External proportions, body and organ masses, and foetal growth of North Sea white-beaked dolphins

Abstract The white-beaked dolphin ( Lagenorhynchus albirostris Gray, 1846), a robust delphinid endemic to shelf areas in the temperate and subarctic North Atlantic, has not been analysed extensively in terms of external proportions while there are no analyses of organ masses or foetal growth. Here, we present analyses of external measurements, data on organ masses and analyses of foetal growth in white-beaked dolphins bycaught or stranded in the southeastern North Sea. White-beaked dolphins grow with similar allometric patterns to other delphinids. There was no significant sexual dimorphism in external proportions, but indications of subtly different growth patterns between the sexes. Mass-length relationships were similar to those of other investigated delphinids, with pronounced positive allometry. However, males showed stronger positive allometry, having relatively lower mass than females as juveniles and greater mass as adults. Distribution of mass among organs was largely similar to other delphinids, except that white-beaked dolphins have proportionally larger hearts. This may be an adaptation to prolonged bouts of swimming. Male white-beaked dolphins had substantially greater proportional muscle mass than females, indicative of potential competition among males for mating opportunities. Females had significantly larger lungs and kidneys than males, smaller lungs being recognised as associated with deep diving in other odontocetes. A model of foetal growth indicated a later birth period, starting in mid-August, compared to the May to August period that has been suggested based on observations of calves.