430 DISCOVERY REPORTS 



in 1939/40 and 1940/41, nor in the early post-war years, but in 1953/54 °f f° ur lactating females ex- 

 amined two were found to be simultaneously pregnant, and in subsequent seasons a further three 

 were recorded. 



It is often said that females cannot be pregnant and suckling a calf at the same time, and the presence 

 of milk has been explained as a premature development of the gland. The Scientific Subcommittee of 

 the International Whaling Commission recently found it necessary to state that ' pregnant whales can 

 be simultaneously lactating and accompanied by a calf (International Commission on Whaling, 

 Seventh Report of the Commission, 1956. Appendix IV, Report of the Scientific Subcommittee, p. 22). 

 Chittleborough (1958, p. 15) also suggests that lactation may be overlooked in pregnant females. It 

 seems probable that lactation was overlooked in pregnant females in the pelagic samples up to 1952. 

 At South Georgia in 1925-28, however, there was probably a real absence of lactating pregnant 

 females (see p. 459). 



Table 19. Proportion of lactating females which are simultaneously pregnant or recently ovulated 



Lactating and pregnant Lactating and ovulation 



* Percentage calculated from columns 2 and 5 only, because lactating and pregnant females do not ovulate. 



If we accept this explanation then the figure of 11 -6% for the proportion of lactating female fin 

 whales which are pregnant is too low. A more realistic figure is obtained if we eliminate the data 

 which are thought to be unrepresentative, and use only those from 1928-31 and 1953-58. In this 

 reduced sample there are 78 lactating females, of which 14, or 17-9%, are simultaneously pregnant. 

 The standard error of this percentage is 4-34, which means that the true percentage of lactating females 

 which are simultaneously pregnant probably lies between 9-2 and 26-6%. It is unfortunate that the 

 small size of the sample does not permit a closer estimate than this. It must also be pointed out that 

 the proportion of lactating females which become pregnant may vary from year to year and may 

 increase in response to exploitation. A discussion of the incidence of corpora lutea of ovulation in 

 lactating females will be postponed until the next section (on the post-lactation ovulation) for reasons 

 which will then be apparent. 



For the 15 lactating females which were pregnant the foetal lengths are known. They range from 



0-31 to 4-4 m.; two-thirds of them are between 1 and 2 m. in length, and the mean foetal length is 



1-71 m., corresponding to a foetal age of 7 months. Mackintosh and Wheeler (1929), on the basis of 



an examination of the sizes of calves and growth of the baleen, concluded that lactation lasts for 



6 or 7 months; it is also shown below on other grounds that the calf is suckled for about 7 months. 



The foetuses of females which are lactating must, therefore, have been conceived at a post-partum 



ovulation. 



Evidence from the sizes of corpora albicantia 



Almost a fifth of lactating female fin whales appear to be pregnant as a result of a post-partum 

 ovulation. The question now arises whether only about one-fifth of mature females experience a post- 

 partum heat, or whether the majority of mature females experience a post-partum heat, but only 

 about one in five become pregnant, possibly owing to low fertility at this stage of the cycle. 



It will be remembered that there is a group of recently mature females in the first lactation or post- 



