Every Shark for Themselves
Updated: Apr 27, 2021
Several species of sharks are often seen in large groups around tropical reefs, where they hunt for prey at night. These groups allow the sharks to search for and pursue prey with more efficiency compared to when they forage alone (this is known as "social foraging'). However, this is not necessarily a "cooperative" form of hunting, where sharks works as a team... in fact, it can be very competitive and become quite a skirmish.
Social foraging differs from "cooperative foraging" whereby all individuals in a group benefits by working together to gain access to food (like lions hunting as a group and sharing the meal). Social foraging is much more an 'everyone-for-themselves' kind of strategy.
Social foraging has multiple benefits, but also several costs. This is known as a "trade-off". Whilst social foraging improves prey detection (lots of sharks looking for prey is better than one) and increases capture success (prey will struggle to escape when sharks are on every side), it also means increased competition amongst the group. So, it can get quite... um... boisterous, which can potentially cause injuries and food stealing.
The most beautiful footage I have ever seen that shows social foraging in reef sharks can be seen in the 'Our Planet' episode called 'Coastal Seas'. Skip to 10:50min for all the action or the complete episode is also available on Netflix.
Social foraging often occurs in single species groups, where "conspecifics" (aka individuals of the same species) hunt within a large group, but scientists have also now discovered that sometimes social foraging occurs amongst "heterospecifics" (aka individuals from different species).
In French Polynesia groups of whitetip reef sharks (Triaenodon obesus) alongside grey reef sharks (Carcharhinus amblyrhynchos) forage together at night. These species occupy the same habitat and target some of the same prey species (Labourgade et al, 2020).
Scientists studying these sharks found that of 406 foraging attempts observed, 7.6 % included both whitetips and grey reef sharks hunting in unison. The whitetip reef sharks often initiated the prey capture and then the grey reef sharks joined in. The scientists noted that as the whitetips are smaller and slimmer, with more flexibility in their bodies, they were able to barge into deeper holes and crevices in the reef compared to the grey reef sharks. Therefore, the whitetips would flush prey out of hiding spots within the coral, which the grey reef sharks could then chase down (Labourgade et al, 2020).
This is clearly beneficial for the grey reef sharks, as it increases their foraging success and allows them to hunt species they may not otherwise have access to. However, it may not always be so good for the whitetips...
The scientists found that there were five possible outcomes of a foraging event when both species were present:
1. the prey escaped (29% of observations),
2. the whitetip reef shark won and the grey reef sharks got nothing (13%),
3. prey was flushed out by a whitetip shark, but consumed by grey reef sharks (36%),
4. the whitetip disturbed a non-target fish which was caught by a grey reef shark (6%) and
5. both species ate some of the same individual prey item (16%) (Labourgade et al, 2020).
The scientists found that in most foraging attempts, the effect of the grey reef sharks was neutral to the whitetips (meaning having them there during social foraging was neither beneficial or detrimental), but sometimes the grey reef sharks were actually stealing the whitetips' prey. The researchers reported that the grey reef sharks successfully stole the prey from the whitetips in 36% of foraging attempts where both species were present (Labourgade et al, 2020).
Therefore, rather than being an example of "mutualism" (whereby an association between two species is beneficial for both parties), this is actually a form of "parasitism", known as "kleptoparasitism" (feeding from a food source that was caught by another animal). This means that the grey reef sharks benefit from this association, much more than the whitetips do! The researchers suggested that the larger size and higher numbers of the grey reef sharks, allowed the grey reef sharks to get away with this... It is more beneficial for the whitetips to occasionally lose some food, but not get injured a fight, than it is to fight harder for their food against a bigger shark with a bigger gang (Labourgade et al, 2020).
Labourgade P, Ballesta L, Huveneers C, Papastamatiou Y, Mourier J. (2020). Heterospecific foraging associations between reef-associated sharks: first evidence of kleptoparasitism in sharks. Ecology, e03117. doi:10.1002/ecy.3117. Access online.