FISHERY BULLETIN: VOL. 74, NO, 2 



Table 7. — Length at which cumulative frequency of calves 

 equals the number of lactating females in eight 1-mo samples of 

 Stenella attenuata. 



higher than that of lactating females and lacta- 

 tion continues after death of a nursing calf. 



A third estimate of length of lactation was de- 

 rived from the ratio of lactating to pregnant 

 females. This analysis included all the material 

 from 1971 and 1972, when only adult females 

 were sampled, as well as the material included in 

 the calf-lactating female analysis above. Females 

 both lactating and pregnant were included in 

 both categories. The assumption is made that 

 samples were unbiased with respect to relative 

 representativeness for lactating and pregnant 

 females. The ratio was 0.95 in the 1971 sample (86 

 adult females), 1.00 in 1972 (455), 0.96 in 1973 

 (573), and 0.97 for the pooled samples {n = 1,114; 

 Table 8). The ratio of lactating to pregnant should 

 equal the ratio of the lactation period to the ges- 

 tation period. Gestation is 11.5 mo, therefore lac- 

 tation is by this method estimated at 0.97 times 

 11.5 mo, or 11.2 mo. Estimated length at this age 

 is 135.5 cm. 



The three estimates of 9.6, 11.6, and 11.2 mo are 

 based on largely independent assumptions and 

 are close enough to each other to indicate that 

 length of lactation is almost certainly between 9 

 and 12 mo. Of the three, the central estimate, 11.2 



mo, is best in terms of sample size and probable 

 validity of assumptions and is used below in es- 

 timating length of the calving interval. 



The basic data used for estimating average 

 length of calving interval were the relative fre- 

 quencies of adult females in several reproductive 

 conditions (Table 8). Adult females were defined 

 as those wath at least one corpus luteum or corpus 

 albicans. Senile females were those with 10 or 

 more corpora albicantia, no corpus luteum or 

 Type 1 or 2 corpora albicantia and ovaries weigh- 

 ing less than 3.5 g. Resting/estrus females were 

 those nonsenile adults that were neither preg- 

 nant nor lactating. Many of these (16 to 31%) had 

 a corpus luteum. The corpus luteum may have 

 represented an undetected very early pregnancy, 

 a very recently aborted pregnancy, loss of a calf 

 shortly after birth (resulting in cessation of lacta- 

 tion), or may have been a corpus luteum of ovula- 

 tion. All of these alternatives may be represented 

 in the samples. 



In calculating the proportions of females in the 

 three phases of pregnant, lactating, and resting 

 (Table 9), senile females were excluded. One-half 

 of the animals simultaneously pregnant and lac- 

 tating were assigned to the pregnant category 

 and one-half to the lactating category. 



The average length of calving interval was es- 

 timated by two methods — 1) using the estimates 

 of gestation and lactation periods and 2) using the 

 percentage of females pregnant. The data for the 

 3 yr are comparable (Table 9), with the exception 

 of possible existence of a trend in proportion rest- 

 ing; therefore, length of calving interval was es- 

 timated from the pooled data. Eighty-four and 

 one-half percent of reproductive females were 

 pregnant or lactating. Pregnancy (11.5 mo) plus 

 lactation (11.2 mo) total 22.7 mo. If the proportion 

 in a phase is equal to the proportion of the total 



Table 8. — Reproductive condition of 1,114 adult female specimens o{ Stenella 



attenuata, collected 1973.' 



'In the resting/estrus category, subcategones A and B (in parentheses) are specimens with and 

 without a corpus luteum, respectively. 

 ^3^10 corpora, no Type 1 or 2 corpora, and ovaries s 3.5 g. 



262 



