Live-bearing shark mums are not the cold-hearted parents they
have been made out to be, according to a new study showing that
sharks in fact provide substantial post-partum investment in their
young.
"Shark pups are born with enlarged ‘super-livers’ that they
feed off during their first few months of life," say the Bangor
University-led international team of researchers, who analysed
sharks captured incidentally by beach protection nets around
KwaZulu-Natal, South Africa.
Their report is the first to demonstrate that live-bearing
carcharhinid sharks provision their young with a maternal
head-start in the form of energy reserves stored in the pup’s
liver. These reserves help the pups through the dangerous first
weeks of life, when prey are difficult to catch and predators most
threatening.
The team, led by Bangor University, and comprised of
researchers from the UK, South Afric and Australia, published its
findings in the Journal of Animal Ecology.
While the use of the liver as an energy store and for buoyancy
is well-documented in adult sharks, this study provided the first
evidence of a decline in liver mass of newborn sharks, from 20 per
cent of body weight at birth to six per cent when they start to
feed themselves. The research shows that during the critical
period after birth, shark pups lose weight by consuming their
liver reserves and that this weight loss is not necessarily an
indication the sharks are in a poor nutritional state, as has been
previously thought.
"It is likely that the liver reserves enable the newborn sharks
to acclimatize themselves to their environment and to develop
their foraging skills," said lead researchers, Nigel Hussey. "We
know that the large sharks use their livers as an energy store,
but we had no idea that the mother provisions her young with
additional liver reserves to enhance their survival."
While sharks have swum the world’s oceans for nearly 400
million years, their reproductive habits appear to be far from
primitive. The study found a dramatic increase in the size of pups
born later in the year, when the risk of predation is lowest. This
suggests mothers have some flexibility in when they give birth,
thereby helping to maximize each pup’s chance of survival.
"Sharks have evolved under continual pressure from their
environment," says Hussey, "and they appear to have developed a
reproductive strategy that is highly attuned to local conditions.
These abilities may be one reason why sharks have had such
evolutionary success."
The study further revealed that the reproductive output of
mother sharks increases with size, but with evidence for a
decline. The largest mothers therefore give birth to smaller pups
than their younger counterparts. Given the widely-reported global
decline of many shark species, the identification of a peak in
reproductive output has substantial conservation implications.
"If we can identify which females are putting out the highest
quality pups, then we can target conservation efforts to those
sizes, directing fishing effort towards capturing smaller or
larger fish, while protecting the pups most likely to survive,"
said co-researcher Aaron MacNeil, of the Australian Institute of
Marine Science.
