The behavioural impact of a life `behind bars' has long been recognised in captive animals. As a result, many modern zoos have gone to considerable lengths to provide stimulating and familiar environments for their animals. But, whilst improved conditions have successfully reduced abnormal behaviour,the anatomical impact of a captive life is often inescapable –particularly in animals that are born and reared in artificial conditions– resulting in a range of shapes and sizes across animals of the same species. For example, captive alligators are typically heavier and have shorter jaws and broader heads than their wild counterparts. But do captivity-induced anatomical differences matter? A new study by Gregory Erickson and his colleagues at the universities of Florida, Florida State and Northern Arizona tackled this question by investigating differences in bite-force performance between long-term captive and wild American alligators. They set out to answer two fundamental questions: (1) do captivity-induced changes in head shape affect biting ability and (2) if present, can these differences be linked to measurable changes in parameters such as jaw length,snout–vent length or body mass?
Erickson and his team bravely set about testing bite force in 47 alligators spanning a fourfold range in snout–vent length and nearly a 150-fold range in body mass. Safely secured to a platform, the animals were encouraged to `open wide' with gentle taps on their snouts before a precision transducer was gently placed on the most prominent tooth at the back of the jaw; the 11th maxillary tooth. Unsurprisingly, this interference triggered extremely aggressive snapping reactions from the animals.
Amazingly, the team measured the highest bite force ever measured in an animal (13172 N), with other animals registering forces ranging from 217 N upwards. The team also discovered that bite force differed significantly between captive-reared and wild alligators when normalised to jaw length;captive alligators bite more forcefully than their wild counterparts.
Physical differences generated by captive conditions therefore can, and do,affect performance. And although the exact mechanism for altered bite performance has yet to be pinpointed, the authors suggest two possible explanations. Firstly, the captive alligators' shortened jaws bring the 11th maxillary teeth closer to the fulcrum of the jaws and may therefore provide greater mechanical advantage. Secondly, the broader heads of captive alligators may give more space for jaw muscles compared with their wild counterparts.
Erickson's study also highlights the importance of normalising to the right parameters when comparing performance measurements between groups. In this study, the team have shown that captive alligators have bites that are either the same force or harder than wild alligators, when the data are normalised to the animal's jaw length or to body mass. If meaningful ecological ties are to be made between studies on wild and captive-reared animals, researchers must be aware that normalising to different parameters can reveal conflicting results. Erickson's work has also shown that differences in biomechanical performance between animals in their natural environment and zoos must also be investigated, as we can't always assume that captive populations have retained their wild forebears' physiological characteristics. And if this feature has triggered your imagination to investigate the effects of captivity physiology,have no fear; less dangerous animals are available too.
