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



100 r es) • ■" ioo 



150 160 170 



BODY LENGTH (cm) 



FIGURE 22.— Estimation of body length at which 50% of female 

 eastern spinner dolphins show ovarian evidence of sexual matu- 

 rity (one or more scars). Fit to central portion of curve (solid line) 

 is linear regression. Dashed portions of curve fitted by eye. Sam- 

 ple sizes in parentheses. 



average age at attainment of maturity at about 5.4 

 growth layers, in close agreement with the esti- 

 mate derived from the age/length equation. A 

 rounded-off average of 5.5 layers is used below. 

 Average age in years at attainment of maturity 

 under the three hypotheses of layer deposition 

 rate are 3.7, 5.0, and 3.7 yr, respectively, with the 

 second estimate being most probably correct. 



Sexually adult females in the sample ranged 

 from 152 to 187 cm and averaged 171.2 cm in 

 length (Figure 24). 



DISTRIBUTION OF CORPORA TO LEFT 

 AND RIGHT OVARIES.— As in all other odonto- 

 cetes so far studied, the left ovary predominates in 

 ovulation. As in the case of S. attenuata, the dis- 

 tribution between left and right side (Table 6) can 

 be accounted for by assuming that about 90 to 95% 

 of the females ovulate the first time from the left 

 ovary, and the remainder from the right, and that 



6 7 8 9 



AGE (growth layers) 



FIGURE 23. — Relationship between proportion of females sexu- 

 ally mature and age, in dentinal growth layers, in the eastern 

 spinner dolphin. Fit is by eye. 



200 1- 



EASTERN 

 FEMALES 

 Ave. = 171.2cm 

 s.d. = 6.08cm 

 n = 560 



145 



155 165 175 185 



BODY LENGTH (cm) 



FIGURE 24.— Length-frequency distribution of 560 sexually 

 adult (possessing at least one ovarian corpus) female eastern 

 spinner dolphins. 



741 



