DNA tests reveal mysterious beluga family trees

Belugas are even harder to study than most whales: it’s difficult to observe a species that vanishes under the Arctic ice. But now DNA analysis has given scientists a precious glimpse into the social life of a beluga population living in Bristol Bay, Alaska. They found that males and females mate with many different partners over the years, which could be keeping this small, isolated population genetically viable. 

“We still know very little about beluga whales, despite their immense popularity,” said Dr Greg O’Corry-Crowe of Florida Atlantic University, lead author of the paper in Frontiers in Marine Science. “The primary reason for this is the difficulty of studying a species that lives beneath the waves in the cold and often frozen north. But this is the challenge that makes discovery, when it happens, more exciting.” 

Secrets of the ice

Over 13 years, a team — which included scientists from Florida Atlantic University and the Alaska Department of Fish and Game, and Alaska Native subsistence hunters from Bristol Bay — collected small tissue samples from 623 whales. Without much existing information about wild belugas’ mating strategies to go on, researchers used evolutionary theory and facts about belugas’ biology to make predictions they could test against this real-life data. For instance, males are significantly larger than females, and females can only have one calf every few years. 

“We predicted that beluga whales had a polygynous mating system where a few of the most competitive and possibly largest males secure most of the matings within a season or even across a few seasons, and that they provide little or no parental care,” said O’Corry-Crowe. 

However, belugas live in large social groupings which split up and reform over time, which could increase females’ access to different potential mates. So the scientists also predicted that females might mate with many different males across breeding seasons. 

Playing the long game?

When the scientists looked at the results, they found that both male and female belugas had calves with different mates over the years. If calves had siblings, they usually only shared one parent. All belugas had a small number of calves, but there was more variation in males’ reproductive success: some males fathered slightly more calves. 

“Beluga males were indeed polygynous, but, surprisingly, only moderately so,” said O’Corry-Crowe. “The three-dimensional aquatic environment likely limits a male’s ability to successfully court or corral multiple females. However, a long life may also be key. Belugas can live 90 years, possibly more. Male beluga whales may, therefore, play a long game of securing a few matings each year over a very long reproductive life! 

“The female story is just as fascinating. The genetic profiling revealed that female belugas regularly switch mates across breeding seasons, also over a long reproductive life. This could be a bet-hedging strategy to limit the risk of mating with low-quality males.”

Surprising resilience

The scientists also found unexpectedly high genetic diversity and low levels of inbreeding, despite a small population of just 2,000 individuals. Comparing the results to other populations and historical samples from Bristol Bay indicates that this population’s genetic diversity is roughly equivalent to larger populations and has remained stable over time. 

“A leading concern for small populations is that they tend to lose genetic diversity faster than large populations and the risks of inbreeding are higher,” explained O’Corry-Crowe. “We expected to find low diversity and high inbreeding, but we found something quite different. The mating system may explain this surprising finding. Frequent mate switching limits the number of highly related offspring in the population. This in turn reduces the risk of highly related individuals mating and producing highly inbred offspring. It also minimizes the risk of diversity loss. We cannot afford to be complacent, but we can be optimistic that beluga whale mating strategies provide evidence of nature’s resilience.”

The scientists warn that other populations could behave differently. Sexual dimorphism is comparatively low in Bristol Bay, which may indicate that mating depends less on competition between these males than it does elsewhere. 

“To me, the differences in sexual dimorphism among populations of beluga whales could indicate that mating systems also vary, and this is something we are currently working on,” said O’Corry-Crowe. “We also can’t determine if females mate with multiple males within a season using genetics, as a female only produces one calf from one lucky male. But we are working on this, using drones at other locations to determine if we can observe mating behaviors in the wild. More on that soon…”