Inferring diet, feeding behaviour and causes of mortality from prey-induced injuries in a New Zealand fur seal

New Zealand fur seals Arctocephalus forsteri are the most abundant of the 4 otariid (eared seal) species distributed across Australasia. Analyses of stomach contents, scats and regurgitates suggest a diet dominated by bony fish and squid, with cartilaginous species (e.g. sharks and rays) either absent or underrepresented because of a lack of preservable hard parts. Here we report on a subadult specimen from south-eastern Australia, which was found ashore emaciated and with numerous puncture wounds across its lips, cheeks, throat and the inside of its oral cavity. Fish spines embedded in the carcass revealed that these injuries were inflicted by chimaeras and myliobatiform rays (stingrays and relatives), which matches reports on the diet of A. forsteri from New Zealand, but not South Australia. Shaking and tearing of prey at the surface may help to avoid ingestion of the venomous spines, perhaps contributing to their absence from scats and regurgitates. Nevertheless, the number and severity of the facial stab wounds, some of which led to local necrosis, likely affected the animal's ability to feed, and may account for its death. Despite their detrimental effects, fish spine-related injuries are difficult to spot, and may be a common, albeit cryptic, type of trauma. We therefore recommend that stranded seals be systematically examined for this potentially life-threatening pathology.

Shearing overbite and asymmetrical jaw motions facilitate food breakdown in a freshwater stingray, Potamotrygon motoro

Many species of fish process their prey with cyclic jaw motions that grossly resemble those seen in mammalian mastication, despite starkly different tooth and jaw morphologies. The degree of similarity between the processing behaviors of these disparate taxa has implications for our understanding of convergence in vertebrate feeding systems. Here, we used XROMM (X-ray reconstruction of moving morphology) to investigate prey processing behavior of Potamotrygon motoro, the ocellate river stingray, which has recently been found to employ asymmetrical, shearing jaw motions to break down its prey. We found that P. motoro modulates its feeding kinematics to produce two distinct types of chew cycles: compressive cycles and overbite cycles. The latter are characterized by over-rotation of the upper jaw relative to the lower jaw, past the expected occlusal limit, and higher levels of bilateral asymmetry as compared with compressive chews. We did not find evidence of the mediolateral shearing motions typical of mammalian mastication, but overbite cycles appear to shear the prey item between the upper and lower toothplates in a propalinal fashion. Additionally, comparison of hyomandibular and jaw motions demonstrates that the angular cartilages decouple jaw displacement from hyomandibular displacement in rostrocaudal and mediolateral directions. The multiple similarities between mammalian mastication and the dynamic processing behavior of P. motoro support the use of sub-family Potamotrygoninae as a model for studying evolutionary convergence of mastication-like processing.