Evidence for a progressively earlier pupping season of the common seal (Phoca vitulina) in the Wadden Sea

Common seals Phoca vitulina give birth in the Wadden Sea area during the summer months. We provide evidence that the pupping season has advanced in date in this region. Analysis of stranding dates of recently-born, orphaned pups admitted for rehabilitation, revealed a shift of, on average, 0.88 days per year over the period 1974-2008, yielding a total advance of 26 days. Although the pupping season has become progressively earlier, there were no indications of any negative impact on the weight of the pups, nor was there an increase in the proportion of seals with a lanugo coat. These observations suggest that the most likely explanation for the change in phenology of the pupping season is a corresponding change in the timing of cessation of the period of delayed implantation. It is suggested that shifts in phenology could reflect an adaptive response of the animals to altered local circumstances. The latter may in turn be induced by larger scale phenomena such as food availability or climate change.