Harbor porpoises (Phocoena phocoena) and bottlenose dolphins (Tursiops spp.) are marine predators that differ significantly in their social system, reproductive strategies and foraging ecology. Like other odontocetes, they use echolocation to navigate, detect and chase prey in environments of various complexity; but they communicate using distinct acoustic signals. These differences make them interesting subjects for exploring how small odontocetes overcome social and foraging challenges in their respective ecological niches. Using droneobservations, underwater acoustic recordings and photo-ID techniques, this thesis investigates how porpoises and dolphins learn to navigate dynamic soundscapes, between newborn period and adulthood, through analyses of mother-calf behaviors and development of foraging skills.

Harbor porpoise females live in the fast lane, reaching sexual maturity at an early age, having high fecundity rates and short longevity. Porpoises live in small, fluid groups and mothers rear their offspring alone for 9 to 11 months before giving birth to a new calf. This brings several challenges for both mothers and calves: Our field studies showed that mothers need to maintain high foraging rates while rearing their calves, urging them to hunt larger prey presumably to compensate for energetic costs of gestation and lactation (Chapters I & II). Calves, on the other hand, developed early autonomy from birth to follow their mothers, showed most effort to maintain proximity within the dyad, and separated more frequently from their mother with age (Chapter I). From the second month of age, calves were able to follow their mothers during foraging and started practicing hunting behaviors, observing their mothers and interacting with prey (Chapters I & II); appearing as preliminary evidence for social learning. As adults, porpoises were foraging in almost 60% of observations, likely compensating for their high metabolic demands (Chapter II). Most tactics were individual,except for one example of group hunting on schooling fish, and targeted small prey. They exhibited foraging flexibility, adapting hunting techniques to prey and habitats (Chapter II).

In contrast to porpoises, bottlenose dolphins exhibit slower life-history processes. Living in dynamic fission-fusion societies, calves benefit early from the group social network, and with this increased transfer of knowledge. A nursing period of 3-4 years allows for gradual learning of social and foraging skills, including the development of specialized foraging tactics. In Shark Bay, Western Australia, Indo-Pacific bottlenose dolphins (T. aduncus) modify or innovate in their behavior to exploit unused ecological niches, sometimes in challenging environments. In this population, daughters stay close to their mother to learn the rare foraging tactics. We describe an innovative foraging strategy in this area where a mother and daughter dolphins modified their behavior to chase fish at high speeds in shallow coastal waters, using the shoreline as a barrier to herd fish (Chapter III). Dolphins echolocated with reduced output level to limit interfering echoes returning from nearby surfaces. They changed their body orientation presumably to improve maneuverability and acoustic detection, some of these behaviors we also observed with harbor porpoises during hunting (Chapter II). Focal dolphins’ calves of 1-2 years old did not stay with their mothers during most chases, except the 2-year-old calf practicing chasing a fish in one case (ChapterIII).

With this thesis, I explore how different life strategies can influence maternal care, calf development, foraging strategies and transfer of knowledge in odontocetes. Our field studies show that porpoises and dolphins can adjust their behavior to different contexts to optimize survival and feeding success, which is likely to depict cognitive flexibility. Ultimately, behavioral flexibility favors resilience, which are both often necessary for adapting to unforeseen environmental changes, such as fluctuations in prey availability, noise disturbance or habitat degradation. 

URL: https://portal.findresearcher.sdu.dk/en/publications/learning-to-mine-the-underwater-soundscape-behavior-and-acoustics/