MYRICK ET AL.: DENTAL LAYERS IN HAWAIIAN SPINNER DOLPHINS 



regions of the dentine) was uniform throughout all of 

 the dentine for a given specimen, GLG-boundary and 

 mid-GLG layers were counted in reverse order of 

 deposition up to the first boundary layer, the neona- 

 tal line, to estimate month and year of birth. 



Table 3 summarizes the month- and year-of-birth 

 estimates made from boundary-layer counts in six 

 specimens and birth dates taken from park records of 

 two captive-born specimens, WFP 670 and the calf of 

 ACM 104. Six were born in late summer/early 

 autumn and two in March. 



TABLE 3.— Estimated birthdates of eight cap- 

 tive Hawaiian spinner dolphins, Stenella long- 

 imstris.. 



'Born in captivity. 



2 Born in captivity, survived 3 d 



DISCUSSION 

 Age-Specific GLG Thickness 



Dentinal GLG thickness appears to be age-specific 

 for the Hawaiian spinner dolphin teeth examined. 

 There was little variability from tooth to tooth or 

 from animal to animal in the sequence of GLG thick- 

 ness through the 11th GLG, despite deposition of a 

 specific GLG in some specimens while still in the wild 

 and in other specimens during their captive lives. 

 This suggests that, to some extent at least, the 

 amount of dentine deposited by animals at a given 

 age may be predetermined and that animals of a 

 given stock, species, or higher common phylogenetic 

 affinity may follow the same or similar pattern of age- 

 specific GLG deposition unaffected by environ- 

 ment. 



Used in conjunction with the GLG component-layer 

 pattern, the regularity in thickness of age-specific 

 GLGs may be useful as an aid in locating GLG boun- 

 daries and counting GLGs in teeth of wild Hawaiian 

 spinner dolphins and dolphins of related species in 

 which GLG thickness and component-layer patterns 

 are found to be similar. When measurements are 

 taken at standard positions in the teeth of such 

 dolphins, one may make fairly rapid age estimates 

 without having to examine each GLG in detail (see 

 Myrick et al. 1983). 



Lunar Monthly Layers (LMLs) 



Laws ( 1 962) was the first to suggest that the system 

 of fine layers within dentinal GLGs of pinniped teeth 

 corresponded to lunar monthly cycles. Putative 

 LMLs have been reported in dentine of dugongs 

 (Kasuya and Nishiwaki 1978; Marsh 1980), in den- 

 tine of beaked whales ("short cycles," Kasuya 1977; 

 "accessory layers," Perrin and Myrick 1980:3, 5), in 

 fossil dolphin teeth (Myrick 1979), and in the man- 

 dibular bone (Myrick 1980b) and dentine of modern 

 dolphins (Myrick 1980b; Hohn 1980a, b). Hui (1978) 

 reported finding no relationship between the fine 

 layers that he counted in a tooth from a known-age 

 bottlenose dolphin and its age in lunar months; but 

 with no prior knowledge of its age, Myrick (1980b) 

 made dentinal LML counts in the same specimen 

 that closely agreed with its known age. 



The present study has furnished verification that 

 LMLs are deposited with lunar-monthly regularity in 

 the animals studied. In the 3.7-yr-old captive-born 

 spinner dolphin (WFP 670), 13 LMLs were counted 

 in each of the three complete annual dentinal GLGs 

 and 9 were counted in the partial fourth GLG. Where 

 LMLs were visible between TCL labels in the den- 

 tine in this and other specimens, they were found to 

 correspond consistently in number to the time in 

 months represented between labeling dates. 



Where LMLs could be seen clearly, no departure 

 from the 13 LML/GLG pattern was detected in the 

 teeth used in the present study. Variability has been 

 reported in studies of other marine mammals. Marsh 

 (1980:197) found only "about 12 [LMLs] per GLG" 

 in the dentine of the deciduous incisor of a dugong. 

 Ten to 15 LMLs/GLG were observed in dugong tusks 

 by Kasuya and Nishiwaki (1978). Kasuya (1977) 

 found between 11 and 13.4 LMLs ("short cycles")/ 

 GLG in teeth of Baird's beaked whales, Berardius 

 bairdii. Hohn ( 1 980b) counted 10-13 LMLs/dentinal 

 GLG in Atlantic bottlenose dolphin teeth. Pre- 

 sumably, LML variability will be found to occur also 

 in Hawaiian spinner dolphins when larger samples 

 are examined. 



Relationship of Cemental GLGs to 

 Dentinal GLGs 



None of the teeth of the studied specimens had 

 reached the stage of pulp-cavity occlusion or dentinal 

 irregularity that necessitated age estimation solely 

 from cemental GLG counts (Kasuya 1976; Myrick et 

 al. 1983). Although the pulp cavities were small in 

 some specimens and some later-administered TCL 



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