whales, narwhals and beluga whales). The graphs (b &c) show that in the species that go through menopause, females have a longer total lifespan given their body size but that their reproductive lifespan is comparable to other species of the same size that don’t go through menopause.


 

Interestingly, recently, one population of chimpanzees was found to experience menopause. This population is unusual because, in all other chimpanzee populations studied to date, females reproduce until the end of life. So what is the difference? Well, it seems that this population of chimps is “living the good life”—they have an unusually high abundance of food and little or no natural predation. Under these conditions, females live beyond their reproductive years. But this population is the exception. The key point is that menopause is very rare. So how has it evolved?


This is where our new study comes in. We asked how menopause in toothed whales evolved and what the potential benefits are for females living beyond their reproductive years. Menopause is an evolved trait—that is, species where females go through menopause evolved from an ancestral species that did not have menopause (i.e., females in the ancestral species reproduced until the end of life). How, then, could menopause evolve? Two pathways could lead to menopause. First, evolution shortened the reproductive period so that females lived for the same lifespan but had a shorter reproductive period. The alternative is that evolution increases the total lifespan while holding the reproductive timeframe. Hence, females reproduce for the same time period but live beyond their reproductive stage (i.e., they have a longer lifespan than their ancestors who did not go through menopause).


If we compare humans’ reproduction and lifespan patterns with those of our closest living primate relatives, the live-longer hypothesis is supported. Women have a reproductive lifespan similar to that of our closest primate relatives, but they can expect to live much longer, with a significant period of life after menopause. But what about killer whales and other toothed whales?


 

We asked how menopause in toothed whales evolved and what the potential benefits are for females living beyond their reproductive years.

 

To understand the evolution of menopause in whales, we needed to measure the lifespan and reproductive lifespan of as many species as possible. To do this, we applied mathematical models to information gathered during whale mortality events such as mass strandings. Scientists studying these mass strandings determine the ages of individual whales—one way they do this is by counting the growth rings on the teeth of whales, which are similar to tree growth rings—and collecting reproductive information for all the whales that strand (e.g., such as patterns of pregnancy). We used this to build the first database of toothed whale lifespan and reproductive lifespan. Using this data, we applied methods originally developed to understand human demography to calculate metrics such as expected age at last reproduction and the expected female (and male) lifespan.


We compared the lifespans of 32 species of toothed whales, finding that in the five species that go through menopause, the females live, on average, 40 years longer, given their body size, when compared to species that don’t go through menopause. Just like humans, menopausal species of whales have a reproductive period similar to that of non-menopausal species. It is the life after reproduction that differs— evolution has selected a longer lifespan.


But what are the benefits of this unusual life history strategy? By extending the total lifespan, post-reproductive females spend more time alive with their offspring and grand offspring. For example, in whale species without menopause, females only live long enough to see one of their grand offspring reach one-third of the age of maturity. In species with menopause, females will live on average until their grand offspring are mature. Post-reproductive females provide significant benefits to their family during this extended life phase and boost the survival of their offspring and grand-offspring. One way female killer whales do this is by acting as repositories for ecological knowledge of when and where to find food. For example, we have shown in our previous work that post-reproductive females lead their family group around the coastal foraging grounds to find salmon, especially when salmon is in short supply. These older females may also help in other ways, and we recently found that they may protect their sons from social disputes. This period of post-reproductive life translates to survival benefits for their offspring and grand-offspring. In a killer whale family, an adult son who loses his post-reproductive mother experiences a 13-fold increase in mortality risk. We find similar patterns for daughters and grand-offspring, but the effects are less pronounced.



Graphical results showing the opportunities for intergenerational help and harm in toothed whales from the Center for Whale Research’s Nature-published paper: The evolution of menopause in toothed whales.


 

So, if living a long time allows females to benefit their offspring and grand offspring, why not reproduce longer? There are many species where older females provide survival benefits to their social groups, so why has the reproductive lifespan also not been extended in some? Our new analysis shows that females avoid reproductive overlap with their daughters by not reproducing in late life. When mothers and daughters in the same group try to reproduce at the same time, there is the potential for conflict over resources because they both want to prioritize resources for their offspring. Such a resource-allocation dilemma would be exacerbated if females reproduced longer. By stopping reproduction, older females avoid this conflict with their daughters.


We have taken a big step forward in understanding how menopause has evolved, and the similarities between humans and toothed whales grow even stronger. But there are also significant differences. In human societies, for example, males also live a long time. What is striking about killer whales is that males only live a fraction of the potential lifespan of females, with males typically dying in their 30s. In contrast, females can live into their 80s or beyond. We currently don’t have an explanation for these differences.


These new findings further highlight the central role that post-reproductive females play in killer whale societies—how they benefit their family group by living a long life supporting their offspring and grand offspring. A worrying trend in the Southern Resident killer whale (SRKW) community is that the number of grandmothers in the population has declined over time. With this decline, the SRKWs are not just losing the direct help that grandmothers can provide their family group (e.g., by catching and sharing fish) but also the ecological knowledge of where and when to find fish. This loss of experience will have consequences for generations to come.


There are still many secrets to unlock in the race to help the Southern Resident killer whales recover.


Read the Center for Whale Research’s Nature-published paper: The evolution of menopause in toothed whales.


 

These new findings further highlight the central role that post-reproductive females play in killer whale societies—how they benefit their family group by living a long life supporting their offspring and grand offspring.

 

Other recent Center for Whale Research scientific papers of interest:


 

48 years ago, we experienced our

first encounter with J pod

April 16, 1976

From left to right: J11, J1, J3, J8 (surfacing), J16, and an unknown juvenile.



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