that she would have a calf that survived until the following year). 

What we found was striking: Each additional surviving weaned son (no longer nursing) reduced a female’s chances of reproducing each year by more than half. For example, a female with no sons has about a 1 in 5 chance of reproducing when she is 21 years old, while a female with a single son has about a 1 in 10 chance of reproducing at the same age.

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The next question was whether this effect might change as males age. Are males still dependent after weaning but more capable as they reach adulthood? After running the analysis, we found no evidence for this; if anything, sons became a bit more costly as they got older! This cost makes some sense, as the effect of maternal loss on male survival actually increases as male killer whales age.

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This “cost” to killer whale mothers says something remarkable about their society. They are not the only species where sons and/or daughters benefit from having their mom around late into life. Similar patterns are found in chimpanzees, hyenas, deer, and elephants, to name a few. However, in the other species studied, females do not seem to pay a cost in providing these benefits to their offspring. In fact, these offspring often help their mother out, as is the case with chimpanzees, where older offspring improve their mother’s reproductive success by helping her care for their younger siblings; this suggests that the mother-offspring bonds in these other species are what we call mutualistic: both parties are receiving a benefit from the relationship. 

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Killer whales seem to be showing something entirely different, which scientists call parental investment: animals sacrificing their future reproductive success to improve the survival of their offspring. Some parental investment is a feature of almost all mammalian life history, but this is the first time we have observed lifetime parental investment, revealing a novel and extreme life history strategy in killer whales, opening up all kinds of avenues and questions for future research.

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Returning to the Southern Resident killer whales, this result also raises questions about the dynamics of this population. Typically, when analyzing the dynamics of animal populations (and forecasting them into the future), we predict population reproductive rates based on population size and composition (i.e., how many reproductive-aged females are there) and environmental conditions (how much food is there for them to reproduce). This result adds a new factor for us to consider: How many reproductive-aged females already care for weaned sons? When more females are caring for sons, we expect the population’s reproductive rate to be lower. Looking at CWR’s long-term data, the portion of females caring for sons has fluctuated from about 30% to over 80%. Currently, there are 28 females in the SRKW population between the ages of 9 and 45. Of those females, 17 have at least one surviving son. Going forward, we hope to incorporate the effects of caring for sons into analyses of the SRKW population’s viability.

Another open question is the role of resource abundance. We think females have lower reproductive rates when they have sons because they share food with their sons. The more they share, the less food they have for themselves, negatively impacting pregnancy success. But what about in populations like the Northern Residents and the Bigg’s killer whales, where there is more food to go around? Do females in these populations experience costs for taking care of sons, and are these costs as high as those for Southern Residents? Or does having more food allow them to take care of their sons while also obtaining enough food to carry pregnancies to term successfully? We do not yet know the answers to these questions, but we are hopeful that future research will start to piece apart this puzzle.