Abstract

The degree of fusion between bones is a useful indicator of skeletal and sexual maturity for cetacean specimens preserved in museum collections. The aim of this study was twofold: first, to examine the degree of fusion between bony elements in skulls of Risso’s dolphins (Grampus griseus Cuvier, 1812) from Taiwanese waters; and second, to analyze the relationship between skull maturity, body length, sexual maturity, and estimated age, with the aim of determining a useful skull predictor for maturity in Risso’s dolphins. The stage of fusion of 20 superficial sutures or joints between selected skull bones was examined on 33 clean, dry skulls, which were salvaged from stranded or bycaught dead Risso’s dolphins in Taiwanese waters during the years of 1994 – 2001. The bones of the caudoventral braincase fused early in development (basioccipital-exoccipital synchondrosis, supraoccipital- exoccipital suture), whereas fusion along the nuchal crest (fronto-interparietal and fronto-parietal sutures) occurred later. Some sutures remained open in some adult specimens (lacrimal/maxilla-frontal, squamosal-parietal, squamosal-exoccipital sutures, and the intermandibular symphysis). Bilateral asymmetry of the fusion process was not detected. Advanced fusion occurred in the fronto-interparietal suture along the medial aspect of the nuchal crest, and in the rostral nasal-frontal and distal maxilla-incisive sutures at total body length > 250 cm, and may be useful skull indicators of sexual maturity.

中文摘要

骨骼間的癒合程度可用以判定博物館收藏之鯨豚標本的性成熟和骨成熟程度。本研究之目標為透過研究臺灣海域瑞氏海豚（Grampus griseus Cuvier, 1812）頭骨各骨片之間癒合程度，分析頭骨成熟度、體長、性成熟及估計年齡間之關係，以尋找可有效估計瑞氏海豚成熟度之頭骨指標。本研究查驗了1994-2001年間於臺灣海域擱淺或誤捕之33隻瑞氏海豚的乾燥頭骨標本、表面可見的20個骨縫（sutures）或關節（joints）之癒合程度。背腹面頭蓋骨處之骨縫與關節最先開始癒合（基枕骨與外枕骨軟骨聯合 basioccipital-exoccipital synchondrosis，上枕骨與外枕骨縫 supraoccipital-exoccipital suture），頸脊部分（頂額縫 fronto-parietal suture，額骨與頂間骨縫 fronto-interparietal suture）為次。在成年個體中仍可見某些骨骼接縫或關節並未癒合（額淚-額頷縫 lacrimal/maxilla-frontal，鱗骨與頂骨 squamosal-parietal，鱗骨與外枕骨縫 squamosal-exoccipital sutures，前下頷韌帶聯合 intermandibular symphysis）。本研究並未發現癒合過程有左右不等的現象。頸脊中央部分的 額骨與頂間骨縫，鼻額縫吻端（rostral nasal-frontal suture）以及上頷骨與前頷骨縫 （maxilla-incisive sutures）先端在海豚體長大於250公分後均達高度癒合程度，應可視為理想的性成熟骨骼指標。

Keyword: Grampus griseus, maturity, Risso’s dolphin, skull, skeletochronology, Taiwan.Grampus griseus、成熟度、瑞氏海豚、臺灣、頭骨、骨鑑齡分析。