FISHERY BULLETIN: VOL. 75, NO. 4 



females truly resting, i.e., not ovulating because of 

 being between cycles, those which have just ovu- 

 lated but did not get pregnant, some with ex- 

 tremely small embryos missed in dissections, 

 those which have recently aborted, and those 

 which have prematurely terminated lactation due 

 to death of the suckling calf. 



The gestation phase of the cycle was estimated 

 above, at 10.6 mo. We estimated average length of 

 lactation by two methods; 1) by assuming that the 

 proportion of a sample of mature females in a 

 particular reproductive phase is directly propor- 

 tional to the relative length of that phase in the 

 overall cycle, i.e., that the samples are not biased 

 with regard to reproductive phase (the length of 

 the "resting" phase was also estimated this way); 

 and 2) by assuming that a suckling calf exists for 

 each lactating female, and the samples are un- 

 biased with respect to suckling calves and lactat- 

 ing females, under which assumptions the length 

 at which the cumulative frequency of calves in a 

 sample equals the number of lactating females 

 should be the average length (and, from the 

 growth equation, age) at weaning. The first esti- 

 mate was based on data for 536 adult females 

 collected 1971-75, classified as pregnant, lacta- 

 ting, pregnant and lactating, "resting," or post- 

 reproductive (Table 7). The resting females were 

 further subdivided into those with and without a 

 corpus luteum. As discussed above, some propor- 

 tion of those with a corpus luteum can be assumed 

 to represent females not truly resting (with a cor- 

 pus luteum of infertile ovulation). Only three 

 females were simultaneously pregnant and lactat- 

 ing (1.44% of lactating females). 



Subtraction of the postreproductive females and 

 allocation of the females both pregnant and lacta- 

 ting to both of the two categories provides esti- 

 mates of the proportions of the reproductive 

 females in the three phases of the cycle (Figure 29) 

 and of the relative lengths of the phases. Estima- 

 ted average length of the phases and the total cycle 

 can then be calculated for each 1-yr sample and for 

 the pooled samples, using the estimated gestation 



o 



h- 



CE 

 O 

 Q. 



O 



or 



Q_ 



FIGURE 29. — Proportions of 532 adult reproductive female east- 

 ern spinner dolphins in pregnant, lactating, and "resting" (not 

 pregnant or lactating) phases of cycle. Based on Table 4. Females 

 both lactating and pregnant alloted to both phases. Postre- 

 productive females excluded. 



period of 10.6 mo (Method 1 in Table 8). The esti- 

 mates of average length of lactation thus derived 

 range from 13.1 to 29.7 mo (the possible causes of 

 this wide year-to-year variation in phase struc- 

 ture of the samples are discussed below in Gross 

 Annual Reproduction), with a pooled average of 

 17.5 mo. 



The second method of estimating length of lac- 

 tation, the "cumulative calf length/lactating 

 females" method yielded estimates for six 

 single-month samples ranging from 7.7 to 16.0 mo 

 and for single-year samples from 9.4 59 10.6 mo 

 (Method 2 in Table 8). The pooled estimate for 

 1973-75 is 10.1 mo. The three yearly estimates 

 are consistent with each other and sharply lower 

 than the estimates yielded by the "proportion-in- 

 phase" method above (compare in Table 9). The 

 first method could be invalid and cause disparate 

 estimates if 1) lactating females (and their nurs- 

 ing calves) were overrepresented in the samples, 

 or conversely, 2) either (or both) pregnant or "rest- 

 ing" females were underrepresented. This situa- 

 tion could obtain if lactating females and their 



TABLE 7. — Reproductive condition of 536 adult female eastern spinner dolphins collected 1971-75. 



1971 



1972 



1973 



1974 



1975 1971-75 pooled 



Condition 



Pregnant only 



Lactating only 



Pregnant and lactating 



"Resting" 

 with corpus luteum 

 without corpus luteum 



Postreproductive 

 Total 



744 



