New Research Reveals Whale Shark Carried Different Aged Babies All From One Dad
Female whale sharks store sperm from one male and fertilise a series of eggs over time, a study of one of the only sets of whale shark embryos available to science reveals.
In 1995 a female whale shark caught near Taiwan was found to be carrying 304 embryos in her uterus ranging in size from ones of 42cm still in egg cases to others up to 64cm swimming freely in the womb.
Scientists at the time froze 29 of those embryos and have recently genetically tested them and much to their amazement discovered they all had the same father.
The study leader Jennifer Schmidt of the University of Illinois in Chicago said the most likely explanation is that the female must have stored one male's sperm and used it over time.
She said: 'If mating occurs randomly when a male and female happen to meet, this would provide the female with a sort of reproductive "insurance policy". She has sperm available to continue to fertilize her own eggs even if she doesn't encounter another male.'
Little is known about whale sharks. Solitary individuals are encountered roaming tropical waters around the globe and aggregations are known to occur at various sites around the world - the largest known to be at Isla Mujeres on the Caribbean coast of Mexico where hundreds of young males gather to gorge on plankton and fish eggs.
Most other aggregations involve immature males except the pregnant females known to gather around the Galápagos Islands and a recent discovery of the only known site where mature males and females regularly get together off the isolated island of St Helena in the South Atlantic. It is thought that this is the only known mating zone for these vast pelagic wanderers.
Schmidt is reluctant to generalize from the study on only one female whale shark's embryos but her explanation that the females store sperm from one a male for later use is the most consistent with what is known about sharks in general and whale sharks in particular.
Most sharks studied give birth to litters of half-siblings after females have mated with lots of male sharks over a relatively short period of time. This is with species which regularly gather in large numbers. But with the exception of St Helena, it seems encounters between mature male and female whale sharks are quite rare.
Whale sharks are known to travel vast distances - but little is known about the purpose and nature of such migrations. Carrying sperm in such a manner would have a number of advantages for these solitary travellers including the' insurance policy' against not bumping into a male and also meaning births can be spread out over time avoiding a feeding feast for predators.