Big livers help young sharks survive
21 October 2009, by Tamera Jones
Sharks abandon their young pups as soon as they're born. But far from being cold-hearted mothers, scientists have shown for the first time that they provide their pups with super-sized livers to live off while they learn to hunt.
Dusky shark (Carcharhinus obscurus)
Researchers from Bangor University made the finding after trawling through shark data records from KwaZulu-Natal in South Africa spanning over 30 years.
'We found this really just by chance. We plotted liver mass against body mass and there was a massive peak in the really young sharks. There was an obvious pattern of the liver being used up,' says Nigel Hussey, who led the research.
The huge livers, which make up around 20 per cent of the pups' body mass, are packed with fats and probably keep them going in the first months of their lives, say the researchers. Once the sharks are fully grown and get better at hunting, the livers make up just six per cent of their body mass.
Until now, researchers hadn't looked in detail at how shark mothers invest in their young.
South Africa is a magnet for sharks. The authorities use beach protection nets, which the sharks inevitably get trapped in. But this means the dataset that Nigel Hussey and his team analysed is one of the largest of its kind in the world.
The team concentrated on dusky and spinner sharks (Carcharhinus obscurus and Carcharhinus brevipinna respectively).
'It looks like this is a normal, natural process and they're not losing weight because they're not getting enough food or any other reason.'
Nigel Hussey, Bangor University
Most shark studies have focused on newborn or juvenile sharks, because they stick to the same nursery grounds, so are easier to study than adult sharks, which hunt over large areas. But scientists began to worry after noticing the young sharks losing weight.
'It looks like this is a normal, natural process and they're not losing weight because they're not getting enough food or any other reason,' says Hussey. 'The liver reserves allow young sharks to move from the protection of the mother environment to independence.'
The study, by researchers from Britain, South Africa and Australia and published in the Journal of Animal Ecology also found evidence to suggest that female sharks choose when to give birth so that conditions are best for the pups. 'If resources aren't good, it's possible that females hold on to pups for longer so that they're born later,' adds Hussey.
The sardine run - a massive gathering of spawning fish off the coast of South Africa - can be a good opportunity for young sharks to cut their teeth, but the huge abundance of fish naturally attracts hundreds of predators. Unfortunately young sharks also make a nice meal for larger predators, so it's also a risky time to be learning how to hunt.
'Instead, if pups are born after the sardine run, they're often bigger and are more able to fend for themselves, because they can swim faster and further,' explains Hussey.
The researchers also discovered that as female sharks get older, they tend to produce larger pups. But as they age, the size of their pups starts getting smaller. 'This is important for shark conservation,' says Hussey.
'If we concentrate our efforts on protecting the sharks that produce the largest pups, maybe we'll have a better chance of increasing stock numbers,' he adds.
Before mating, females feed themselves up to increase the size of their livers. Sharks carry an average of eight to 12 pups during pregnancy, but when they're close to giving birth they stop feeding. 'This may be because there simply isn't room,' explains Hussey. During this period, they use the liver to feed the pups.
Fish - other than sharks - provide their young with yolk reserves within the egg sac. Very young fish can't use jaws, so once young fish have used their reserves, they reach a critical point where they have to search for food, otherwise they'll die. On the other hand sharks can use their jaws, but they have to learn how to chase and capture prey before they get a chance to do so.
The liver acts like a buffer system, because they can fall back on the fat reserves in their livers while they learn how to find food and get used to their new environment.
'The stomach contents of young sharks are fairly limited, meaning they must be using up their liver reserves. But as they get older the prey they feed on are generally small versions of the adults' prey,' says Hussey.
Maternal investment and size-specific reproductive output in carcharhinid sharks
Nigel E Hussey et al
Journal of Animal Ecology, 2009
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