Causes of individual variation in fasting fuel allocation and consequences for diving behaviour in grey seal pups (Halichoerus grypus)

Abstract

Grey seal pups are weaned abruptly after a short, intensive nursing period, during which they lay down fat and protein. Fat stores are crucial for insulation when they first go to sea and protein is essential for muscle and oxygen store development. Both components also provide them with energy to sustain them during an extended postweaning fast and their first days and weeks at sea until they can forage effectively. There is thus a trade off between the requirements for fat and protein in metabolism and for other functions. It is crucial for the survival of grey seal pups that they begin to feed before these reserves become critically depleted. The ability to do so depends on the size of their fat and protein stores at weaning, energy partitioning during the postweaning fast and the development of adequate diving and foraging capabilities. The management of fat and protein stores through energy partitioning and appropriate timing of departure are key to survival but the mechanism by which this management is achieved is unknown.

To investigate the mechanism that controls energy partitioning and timing of departure from the colony, changes in body mass and composition and levels of hormones that are involved in energy balance in other mammals were measured during the postweaning fast of wild grey seal pups. The impact of initial energy reserves and hormone levels on energy use and departure was investigated directly, by manipulation of energy reserves and hormone levels, using supplementary feeding and hormone treatment. This is the first time such intervention studies have been performed in wild fasting pups. The impact of the stress of repeated handling on hormone levels and energy utilisation was also examined. The movement patterns and development of maximum diving capabilities of pups after departure were investigated using satellite telemetry.

Leptin was present in grey seal serum but could not be measured accurately using two currently available immunoassays. Prolactin was not detected in postweaned pup serum and was eliminated as a potential candidate for control of fuel use and timing of departure from the colony. Cortisol and thyroid hormones (TH) were measured in serum but Cortisol could not be measured in grey seal saliva.

Cortisol and TH are potentially involved in energy expenditure and fuel partitioning during fasting. Cortisol and total thyroxine levels decreased from weaning to midway through the fast. Both hormones increased in response to both natural and artificially induced changes in fuel availability. Dexamethasone, a potent Cortisol analogue, caused a short-term increase in mass loss, and induced reversible and short-lived changes in Cortisol levels and immune function, but its impact on energy partitioning was unclear. The effects of the drug were not mediated through TH. There were no changes in TH or Cortisol consistently associated with the timing of departure and dexamethasone did not induce departure. Neither hormone therefore seemed likely to be involved directly in the cue to leave the breeding colony. Handling regime did not impact significantly on Cortisol secretion, TH levels or long-term fuel use in grey seal pups.

Fatter animals, or those provided with additional food, relied more heavily on fat to meet energetic requirements than leaner pups. There was no difference in energy partitioning between the sexes. Grey seal pups had a greater tolerance to protein depletion than terrestrial mammals but may have reached critical protein levels soon after departure from the colony. They were therefore under considerable pressure to begin to feed very soon after they have gone to sea, despite their extensive fat reserves.

Pups showed marked inter-individual differences in movement patterns, which were very different from those of adults. Initially they undertook coastal movement, but showed wide dispersal from the colony within their first few months at sea. They began to exhibit adult-like repeated trips from known haul-outs to discrete offshore areas 4-5 months after departure. They were able to reach almost all areas and depths available in the North Sea, but were constrained in their ability to remain submerged, in terms of dive duration, post-dive surface interval and percentage of time spent diving over extended periods, compared with adults. Their maximum ability to remain submerged when they first went to sea was related to the duration of the postweaning fast and thus the degree of development on land. In contrast, maximum diving capabilities achieved during the first three months at sea did not vary substantially between animals and increased with time since departure, and were thus likely to be a product of diving-induced development of oxygen stores and cardiovascular control. Overall, larger body size and longer fast duration conferred increased diving capabilities, which may present one mechanism for increased survivorship in bigger animals.