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Dusky Sharks: Whale Killers

It’s generally thought that baleen whales are too large to be successfully attacked by most marine predators.  Orcas are typically considered the only real predatory threat to large whales, and even they have to use teamwork to take down a young whale.  Large sharks, which also sit near the top of the marine food web, are known to scavenge on whale carcasses as a nutritious and blubbery supplement to their usual diet of fishes and smaller marine mammals.  However, evidence has been found that white sharks actually take a proactive approach to increasing the whale carcass supply by attacking live northern right whale calves.  Now researchers in South Africa directly observed dusky sharks actively teaming up to bring down a humpback whale calf.

Dusky sharks are known for being one of the largest species in the Carcharhinus genus and their populations have the dubious honor of being among of the most depleted.  Being big sharks, they function as apex predators and are classified as “occasional predators” of smaller whales thanks to the remains of dolphins and other marine mammals found in their stomachs.  There have been no direct observations of this species attacking any cetacean and these sharks were thought to live a solitary lifestyle feeding on fish, other sharks, and the occasional dolphin.

Carcharhinus obscurus: Whale killer?

The attack reported by Dicken et al. (2014) was observed by researchers and divers checking out South Africa’s annual sardine run, a marine predator party that has been featured in many nature documentaries.  They found a lone humpback whale calf being followed by a group of 10-20 adult-sized (2-3 m/6-10 ft) dusky sharks.  The whale was covered in superficial shark bites, and individual sharks were observed (and apparently filmed) charging at the whale and taking bites out of its left side.  The attack went on for 6.5 hours after being first observed, when the whale finally dove and did not surface for an hour.  At this point it was assumed the whale had finally died of its injuries or exhaustion.

Dusky shark bites on a humpback whale calf. From Dicken et al. (2014).

Dusky shark bites on a humpback whale calf. From Dicken et al. (2014).

This observation has a lot of interesting implications.  The most obvious is that it can’t be taken for granted that whales are protected from shark predation by size alone.  The dusky shark is an apex predatory species, but is physically similar to many other large Carcharhinid sharks.  If duskies can take down a humpback whale calf, then other large sharks like bull sharks, tiger sharks, and oceanic whitetips may also be physically capable of threatening large baleen whales.  Sharks may be an important part of whale ecology.

Perhaps more interestingly, this is a case of a normally solitary shark working cooperatively to take down large prey.  A single humpback whale calf is far too big a meal for a single dusky shark, so there is clearly a shared benefit to all the members of the pack.  The idea of sharks forming packs isn’t that far-fetched: cooperative hunting has been observed in other shark species and shark social behavior has proven to be much more complex than previously thought.  Dusky sharks normally keep to themselves but do gather in large numbers during the sardine run.  Did this group form incidentally or is this something dusky sharks do on a regular basis?  As dusky shark populations rebuild, will we start seeing this sort of thing more often?

If nothing else, this gives me more ammunition the next time I get into a “my study organism can beat yours up” conversation.  Yes, ecologists have those.  Often.

Reference

Dicken, M. L., A. A. Kock, and M. Hardenburg. 2014. First observations of dusky sharks (Carcharhinus obscurus) attacking a humpback whale (Megaptera novaengliae) calf. Marine and Freshwater Research. http://dx.doi.org/10.1071/MF14317