Are porpoises sexist? Effects of dolphin sex on porpoise avoidance behavior

Top predators affect ecosystems by controlling prey populations both directly through consumption and indirectly through fear. The threat of predation affects the spatial behaviour of prey species and can even drive emigration from areas with higher predation risk.
The most common cetacean species in the Danish waters is the harbour porpoise. A few years ago, several bottlenose dolphins arrived from Scotland and settled in different areas around Denmark. Bottlenose dolphins are known to harass and kill harbour porpoises. This behaviour is usually observed in males during breeding season and can be a form of aggressive play in frustrated males with high testosterone levels and limited access to receptive females. Indeed, among five Danish dolphins observed in 2022-2025, two males were reported to kill porpoises, while there were no reliable observations of the three female dolphins performing this behaviour. It is however unknown whether porpoises are aware of the lower risk coming from female dolphins compared to males, and whether this affects their avoidance behavior. Here we report the results of passive acoustic monitoring of the dolphin and porpoise presence in areas where the dolphins occurred: Svendborg, Thyborøn and Lillebælt. We used underwater dataloggers (F-PODs) to detect click trains of porpoises and the dolphins. We calculated the detection-positive minutes (DPMs) of porpoise clicks during three periods: half an hour before the appearance of dolphin clicks, throughout the period of dolphin acoustic presence, and half an hour after.
In our presentation we will compare the differences in porpoise avoidance behavior in response to the presence of male and female bottlenose dolphins and discuss the implications for porpoise habitat exclusion in case if more dolphins colonize Danish waters.
This study was supported by Aage V. Jensen Naturfond.