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Why are there differences in post-reproductive lifespans and what are the evolutionary causes and benefits of post-reproductive female cooperation?

We found this interesting because it challenges the conventional notions about the purpose of life beyond reproduction. Exploring the significance of post-reproductive females in animal societies reveals how longevity can be adventageous for social cohesion and knowledge transfer. It's fascinating to see how individuals contribute to their communities in diverse ways, highlighting the complexity of evolutionary strategies beyond mere reproduction.

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Ellis, S., Franks, D. W., Nattrass, S., Cant, M. A., Bradley, D. L., Giles, D., Balcomb, K. C., & Croft, D. P. (2018b). Postreproductive lifespans are rare in mammals. Ecology and Evolution, 8(5), 2482–2494. https://doi.org/10.1002/ece3.3856

In their 2018 study, Ellis et al. explore the rarity of post-reproductive lifespans in mammals. They address the intriguing question of why, despite the common occurrence of menopause and reproductive senescence in female mammals, few species exhibit extended post-reproductive lifespans akin to humans. The authors employ a comprehensive approach, analyzing a large set of data spanning 30 species to investigate the factors influencing post-reproductive lifespan. Their findings challenge common theories, suggesting that PRLS (post-reproductive lifespan) is not necessarily linked to increased fitness. Instead, they propose that environmental variability and the associated unpredictability in survival likelihood may play a crucial role in the evolution of extended PRLS. This reading offers valuable insights into the selective pressures shaping mammalian life histories.

However, the study has its limitations. The authors primarily focus on the presence or absence of PRLS without delving deeply into the mechanisms driving its evolution. Regardless, this paper significantly contributes to our understanding of PRLS evolution, shedding light on the interplay between life history traits and environmental factors.

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Garrett Jostworth, Rachel Robinson, Emmie Williams, Natasha Vaughn

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We believe that it is advantageous to the social structure of a species to maintain life after reproduction.

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Grimes, C., Brent, L. J. N., Ellis, S., Weiss, M. N., Franks, D. W., Ellifrit, D. K., & Croft, D. P. (2023). Postreproductive female killer whales reduce socially inflicted injuries in their male offspring. CURRENT BIOLOGY, 33(15), 3250–+. https://doi.org/10.1016/j.cub.2023.06.039

Contributor: Rachel Robinson

Grimes et al. (2023) delve into the evolutionary question, why did menopause evolve for some species? They investigate why females cease ovulation before the end of their natural lifespan. They propose that in human societies, intergenerational resource transfers, like food sharing and caregiving, may have influenced the evolution of menopause. The study shifts focus to resident killer whale populations, hypothesizing that post reproductive female killer whales may provide late life contributions to their kin. Through empirical testing, they discover that post reproductive female killer whales indeed offer social support to their offspring by reducing socially inflicted injuries, particularly in male offspring. However, this effect is not observed in female offspring, grand offspring, or non-offspring social unity members. Their findings help us understand the evolution of menopause in killer whales, which we can pull meaning from for humans as well. The significance of directed late-life support and kinship dynamics in shaping reproductive strategies and social behaviors within not only killer whales but humans too. While their data was widespread,

They used tooth rake marks–thin, white, parallel lines that appear as the result of the teeth of another animal puncturing the skin–to quantify socially inflicted injuries, since the direct observation of behavior in marine animals is difficult, they say that these markings are indirect evidence of social interactions. However, they say that killer whales have no predators other than humans. So, where do these marks come from? Their whole idea is that mothers protect their kin, but if they have no predators, what are they protecting them from and is it from intergroup aggression? I would think that data might taint some of the results. They mention they can get them with just play behavior, not just aggression. This could also interfere with results. How many tooth marks are from just play rather than aggression? While useful information, I would be interested if it were possible to watch them with video results, rather than tooth marks. Albeit, long and expensive work.

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Jack L McCormack, Kevin Arbuckle, Karen Fullard, William Amos, Hazel J Nichols, Lack of intergenerational reproductive conflict, rather than lack of inclusive fitness benefits, explains absence of post-reproductive lifespan in long-finned pilot whales, Behavioral Ecology, Volume 34, Issue 6, November/December 2023, Pages 950–959, https://doi.org/10.1093/beheco/arad062

Female long finned pilot whales have many similarities to other species that have long post reproductive life spans, but they continue to reproduce as they age. The mothers continue to reproduce throughout their lives while still assisting their daughters. Scientists wanted to use this species to assess if having a mother present increases the daughter’s and, in turn, her own fitness, despite the mother continuing to reproduce. A sample of 1804 individuals from 25 pods was collected through hunting. These long finned pilot whales were assessed for their sex, body size, relative age, and genetic information. Of the pods in this research, only 13 had grandmothers present. There was also no significant increase in size if a mother or grandmother was present. Females under 20 had higher reproductive success if a mother was present, but when the sample was expanded to include all females, a mother being present did not affect the number of offspring. This data shows a benefit from having older females present without suffering if there is not. Alloparenting with other mothers in the group may explain why grandmothers are not as beneficial in this group. This sample does face the problem of it being a “snapshot” of the population rather than a longitudinal study. Relatedness was detrimental through genetic analysis instead of observation, which limited the knowledge. Behavioral observations of the rearing process of these whales would benefit this study.

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Lynch, E. C., Lummaa, V., Htut, W., & Lahdenperä, M. (2019). Evolutionary significance of maternal kinship in a long-lived mammal. Philosophical Transactions: Biological Sciences, 374(1780), 1–10. https://www.jstor.org/stable/26916750

In this study researchers wanted to see how maternal kinship benefits the fitness of long living mammals, in this case Asian elephants. Elephants have incredibly long life spans and typically stay in matriarchal family groups. This study measured a group of elephants across their different life spans to see if they benefited from having their sisters around. They observed young, middle-aged, and older elephants to measure how many sisters were nearby and what that group's reproductive output was at that time. They then tested how sisters nearby and the age difference between sisters affected their reproductive output. In the younger generation they found that having a sibling around significantly improved their reproduction. The opposite effect was found in middle-aged elephants, the only reliable predictor of reproductive success for them was if their mother was alive or not. No relationships were found for the older elephant groups. Only young elephants were found to have a significant increase in reproductive success when they had a younger sibling present. This study showed that there are benefits to having siblings around that improve reproductive success. This study does bring up questions about the relationship age plays in this sibling benefit, as middle-aged and older elephants did not have any significant changes. This study had an incredible database on this group of elephants, but there are questions as to the semi-captive, working group of elephants like the ones they used. Could the results be different in a purely wild group of elephants, as these elephants sometimes are separated and moved around. But this is a big breakthrough in the study of kin selection and post reproductive maternal relationships.

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Grimes, C., Brent, L. J. N., Ellis, S., Weiss, M. N., Franks, D. W., Ellifrit, D. K., & Croft, D. P. (2023). Postreproductive female killer whales reduce socially inflicted injuries in their male offspring. CURRENT BIOLOGY, 33(15), 3250–+. https://doi.org/10.1016/j.cub.2023.06.039

Ellis, S., Franks, D. W., Nattrass, S., Cant, M. A., Bradley, D. L., Giles, D., Balcomb, K. C., & Croft, D. P. (2018b). Postreproductive lifespans are rare in mammals. Ecology and Evolution, 8(5), 2482–2494. https://doi.org/10.1002/ece3.3856

Jack L McCormack, Kevin Arbuckle, Karen Fullard, William Amos, Hazel J Nichols, Lack of intergenerational reproductive conflict, rather than lack of inclusive fitness benefits, explains absence of post-reproductive lifespan in long-finned pilot whales, Behavioral Ecology, Volume 34, Issue 6, November/December 2023, Pages 950–959, https://doi.org/10.1093/beheco/arad062

Lynch, E. C., Lummaa, V., Htut, W., & Lahdenperä, M. (2019). Evolutionary significance of maternal kinship in a long-lived mammal. Philosophical Transactions: Biological Sciences, 374(1780), 1–10. https://www.jstor.org/stable/26916750

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Post-reproductive life, a phenomenon observed in select species such as humans, elephants, and whales, continues to intrigue scientists probing the depths of evolutionary biology. Post-reproductive life is defined as the stage in an organism's life cycle where reproduction ceases but lifespan continues, post-reproductive life presents a paradox: if it confers benefits, why isn't it more widespread across the animal kingdom? This question brings a wider scope of inquiry into the evolutionary mechanisms shaping life histories and social structures. To delve into this mystery, researchers have looked into diverse species, from the elephants to the killer whales, seeking insights into the drivers behind the evolution of post-reproductive life. As the research unfolds, the significance of kinship, social support, and intergenerations cooperation, the quest to comprehend the origins and implications of post-reproductive life continues to captivate scientific inquiry.

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Cost/Benefit of Post-Reproductive Life:

a. Costs:

- Energy Allocation: Post-reproductive females still invest energy in offspring care or other social activities, which could otherwise be allocated to their own survival or future reproductive efforts.

- Increased Vulnerability: Aging individuals may become more susceptible to predation or diseases due to decreased agility or immune function.

- Competition for Resources: In species where resources are limited, post-reproductive individuals may face increased competition from younger, reproducing females for food or shelter.

b. Benefits:

- Enhanced Offspring Survival: Post-reproductive females often contribute to the survival of their offspring or grandchildren through assistance in rearing, protection, or knowledge transfer.

- Social Cohesion: Their presence can enhance social cohesion within groups or herds, providing stability and cooperation among members.

- Knowledge Transfer: Older females may possess valuable knowledge about foraging strategies, migration routes, or predator avoidance, which can benefit the entire group.

- Altruistic Behavior: Post-reproductive females may engage in altruistic behaviors, such as babysitting or defending younger individuals, which can increase overall group fitness.

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a. Inclusive Fitness: Kinship Theory suggests that individuals may increase their genetic success by helping relatives, even if it comes at a cost to their own reproduction. This theory predicts that post-reproductive females, who are genetically related to younger individuals in their group, may enhance their inclusive fitness by assisting in the care of relatives.

b. Cooperative Breeding: In species where cooperative breeding occurs, such as certain bird species or mammals like meerkats, post-reproductive females may play crucial roles in providing care for offspring that are not their own, thereby promoting the survival of their genetic relatives and indirectly increasing their inclusive fitness.

c. Grandmother Hypothesis: This hypothesis proposes that the presence of post-reproductive females, particularly grandmothers, enhances the survival and reproductive success of their grandchildren. By providing assistance in rearing, provisioning, or protection, grandmothers contribute to the overall fitness of their genetic lineage, thus supporting the predictions of Kinship Theory.

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Semelparity v. Iteroparity:

The evolutionary strategies of semelparity (death after first reproduction) and iteroparity (continued life after reproduction), observed in various animal species, reflect adaptations to environmental conditions and reproductive challenges. Semelparous animals, like certain insects and salmon, invest all their energy into a single reproductive event, maximizing their chances of passing on their genes before potentially succumbing to unpredictable or harsh environments. In contrast, iteroparous species, such as mammals and birds, spread their reproductive efforts across multiple breeding seasons, benefiting from stable or predictable environments where survival to reproductive age is more assured. These strategies highlight the trade-offs between reproduction and survival, with each species evolving the most effective approach to ensure the continuation of their genetic lineage over time.

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Killer whales have a strong social structure and the grandmothers play a very important role.

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Inside the killer whale matriarchy - Darren Croft

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Killer Whales: evidence for social support

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Killer Whales: evidence for social support

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Grimes et al. (2023) took on a fascinating question: why do some species, like humans and killer whales, stop having offspring before they grow old? They started by looking at humans and suggested that things like sharing food and taking care of each other across generations might be part of why women go through menopause. They turned their attention to killer whales, hypothesizing that older female whales might still help out their family even after they stop having offspring. Their research showed that older female killer whales do seem to provide support to their offspring by reducing the chances of them getting hurt by other whales, especially the males. But strangely, this wasn't the case for female offspring, grandchildren, or other members of the whale groups. They looked at marks left by teeth on the whales' skin to figure out how often they were getting hurt. However, their approach has its challenges since killer whales don't have natural enemies other than humans, and they wouldn't be protecting them from us. So, if not fighting marks, then what are they? Despite the difficulties, it still gives evidence for how killer whales have a complex social structure that includes for post-menopausal protection from females and show that it may have adapted for higher thinking social creatures like whales and therefore humans.

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Brian M. Wood et al.Demographic and hormonal evidence for menopause in wild chimpanzees.Science 382, eadd 5473 (2023).DOI:10.1126/science.add5473

Thompson, J. (2024, February 20). Wild chimps shown to undergo menopause for the first time. Scientific American. https://www.scientificamerican.com/article/wild-chimps-shown-to-undergo-menopause-for-the-first-time/

Locke, D. P., Hillier, L. W., Warren, W. C., Worley, K. C., Nazareth, L. V., Muzny, D. M., Yang, S. P., Wang, Z., Chinwalla, A. T., Minx, P., Mitreva, M., Cook, L., Delehaunty, K. D., Fronick, C., Schmidt, H., Fulton, L. A., Fulton, R. S., Nelson, J. O., Magrini, V., Pohl, C., … Wilson, R. K. (2022). Author Correction: Comparative and demographic analysis of orang-utan genomes. Nature, 608(7924), E36. https://doi.org/10.1038/s41586-022-04799-7

YouTube. (2021c, October 4). Semelparity vs Iteroparity | Evolutionexamples. YouTube. https://www.youtube.com/watch?app=desktop&v=RrbAeS4OdIk

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Three post-reproductive female chimpanzees 

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Some wild chimps live beyond their reproductive period. Just like in humans their fertility decreases after 30. There has also been no recorded births after the age of 50. Though in many populations few reach this age. (Wood et. al, 2023).

Adult females move to spectate families when they reach sexual maturity which does not allow for mothers to help raise their daughter's children (Thompson, 2023).

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These chimps are able to survive long due to special circumstances. This population live longer and in a comparable way to hunter gather humans.

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Welcome to the club, chimps! #sciencetok #menopause #evolution #grandma #learnontiktok #biology #ape #chimp #primate #human #humanevolution #edutok #stem #ngogo #whale #edutok 

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@zekedarwinscience
Welcome to the club, chimps! #sciencetok #menopause #evolution #grandma #learnontiktok #biology #ape #chimp #primate #human #humanevolution #edutok #stem #ngogo #whale #edutok
♬ Stories 2 - Danilo Stankovic

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Chimps and humans diverged relatively recent in regards to evolutionary history. They shared the last common ancestor around 6 million years ago through it is hard to pinpoint (Locke et al, 2022).

On of the theories for menopause is that it just an artifact of some common ancestor and all mammals that live long enough should experience it.

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Long Finned Pilot Whales and Grandmothers' Care

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Long Finned Pilot Whales and Grandmothers' Care

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One study worked to record the size and relationships in multiple pods in long finned pilot whales. The data was collected through subsistence hunting. A sample of 1804 individuals from 25 pods were collected and were assessed for their sex, body size, relative age, and genetic information.

Long-finned pilot whales continue to reproduce until they die. Their close relatives who are closely related to short finned pilot whales go through menopause. In long finned pilot whales grandmothers often help their daughters even through they continue to reproduce themselves. This is not present in all pods because many only have 2 generations present (McCormack et. al, 2023)

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The blue line represents whales who had a mother present. The red line represents whales who did not have a mother present. 

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Reproductive Success of Long Finned Pilot Whales 

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Reproductive Success of Long Finned Pilot Whales 

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When a grandmother was present her daughter had a higher level or reproductive success. This effect is present even though the grandmothers still reproduce (McCormack et. al, 2023).

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A graph showing the differences in rake marks of when there is a post-menopausal female or no reproductive mother and a few things in between

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Elephant’s have incredibly long life spans, an average of 60 to 70 years depending on the species. They live on much longer than their reproductive life, but why? Are there benefits to having these post reproductive females around? Does it have anything to do with kinship? Elephant's live in matriarchal societies, and stay with their close relatives their whole lives. This study wanted to attempt to find reasoning behind these relationships, and why this species has such long post-reproductive life spans.

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Figure 2. Showing how the annual reproduction changes based on age and sibling condition.

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The study found that having a sister around is actually most beneficial to the younger elephants. Their reproductive success was much higher. Only young elephants were found to have a significant increase in reproductive success when they had a younger sibling present. This study does bring up questions about the relationship age plays in this sibling benefit, as middle-aged and older elephants did not have any significant changes. There must be bigger benefits to helping younger siblings for young elephants as they are the only ones who showed a significant relationship between the two.