Scaling of Bite Force in Sharks Impacts Their Ecological Niche
Many animals undergo ontogenetic dietary shifts over their life history, especially predatory species. This is often a result of greater physical performance, prey availability, and energetic requirements. Many shark species have been documented to shift species that they consume over ontogeny, but the mechanisms facilitating this shift have not been directly addressed. One primary driver of this shift may be due to changes in bite force, which would allow sharks to potentially consume more functionally difficult prey (more evasive, larger, harder). Additionally, coastal shark species often overlap in habitat and potentially in diet. Dietary tracers, such as stable isotopes of carbon and nitrogen, have often been used to determine the extent of niche space and overlap among co-occurring species. Bull (Carcharhinus leucas), blacktip (Carcharhinus limbatus), and bonnethead sharks (Sphyrna tiburo) were measured evaluate how the scaling of bite force over ontogeny influences ecological niche and overlap since these species have been previously shown to undergo dietary shifts and have distinct ecological niches.
While many studies that measure the scaling of bite force over ontogeny often analyze all conspecifics simultaneously, this may mask any changing patterns in bite force. Therefore, young-of-the-year and juvenile sharks of each species were analyzed separately from sub-adults and adults. This study found that the smaller shark all underwent positive allometric scaling of bite force, whereas larger sharks exhibited isometry. While rapid increases in bite force did not appear to necessarily facilitate a diet shift in blacktips, it appears that bull and bonnethead sharks may show a relationship between positive scaling and diet shifts. Bull sharks exhibited the greatest niche width, which supports their classification as a generalist predator. There was also a relatively high level of overlap of bull sharks with blacktips and bonnetheads. This was expected for blacktips, but was surprising for bonnetheads given their very little overlap in known prey species. The inclusion of a third tracer, such as sulfur, may improve the resolution of overlap among these species.