Every Shark for Themselves

It is a truly spectacular sight... hundreds of sharks looming out of the darkness, all swarming around a reef, hunting for prey amongst the corals. It is busy and intense, and often seems quite violent. So why do these sharks hunt like this? Why is it beneficial? And does every shark manage to find enough food for themselves?

Grey reef and whitetip reef sharks hunt in mixed species groups on coral reefs at night (Image Credit: Laurent Ballesta / Labourgade et al, 2020)

Psycho Social

Several species of sharks are seen in large groups around tropical reefs, where they hunt for prey at night. It might look manic, but actually these big groups allow the sharks to search for and pursue prey efficiently compared to when they forage alone. This is known as "social foraging' (Robbins & Renaud, 2016; Labourgade et al, 2020).

Social foraging in grey reef sharks is much more competitive than cooperative foraging that we see in groups of social animals like lions (Robbins & Renaud, 2016)

What is interesting though is that, in sharks this foraging strategy is not necessarily "cooprative". Social foraging differs from "cooperative foraging" (where all the sharks are working together as a team and all the individuals in the group benefit by gaining access to food, like lions hunting as a group and sharing the meal) because the sharks are much more into an everyone-for-themselves kind of strategy. In fact, it can be very competitive and become quite a skirmish (Robbins & Renaud, 2016; Labourgade et al, 2020).

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 coming from every angle), it also means increased competition amongst the group. So, it can get quite... um... boisterous, which can potentially cause injuries and food stealing (Robbins & Renaud, 2016; Labourgade et al, 2020).

Grey reef sharks and whitetip reef sharks hunt in mixed groups in French Polynesia (Image Credit: Laurent Ballesta / Labourgade et al, 2020)

A Mixed Bag

Social foraging often occurs in single species groups, where "conspecifics" (individual sharks of the same species) hunt within a large group, but scientists have also now discovered that sometimes social foraging occurs amongst "heterospecifics" (individuals from different species) (Robbins & Renaud, 2016; Labourgade et al, 2020).

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).

Whitetip reef sharks are nocturnal; hunting in large groups at night (Image Credit: Kris Mikael Krister / WikimediaCommons)

Scientists observed hundreds of foraging attempts in these sharks and found that it was actually not common for the two species to hunt in unison (only 7.6% of foraging attempts). They noted that in these rare instances, whitetip reef sharks often initiated the prey capture and then the grey reef sharks joined in afterwards. 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).

In mixed species groups, whitetip reef sharks often lose out to the bigger grey reef sharks, after they have flushed prey out of crevices (Image Credit: Laurent Ballesta / Labourgade et al, 2020)

Life's not Fair!

What was really interesting about this study, is that the scientists discovered that in these mixed groups, social foraging is actually not beneficial to the whitetip reef sharks, as sometimes the grey reef sharks were actually stealing their prey. This means that the grey reef sharks benefit from this association much more than the whitetips do (Labourgade et al, 2020).

When foraging in a group the whitetip reef shark will burrow into crevices to flush out prey (a), which grey reef sharks often steal (b)(Labourgade et al, 2020)

This means that, 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" (where one animals feeds from a food source that was caught by another) (Labourgade et al, 2020).

So why do they whitetips keep going along with this even though it seems bad for them?

The scientists concluded that the grey reefers could get away with this because they were both bigger and higher in numbers, so from the whitetip reefers' perspective it was actually more beneficial to them to allow them to steal some of their food occasioanlly, rather than risk getting into a fight that they are not likely to win. The whitetips still caught sufficient food faraging in this way, so it seems they have just decided to pick their battles (Labourgade et al, 2020).

The most beautiful footage I have ever seen that shows social foraging in reef sharks is from the 'Our Planet' episode called 'Coastal Seas''. The whole episode is stunning and it is well worth your time to check it out on Netflix. If you would like to skip straight to the action, jump to 10:50min in the video below.

References

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. Image Credits: Laurent Ballesta. Access online.

Robbins WD & Renaud (2016). Foraging mode of the grey reef shark, Carcharhinus amblyrhynchos, under two different scenarios. Coral Reefs, 35, 253-260. Access online.

By Sophie A. Maycock for SharkSpeak.