New evidence of menopause in Bigg’s Transient killer whales

||| FROM CENTER FOR WHALE RESEARCH |||

Menopause has long been a puzzle for scientists. In most animals, it will be favorable for an individual to continue reproduction throughout their entire adult lifespan. However, in humans and a handful of toothed whales, females stop reproduction well before the end of their natural lifespan.

Bigg’s Transients, T101s, swimming along the Vancouver Island, British Columbia shoreline on April 24, 2021.

Previous research on the Southern Resident killer whales (SRKW) has shown that females in this population typically have their last calf in their late 30s but can live for decades as a post-reproductive female (Ellis et al. 2017). What we did not know was if menopause was a general trait of killer whales or something unique to the SRKW population?

By combining the incredible long-term data collected by the Center for Whale Research (CWR) and Fisheries and Oceans Canada, we have investigated whether menopause is a shared trait among the Southern Resident and Bigg’s Transient killer whale (orca) ecotypes of the Salish Sea.

Bigg’s Transients T65A2, T65A4, and T34A1 swimming north in Haro Strait on June 17, 2021.

So why has menopause evolved in Southern Resident orcas? In this ecotype, neither sons nor daughters disperse, and as a result, females become more and more related to their group as they begin producing offspring of their own.

Under these conditions, theory predicts that post-reproductive females should invest more time and energy into helping children and grandchildren than younger females. In support of this, we see from previous research that post-reproductive Southern Resident females provide survival benefits to their offspring and grand offspring (Nattrass et al. 2019; Foster et al. 2012).

Frequently, the oldest Southern Resident female will take the lead in guiding the pod between hunting grounds. And she will often share her Chinook salmon catch with individuals in the group.

In addition to this, because daughters stay with their mothers, a mother will compete for reproduction with her daughters. This competition can be costly when resources are limited and shared among group members. In the Southern Resident community, such a conflict between a mother and her daughters over reproduction favors the younger females, meaning that calves from mothers of the older generation are less likely to survive when in conflict with calves from mothers of the younger generation (Croft et al. 2017).

In Southern Resident orcas, older mothers can avoid this reproductive competition by stopping reproduction and instead help their already existing children and grandchildren to survive and reproduce.

Above: Post-reproductive Southern Resident orca, J19, with her grand offspring, J58, on March 22, 2021.

Bigg’s Transients, T124A4 and T124A4A, on April 17, 2021.

In contrast, Bigg’s Transient killer whale sons and daughters don’t all stay with their mothers. Some will disperse and start groups of their own, possibly diluting the mechanisms that drive the evolution of a long post-reproductive period like in the Southern Resident killer whale population.

Comparing the age of last reproduction and length of lifespan between the Southern Resident and Bigg’s Transient orcas, the life histories of the Salish Sea’s mammal-and fish-eating orcas are very similar. Females cease reproduction in their late 30s, where they can expect to live more than 20 years as post-reproductive. The whales have a substantial period to focus their time and energy on helping their existing children and grandchildren rather than reproducing.

Adult Bigg’s Transient, T124D, attacking a Harbor porpoise on April 17, 2021.

The similarities between the life histories of the two orca ecotypes suggest that this trait evolved in a common ancestor and that it is robust to the substantial changes observed in behaviors and social structure. Gaining a better understanding of the life history of both ecotypes is essential to inform conservation and management decisions.

Bigg’s Transient orcas’ preferred prey is Harbor seals and porpoises; the orcas work together in small family groups to catch their marine-mammal prey.

Our new findings suggest that older females play a crucial role in supporting the family group in Bigg’s Transient killer whales, just like Southern Residents. There is exciting work ahead to investigate the role of grandmothers in Bigg’s Transient killer whales. The long-term datasets of the Center for Whale Research and Fisheries and Oceans Canada, combined with new drone observations conducted by CWR in 2021, will allow researchers to examine whether post-reproductive Bigg’s Transient killer whales take critical leadership and helping roles, similar to the Southern Resident orca elders.