Visual and behavioral evidence indicates active hunting by sperm whales

Abstract

It is hypothesized that sperm whales employ active pursuit strategies for hunting prey, mainly deep-sea squid at great depths, but no visual evidence has been obtained to confirm this. We recorded the hunting behavior of sperm whales using animal-borne cameras and accelerometers simultaneously deployed on 17 whales, and obtained 42.8 h of diving data, including 17 715 images. A statistical comparison indicated no clear effect of light (with or without flashing white lights from cameras) on diving behavior of tagged whales. Although 98.5% of the still images were of empty water and uninformative, 5 classes of images with visible material were identified: (1) suspended material, possibly squid ink (n = 17), (2) unidentified particles (n = 4), (3) possible animal body parts (n = 2), (4) other sperm whales (n = 221), and (5) the seafloor (n = 8). All image classes were recorded at deeper depths (mean ± SD = 785 ± 140 m), except Class 4 images, which were recorded only at depths <339 m, suggesting that tagged whales swam alone while foraging at great depths. Simultaneous use of speed and image sensors revealed that Class 1 images were associated with bursts of speed up to approximately twice (3.3 ± 1.0 m s-1, max. 6 m s-1) the mean swim speed (1.8 ± 0.4 m s-1). These images, likely derived during chasing prey, support the hypothesis that sperm whales actively hunt to capture prey.