A comparison of the cranial morphology of bottlenose dolphins (Tursiops truncatus) in the Gulf of Mexico

Abstract

The current taxonomic status of the bottlenose dolphin ics. in the Gulf of Mexico is that of a single species, Tursiops truncates, although significant variability has been described on global and local scales. One hypothesis for this phenomenon is that the genus is locally divided into coastal populations comprised of small, elongate animals, and onshore populations made up of large, robust animals. To test this hypothesis, 220 skulls were collected from stranded animals throughout the Gulf of Mexico, encompassing Mississippi (n=1), Louisiana (n=9), Florida (n=59) and six stranding areas from Texas (n=151). Data on standard length, sex, location of stranding, and a sample of teeth for aging were collected. Though most specimens stranded from unknown populations, 37 animals were taken from known inshore communities. The inshore sample was comprised of specimens collected from coastal gillnets (n=3) as well as from die-of events within local bay communities (n=34). Thirty-five cranial measurements, from Perrin 1975 and Walker 198 1, were examined along with age, sex, standard length, skull maturity, and geographic location of stranding. Statistical analysis was limited to only adult animals (n=170) as determined by fusion of the skull and age. Characters were grouped based upon ecological functionality and used to compare groups from Texas and Florida populations. A group of Pacific animals was used as an subgroup, due to their genetic similarity to offshore animals from the Gulf of Mexico. Cluster analysis identified a small group of animals (n=3) that were significantly different from the rest of the dolphins. Due to the shape and size of the characters exhibited by the skulls of these animals, they were hypothesized to be ''offshore'' animals. Discriminant analysis determined that traits defining skull length, feeding musculature, and nares shape identified separation between Texas inshore and Texas "offshore'' populations, while traits defining width of-rostrum and internal nares identified separation between Texas and Florida populations. Further investigation found that separation between the inshore and offshore dolphin populations could be determined by as few as three skull characters, consistent with findings from populations in the western Atlantic Ocean.