Paralysis as a defence response to threatening stimuli in harp seals ( Phoca groenlandica )

• Published in: Canadian Journal of Zoology
• Main Authors: Lydersen, Christian, Kovacs, Kit M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1995
• Subjects: Animal Science and Zoology; Ecology, Evolution, Behavior and Systematics; Phoca groenlandica
• Online Access: http://dx.doi.org/10.1139/z95-055; http://www.nrcresearchpress.com/doi/pdf/10.1139/z95-055

In this study we describe the behavioural and physiologial characteristics of a fear-induced paralysis response in harp seals (Phoca groenlandica). Of 382 pups tested, 328 (86%) displayed this passive defence response, and of 46 adult seals tested, 26 (57%) individuals exhibited paralysis. Gender had no effect, but among pups there was a signifiant increase with age in the proportion of individuals that performed the response (contingency table; G = 21.46, p = 0.002). During paralyses seals became rigid, the foreflipprs were brought into the sides of the body, the hind flippers were pressed closely together, and the neck was retracted so that the forehead was drawn into the fat sheath that encases the rest of the body. The animals would always urinate and often defecate. Physiologically speaking, paralyses had two phases. In pups, an apnoea phase preceded a period of hyperventilation. Adults alternated between these two phases more sporadically during a paralysis bout. Mean heart rates of pups during paralysis apnoeas were approximately 30 beats/min. During the hyperventilatory phase their heart rates escalated to values in excess of 150 beats/min. Respiration rates for the pups during hyperventilation ranged from 26 to 66 breaths/min. Heart rates of adults during apnoeas were more viable than those of pups, but the average value was similar (32 beats/min). The lowest heart rate recorded was 10 beats/min. Average heart rates of adults during breathing phases were lower than those of pups, ranging from 118 to 130 beats/min. A suite of lactation-related adaptations and ecological circumstances may explain the paralysis response in harp seals. Harp seals have evolved in the presence of polar bears. They are afforded some protection by giving birth in dense aggregations on unstable ice that is variable in geographical location, but pups are extremely vulnerable when a surface predator does enter the whelping patch, and available defence options are limited. The paralysis response may serve to disinterest the ...