Patterns of growth and maturation of the skeleton in Risso''s dolphins, Grampus griseus, from Taiwanese waters
Date Issued
2009
Date
2009
Author(s)
Chen, Ing
Abstract
Risso’s dolphins (Grampus griseus) are moderately small odontocetes, widely distributed throughout temperate to tropical pelagic regions of the world’s oceans and are one of the most frequently encountered cetaceans in eastern Taiwanese coastal waters. Since their life history is poorly known, the goal of this study was to investigate the maturation of the skull and flipper bones, and to estimate age based on growth layers in tooth sections, and to determine the relationship between the age, body length and skeletal maturity for this Taiwanese population. Ninety-nine carcasses of dead-stranded or incidentally bycaught dolphins were measured and dissected during 1994 – 2008 (total body length, TBL: 124 – 290 cm). Skeletal maturity was assessed by a radiographic examination of the fusion of the distal epiphyseo-metaphyseal junction in the distal radius and ulna of 67 flippers (from 34 dolphins), and by an examination of the fusion stages of 20 sutures/synchondroses/symphysis on 33, cleaned dry skulls. Age was estimated by counting growth-layer-groups in routine histologically prepared tooth sections from 28 dolphins. Preliminary analysis revealed no sexual dimorphism in body length, and the onset of sexually maturity occurred at 240 – 255 cm in females and 253 – 265 cm in males, which was at about 10 years of age for both sexes. Body length at sexual maturity was slightly shorter than those from Japan and South African waters, but much shorter than those from northeast Atlantic Ocean. Cessation of longitudinal flipper growth in epiphyseo-metaphyseal ossification in the distal radius and ulna occurred at 248 – 255 cm TBL, about the age at sexual maturity. The distal radius closed earlier than distal ulna; bilateral asymmetry and sexual dimorphism was not detected. On the skull, the bones of the caudoventral brain case fused early in development (basioccipital-exoccipital synchondrosis and supraoccipital-exoccipital suture), whereas fusion along the nuchal crest (fronto-parietal, fronto-interparietal sutures) occurred later; bilateral asymmetry of the fusion process was not detected. At TBL > 250 cm, advanced fusion occurred along the nuchal crest, rostral nasal-frontal, and distal maxilla-incisive sutures which may be useful indicators of sexual maturity.
Subjects
Grampus griseus
Risso’s dolphin
Taiwan
skull
flipper
teeth
GLG
life history
age estimation
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