categories provides estimates of the proportions 

 of reproductive females in the three phases of the 

 cycle and, comparing the proportions, of the rela- 

 tive lengths of the phases. Estimated average 

 length of the phases and the total cycle can then 

 be calculated for each 1-yr sample and for the 

 pooled samples, based on the relative lengths of 

 the phases and on the estimated gestation period 

 of 11.5 mo (Method 1 in Table 3). The estimates of 

 average length of cycle thus derived trend from 

 27.3 mo in 1973 to 42.3 mo in 1975, due to increase 

 in the estimated length of lactation from 11.2 mo 

 to 23.3 mo. 



Annual pregnancy rate under Method 1 (also in 

 Table 3 ) is calculated as proportion of reproductive 

 females pregnant divided by the length of gesta- 

 tion (0.958 yr). The reciprocal of annual preg- 

 nancy rate is the estimate of average calving 

 interval. 



In the second method of calculating length of 

 calving cycle, we estimated length of lactation 

 by assuming that a suckling calf existed in the 

 samples for each lactating female. Under this 

 assumption, the length at which the cumulative 

 frequency of calves in a sample equals the number 

 of lactating females should be the average length 

 at weaning (from which, using the length-age 

 equations published by Perrin et al. (1976), the 

 average age at weaning can be calculated). If the 

 length of lactation increases, the average length 



TABLE 3. — Estimates of lengths of reproductive phases, preg- 

 nancy rate, and calving interval under two methods of estimat- 

 ing length of calving cycle (see text) of the offshore spotted 

 dolphin, Stenella attenuata, 1973-75. 



Item 



1973 1974 



1975 



1973-75 

 pooled 



Sample size (no.) 

 Pregnancy (mo) 

 Lactation (mo): 

 Method 1 



Method 2 (Hyp. II) 

 "Resting (Method 1) 

 Sum of phases: 

 Method 1 

 Months 

 Years 

 Method 2 

 Months 

 Years 

 Annual pregnancy rate 

 (APR): 

 Method 1 

 Method 2 

 Calving interval 

 (1/APR): 

 Method 1 

 Years 

 Months 

 Method 2 

 Years 

 Months 



574 

 11.5 



481 

 11.5 



497 

 11.5 



11.2 219 23.3 



11.2 12.4 12.1 



4.6 6.4 7.5 



27.3 

 2.28 



27.3 

 2.28 



0.452 

 0.472 



2.21 

 26.5 



2 12 

 25.4 



39.8 

 3.32 



28.5 

 238 



0314 

 0459 



3.18 

 38.2 



2.18 

 262 



42.3 

 3.53 



28.2 



235 



296 

 0.461 



3.38 

 40.5 



2.17 

 260 



1,552 

 11.5 



17.4 

 11.9 



5.9 



348 

 2.90 



28.0 

 2.33 



0.359 

 0463 



279 

 33.4 



2.16 

 259 



at weaning estimated by this method should in- 

 crease concomitantly. The calculated length at 

 weaning did not increase sharply between years 

 (Table 4). Under Hypothesis II of Perrin et al. 

 ( 1976 1 of the rate of deposition of dentinal growth 

 layers (two in first year and one per year there- 

 after — the most likely alternative), the estimated 

 length of lactation ranges from 11.2 mo in 1973 

 to 12.4 mo in 1974. To arrive at estimates of the 

 total length of the calving cycle under Method 2, 

 we used the estimate of time spent in the "resting" 

 phase under Method 1 for 1973 (the year for which 

 the two estimates of length of lactation coincide 

 exactly) or 4.6 mo, for each of the three annual 

 estimates. This estimate is based on the assump- 

 tions under Method 1 but must suffice as a first 

 approximation. In estimating pregnancy rate (as 

 reciprocal of calving interval) — Table 3 — over- 

 lapping cycles were taken into consideration by 

 adjusting the effective length of lactation down- 

 ward by a factor equal to the percentage of lactat- 

 ing females also pregnant. 



The 1973 estimate of length of lactation (and 

 length of cycle, pregnancy rate, and calving inter- 

 val) is very close to that obtained by Method 1 

 above (11.2 mo), but the two sets of estimates 

 diverge sharply thereafter. The first method could 

 be invalid and cause diverging estimates if 1 ) 

 lactating females (and their nursing calves) were 

 overrepresented in the samples for 1974 and 1975 

 or, conversely, 2) either (or both) pregnant or 

 "resting" females were underrepresented. The 

 first situation could obtain if lactating females 

 and their accompanying calves are more likely to 

 be captured and killed in the net because of lim- 

 ited endurance and ability to escape of the calf, 

 certainly less than those of adults, and the 

 strength of the mother-calf bond. The second 

 method could yield erroneous estimates if 1 ) nurs- 

 ing calves were overrepresented in the samples or, 



TABLE 4.— Estimates of length of lactation in the offshore 

 spotted dolphin, Stenella attenuata, based on the cumulative 

 calf length /lactating females method (see text) 1973-75. 



1973 

 1974 

 1975 

 1973-75 

 pooled 



259 

 301 

 376 



936 



1358 

 138.5 

 1382 



137.8 



1.86 

 2.03 

 2.01 



1.98 



11.2 

 12.2 



12.1 



11.9 



11.2 

 12.4 



12.1 



11.9 



11.2 



122 

 12.1 



11.9 



'Includes mature females (s177 cm) without lactation data prorated to 

 lactating and nonlactatinq based on proportions in sample with lactation data. 

 2 Length at which cumulative number of calves = number of lactating females. 



631 



