Endangered killer whales off the west coast of Canada and the U.S. are facing an additional threat—inbreeding.
A research team headed by the U.S. National Oceanic and Atmospheric Administration (NOAA) has found that orca whales that frequently visit Washington’s Puget Sound and the coast of British Columbia are mating within the same pods, or the same social groups.
This could lead to a significant reduction in the genetic diversity of the already perilously small population of southern resident killer whales, which currently numbers only about 85 animals.
Observations of other groups of whales have shown that inter-pod breeding is a rare and abnormal occurrence, an example being the northern resident killer whales—whose habitat ranges from Washington to southern Alaska—in which inbreeding has never been detected. The two groups are distinct and do not socialize with each other.
Although southern residents do not mate outside their population, the whales do tend to avoid mating with their more close relatives, found the study, which was published in the Journal of Heredity.
“Even though some of the fathers were in the same pod as the mothers, none of them were really closely related to each other. Our results suggest that Southern Residents avoid mating with their siblings or parents, but we aren’t really sure of the social process that results in this avoidance,” Ford said.
Males within the southern resident community were also found to have a relatively low variance of reproductive success compared to other mammals, meaning several males are responsible for offspring production in the population rather than just one or two.
This may reflect the difficulty male killer whales have in controlling other males’ access to females.
However, the study also showed that the older and larger males appeared to be responsible for most of the successful matings, indicating that females may choose to mate only with older males, or that older males may prevent the younger males from breeding successfully.
The southern resident group was listed as endangered under the U.S. federal Endangered Species Act in 2005.
According to Fisheries and Oceans Canada, southern residents are heavily contaminated with toxic chemicals such as organochlorine, which can lead to reproductive problems. Water pollution is also affecting the whales’ food supply, and there is the ongoing problem of ocean traffic near major centres like Seattle and Vancouver that interferes with the animals.
