21 October 2009
Live-bearing shark mums are not the cold-hearted parents they have been made out to be, concludes 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 few 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 Africa, and Australia, publishes its findings today, 21 October 2009, 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 provides the first evidence of a decline in liver mass of newborn sharks, from 20% of body weight at birth to 6% 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 indicaton that 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" says lead researcher Nigel Hussey, "We know that 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 primative. 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 chances 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" says co-researcher Aaron MacNeil, of the Australian Institute of Marine Science.
The results of this study raise important questions over the reproductive success of sharks and the survival of their newborn pups. "Sharks are under severe pressure by human activity, but our current understanding of their reproductive potential remains limited. We have much work to do to improve our basic understanding of shark biology if we are to implement effective management plans" said Nigel Hussey.
The team comprised researchers from the Bangor University, the KwaZulu-Natal Sharks Board, the University of KwaZulu-Natal, and the Australian Institute of Marine Science.
The work was supported by grants from the Natural Environment Research Council of the UK and operating funds from the Australian Institute of Marine Science.
For further information, please contact:
UK INTERVIEWS: Mr Nigel Hussey , School of Ocean Sciences, University of Bangor.
Tel. +44 (0) 1248 382900 Email:firstname.lastname@example.org
NORTH AMERICA/AUSTRALIA INTERVIEWS: Dr Aaron MacNeil , Australian Institute of Marine Science/Bedford Institute of Oceanography.
Tel.07 5641 4787or +1 902 426 4435 Email:email@example.com.
SOUTH AFRICA INTERVIEWS: Dr Sheldon Dudley , KwaZulu-Natal Sharks Board. Tel. +27 (0) 31 5660400 Email:
Ms Wendy Ellery, AIMS media liaison, 07 4753 4409, 0418 729 265 ,firstname.lastname@example.org
Source information: ‘Maternal investment and size-specific reproductive output in carcharhinid sharks', Hussey,N.E. et al , Journal of Animal Ecology. Copies available from http://www.interscience.wiley.com/journal/122658402/abstract?CRETRY=1‘SRETRY=0