TABLE 1. — The sample of the offshore spotted dolphin, Stenella 

 attenuata, by sex, year, and month, 1973-75. Date of capture was 

 not available for 18 of the total 6,243 specimens collected. 



x annual pregnancy rate), Perrin et al. (1976) 

 estimated the proportion of adult females which 

 were reproductive from coloration phase data, 

 based on a subsample of data on percentage 

 mature in the various coloration phases 

 ("mottled" and "fused-adult"). In the present 

 study, a much larger sample of complete reproduc- 

 tive data was available; therefore, the proportion 

 of total females which were reproductive was 

 estimated directly from that sample. Specimens 

 for which ovarian data were lacking or incomplete 

 were allocated to mature or immature categories 

 based on a length criterion. Average length at 

 attainment of sexual maturity was estimated as 

 that length (177 cm) at which the number of 

 shorter but mature specimens in the sample 

 equals the number of longer but immature 

 specimens. 



Results and Discussion 



Calving Cycle and Pregnancy Rate 



The calving cycle, for purposes of analyzing 

 field data, can be divided into three phases: 1) 

 pregnancy, 2) lactation, and 3) "resting" (a catch- 



all phase for animals neither pregnant nor lactat- 

 ing, which includes females truly resting, i.e., not 

 ovulating because of being between cycles, those 

 which have just ovulated but have not become 

 pregnant, some with extremely small embryos 

 missed in dissection, those which recently 

 aborted, and those which have prematurely termi- 

 nated lactation due to death of the suckling calf). 



We estimated the length of the cycle (and preg- 

 nancy rate) in two ways: 1) based on the reproduc- 

 tive structure of the sample of adult females, i.e., 

 based on the assumption that the samples are not 

 biased with respect to reproductive phase, and 

 that the proportion of a sample of mature females 

 in a particular phase is directly proportional to 

 the relative length of that phase, using the pre- 

 viously estimated (Perrin et al. 1976) length of 

 gestation (11.5 mo) as a time calibration, and 2) 

 based on the estimate of length of gestation and 

 a largely independent estimate of length of 

 lactation. 



The first estimate was based on data for 1,876 

 females classified as pregnant, lactating, preg- 

 nant and lactating, "resting," or postreproductive 

 (Table 2). The "resting" females were further 

 subdivided into those with and without a corpus 

 luteum. As discussed above, some proportion of 

 those with a corpus luteum can be assumed to 

 represent females not truly resting (with a corpus 

 luteum of infertile ovulation). In the total sample 

 of 3,443 females, 61 were simultaneously preg- 

 nant and lactating (6.1% of the lactating females). 

 Minor differences between the numbers in Table 2 

 and in table 8 of Perrin et al. (1976) reflect in- 

 crease of the 1973 sample by eight specimens and 

 reexamination and reevaluation of the materials. 



Subtraction of the postreproductive females 

 from the aggregate of mature females of deter- 

 mined reproductive condition and allocation of 

 the females both pregnant and lactating to both 



TABLE 2. — Reproductive condition of 3,469 female offshore spotted dolphins, Stenella attenuata, 



collected 1973-75. 



630 



