Diurnal and seasonal variations in the duration and depth of the longest dives in southern elephant seals (Mirounga leonina): Possible physiological and behavioural constraints

This study seeks to understand how the physiological constraints of diving may change on a daily and seasonal basis. Dive data were obtained from sout hern elephant seals (Mirounga leonina) from South Georgia using satellite r elay data loggers. We analysed the longest (95th percentile) dive durations as proxies for physiological dive limits. A strong, significant relationsh ip existed between the duration of these dives and the time of day and week of year in which they were performed, The depth of the deepest dives also showed a significant, but far less consistent, relationship with local time of day and season. Changes in the duration of the longest dives occurred i rrespective of their depth. Dives were longest in the morning (04:00-12:00h ) and shortest in the evening (16:00-00:00h). The size of the fluctuation v aried among animals from 4.0 to 20.0min. The daily pattern in dive depth wa s phase-shifted in relation to the diurnal rhythm in dive duration. Dives w ere deeper at midday and shallower around midnight. Greater daily changes i n duration occurred in seals feeding in the open ocean than in those foragi ng on the continental shelf. The seasonal peak in the duration of the longe st dives coincided with austral midwinter. The size of the increase in dive duration from autumn/spring to winter ranged from 11.5 to 30.0 min. Change s in depth of the longest dives were not consistently associated with parti cular times of year. The substantial diurnal and seasonal fluctuations in m aximum dive duration may be a result of changes in the physiological capaci ty to remain submerged, in addition to temporal changes in the ecological c onstraints on dive behaviour. We speculate about the role of melatonin as a hormonal mediator of diving capability.