Scientists at the Sea Mammal Research Unit, at St Andrews University, are mystified by their latest findings, which seem to show that the weight of a seal after it is weaned from its mother does not affect its chances of surviving its first year.
Evolutionary theory would suggest that, if weight makes no difference, then the mothers should not invest the extra time and energy required to feed pups with their incredibly fatty milk - one of the richest produced by any animal, which in just one month can quadruple a pup's weight.
But Ailsa Hall, of the St Andrews research unit, said yesterday that though the weights of pups after weaning could vary from 25kg to 50kg around an average of 39kg, "size isn't everything".
About 60 per cent of pups survive their first year, during which they must learn to fish, evade predators and cope with disease and injury.
To track the seals, from the Isle of May in the Firth of Forth, the scientists stick bright yellow or green plastic caps, each with two letters, to the fur on their heads.
The caps allow researchers to observe the seals from some distance. Members of the public can also jon in by taking tour boats to the seals' sites and reporting the codes on any caps they see.
The caps do not in themselves seem to affect survival, Dr Hall said: "We have animals from the same birth group with flipper tags, and they have the same survival rate."
Nor do the caps affect their social standing - as far as the researchers can tell. The hat falls off after a year when the pup moults its fur.
So far, the St Andrews scientists have data on about 40 per cent of the 209 pups born last December. Dr Hall said: "We have done autopsies on the few bodies washed up and the cause of death isn't starvation.
"In fact, they're in pretty reasonable condition, except that they're dead."
Other factors may be at work, and Dr Hall is now investigating the possibility that pollutants or other environmental factors could affect the survival of pups.