Sharks are among the most misunderstood animals in the ocean, thanks in large part to the Jaws movie franchise. Even scientists aren’t immune to some of these misconceptions — but with the help of a new study from the Georgia Aquarium, marine biologists are now beginning to challenge one long-held misconception in particular.
The study, which examined mating patterns in shark and ray species, included contributions from Dalhousie University and was published in Molecular Ecology.
Do shark reproduction patterns benefit males or females?
Many shark species breed using a strategy known as multiple paternity, where the female shark gives birth to a litter of pups from multiple different fathers. The exact biological driver behind this breeding pattern is unknown, but traditional explanations paint female sharks as unwilling participants who are unable to fend off advances from males — males who are assumed to be aggressive, like the infamous shark in the movie Jaws. This explanation is referred to as convenience polyandry.
Given that female sharks invest a great deal of energy into producing their young, however, you might wonder why the female doesn’t have more of a say when it comes to her mate. This question was what led Kady Lyons, a research scientist at the Georgia Aquarium and lead author of the study, to investigate whether or not convenience polyandry really is the best explanation for multiple paternity.
“If convenience polyandry is the only reason we talk about, it takes the agency away from females,” Lyons said in a press release. “If she’s investing all this energy into making big babies, why doesn’t she get a say in who will be the sire of her offspring?”
While the majority of previous studies have only considered this breeding strategy from the male point of view, Lyons and her colleagues decided to investigate the female perspective as well.
They started by listing different biological behaviours that could contribute to increased rates of multiple paternity, and categorizing these based on whether they were more influenced by males or females. For example, female sharks could be actively seeking out multiple mates as a way to increase genetic diversity and strengthen their offspring’s chances of success.
The team then created models to predict the outcomes of these behaviours. If female sharks have a biological mechanism that works to reduce competition between sperm and increase genetic diversity, for example, you might expect to see large litters from many different fathers. If competition between male sharks is the main biological driver behind multiple paternity, however, litters would more likely be skewed towards just one or two fathers with the most competitive sperm.
Lyons and her colleagues compared these models to previous observations of shark litters from multiple species. For the most part, the observations indicated equal advantages for both male and female sharks, suggesting that multiple paternity is not just beneficial for males.
“Male or female drivers may sometimes produce the same multiple-paternity end result, but more often than not it is the male factors that get the lion’s share of the credit,” Lyons said.
“This seemed odd to me considering how complex and energetically taxing female reproduction is.”
Diverse perspectives can enhance our science
The researchers go on to describe the wide diversity of shark and ray species, and reiterate that one single explanation — such as convenience polyandry — can’t be assumed for such a broad range of animals.
They hope that diversifying the field of marine biology as a whole will provide additional insights going forward. The majority of previous studies on shark reproduction had been led by male scientists, which could be one reason why the male-dominated convenience polyandry explanation — which paints male sharks in a stereotypically aggressive light — has been accepted for so long.
“Perspective is completely shaped by background,” Lyons said. “If you don’t have a diverse background, your perspective will be limited.”