A small group of leopard sharks has been seen – and caught on film – engaging in a three-way bout of reproductive hanky-panky in the wild.
It's the first time scientists have documented this mating behavior in the animals, as first one male and then a second mated with a female leopard shark (Stegostoma tigrinum) in quick succession. The stunning observation, recorded from beginning to end, could shed light on the reproductive strategies of this endangered species.
"It's rare to witness sharks mating in the wild, but to see it with an endangered species – and film the event – was so exciting that we just started cheering," says marine biologist and ecologist Hugo Lassauce of the University of the Sunshine Coast in Australia.
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There's actually quite a lot we don't know about shark reproduction. The animals live complex, elusive lives in a habitat inhospitable to humans. In addition, they have complicated reproductive anatomy and strategies.
Leopard sharks (also known as zebra sharks for the stripes seen on juveniles) in particular have very complicated strategies, with the ability to switch between sexual and asexual or 'parthenogenic' reproduction. Parthenogenesis has some downsides, such as a drop in genetic diversity, but it can be a practical fallback option if population numbers decline.
The new observations, recorded by Lassauce in the course of his work monitoring sharks off the coast of New Caledonia, could hold clues about other mating strategies. It was the first time he'd ever managed to observe the entire act.
"I'd seen males swimming fast after females before and I'd arrived 'on the scene' just after a male and female separated, but I'd never seen the whole sequence," he says.
"Then, while I was surveying this particular aggregation of leopard sharks, I spotted a female with two males grasping her pectoral fins on the sand below me. I told my colleague to take the boat away to avoid disturbance, and I started waiting on the surface, looking down at the sharks almost motionless on the sea floor.
"I waited an hour, freezing in the water, but finally they started swimming up. It was over quickly for both males, one after the other. The first took 63 seconds, the other 47. Then the males lost all their energy and lay immobile on the bottom while the female swam away actively."
All three sharks were about the same size, around 2.3 meters (7.5 feet) in length. Leopard sharks are slow and unaggressive, feeding mainly on gastropods and crustaceans. They are not a danger to humans.
The implications of this mating style are not yet clear, but we can make some inferences. Where parthenogenesis decreases genetic diversity, multiple fathers would increase it.
It's also interesting to note, although it may not be relevant here, that female sharks of some species can store sperm in special pockets called oviducal glands, a strategy that allows the female to choose the optimal time and conditions for making babies. At least one species can store sperm for years.
"This evidence suggests the site in New Caledonia is a critical mating habitat, which can inform management and conservation strategies as well as help us understand population dynamics and reproductive behaviours more widely," says marine biologist Christine Dudgeon of the University of the Sunshine Coast.
"It's surprising and fascinating that two males were involved sequentially on this occasion. From a genetic diversity perspective, we want to find out how many fathers contribute to the batches of eggs laid each year by females."
The observations are detailed in the Journal of Ethology.