PERRIN ET AL.: GROWTH AND REPRODUCTION OF THE EASTERN SPINNER DOLPHIN 

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LAYER (no) 



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FIGURE 6. — Thickness of growth layers, beyond second post- 

 natal layer, in teeth of the eastern spinner dolphin as proportion 

 of thickness of next older, adjacent zone. Box is one SD on each 

 side of mean; vertical line is range; sample size in parentheses. 



RESULTS 

 Growth 



Length at Birth 



The largest fetus encountered was 84 cm long. 

 The smallest free-swimming calf was 70 cm long. 

 Estimated average length at birth is 76.9 cm. The 

 estimate is based on a weighted linear regression 

 of percent postnatal on body length, for 3-cm 

 groupings, of 101 specimens (54 fetuses and 47 

 neonates) between 67 and 99 cm long (Figure 7) 

 collected in random samples. Because of the small 

 sizes of the available samples, 23 specimens of the 

 whitebelly form (11 neonates and 12 fetuses) and 

 23 specimens unidentified to geographical form 

 (16 neonates and 7 fetuses) were included. This is 

 justified because of the small difference in length 

 of adults of the two forms ( <5 cm — Perrin 1975a). 

 Such a difference could be expected to translate 

 into a probable error in the estimate of length at 

 birth, based on the present sample composition, of 



• (9) 



Y = 6083l X -4176 

 r = 0.970) 



70 73 76 79 82 85 88 



LENGTH (cm) 



FIGURE 7. — Estimation of average length at birth, based on 

 weighted linear regression of percent postnatal on body length, 

 in 3-cm increments, for 101 specimens of Stenella longirostris (54 

 fetuses and 47 neonates) between 67 and 88 cm long. 



<0.5 cm, less than that to be expected to be intro- 

 duced by reduction of the sample size (by 47'' I 

 through restriction to specimens known to be 

 eastern spinner dolphins. The estimate is rounded 

 off to 77 cm in analyses below. 



This method of estimating average length at 

 birth assumes that pregnant females and calves 

 are 1) equally vulnerable to capture in the purse 

 seine, 2) equally likely to die once captured, and 3 ) 

 equally represented in the sample of dead animals 

 measured. It also assumes equal rates of prenatal 

 and postnatal natural mortality and assumes that 

 the stresses imposed by pursuit and capture do not 

 cause premature births during the chase or in the 

 net. It was not possible to test these assumptions 

 although some evidence indicates that the last 

 may not be justified (see discussion below in The 

 Calving Cycle and Pregnancy Rate). 



Length of Gestation and Fetal Growth 



The usual method used to estimate length of 

 gestation is that of Laws ( 1959), in which progres- 

 sion of a mode in fetal lengths is followed through 

 the seasons. This method was used to estimate 

 length of gestation for the spotted dolphin (Perrin 

 et al. 1976). The method could not be applied to the 

 present samples of data for the eastern spinner 

 dolphin, however. Although breeding is perhaps 

 synchronous at some level (e.g., note peaks in the 

 length-frequency distributions for postnatal 

 males and females in February and April 1973, 

 and February 1974— Figures 2, 3), progression of 



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