Killer Whale Grandmothers Boost Survival of Calves

Introduction

Along with being one of the world’s most powerful predators, Killer whales are also highly intelligent and social creatures with intricate techniques of interaction. They can be found almost all around the world in highly productive areas of cold water upwelling. They are usually found in ‘pods’ which consist of various sexes and ages. It usually contains the mother and the one or two offspring. The strong associations between the pod members carry on into adulthood, therefore family associations are very strong, where each member has certain roles. This article looks at the effect post-menapausal grandmothers have on their offspring compared to breeding grandmothers. They do this by looking at salmon population data as they believe grandmothers play a large role in collecting prey. 

Background

Besides humans, only 4 species are known to experience menopause: belugas, narwhals, killer whales and short-finned pilot whales. According to one of the authors of the study, Sam Ellis, in killer whales, the reason to stop reproducing is because both male and female offspring stay with the mother for life. If the mother keeps having young, they would have their own descendants competing for resources. Older females share their knowledge in order to help their group survive. Killer whales have “the longest post-reproductive life span of all nonhuman animals: Females stop reproducing in their 30s to 40s but can survive into their 90s.” This allows the mothers to maximize the fitness of her offspring by ensuring their survival and that the offspring achieve reproductive success.

Methods (Models and Variables)

With regard to methodology, this research was comprised of 3 interconnected methods: Study populations, Survival Model with Time-Dependent Effects, and Interbirth Interval Model. In the Study Populations, demographic records were collected manually using photographic censuses for 2 resident killer whale populations: Southern (1976-2016) and Northern (1973-2016) populations in the inshore coastal waters of Washington State and British Columbia, Canada. Individuals were identified by their unique fin shapes, saddle patches, and the presence of any nicks or scratches, and were sexed using distinctive pigmentation patterns around the genital slits and, in adults, differences in fin size. Genealogical relationships were inferred from long-term observations of social organization, and mothers were identified by their repeated association with young calves.

Overall, the Data collected for the analysis was comprised of the following variables: Year of Birth (YOB), Year of Death  (YOD), ID of Mother. The aforementioned variables were used to deduce the Age of Death, and Maternal Grandmother ID in order to provide enough variables to run the models. In the Survival Model with Time-Dependent Effects, a Cox Proportional Hazards model was created to examine the consequences of a grandmother’s death on grand offspring survival. Lastly, an Interbirth Interval Model was used. A Generalized additive model was used to examine the consequences of a grandmother’s status on her daughter’s interbirth intervals. To test whether grandmothers decrease their daughters’ interbirth intervals, the researchers regressed a number of covariates on each interbirth interval. 

Results

Considering the above methods to compile forty years’ of data into a survival model and an interbirth interval model, numerous specific models were considered for each task. The best model for the survival model was given by the following equation which reflects a general grandmother and post-reproductive grandmother effect.

h(t)=h0(t)exp{2.9sMR+4.3GMR+0.4GMo45−2.8(slm×GMR)}

Where sMR is 1 for male, 0 for female (if mother died in past two years) GMR is 1 if grandmother died in past two years, and GMo45 = 1 if grandmother was post-reproductive, slm is salmon index.

From this we see that the loss of a grandmother within the last two years increases mortality hazard by 4.5, while the loss of a post-reproductive grandmother increases mortality by 6.7, if the salmon index is at the norm of 1. If the salmon index is lower (meaning food is scarcer), the grandmother effect becomes even more pronounced. The sex impacted the mother effect, but not the grandmother effect.

The best interbirth interval model demonstrated that living grandmother’s did not impact the interbirth interval for mothers. However, two alternative equations with low AIC relative to the best model, both included grandmother effects on interbirth interval but shockingly showed that the presence of a grandmother tended to increase the interbirth interval of a mother.

Significance

The purpose behind menopause has been an evolutionary puzzle, especially as only humans and four species of whales experience menopause. Female killer whales in particular have evolved the longest post-reproductive life span of all nonhuman animals. By demonstrating that post-reproductive grandmothers reduce grand offspring mortality more than grandmother killer whales that are still reproducing, this research reveals the measurable positive influence that menopause has on killer whales’ grand offspring. Knowing the benefits that menopause brings to the killer whales is essential to understanding why female killer whales have evolved to live long lives post-reproduction.

Chosen News Article

 www.sciencedaily.com/releases/2019/12/191209161339.htm

University of York. “Killer Whale Grandmothers Boost Survival of Calves.” ScienceDaily, ScienceDaily, 9 Dec. 2019. 

References

Foster, E. A., Franks, D. W., Mazzi, S., Darden, S. K., Balcomb, K. C., Ford, J. K. B., &
Croft, D. P. (2012). Adaptive Prolonged Postreproductive Life Span in Killer Whales. Science,
337(6100), 1313. https://doi.org/10.1126/science.1224198

Imster, E. (2018, August 31). Beluga whales and narwhals go through menopause. Retrieved
February 16, 2020, from
https://earthsky.org/earth/beluga-whales-narwhals-go-through-menopause

Nattrass, Stuart, et al. “Postreproductive Killer Whale Grandmothers Improve the Survival of Their Grandoffspring.” PNAS, National Academy of Sciences, 26 Dec. 2019,   www.pnas.org/content/116/52/26669

Nationalgeographic.com. (2010). Orcas: Killer whales are the largest dolphin species. [online]
Available at: https://www.nationalgeographic.com/animals/mammals/o/orca/ [Accessed
17 Feb. 2020].

Robin W Baird and , Hal Whitehead ” Social Organization Of Mammal-Eating Killer Whales:
Group Stability And Dispersal Patterns – Canadian Journal Of Zoology “. 2020. Canadian
Journal Of Zoology.
https://www.nrcresearchpress.com/doi/abs/10.1139/z00-155#.XksFWxNKg1L

Yong, E. (2017, January 12). Why Killer Whales (and Humans) Go Through Menopause. Retrieved February 16, 2020, from https://www.theatlantic.com/science/archive/2017/01/why-do-killer-whales-go-through-menopause/512783/