Diving ontogeny and lunar responses in a highly migratory mammal, the northern fur seal Callorhinus ursinus

Diving ontogeny studies enable the examination of both the evolution of diving strategiesand the physiological constraints and environmental factors determining foraging behaviour.Northern fur seal (NFS) Callorhinus ursinus pups that undertake far-ranging migrations in their firstyear are an ideal species for examining such factors. The diving behaviour of 64 NFS pups from 4North American breeding sites was studied using satellite-dive recorders deployed on pups prior toweaning. Summarised diving activity (6 h histograms of dive depth and duration) was recorded duringthe pups first 8 mo at sea and transmitted via satellite. During the first month at sea, pupsadopted the nocturnal diving patterns characteristic of adults, with average maximum nightly andcrepuscular dive depths and durations exceeding daytime values by a factor of from 4 to 4.5. Divingcapacity in terms of maximum depths (112 m) and durations (285 s) attained also increased linearlywith age until ~8 to 10 mo of age. Overlaid on diving capability development was the significantinfluence of environmental cues, such as lunar phase, on migratory diving behaviour. During fullmoons, pups dived deeper and for longer periods than during other lunar phases, as pups likely mimickedthe behaviour of their vertically migrating prey. These findings indicate that prey accessibility,particularly for younger pups with reduced diving capacity, may prove more challenging duringhigher lunar illumination periods.