|MadSci Network: Zoology|
The short answer to your question would be, it's more likely a sensory confirmation of injury to a conspecific rather than cannibalism, given the scenario you describe. The more detailed answer: Remarkably little is known about hippopotamus behavior and biology. Walker's Mammals of the World Online probably provides the most comprehensive listing of hippo behavioral citations: http://www.press.jhu.edu/books/walkers_mammals_of_the_world/artiod actyla/a rtiodactyla.hippopotamidae.hippopotamus.html Hippos are reliably considered to be obligate herbivores, and hence meat woudl not be part of their diet under normal conditions. However, under sufficiently stressful or food-limited conditions conditions, most organisms can or will eat meat, even that from members of their own species. As the owner of any small "herbivore" pet such as a rabbit will tell you, a stressed mother will even consume her own living offspring. However, there are several factors here which call such a scenario into question. Your description of the subadult "nibbling and licking" indicates that it was engaged in relatively fine oral manipulation of the conspecific, rather than clearly rending flesh for consumption. This sound very much like an animal gathering chemical cues about the degree of injury to the conspecific. Numerous animals (including possibly humans - Bhatnagar,KP et al, 2002, Am.J.Rhinol., 16: 343-350 - although this is highly controversial) have a highly sensitive olfactory and chemosensitive structure called the vomeronasal organ (VNO) which is located in the roof of the mouth. There is a very good overview of the mammalian VNO in a web page here: http://athena.neu ro.fsu.edu/research/vomeronasal/ This structure is highly responsive to pheromones and other biologically relevant chemical cues, including, in many species, chemical markers which indicate degree of injury or death, particularly of conspecifics. A typical behavior for vomeronasal sensing is rapid "nibbling" around an area of interest, which can often mix air with aerosolized chemical cues to transport it to the VNO more effectively. From your description, I would agree that there is no evidence, nor any need to impose some "emotional state" on the subadult, especially since you have no idea the relatedness between the survivor and the cadaver, the emotional "potential" of a hippo, or a detailed record of the event. However, it would be highly adaptive, under all circumstances, to determine how badly injured a conspecific is, especially if there are issues of territory distribution or defense from the crocs) to work out.
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