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Fishery Bulletin 88(2). 1990 



attributed to stress and blubber mass loss through lac- 

 tation (Cockcroft and Ross 1990a, b). Some of this mass 

 difference between the sexes may be a direct conse- 

 quence of the male mass growth spurt between 10 and 

 12 years of age. It is unclear why this growth spurt 

 is not reflected in the length of males; it may be that 

 robusticity and not length is important in male and 

 female interaction. 



Despite this, the male growth spurt at the onset of 

 puberty may be similar to the two-stage growth that 

 Perrin et al. (1976, 1977) described for male and female 

 spotted and spinner dolphins in the eastern tropical 

 Pacific where growth showed a pubertal secondary 

 growth spurt. In male bottlenose dolphins, this spurt 

 occurs 4-5 years later than in spotted or spinner dol- 

 phins but also at the onset of puberty, suggesting that 

 such growth spurts may be directly related to the at- 

 tainment of sexual maturity. It is possible that this 

 growth spurt may also be evident in female bottlenose 

 dolphins but is not discernible owing to the small sam- 

 ple size. 



Females mature sexually some 2 or 3 years prior to 

 the attainment of physical maturity in contrast to 

 males, where sexual maturity is attained just before 

 physical maturity. In both sexes, however, physical 

 maturity occurs almost in concert with the occlusion 

 of the tooth pulp cavity, supporting previous sugges- 

 tions that animals with occluded pulp cavities are sex- 

 ually and physically mature (Ross 1977, 1984). Females 

 attain sexual maturity at least 2 years earlier and at 

 a lesser length and mass than do males, although the 

 reduced number of first-time ovulators and high occur- 

 rence of females with multiple ovulations will have 

 biased this upwards. It is not unusual for female del- 

 phinids to attain sexual maturity sometime before and 

 at a smaller size than males. Female Tur slops in the 

 western North Atlantic also appear to follow this 

 pattern (Sergeant et al. 1973). Female spotted and 

 spinner dolphins reach sexual maturity about 3 or 3-4 

 years, respectively, before males and both are smaller 

 than their male counterparts (Perrin et al. 1976, 1977). 

 It has been proposed that this disparity ensures more 

 sexually mature females than males in the population 

 (Bryden 1972). Intensive behavioral field work is 

 needed before this suggestion can be confirmed. 



Although no direct estimate of gestation period was 

 available from the fetal growth data, both derived 

 estimates were in excellent agreement, about 373 days. 

 A 373-day gestation period is slightly longer than 

 previous estimates for Indian Ocean Tursiops, which 

 range from minimums of 342 and 341 days (Saayman 

 and Tayler 1977) to maximums of 364 and 368 days 

 (Ross 1984) in captive bottlenose dolphins. Similar 

 estimates of the gestation period in captive Tursiops 

 from other areas have been given by Tavolga and 



Essapian (1957), McBride and Kritzler (1951), and 

 Kasuya (1985). 



Although the sample was too small for an assessment 

 of the age at first ovulation, the onset of ovulation in 

 females is apparently rapid. Thereafter, there was con- 

 siderable variability in the ovulation rates. Some 12- 

 or 13-year-old females had particularly high ovulation 

 rates, possibly a result of several initial infertile ovula- 

 tions (Harrison et al. 1972). Others of the same age had 

 low corpora counts, were all lactating, and one was 

 pregnant, indicating that fertilization occurred on the 

 first or second ovulation. Indirect evidence, which 

 shows that lactational transfer of organochlorines in 

 female bottlenose dolphins occurred after one or two 

 ovulations (Cockcroft et al. 1989), supports the view 

 that the majority of females conceive after one or two 

 ovulations. The variation in ovulation rates of older 

 females may be due, in part, to the same factors which 

 apply to younger females and to additional reasons such 

 as calf mortality or aborted pregnancies. 



Overall, the calculated annual ovulation rate for 

 female bottlenose dolphins was 0.28, a substantially 

 lower rate than that observed for Tursiops from north- 

 east Florida (Sergeant et al. 1973). There was little sign 

 of reproductive senescence in females from Natal, as 

 ovulation rate did not appear to decline with age, and 

 there was no reduction in ovary mass with an increas- 

 ing number of ovarian scars. Also, the oldest captured 

 female was lactating, and a number of older females 

 were captured with calves and had enlarged corpora 

 in their ovaries. Results from organochlorine residue 

 studies in these females also indicate that older females 

 do not become senescent (Cockcroft et al. 1989). These 

 data imply that older females do not act as "wet 

 nurses," which is contrary to suggestions for several 

 other species of odontocetes that manifest age-related 

 declines in fecundity. Senescent females invest more 

 in quality calf-rearing and a longer lactational commit- 

 ment than in quantity calf-bearing, as their reproduc- 

 tive potential fails (Kasuya and Marsh 1984, Marsh and 

 Kasuya 1986). However, sample numbers in this study 

 were small and certain female-calf pairs showed an 

 extended relationship, though there was no indication 

 that this was restricted to older females. 



Taken in combination, these facts indicate that some 

 Indian Ocean bottlenose dolphin females are probably 

 reproductively active until an advanced age. A similar 

 conclusion was reached by Kasuya (in Marsh and 

 Kasuya 1986) who found that although the annual 

 pregnancy rate and the number of resting females in 

 a sample of Tursiops from the Pacific declined with 

 age, pregnant and lactating females were present in 

 all age groups, presenting no conclusive evidence of 

 senescence. 



