Cockcroft and Ross. Tursiops truncatus off southern Africa 



297 



320 g and 310 g, respectively. Mean age, mass, and 

 length of males according to testis development stage 

 are given in Table 4. Puberty in male Tursiops from 

 the Indian Ocean may begin as early as 9 years of age 

 but primarily between 10 and 12 years (Fig. 11). How- 

 ever, sexual maturity (50% each of stages 2/3 and 4) 

 occurs only at about 14.5 years of age, a length of 240 

 cm and a mass of 165 kg. 



There was no evidence of a seasonal pattern in either 

 combined testis mass and tubule size (Fig. 13) or the 

 presence of sperm in the epididymis. 



Discussion 



The similarity of dentinal GLG counts to "Dolfie's" 

 actual age indicates an annual deposition of dentinal 

 GLGs for at least the first 6 years. Ross (1984) superim- 

 posed the growth rate of this animal on a length-versus- 

 age (dentinal GLGs) relationship in bottlenose dolphins 

 from Natal and the Eastern Cape, and concluded that 

 "the best fit of the points to the curve is reached when 

 the accumulation rate of dentine layers is equal to one 

 per year." Similarly, when the growth rate of another 

 captive Indian Ocean animal (Cockcroft and Ross 

 1990b) is fitted to the curve, the relationship is best 

 explained by an annual deposition of dentinal GLGs, 

 also proposed for Tursiops in other areas (Sergeant 

 et al. 1973, Hui 1980). 



In contrast to apparently continuously deposited 

 dentinal GLGs, cemental GLGs appear to be rapidly 

 deposited and only accumulate as whole layers in the 

 teeth of Tursiops. Nevertheless, the strong correlation 

 between dentinal and cemental age estimates up to 

 occlusion of the pulp cavity, and the similarity of den- 

 tinal and cemental GLG counts in teeth of the captive 

 animal, suggest that cemental GLGs also accumulate 

 annually and are reliable estimators of age in bottlenose 

 dolphins from the Indian Ocean, at least up to an age 

 of 12 years. Despite the lack of direct evidence, it seems 

 likely that cemental GLGs are deposited annually even 

 after 12 years. 



Male and female Indian Ocean bottlenose dolphins 

 may attain an age greater than 40 years (cf. Ross 1977) 

 with little difference in the apparent maximum ages 

 of the sexes. In excess of 20% of the Natal net catch 

 were older than 20 years, indicating a long-lived spe- 

 cies. Few data are available on the longevity of Tur- 

 siops elsewhere, and most existing studies have used 

 dentinal age estimates. Sergeant et al. (1973) estimated 

 the longevity of Tursiops from northeast Florida to be 

 about 25 years, with no apparent differences in the life 

 expectancy of males and females. Hohn (1980), in a 

 study of Tursiops from the southeast coast of the 

 United States, found animals with up to 27 dentinal 



GLGs, males and females reaching similar ages. In con- 

 trast, the maximum age of spotted dolphins, estimated 

 from cemental GLGs, is in excess of 45 years (Kasuya 

 1976). Thus, the use of cemental GLG age estimates 

 in future studies of bottlenose dolphins may yield 

 greater estimates of maximum age. 



The estimated mass and length at birth of Indian 

 Ocean bottlenose dolphins in this study are comparable 

 with those calculated by Ross (1984). Subsequent to 

 birth, growth is rapid, particularly in terms of mass, 

 but decreases gradually with age. The proportional 

 length increase is similar to that previously recorded 

 for bottlenose dolphins from Natal and the eastern 

 Cape (Ross 1977, 1984; Cockcroft and Ross 1990b) and 

 also those from other areas such as northeast Florida 

 (Sergeant et al. 1973), the western North Atlantic 

 (Hohn 1980) and captive animals from the Pacific coast 

 of Japan (Kasuya et al. 1986). An enormous increase 

 in mass during the first (suckling) year is well known 

 in seals and balaenopterid whales but has not often 

 been recorded for delphinids. Presumably, a large ini- 

 tial mass increase reflects the rapid development of the 

 calf and its need to reach thermoregulatory equilibrium 

 as well as some social and motor independence from 

 its mother (Cockcroft and Ross 1990b) before the 

 female's involvement with the next pregnancy and calf. 



The asymptotic length and mass values obtained 

 in this study and those of Ross (1977, 1984) are less 

 than those for bottlenose dolphins from the western 

 North Atlantic (Hohn 1980), Florida (Sergeant et al. 

 1973), and the Japanese Pacific (Kasuya et al. 1986). 

 It is unclear why bottlenose dolphins from different 

 areas have varying asymptotic sizes. Some authors 

 have suggested that this may warrant the separation 

 of the various populations to the specific level (Ross 

 1977), although Ross and Cockcroft (1990) have sug- 

 gested that there is little morphometric evidence to 

 suggest this and that such differences may only have 

 resulted from environmental conditions, particularly 

 temperature. 



The asymptotic lengths of male and female Indian 

 Ocean bottlenose dolphins are only slightly different 

 (243 cm and 238 cm, respectively). In the western 

 North Atlantic, Hohn (1980) found no difference in the 

 maximum lengths of males and females. These results 

 support the findings of Sergeant et al. (1973) that the 

 total lengths of male and female delphinids, in general, 

 do not appear to be different, although they found that 

 the asymptotic length of male Tursiops from Florida 

 was 20 cm greater than that of females. 



In contrast, fully grown male Indian Ocean bottle- 

 nose dolphins are considerably heavier (9%) and more 

 robust than females (176 kg and 160 kg, respectively). 

 Lactating and nonlactating females had a similar mean 

 mass, indicating that this mass difference cannot be 



