It’s difficult to imagine the voracious and predatory great white shark as prey. Could orcas really be overpowering them and removing their livers?
Comparing the statistics of the two predators reads like a game of Top Trumps
The great white shark, Carcharodon carcharias, is considered the most voracious apex predator in temperate marine ecosystems worldwide, playing a key role in controlling ecosystem dynamics.
As a result, it is difficult to imagine a great white as prey. And yet, earlier this year the carcasses of five great whites washed ashore along South Africa’s Western Cape province. Ranging in size from 2.7 metres (9ft) to 4.9 metres (16ft), the two females and three males all had one thing in common: holes puncturing the muscle wall between the pectoral fins. Strangest of all, their livers were missing.
The bite marks inflicted, together with confirmed sightings indicate that orcas, Orcinus orca, were responsible for this precisely-targeted predation. Although the opening scene from Jaws II immediately springs to mind, in which an orca washes up with huge bite marks on it, the reality has turned out to be the exact opposite.
Orcas are apex predators whose diet is often geographic or population specific
When comparing these two apex predators alongside each other, the stats read like a game of Top Trumps. Max length: great white 6.4 metres, orca 9.6 metres; max weight: great white 2,268kg, orca 9,000kg; burst swim speed: great white 45km/h, orca 48km/h. On paper, at least, it does seem that orcas have the edge.
The diet of orcas is often geographic or population specific. Those populations predating in South African waters have been documented targeting smaller shark species for their livers. Cow sharks, blues and makos caught on longlines have had their livers removed by orcas, alongside the brains of the billfish also caught. Cow shark carcasses without livers have also washed ashore near Cape Town, and again, this followed nearby orca sightings.
With no doubt that orcas are using highly specialised hunting strategies to target the liver; the real question is: why?
Shark livers are large, typically accounting for 5% or more of a shark’s total body weight. They are oil rich, with a principal component, squalene, serving as an energy store and providing buoyancy in the absence of the swim-bladder found in teleosts (bony fish).
Analysis of white shark livers in particular shows an extremely high total lipid content, dominated by triacylglycerols (>93%). This results in an energy density that is higher than whale blubber. For the sharks this serves as an energy storage unit to fuel migrations, growth and reproduction (Pethybridge et al 2014). For the orcas this is like eating a deep fried Mars Bar with added vitamins. Generally speaking, livers contain vitamin C, vitamin B12, folate, vitamin B6, niacin, riboflavin, vitamin A, iron, sodium and of course fat, carbohydrate and protein energy sources.
Since the attraction of this delicacy to the orca is clear, how exactly does an orca go about removing a great white shark’s liver? The evidence we have shows that it is done with some precision – the shark carcasses were not obliterated.
The examination of one of the great whites begins. The incision under the pectoral fin was made by the orca
What the orcas were exploiting to their own advantage is a curious phenomenon known as “tonic immobility” (TI). This is a natural state of paralysis, which occurs when elasmobranchs are positioned ventral side up in the water column. For certain species of shark like the great white, which is unable to pump water across its gills unless it keeps swimming, the consequence of being maintained within this ‘tonic’ state for too long is final. Effectively, the orcas have learned how to drown their prey whilst minimising their own predatory exertion.
Sub-adult lemon shark held in tonic immobility for hook removal
Researchers often use this reflex to help with the surgical implantation of acoustic tags. The rapid induction and recovery of the animals optimises the surgical procedure, which is particularly desirable during what are often complex field work conditions (Kessel & Hussey 2015).
Helpful to researchers and predatory orcas it may be, but the evolutionary benefit to those elasmobranchs exhibiting TI is less certain. It may serve as a defence strategy, but the advantage of sharks being able to “play dead” is not clear. An alternative theory suggests that TI may be related to mating, with males using it as a technique to temporarily immobilise the female.
Whatever the primary use of TI is in the wild for elasmobranchs, orcas are known to exploit this to aid their predation. The sharks however, also learn, and sightings of great whites off the South African coastline rapidly declined whilst the orcas were in the area. Once the orcas moved on, the great whites slowly began to return.