Abstract
Few studies have been conducted on the visual capabilities of large cetaceans, such as the humpback whale (Megaptera novaeangliae), and understanding these capabilities provides insights into the natural history and anthropogenic vulnerabilities of these animals, which are otherwise difficult to study in situ. Here, we performed an anatomical and histological study of a subadult humpback whale eye to estimate visual acuity (i.e. spatial resolution of vision) for modelling the perception of relevant visual targets in an open ocean environment. Visual acuity was estimated at 3.95 cycles per degree (CPD)-a value that is an order of magnitude lower than what is predicted by absolute eye size. Perceptual models based on this acuity indicated that low frequency spatial information (e.g. large silhouettes) remains discriminable over ecologically relevant distances, while high frequency spatial information, perhaps critical for target identification, appears lost at 3-4 average humpback whale body lengths away. Models of horizontal sighting distance provided detection-distance thresholds that encompassed the effective range predicted for visual acuity. This study provides new insight into the visual capabilities of humpback whales, suggesting spatial vision that is suited to their open ocean ecology, but challenged by visual targets with more nuanced characteristics unless viewed at close range.