Shark pups are born with super-sized livers, providing them with nutrients to help them survive early life.
The researchers who made this discovery say it shows how the sleek fish have evolved to “invest” in their newborns.
Previously, shark mothers were believed by some to to be selfish or apathetic, leaving pups to fend for themselves from the moment they are born.
But the research team found that pregnant dusky sharks expend energy to provide this early food supply.
Nigel Hussey, the biologist from the Bangor University, UK, who led the study, described the enlarged liver as a “buffer” for the newborns.
“All live-bearing sharks birth their young, turn round and disappear, or in some cases eat them,” he told BBC News.
“So they have a reputation as very cold-hearted mothers.”
He and his colleagues described in the Journal of Animal Ecology how it took 30 years of data collection to finally reveal their maternal secret.
This data consisted of measurements gathered since 1978 from sharks accidentally caught in beach protection nets in KwaZulu-Natal, South Africa.
The size of shark livers was already of great interest to researchers – the organ is understood to be an energy store and to play a role in the animals’ buoyancy.
“But when we looked at the data, a very strange pattern emerged,” said Mr Hussey.
“There was a massive peak in liver size in the smallest sharks.”
His team found that, just after birth, the shark pup’s liver made up about 20% of its body mass. At a few weeks or months of age, this reduced to just 6%.
Dusky sharks belong to the carcharhinid family, all of which produce live young.
As soon as the pups are born, they are able to feed themselves. But the nutrient supply from such a large liver could help buy them time to find the best feeding grounds.
This vast set of data held other intriguing clues about the animals.
From studies of pregnant sharks, it emerged that there may be an “optimum size range” for the mothers.
“Generally, the larger the mother, the larger the pup,” explained Mr Hussey.
“But in the largest mothers [of the two species we studied] there was a slight decline in the size of the pups.”
Larger young have a better chance of survival, so knowing which sharks produce the most viable young “could have important consequences for conservation,” he explained.
Matt Rand, director of Global Shark Conservation for the Pew Environment Group said that this was the latest of a number of studies “confirming the importance of preserving the parts of the ocean where sharks breed and raise their pups”.
He continued: “If we can establish more marine reserves and fishing policy based on this science, we can let the sharks recover from decades of overfishing and hopefully save some of the species that are threatened with extinction.”
Dr Hussey led an international team of researchers from the University of KwaZulu-Natal and the Australian Institute of Marine Science in Townsville.