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5 3 



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PREGNANT 



LACTATING 

 NON-PREGNANT 



r-N-F 



I 



FISHERY BULLETIN; VOL. 74, NO. 2 



-ill 



10 



RESTING/ 

 ESTRUS 



11% 



♦aar ave 



- 7 



- 6 



- 5 



-AVE. 



? 62 % 







100 200 300 400 500 600 700 800 900 



LENGTH OF FETUS (mm) 

 Figure 34. — Diameter of largest follicle in pregnant, lactating, and resting females of Stenella attenuata. 



(Figure 35) increases from about 0.1 at 8 layers to 

 about 0.6 at 12 layers and then again decreases to 

 about 0.5. 



The initial very low lactation rate compared to 

 pregnancy rate, of course, reflects the fact that a 

 very high percentage of the young females are 

 pregnant for the first time and thus cannot be 

 lactating. The lactation rate climbs rapidly to a 

 level about equal to the pregnancy rate (at about 

 12 layers) and behaves like the pregnancy rate 

 thereafter. The apparent decline of reproductive 

 rates in older females may be related to the 

 physiological or social mechanisms that cause the 

 appearance of postreproductive females in this age 

 group (see above; not included here). 



Sex Ratios 



The overall sex ratio was 44.9% males and 

 55.1% females (Figure 36). Many large samples 

 examined were predominantly female. Fourteen 

 of 32 single-school samples of 50 or more speci- 

 mens were more than 60% female, whereas none 

 was more than 60% male. The largest single- 

 school sample examined (342) was almost half and 

 half males and females. 



Sex ratio changes with age (Table 10). This is, of 

 course, making the assumption that the samples 

 examined were representative of the population. 

 Neonates and two-tone animals were almost 

 equally divided between the sexes, but only about 



264 



