Cockcroft and Ross: Tursiops truncatus off southern Africa 



299 



Although births apparently occur throughout the 

 year, there is a peak in summer, between November 

 and February, when over 60% of births occur. How- 

 ever, as birth dates were back-calculated, this may 

 reflect the greater catch of dolphins in these months 

 and the bias of the net catch for larger calves (Cock- 

 croft and Ross In press), although previous work in this 

 region noted that births occurred predominantly in late 

 spring and summer (Ross 1977). No seasonal cycle of 

 either testis mass, tubule diameter, or occurrence of 

 sperm in epididymis was evident in mature males. 

 These data also imply no distinct mating or breeding 

 season in Indian Ocean Tursiops. In Florida waters, 

 the main mating and calving season is apparently 

 February to May (Essapian 1963), or spring to early 

 fall (Irvine et al. 1981), which is similar to that found 

 in the present study. In contrast, bottlenose dolphins 

 off Argentina show a distinct summer calving and 

 mating season (Wursig 1978). These geographical vari- 

 ations, however slight, indicate the adaptability of 

 coastal Tursiops to local conditions. 



Although it has been suggested that suckling as a 

 nutritional source probably only lasts 1 year (Kasuya 

 and Marsh 1984, Cockcroft and Ross 1990b), there is 

 evidence that suckling may last at least 18 months in 

 Tiirsiops and that non-nutritional suckling may con- 

 tinue for as long as 3 years for some mother and calf 

 pairs. An estimate of the duration of lactation is dif- 

 ficult, where suckling extends over long periods and 

 may serve a non-nutritional purpose such as enhanc- 

 ing the cow-calf bond (Brodie 1969). Of the calves from 

 lactating female-calf pairs, 25% were over 1 year old 

 and a further 25% were over 2 years old, and some of 

 the latter had both milk and solids in their stomachs. 

 The mammary glands from these lactating females, 

 only one of which was pregnant, increased in size with 

 calf size, until calves were at least 18 months old. In 

 combination, these data indicate that lactation in In- 

 dian Ocean bottlenose dolphins lasts more than 1 year 

 and in some instances may extend to more than 2 years. 



This is slightly longer than previous estimates of lac- 

 tation length and age at weaning based on studies of 

 captive and captured free-ranging bottlenose dolphins 

 (McBride and Kritzler 1951, Gurevich 1977, Saayman 

 and Tayler 1977, Kasuya 1985, Cockcroft and" Ross 

 1990b) and suggests a prolonged mother-calf associa- 

 tion that may extend in free-ranging bottlenose dol- 

 phins for at least 15 months (Irvine et al. 1981). Such 

 extended mother-calf interaction may indicate a stable 

 school structure, such as that postulated for short- 

 finned pilot whales off the Pacific coast of Japan, which 

 may be indicative of late maturing, long-lived animals 

 (Kasuya and Marsh 1984). This may equally apply to 

 Tursiops where a lengthy mother-calf bond may be im- 

 portant in the calf's development and be a reflection 



of the smaller school size and inshore habitat, mastery 

 of which may require greater maternal care and a 

 longer learning period (Cockcroft and Ross 1990b). 



During a study of captive bottlenose dolphin mother 

 and calf association, Cockcroft and Ross (1990b) have 

 shown that the calf's suckling rate decreased with age, 

 although its energy requirements probably grow with 

 its level of independence and activity. As there was no 

 evidence of energy changes in delphinid milk during 

 lactation (Arvy 1974) to compensate for this, the 

 authors proposed that the quantity of milk ingested 

 may increase as the calf's stomach volume increased 

 (Cockcroft and Ross 1990b). This explanation is sup- 

 ported by the present findings that a female's mam- 

 mary glands increase in size, probably increasing the 

 volume of milk produced, during lactation. 



In view of the extended lactation period of female 

 bottlenose dolphins and the early and probably increas- 

 ing intake of solid food by the calf, it is unlikely that 

 females require a substantial interval between the end 

 of lactation and the next pregnancy. A 1-year resting 

 period, estimated from the catch statistics data, is 

 almost certainly an overestimate due to catch bias. 

 Kasuya (1985) estimated a 3-month resting period for 

 Tursiops in the western north Pacific, and it is probable 

 that Indian Ocean Tursiops are similar. Considering 

 that gestation lasts about 1 year and that lactation 

 probably lasts 18 months to 2 years, a calving interval 

 of around 3 years can be estimated for Indian Ocean 

 bottlenose dolphins. This estimate is in good agreement 

 with the projected ovulation rate of one every 3 years, 

 but assumes that all calves survive and ignores the 

 effects of differential calf mortality (Perrin and Reilly 

 1984) that would lower the mean calving interval 

 considerably. 



Nothing is known of the age and sex structure of the 

 Natal bottlenose dolphin population. The only available 

 information is from the catch of these animals in the 

 Natal shark nets, the sex, size, and age structure bias 

 of which have been discussed (Cockcroft and Ross In 

 press). Given these biases, attempts to calculate repro- 

 ductive parameters from these data are flawed but pro- 

 vide the only means of calculating the reproductive 

 potential of this population. 



The relevant proportions in the net catch of females- 

 mature, lactating, pregnant, and resting— are 56%, 

 43%, 27%, 5.2%, and 5.8%, respectively (Cockcroft and 

 Ross In press). Annual pregnancy rates (APR; Perrin 

 and Reilly 1984) calculated from these catch data and 

 lactation period (either 1 or 2 years) range between 

 5.2% and 27%. Changes in either the proportion of 

 females lactating or the length of lactation greatly in- 

 fluence this calculation, but even the highest estimate 

 is low in comparison with values calculated for Tursiops 

 in other areas; 63% in the Black Sea (Danilevsky and 



