Long-term observations reveal short-term mother–calf affiliation in wild harbour porpoises

Abstract

In mammals, the maternal bond is essential for the development of newborns. Compared with most terrestrial mammals, cetacean newborns are extremely precocious in motor and sensory development, but remain dependent on their mothers for nutrition. Little is known about the mother–calf relationship in harbour porpoises, Phocoena phocoena, where the high fecundity of females, short duration of lactation and precocious development of calves may challenge the amount of maternal care and the nature and intensity of interactions. We used photo-identification and drones to monitor a wild female harbour porpoise that had nine calves over 9 years and quantified her interactions with her calves aged between 1 and 5 months as a cross-sectional study. We compared their behaviour with other mother–calf pairs at 9 months postpartum. Between 1 and 9 months postpartum, the calves showed the most effort in maintaining proximity, in that the mothers were frequently observed to leave and the calves to return after separation. Across all months, the calves spent about half their time within a 2 m range from their mothers, synchronizing their respirations. While in close proximity, the calves mostly stayed in echelon position in their first month and in infant position at 2–9 months postpartum. The mothers increased distances from their calves when travelling or foraging, and the calves increasingly departed from their mothers with age for foraging, resting or exploring. The mother–calf relationship in harbour porpoises appears to be short-term and primarily maintained by calves from the first month of life. We suggest that maternal care in harbour porpoises is less pronounced than in many odontocetes because of the high fecundity of females, short interbirth interval and high energetic costs associated with gestation and lactation.