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At the southern end of Fakarava Atoll, a reserve in French Polynesia, is a narrow channel connecting the lagoon to the ocean. It is the hunting grounds of up to 900 reef sharks, including grey reef, whitetip reef, silvertip, and blacktip reef sharks.
Researchers studying this nearly pristine ecosystem recently documented a new phenomenon during their nighttime dives: The sharks hunt in packs.
They do not cooperate with one another or share the prey; the first shark to catch the fish gets to eat it. It’s more of an opportunistic relationship between two shark species, the grey reef and the whitetip reef shark. The grey shark appears to exploit the assets of the whitetip to increase its predatory success rate.
Behavioral ecologist Johann Mourier, of Marbec (MARine Biodiversity, Exploration, and Conservation), says that diving among hundreds of feeding sharks was exciting, but not dangerous.
“The sharks would approach us, but we were never bitten,” he says. “I think they saw us as an obstacle and not a prey item.”
Mourier and his colleagues observed an unusual hunting strategy among the grey reef sharks of Fakarava Atoll. Grey sharks tended to follow foraging whitetip sharks. As the whitetips searched for hidden prey in the crevices of the coral reef, the grey sharks would attempt to capture fishes the whitetips chased out of the reef.
Whitetip reef sharks are built differently than grey reef sharks. Whitetips are smaller, slimmer, more flexible, and, unlike grey sharks, they are able to breathe without swimming. These characteristics allow whitetip sharks to extract fishes that hide within the reef. Since grey sharks cannot maneuver as well, it appears they use the whitetip sharks to access additional prey.
Mourier and his colleagues documented five possible outcomes for these inter-species hunting parties:
- The targeted prey escaped unharmed (29 percent of hunts)
- The whitetip reef shark caught the prey and ate it, while the grey sharks got nothing (13 percent of hunts)
- The whitetip shark disturbed another fish it did not target, which was consumed by a grey reef shark (6 percent of hunts)
- Prey was disturbed by a whitetip shark but caught and consumed by grey sharks – a phenomenon known as “kleptoparasitism” (36 percent of hunts)
- Both species competed for the same prey item and ended up “sharing” it – they both bit on the prey and each got a piece (16 percent of hunts)
While in most cases this association is neutral for the whitetip reef sharks, grey reef sharks usually benefit. Sometimes, when they steal a fish that a whitetip shark disturbed, the grey sharks win at the expense of the whitetips.
“Grey reef sharks benefit from hunting with whitetip reef sharks because it increases the number of prey species the grey sharks can access,” says Mourier. “The association with whitetips also increases grey shark predation success by almost 25 percent.”
Mourier says that predation efficiency in sharks tends to be low. Sharks of different species might associate because hunting collectively increases the chances to catch prey.
Although we tend to think of sharks as territorial predators, Mourier says this collective hunting strategy may not be unusual.
“Where the density of sharks is low, where they have been overfished or their habitat has been diminished, we might not see this behavior,” he says. “But in this pristine place, we have so many sharks that we are able to see this kind of phenomenon. The behavior is new to us because there are not many places like this left in the world.
“If we change the density of sharks, it could make changes to this interaction and have some cascading effects on the food web.”
Mourier says this finding can also inform our understanding of behavior and cognition.
“We show that this type of collective behavior can arise in so-called ‘primitive’ species, just using simple rules,” he says. “There is no strong social structure, but the sharks can still use collective behavior with simple rules to benefit and increase their fitness.”
References
Labourgade P, Ballesta L, Huveneers C, Papastamatiou Y, Mourier J. Heterospecific foraging associations between reef-associated sharks: first evidence of kleptoparasitism in sharks [published online ahead of print, 2020 Jun 10]. Ecology. 2020;e03117. doi:10.1002/ecy.3117.
