The critically endangered Southern Resident killer whale population is having too many baby boys, threatening its chances of recovery. Now a new scientific consortium may help explain the skewed sex-ratio mystery.

The Southern Residents, which spend much of their lives in the inland marine waters of Washington state, have experienced an encouraging baby boom in recent months. Of nine calves born since late 2014, eight have survived, bringing the population’s total number to 84.

But to the dismay of conservationists, at least five of the calves have been confirmed as male, the Washington state–based Center for Whale Research said this week, and only one as female. Of the remaining two, one is believed to be male and the gender of the other is still undetermined.

Typically, killer whales give birth to offspring with a 50-50 ratio between males and females.

“From a population perspective, this sex ratio does not bode well,” the center said in a statement. “Given that female killer whales don’t tend to give birth until after the age of 10 and are only able to give birth roughly every three years, it is not hard to see how slowly the population will grow” if low numbers of female offspring lead to a reproductive bottleneck in the future.

Eric Ward, a scientist at the National Oceanic and Atmospheric Administration, does not consider the male-to-female ratio of recent births that unusual.

“The sex ratio is still well within what we’d expect if it’s truly 50:50, and there’s no evidence of a trend or change over time,” Ward wrote in an email. “How unusual is six of eight calves [being male]? We'd expect to see that or something more extreme about one in six times. We’d expect to see a skewed sex ratio in general one in three times.”

“To put things in perspective, of the last 10 human babies [or so] born in Seattle that I know, eight or nine are male,” he added. “One might look at this and say ‘Eek—there’s something in the water!’ But given the sample sizes, it’s a very common result and nothing to draw larger inference from.”



When it comes to the Southern Residents, however, other scientists believe it really could be something in the water, as well as something missing. An ongoing shortage of the whales’ preferred prey, Chinook salmon, as well as high levels of PCBs, heavy metals, and other pollutants in Puget Sound, are known problems with the species’ recovery.

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“In some animal populations, there may be a food-shortage link to male bias. It may have to do with a reduction in carbohydrates,” said Deborah Giles, research director at the Center for Whale Research. “Or it could be environmental toxins or a combination of factors.”

Some studies have shown a link between toxins and skewed sex ratios, though most of those found a bias toward female offspring. At least two pollutants commonly found the endangered orcas’ habitat have been associated with more male offspring in fish: discharges from paper and pulp mills, and tributyltin, a compound once used to prevent the growth of marine organisms on ship hulls. Although an international ban on the highly toxic chemical was enacted in 2008, tributyltin can persist for up to 20 years in the environment.

It is not yet known if these pollutants would have a similar impact on marine mammals. But a new scientific collaboration, announced on Tuesday, may help figure that out.

Researchers from the SeaDoc Society, the University of California Davis School of Veterinary Medicine, NOAA’s marine fisheries service, and the National Marine Mammal Foundation have agreed to pool vast amounts of information on the Southern Resident killer whales into a single database and create a “medical chart” for each animal.

“I’m excited about the process of pulling multiple data sets together on behavior, family relationships, contaminant levels, skin diseases, and stranded animals,” said Lynne Barre, a marine biologist at NOAA Fisheries who works on the recovery of the Southern Residents. “It will be a very powerful tool to rule things out and focus on what’s important.”

Joe Gaydos, a veterinarian with UC Davis and chief scientist at the SeaDoc Society, hopes that once the data are compiled, “you can look at babies, and what their sex is, and how contaminants have changed over time.”

Still, the skewed sex ratio among the Southern Residents could remain enigmatic for years.

“Everyone’s shaking their heads about this,” Gaydos said. “When there’s a small population and so many variables, it takes time to work out whether things are happening by chance, and this could be pure chance.”