Killer Whale Grandmothers Improve Survival Rates for Grandchildren

(CN) – Killer whale grandmothers that no longer reproduce can help their grand-offspring survive longer, according to a study published Monday in the journal Proceedings of the National Academy of Sciences.

Ross Sea killer whales from the Antarctic are distinguished by their narrow angled eye patch. The smallest of the three Antarctic types, these orcas eat fish that are found primarily under the ice pack, so they follow leads deep into the ice as it breaks up in the summer months. (Photo Credit: Dave Ellifrit, NOAA Alaska Fisheries Science Center)

Researchers led by postdoctoral researcher Stuart Nattrass in the York Centre for Complex Systems Analysis at the University of York have recently made a surprising discovery on the roles that killer whale grandmothers continue to play in their family network long after their reproductive years have passed.

Using nearly four decades of census data on 378 killer whale grand-offspring with confirmed grandmothers off the coast of Washington state and British Columbia, researchers discovered that in the years following a grandmother’s death, killer whale grand-offspring experience a notable drop in their own survival rates.

Researchers suggest that these observations imply that once a grandmother is no longer present in a killer whale’s life, survival for its grand-offspring become much more challenging.

Scientists report that this phenomenon, known as the grandmother effect, only arises from killer whale grandmothers that have already experienced menopause and ceased all reproductive-related activities and behaviors.

“Here we have shown evidence that grandmother killer whales provide support to their grand-offspring, and that this is especially the case when the grandmothers are post reproductive,” the study states. “By stopping reproduction, post-reproductive grandmothers not only avoid reproductive conflict with their daughters but also offer increased benefits to their grand-offspring above that provided by reproductive grandmothers.”

Data suggest the grandmother effect has been observed before in humans, where grandmothers gain and offer unique advantages to their grandchildren, but that such an effect has not been fully witnessed before in killer whales.

Researchers suggest that killer whales may be more susceptible to this effect because, like humans, killer whales possess a lifespan that far exceeds their reproductive cycle – allowing a grandmother to support the livelihoods of their grand-offspring for years after her own reproduction has ended.

Researchers say there are several possible ways that killer whale grandmothers help the livelihood of their grand-offspring. One notable theory is that when a killer whale is still reproductively active and is caring for their immediate offspring, their movements and behaviors become more limited, as their attention is fixed on caring for their offspring.

Caring for those needs as well requires more food, meaning that a reproductive killer whale is allocating more of their food resources toward those necessities.

The study says that a post-reproductive grandmother killer whale, however, is no longer concerned with such considerations, and their priorities shift towards the protection and wellbeing of their grand-offspring.

“There are a number of potential mechanisms that may explain this finding. For example, it is possible that, when grandmothers are supporting their own calves, their movement and activity patterns are constrained and they are not able to act as leaders in the same way as post-reproductive females. Moreover, grandmothers with their own calves will require more food for lactation and thus are perhaps less likely to share food with other group members,” according to the study.

Scientists say that while these findings help to shed much light on how the grandmother effect influences mortality rates among killer whales, there is still much to be learned on the subject has a whole. Specifically, researchers continue to ponder how killer whales have come to place such an evolutionary stress on living so long past the end of their reproduction cycles, which is what makes the grandmother effect among killer whales possible in the first place.

The study says that further research is needed that will consider a more integrated and detailed perspective on how these reproduction behaviors and the ramifications of the grandmother effect influence killer whale communities.

“Only with this integrated approach can we fully explain why killer whales have evolved one of the longest post-reproductive life spans recorded for all nonhuman animals,” the study states.

Nattrass did not immediately respond to a request for comment by press time.

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