The tide as zeitgeber for Weddell seals

The haul-out and underwater activity patterns of five Weddell seals were in vestigated in relation to the tide. Electronic recorders measured the seals ' dive depth and internal body temperature simultaneously. Diving and tempe rature data were analysed with the help of time series analyses tools. We i dentified a half-day (semicircadian) rhythm in underwater activity and body temperature. A close correlation was Found between the rhythmicity of the tide and the timing of the seals' underwater activity and oscillations of t heir body temperature. We propose that the tide is the principal zeitgeber for the Weddell seals' ultradian cycle of behaviour during the austral summ er.