Probably everything you wanted to know about killer whale reproduction

— from Michael Weiss, CWR Field Biologist/PhD Candidate —

L103 and L123 (Photograph by Dave Ellifrit, CWR)

With the latest announcement from the SR3 drone photogrammetry team that several Southern Residents are pregnant, we thought it was a good time to discuss killer whale pregnancy and reproduction, both for killer whales as a whole and for the Southern Residents in particular.

Killer whale pregnancies last 18 months, one of the longest gestations of any mammal. Newborn calves are around 8 feet long and weigh about 400 lbs. These newborn calves suckle for short periods dozens of times a day. Their mothers’ milk is extremely rich, possibly containing 40-60% fat. While calves may start experimenting with solid food at a young age, they likely do not fully wean until around the age of 3.

The Southern Resident killer whales have unusually low reproductive output, lower even than their nearby neighbors, the Northern Residents. Work using hormones derived from fecal samples has shown that approximately 69% of Southern Resident pregnancies result in spontaneous abortion.

Even if the pregnancy is successful, life isn’t easy for the calf. Of the whales that have been assigned an ID by the Centre for Whale Research since 1976, about 1 in 6 died before their first birthday. Given the high rate of pregnancy failure and the low survival of calves, the story is grim: any given pregnancy has about a 1 in 5 chance of resulting in a calf that survives for more than a year.

What is driving this low reproductive output? Analyses of demography and hormones point towards reduced prey availability. Controlling for age, females are more likely to reproduce in years following years of high Chinook salmon abundance, and spontaneous abortions are correlated with hormonal evidence of nutritional stress. The survival of calves (and all Southern Residents) is also correlated with salmon abundance.

Additional stress comes from persistent organic pollutants, such as polychlorinated biphenyls (PCBs). These toxins get passed from the mother to the calf during gestation and nursing, which could cause pregnancies to fail and young calves to die. There is evidence that these two threats interact; PCBs become more of a threat when salmon abundance is low, and the whales’ body condition is poor.

Killer whales as a species are already slow to reproduce, with their long interbirth interval, old age of first reproduction, and long post-reproductive lifespan. With the addition of nutritional stress and toxins, the Southern Residents’ reproductive output is slowed to a crawl. To get the Southern Resident population growing, it is imperative that they are allowed to have an adequate food supply, which means increasing the abundance of Chinook salmon in our waters.