Growth of the skull of the bottlenose dolphin, Tursiops truncatus, in the Southwest Atlantic Ocean

Abstract

Defining the age of attainment of physical maturity is important for many studies, including identification of stocks, populations or species. In order to identify the age when the skull of the bottlenose dolphin, Tursiops truncatus, reaches maturity, skulls of fifty-three specimens found stranded along the coasts of southern Brazil, Uruguay and northern Argentina (27o35’S, 48o34’W-36o49’S, 55o19’W) were analyzed. Sixty skull measurements were taken to compare the growth rate of the different functional apparatuses. Age was estimated by counts of growth layer groups in the dentine of decalcified, stained longitudinal sections of teeth. Von Bertalanffy’s equation was applied to assess the growth and determine the age at maturity of each apparatus. Generally the maturation of skull starts at age two and stabilizes at age five, and the age of reaching the mature size varies amongst different characters. The braincase is the most precocious apparatus, while the feeding is the one that last stabilizes. The development patterns observed for the hearing, vision and breathing apparatuses were similar. Statistic analysis revealed significant differences among the ages at maturity, but not for von Bertalanffy’s growth equation parameters for each functional apparatus. For the studied population it is suggested that skulls can be considered mature in animals with more than five years.

Biology and ecology of bottlenose dolphins (Tursiops truncatus) stranded and sighted at 25°S in the south-western Atlantic Ocean during 1989–2016

“…As expected, there was a positive relationship between TL and CBL for T. truncatus observed here, but also substantial variation, with two individuals having the same TL (277.00 cm) but quite different CBLs (532.00 and 585.00 mm, respectively). Notwithstanding the above uncertainty, typically T. truncatus from southern Brazil, Uruguay and Argentina begin their cranial maturity at age two, with stabilization at age five and with a CBL of ~575.00 mm (Barreto, 2017), but only three specimens (TL>230.00 cm) stranded at Paraná had a CBL >575.00 mm. One of these individuals had differences in the inclination and degree of pterygoid separation, cranial width and maxilla length and width, characterized by the southern/coastal morphotype (Costa et al ., 2016; Wickert et al , 2016).…”

“…The mean condyle-basal length (CBL) of adults varied between 511.00 and 585.00 mm (538.60 + 19.90 mm) ( Table 1). One specimen had a clear southern skull morphotype ( Figure 3A) and was included in the group of three individuals with a CBL larger than 575 mm (proposed as cranial maturity for the southern form by Barreto (2017)). These animals were collected during winter (July and August) and their TL varied between 230.00 and 305.00 cm.…”

Skull morphology of bottlenose dolphins from different ocean populations with emphasis on South America

“…A sample of 201 well‐preserved adult skulls of bottlenose dolphins from different oceanic regions was analyzed. Skulls were considered adults when the specimens' age, assessed through growth layer groups in teeth (Hohn, Scott, Wells, Sweeney, & Irvine, 1989), was higher than 5 years (Barreto, 2016) or had fused bones, closed sutures and closed alveoli (Tavares et al, 2010). A full list of specimens, sex, locality, and museums is available in supplemental online material, Appendix I.…”

Report of the Working Group on Population Parameters and Demography of Tursiops truncatus in the Southwest Atlantic Ocean

“…The data presented by Barreto (2000;2016 this volume) and Siciliano et al (2007) suggest significant different patterns in the age structure of stranded bottlenose dolphins between southern and southeastern Brazil (Fisher Exact test, p < 0.05). The mortality in the southern region seems to be skewed towards young male individuals.…”