FISHERY BULLETIN: VOL. 75, NO. 4 



100 T (51) (83) (89) (101) (134) (175) (80) 



(37) 



155 



165 170 175 180 



BODY LENGTH (cm) 



185 



195 



FIGURE 20. — Proportion of males sexually mature in relation to 

 body length in the eastern spinner dolphin under three criteria of 

 testis-epididymis weight. Sample sizes in parentheses. 



TABLE 4: — Body length of adult male eastern spinner dolphins 

 under three criteria of sexual maturity. 



Testis-epididymis weight is more closely corre- 

 lated with age (in dentinal growth layers) than 

 with body length (Figure 21). The 94-g level is 

 reached on the average at about 9 growth layers 

 and attained by all males with more than 12 

 layers. The 150-g level is reached at about 10 

 layers on the average and by all males at about 13 

 layers. The 400-g level is reached on the average 

 at about 12 layers, but the oldest male examined 

 (16.5 layers) had a testis-epididymis weight of 

 only 333 g. Estimated average age in years at 

 sexual maturity under the three criteria and 

 under the three layer/year hypotheses ranges 

 from 6.0 to 11.5 yr (Table 5), with the most likely 

 estimates (Hypothesis II) 8.5 to 11.5 yr. 



The question of age at attainment of social 

 maturity (sense of Best 1969) in the spinner dol- 

 phin must await studies of social structure and 

 breeding patterns. Other (larger) odontocetes, 

 such as the sperm whale, Physeter catodon, and 

 the long-finned pilot whale, Globicephala melae- 

 na, are known or thought to be polygynous, to 

 varying degrees, but the social structure of the 

 spinner dolphin is as yet unknown. 



No systematic seasonal variation in testis 

 weight or condition was detected, although the 

 heavy bias in seasonal coverage of the sample pre- 

 cludes an adequate evaluation of this factor. 



740 



1 



6 8 10 12 14 



DENTINAL GROWTH LAYERS (no) 



18 20 



FIGURE 21. — Scatterplot of testis-epididymis weight on age (in 

 dentinal growth layers) for 106 eastern spinner dolphins. 



TABLE 5. — Estimated average age in years at attainment of 

 sexual maturity in male eastern spinner dolphins under three 

 criteria of maturity and three growth layer hypotheses. 

 [See text for definition.] 



The Female 



ATTAINMENT OF SEXUAL MATURITY.— 

 The smallest sexually mature female in the pre- 

 sent sample was 152 cm long. The largest imma- 

 ture female was 182 cm long. One estimate of 

 average length at attainment of sexual maturity 

 is the length at which 50% of the females show 

 evidence of having ovulated, i.e. possess ovaries 

 with one or more scars (corpus luteum or corpus 

 albicans). This length in the present sample of 

 eastern spinner dolphins is estimated at 164.1 cm 

 (Figure 22). 



The sigmoid curve in Figure 22 is slightly 

 asymmetrical, that is to say, there are more imma- 

 ture animals (91) above the 50%-mature length of 

 164.1 cm than there are mature animals below it 

 (62). At 165 cm, the numbers are 80 and 77, respec- 

 tively. This length, 165 cm, is used below to clas- 

 sify as sexually mature or immature specimens for 

 which ovarian data are lacking. The predicted 

 number of growth zones (from the growth equa- 

 tion) at this length is 5.5. 



Average age at attainment of sexual maturity 

 can also be estimated directly from the smaller 

 sample of females for which teeth were sectioned 

 (n = 247). This analysis (Figure 23) estimates 



