Friday, February 3, 2017

American Alligator

The Organ Lab is excited to feature alligators from the chest freezer of the newest member of the IUSM Department of Anatomy and Cell Biology, Dr. Margaret McNulty. Dr. McNulty previously served on the faculty of the Louisiana State University School of Veterinary Medicine. She is a comparative anatomist with a strong research interest in comparative anatomy, and a fascinating collection of anatomical specimens. Here is the story of some of them...

The vast majority of veterinarians (DVMs) will go into small animal, large animal, or mixed practice, treating one or all of the “Big 4” taxa (dogs, cats, horses, and cattle) or other domestic animals like poultry, pigs, and goats. However, there are some who pursue more non-traditional career paths, including exotic medicine, zoo medicine, wildlife medicine, or research, such as our friends the Gorilla Doctors of the Moutain Gorilla Veterinary Project at UC-Davis. As students, veterinarians with interests in non-traditional careers must find opportunities to get exposure to the animals that they do not encounter regularly in the traditional DVM curriculum. Therefore, many student groups organize wet labs or seminars outside of normal course hours to enhance their learning and skills necessary to practice good medicine upon graduation. A couple years ago, Dr. McNulty, in conjunction with a wildlife veterinarian and a veterinary anatomical pathologist, was invited to participate in a wet lab for professional veterinary students focusing on proper handling, routine medical procedures (e.g. blood draws), euthanasia, and necropsy procedures for alligators.

In Indiana, alligators are obviously very rare. But along the Gulf Coast, from North Carolina to Texas, alligators are almost as common as white-tail deer in the Midwest. The American alligator (Alligator mississippiensis) is the largest reptile in North America. When they hatch, they are approximately 8-12 inches in length, and they can grow between 2 and 12 inches per year. Females will usually top out at approximately 9 feet in length (200+ pounds) and males can be over 13 feet long and 500+ pounds. Alligators are found most commonly in Louisiana, and are a large part of the heritage and economy in that state. In fact, the alligator industry in Louisiana is valued at over $700 million dollars. To that end, Louisiana started an alligator ranching program in 1986; this program allows incubation and hatching of eggs as well as raising and harvesting alligators for various purposes (e.g., meat for human consumption, hides). Alligators that are not harvested are tagged and released back into the wild, to be tracked by the Louisiana Department of Wildlife and Fisheries. These released alligators provide data regarding growth, survival, and dispersal rates that are used to monitor and adjust regulations.

For a veterinarian wishing to practice any sort of wildlife or exotic medicine along the Gulf Coast, especially Louisiana, knowledge of how to handle these animals properly is critical to both the safety of the alligator as well as the veterinarian. Outside of the normal daily interaction with these animals, alligators are also commonly used in a research setting, as they have virtually remained untouched from major evolutionary change for several million years. Therefore they provide a unique opportunity to study anatomical adaptations that have persisted for long stretches of time.

3D rendering of a microcomputed tomography scan of the skull (gray), brain endocast (blue), and trigeminal nerve (yellow) of a juvenile alligator. of a juvenile alligator. From George and Holliday, 2013.
One of the most fascinating characteristics of modern crocodylians (Order Crocodylia, to which alligators belong) is their derived sense of face touch. Sensory information to the face in vertebrates is conveyed through the fifth cranial nerve (Trigeminal nerve). In crocodylians, numerous trigeminal-nerve innervated pressure receptors are present in a speckled pattern across the face and lower jaw. These dome-shaped receptors sense mechanical stimuli like splashing in water while the face is partially submerged. This is one of the mehcanisms by which alligators (and other crocodylians) can sense their prey. Interestingly, the number of axons in the trigeminal nerve is negatively correlated with body size (or skull size), indicating that smaller crocodylians have higher axon density, and presumably finer sense of touch, than larger ones.

Contributed by Margaret McNulty, PhD, and Jason Organ, PhD

Have you ever wondered about the differences between alligators and crocodiles? You can learn about those differences here.

George, I., & Holliday, C. (2013). Trigeminal Nerve Morphology in and Its Significance for Crocodyliform Facial Sensation and Evolution The Anatomical Record, 296 (4), 670-680 DOI: 10.1002/ar.22666

Holliday, C., & Witmer, L. (2007). Archosaur adductor chamber evolution: Integration of musculoskeletal and topological criteria in jaw muscle homology Journal of Morphology, 268 (6), 457-484 DOI: 10.1002/jmor.10524