Shark Mothers Provide for Their Pups
Maternal Sharks Ensure New-Borns Have Internal Food Supply
Nov 2, 2009
Female sharks have a reputation for lacking the maternal instinct, as they swim off once their pups have been born. When the pups are first born, they are very poor at swimming and foraging for food, yet the mother doesn’t stay around to provide food.
Now Australian, British and South African marine researchers have shown that while the mother doesn’t stick around, she has already provided her pups with a food source – a greatly enlarged liver.
The team, led by Dr Nigel Hussey from the School of Ocean Sciences, Bangor University, UK, observed dusky sharks (Carcharhinus obscurus) and spinner sharks (Carcharhinus brevipinna) in the waters around South Africa.
The sharks had been caught in beach protection nets around KwaZulu-Natal, and were examined by the researchers before being released.
Their conclusions, that “live-bearing carcharhinid sharks provision their young in the form of energy reserves stored in the pup's liver”, are reported in the Journal of Animal Ecology.
‘Super-Liver’ Food Source as well as Buoyancy Device
Dusky and spinner sharks have swum the world’s oceans for nearly 400 million years. They are found in the warmer waters around South America, Southern Africa, Spain, the Southern Mediterranean, Japan, Indonesia and Australia.
While adult sharks have been known to use an enlarged liver as a buoyancy device and an energy store, Dr Hussey said this was the first time that it was recognised that new-born pups also gained energy from their ‘super-livers’.
“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," he said.
Although the pups are born in safe calm waters, away from adult sharks and other predators, for the first few days they are unable to swim much or find food for themselves.
Dr Hussey said the study provided 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.
“During the critical period after birth, shark pups lose weight by consuming their liver reserves and this weight loss is not necessarily an indication that the sharks are in a poor nutritional state, as has been previously thought,” he commented.
“It is likely that the liver reserves enable the newborn sharks to acclimatize themselves to their environment and to develop their foraging skills.”
Mother Sharks Time Their Pup’s Birth
The researchers also found that female sharks could choose when to give birth to their live young at the safest time for the pups. They 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,” Dr Hussey said.
“Sharks have evolved under continual pressure from their environment, 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.”
At the same time, the team observed that as female sharks got older, they began producing smaller pups, “almost as if they were reaching menopause or senescence,” said the Australian researcher with the team, Aaron MacNeil, of the Australian Institute of Marine Science.
“If we can determine when the sharks are producing their best pups, we might have a window for understanding how to conserve the species,” he said.
Observation Key To Understanding Sharks
Dr MacNeil said the information about the sharks’ reproductive methods had been gained by observing the fish living in the wild, and that thousands of observations were necessary to understand how sharks evolved and thrived.
“We need thousands of observations to get the very large data sets we need to draw our conclusions. And as we know more about the sharks’ life cycle, we’ll become better at protecting them.”
He said he was encouraged about people’s increasing awareness "about what we have done to shark populations in the past" and the importance of protecting sharks.
“I hope we’ll see many more studies into their life cycles, and into the management and protection of threatened species.”
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