Social behavior of the Cuvier’s beaked whale (Ziphius cavirostris) investigated by telemetry and photo-identification

Studies of the social behavior of Cuvier’s beaked whales (Ziphius cavirostris) are challenging because of their pelagic habitat and the limited time they spend at the surface. The sociality of these deepest diving mammals is of great interest, however, including how social behavior might influence responses to anthropogenic disturbance as this species has a history of stranding in association with certain types of human-created noise. Beyond conservation concerns, the beaked whales (Family Ziphiidae) are a valuable group for the comparative study of the ecological influences on mammalian social evolution since they are an “out group within an out group”, including some of the most extreme diving mammals within the Cetacea which are themselves one of the few mammalian groups to become secondarily fully aquatic. The beaked whales are also the second most speciose family in the cetaceans and so can lend statistical power to formal comparative analyses. One unifying characteristic of most beaked whale species is sexually dimorphic dentition, where only adult males have erupted tusks which they appear to use as weapons in contests with each other as evidenced by the high level of scarring in adult males. Despite this, previous studies suggest a substantial amount of variation in beaked whale social structure and mating systems.

In this work, I investigated the social behavior of a relatively accessible, high density population of Cuvier’s beaked whales off the coast of Cape Hatteras, North Carolina, USA. I used data from photo identification surveys from a longitudinal study, as well as satellite-linked depth-recording telemetry tags to investigate within and between group behavior. In addition, I present a case study and assessment of how I optimized the programming of telemetry instruments to increase their value in behavioral studies.

I tested the stability of adult male-male interactions over the medium term using diving synchrony as a proxy for group membership and found that adult male-male pairs stayed remarkably synchronous for days to weeks, contrary to expectations that individuals would quickly competitively exclude each other. Using photo-identification records of whole groups, I did not find strong evidence for sexual segregation and instead found a high level of fission-fusion behavior between groups. In addition, I did not find strong evidence for long term stable group formations, although given the large population size and a relatively small sample size infrequent but important bonds among individuals may have been missed. Using a new programming regime to collect up to 14 days of time-series depth data on an individual, I was able to confirm that diel differences in dive depth were almost entirely explained by bouts of shallow diving which occur only a night. These shallow dives appear to be a separate mode of diving behavior not previously well described in the literature, and although this behavior may have multiple functions, it may be a useful behavioral modality to investigate further in relation to social behavior.

Finally, I argue that the application of similar methods used here combining photo-identification and telemetry tagging could lead to rapid discovery of social parameters in other hard to study pelagic Cetaceans including other Ziphiidae with a view toward building a comparative dataset.