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.
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.
During a 1997 encounter off the Farrallon Islands off the coast of San Francisco, a group of whale watchers witnessed an orca ramming into the side of a great white shark, momentarily stunning it and allowing the orca to flip it over and holding it in place (ventral/belly up) for around 15 minutes, after which the orca began consuming its prey, much to the surprise of the whale watchers on board. A similar incident was captured on film off Costa Rica in 2014 – this time the orca’s prey was a tiger shark. And it’s not just sharks; orcas have been observed doing the same to stingrays too.
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.
by Lauren Smith, The Guardian | Read more:
Image: Composite: Rex Features and Getty Images
