DNA testing has revealed that beluga whales have several different mates over their lifetime and this could be important for protecting the population, according to a new study published in Frontiers in Marine Science.
Studying beluga whales is incredibly challenging because these white whales are often hard to spot when they disappear under the safety of the ice.
This has made it difficult for scientists to learn about their mating habits. Researchers knew that males grow much larger than females and that females typically have one baby every few years. This led them to believe that beluga whales are polygynous.
Polygynous mating – when a male mates with several females – is thought to be the norm for many cetacean species (whales, dolphins and porpoises) based on what is known about land-based animals, which are better studied.
“One pattern that tends to hold in many terrestrial animals, for example, is that species where males are much larger than females and do not provide much parental care tend to have polygynous mating systems,” says the study’s lead author Dr Greg O’Corry-Crowe, a researcher at Florida Atlantic University.
They expected that the result would show that, each season, the largest or most dominant males produced most of the calves. The scientists’ theory was that “beluga bulls might put their energies into competing intensely for access to large numbers of females within a breeding season,” says O’Corry-Crowe. “In extreme cases it could even result in a winner take-all-type of scenario.”
But, when they tested this theory, what they found surprised them all.
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The scientists worked with native subsistence hunters to collect 623 tissue samples from one population of belugas in Bristol Bay, Alaska. Their data showed that both males and females mate with lots of different partners over their lifetime.
“Some males did fare better than others, but none secured very high numbers of successful matings over the course of the study,” says O’Corry-Crowe.
This could be down to how hard it is to court several females in the ocean or because beluga whales live for such a long time – sometimes reaching the ripe old age of 90. “Beluga males seem to play the long game over a very long reproductive life,” he adds.
And it wasn’t just the males. Females also switch mates across different breeding seasons. “This could be a bet-hedging strategy to limit the risk of mating with low-quality males,” says O’Corry-Crowe.
Although this finding might be unique to the whales in Bristol Bay – other beluga populations might have different mating strategies – it could explain why this small population has not lost genetic diversity.
“In polygamous species, because some individuals are more successful at passing on their genes than others, overall genetic diversity within the population can be lost quite quickly,” O’Corry-Crowe explains. When only a few dominant males are producing calves, there’s more chance of whales in the next generation mating with a highly related beluga, leading to inbreeding.
When a population has more inbreeding, it is less genetically diverse and so less resilient to potential threats.
“This is especially true in small populations, like the one we investigated in Alaska,” which has just 2,000 whales, he says, so they were surprised by the low levels of inbreeding they found. “It appears that regular mate switching may be a good way to temper the negative effects of polygamy in a small population.”
O’Corry-Crowe and the team want to take this further and plan to use drones to spy on mating behaviours, he says: “We must head back out into the wild and learn more about beluga whale mating systems and how they have perfected a long life among the ice floes.”
Top image credit: Photo taken by Lisa Barry with permission from NOAA Fisheries
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