OFFSPRING SEX-RATIO IN RELATION TO FEMALE SIZE IN SOUTHERN ELEPHANT SEALS, MIROUNGA-LEONINA

Southern elephant seals Mirounga leonina display extreme sexual dimorp hism. In addition females show great variation in size and stored reso urces at parturition. Therefore they present an excellent opportunity for examination of responses of sex ratio to resource availability. We studied the relationships between the size of southern elephant seal females at parturition and the size and sex of their pups at South Geo rgia over four breeding seasons. We found a large individual variation in maternal post-partum mass (range 296-977 kg, n=151). Larger mother s gave birth to larger pups, irrespective of the sex of their pup. Mal e pups were on average 14% larger than females at birth and consequent ly more costly to bring to parturition. Our results suggest that femal e southern elephant seals must weigh more than 300 kg if they are to b reed at all, and more than 380 kg if they are to give birth to a male pup. Above this threshold the proportion of males among offspring rapi dly increases with maternal mass, and stabilizes at a level not signif icantly different from parity. These results show that smaller females of southern elephant seals vary offspring sex ratio in a way that is consistent with theories on adaptive offspring sex ratio. A smaller mo ther with a male foetus may benefit from terminating her pregnancy and allocating the resources she saves to her own growth. She could then give birth to and raise a larger pup in the subsequent season.