Cockcroft and Ross: Tursiops truncatus off southern Africa 



295 



Figure 9 



Relationship between the length of a bottlenose dolphin calf and an 

 index of the volume of the most recent or largest corpus in the 

 mother. The fit of a linear regression (calf length in cm = 209.61 

 - 0.032 X corpora volume index) to these data is shown. 



Figure 10 



Relationship between the mass of a bottlenose dolphin calf and an 

 index of the volume of the mother's mammary glands. 



Two other females, 12 and 13 years old, which had 

 each undergone two ovulations, were iactating and the 

 index volumes of their largest corj)ora albicans were 

 900 mm'' and 2100 mm-\ respectively. Extrapolation 

 from Figure 9 suggests that the calves of these females 

 were approximately 181 cm or 18 months old, and 143 

 cm or 6 months old, respectively. These data imply that 

 at conception of the calves, these two females were 

 about 10.5 and 9.5 years old, respectively. The above 

 data, and the presence of 10-year-old females that had 

 not ovulated, imply that first ovulation occurs between 

 9.5 and 11 years of age. One 17-year-old female had 

 undergone two ovulations, was Iactating, and had a 

 170-cm calf of approximately 1-year-old. This suggests 

 that this female was 15-years-old at the time of con- 

 ception of this calf, although she may have undergone 

 a previous pregnancy. 



A regression fitted to the number of ovarian scars 

 on age of sexually mature females has a slope of 0.29 

 (r = 0.6, A'^ = 32), implying that mature females, in 

 general, ovulate at least every third year (Fig. 8). A 

 similar regression of log of age on log of number of cor- 

 pora albicantia. was linear (log age = -0.61 + 1.03 

 • log # scars, r = 0.62), indicating no decrease in ovula- 

 tion rate with age. 



Length of lactation calculated from the catch statis- 

 tics of animals on the Natal coast (Cockcroft and Ross 

 In press) where 27% and 5.2% of females were Iac- 

 tating and pregnant, respectively, gave an estimate of 

 5.2 years (proportion lactating/proportion pregnant; 



Perrin and Reilly 1984). As this is obviously exagger- 

 ated by the overabundance of Iactating females and 

 dearth of pregnant females, alternative means of 

 estimating lactation period are required. The relation- 

 ship between calf mass and an index of mammary gland 

 volume (length x height x depth) in 13 mother and 

 calf pairs suggests that the mammary glands increase 

 in size during lactation, until the calf's mass is between 

 60 and 70 kg, after which the mammary volume 

 decreases (Fig. 10). Extrapolation from the growth 

 curves suggests that calves of this mass are about 

 18-months-old. An examination of the stomach contents 

 of captured calves and juveniles showed that solids first 

 appear in stomachs at about 6 months of age, although 

 milk remains were still evident in calves of up to 3 years 

 of age. 



Of the 20 known mother and calf pairs, ten calves 

 were 1 year old or less, five were 1-2 years old and 

 a further five were greater than 2 years old; the mother 

 of one of the latter was pregnant with a fetus of only 

 38.5 g. Although only solids were found in the stomach 

 of her 69-kg calf, she was still Iactating, the only one 

 of six pregnant females simultaneously Iactating. These 

 facts imply a mother and calf association of up to 3 

 years before a subsequent pregnancy. 



Female resting period calculated from gestation 

 period (1 year), proportion of females resting (5.3%) 

 and the proportion of females pregnant (Perrin and 

 Reilly 1984) yielded an estimate of about 1 year. 



