Nature
PUBLISHED
An isolated Alaskan population of beluga whales have a population small enough that inbreeding could be a major threat. Yet they remain genetically healthy, and new evidence suggests this is a consequence of rejecting both monogamy and polygyny. Instead, males and females are breeding on reasonably equal terms, to the benefit of their offspring.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Many discussions of mating patterns focus on just two: long term monogamy such as is common in many seabirds, and polygyny where the strongest or most attractive males have many mates, while most females have just one. Indeed, there’s a strand of evolutionary psychology that wants people to believe these are really the only two that exist, and any deviation from monogamy in humans will devolve to a few males with vast harems.
Perhaps all those finger-wagging books and videos subconsciously affect scientists as well, because when marine mammal researchers investigated the genetics of beluga whales living in Bristol Bay, Alaska, they expected to find a few males fathering most of the young. The outcome was quite different.
“We still know very little about beluga whales, despite their immense popularity,” said Dr Greg O’Corry-Crowe of Florida Atlantic University in a statement. “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.”
The team used DNA from tissue samples collected from 623 Bristol Bay whales collected over 13 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,” O’Corry-Crowe added.
Instead, the team found that no whales dominated the next generation’s gene pool. “Beluga males were indeed polygynous, but, surprisingly, only moderately so,” said O’Corry-Crowe. In other words, the most successful males didn’t have many more offspring than the least. No male in their sample had more than four offspring among those whose tissue was collected.
That didn’t mean the belugas were picking and sticking with a specific partner – there were few full siblings, with most calves from the same mother having a different father.
“The three-dimensional aquatic environment likely limits a male’s ability to successfully court or corral multiple females,” O’Corry-Crowe said. “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.” Or maybe they just get bored with the same mate over and over.
Probably as a result, the genetic diversity of the bay was high for a population of just 2,000 that have been isolated for a long time. The team also found historical evidence that this is a fairly stable trait, making the risk of inbreeding low.
The authors stress that we shouldn’t assume this finding applies to all beluga populations, noting that Bristol Bay’s male and female belugas are more similar in size than in other locations. Sexual dimorphism, which is higher in most beluga populations than almost any other whale, is often associated with polygyny, as it usually occurs when males compete aggressively to control harems. Although reports of scarring caused by other beluga whales’ teeth have been interpreted as evidence that males fight aggressively for females, the team found this was rare in Bristol Bay. Perhaps the bay’s whales just don’t do toxic masculinity.
“The differences in sexual dimorphism among populations of beluga whales could indicate that mating systems also vary,” O’Corry-Crowe said. The team hope to study this by expanding their research to other populations. They also want to see whether females mate with multiple males in a single breeding season, or if they are serially monogamous, moving on in subsequent years. Solving the latter question will be done using drones to try and watch mating encounters, since privacy laws do not protect marine mammals.
It's not entirely surprising beluga whales might have a try-anything-once attitude to sex. Skulls have confirmed Inuit accounts of beluga whales mating successfully with their nearest relatives, narwhals, so they might not be that picky.
The study is published open access in Frontiers in Marine Science.
