MULTIPAROUS FEMALES 445 



criteria showed that a substantial proportion were either not lactating or were in a doubtful condition. 

 Chittleborough (1958) has also distinguished on histological grounds two groups of females, each 

 with apparently normal milk in the glands. When the cells lining the alveoli were actively secreting 

 the female was classified as still suckling a calf, but if the lacteal ducts contained milk and the alveolar 

 cells were no longer secreting, then it was considered that ' the female had either recently lost, was 

 weaning, or had just finished weaning the calf, and that the apparently normal milk present in the 

 ducts was a residuum which would shortly have disappeared' (p. 7). 



Of the 69 females examined by van Lennep and van Utrecht (1953) four humpback whales and 

 a sperm whale should be excluded from the present discussion because in these species the calf is 

 usually weaned in low latitudes. Of the remaining 64 fin and blue whales, 41 were held to be lactating, 

 six definitely not lactating, and 17 could not be definitely assigned to either of the first two groups. 

 In some of the material in the third group fixation was bad; some of these whales were thought to 

 be weaning the calf; and at least two were thought to be pathological. We are not likely to be far 

 wrong if we assume that about half of the females in the doubtful group were weaning the calf. Then 

 for 41 females known to be lactating there are 14 which are thought to have just ceased to secrete 

 milk, that is about 25 %. Even if the doubtful group is excluded, there is a minimum of 6 out of 47 

 (or 12-8%) which were no longer secreting milk. On the west Australian coast Chittleborough (1958) 

 found 23 % (5 out of 22) of female humpback whales with milk in the glands to be in this condition. 

 Lactating females appear in the catches off the west Australian coast shortly before weaning, and the 

 period after secretion has ceased when normal milk is still present in the glands, is almost certainly 

 very short. It is interesting to note that the lactating female fin whales taken in antarctic waters have 

 probably been on the whaling grounds for less than a month, possibly only for 2 weeks. This con- 

 clusion receives support from the results of the analysis of the incidence of diatom infection (Table 21). 

 In some 71 % of lactating females diatoms are absent or incipient, showing that the majority of lactating 

 females in the sample have entered the Antarctic recently, probably less than a month before their 



capture (Hart, 1935). 



Further support comes from an analysis of the foetal lengths of those lactating females which are 

 also pregnant. The work of Mackintosh and Wheeler (1929) strongly suggested that the lactation 

 period was about 6-7 months, and this estimate cannot be very far out. In fact it is in very close 

 agreement with the conclusions reached below. The average foetal length will give us the average 

 foetal age for this class of females, and will enable us to see how near this group of females is to 

 weaning the calf. It is reasonable to assume that the lactation period is no longer in pregnant lactating 

 females than in non-pregnant lactating females, and that in other respects the behaviour of these two 



groups is similar. 



There are 15 fin whale females in the material which are concurrently pregnant and lactating. The 

 foetal lengths range from 3-1 to 4-4 m. with a mean length of 1-69 m., and a median length of 1-58 m., 

 corresponding to foetal ages of 7-3 and 7-1 months. Of these 15 foetuses, 12 are between 1 m. and 

 3 m. in length; the largest at 4-4 m. is equivalent to a foetal age of 10 months, and the two smallest 

 measuring 31 cm. and 76 cm. represent ages of 2-3 months and 4 months respectively. If these three 

 foetuses are excluded then the mean length, i-66 m., corresponds to a foetal age of 6-95 months, which 

 is probably a better estimate. 



The foetuses of lactating females are conceived at a post-partum ovulation, which on average 

 follows closely on the mean calving date at the end of May (p. 403). The lactating females are very 

 near weaning time and allowing about 7 months for the length of the lactation period (to account for 

 the mean foetal age) suggests weaning at the end of December. Mackintosh and Wheeler (1929, 

 pp. 431-7) estimated that the average length of the calf at weaning is probably about 12 m. in the 



