2go DISCOVERY REPORTS 



estimates. The assumption (from rather sparse data) that L/q is 90% of Wt^ appears to be justified 

 and the length of the estimated period Lt^ varies from 34 days in Physeter to 50 days in Phocaena. 



Huggett and Widdas (1951, p. 413) remark that 'among mammals in the intermediate range of 

 [their] Fig. 8, the period of linear growth is determined by the size of the foetus at birth. Thus, as 

 the birth weight of the young is increased the mammal does not grow its young quicker, along a 

 steeper slope, but must grow its foetus for a longer time.' This is now seen not to be entirely true of 

 the toothed whales in which the slope of linear growth may be steeper and also continued for a longer 

 time, as for instance in the sperm whale compared with the porpoise. This answers the question put 

 by Huggett and Widdas (195 1 ) (see p. 287), and shows that Rubner's finding (1908) that, in all species 

 except man, the birth weight is proportional to the gestation time, is not true of the toothed whales. 

 As will be apparent when the position in baleen whales has been established, it is even less applicable 

 to whales in general. 



BALEEN WHALES {MYSTICETI) 

 The only group of whalebone whales in which foetal growth has been studied in any detail are the 

 Balaenopterids. The principal papers are listed in an earlier section of this paper. 



In all species of Balaenopterids and in the grey whale, Eschrichtius gibbosus Erxleben (Hubbs, 1958), 

 the gestation period has been fixed at a year or less. Records of mean monthly foetal lengths, evidence 

 of the pairing season from examination of male and female reproductive tracts, of the occurrence of 

 calves, and of the proportion of mature females which are pregnant, all point to this conclusion. 

 Indeed, it is evident that in some species females commonly undergo two pregnancies in 2 years 

 (Jonsgard, 1951 ; Omura and Sakiura, 1956) and in others it is not uncommon (Chittleborough, 1958 ; 

 Laws, 1958; Hubbs, 1958). 



The neonatal lengths are known fairly accurately for most species and adequate foetal length records 

 cover several months of the gestation period in these species (Text-fig. 12). When the mean monthly 

 foetal lengths and the neonatal lengths are plotted, it is immediately apparent that if the gestation 

 period for these species is a year or less, foetal growth in length cannot be described by a straight line 

 as in the Odontocetes discussed above. Instead it appears that the slope of the growth curve,' gradually 

 increases throughout pregnancy' as Mackintosh and Wheeler (1929) showed for blue and fin whales. 

 It should be pointed out that even in these two species, which are the fastest growing whales, more 

 than two straight lines are necessary in order to fit the points and at the same time to give a gestation 

 period of a year or less. 



A number of trial plots were made at first and these suggested that the first half of foetal growth was 

 linear (as in the toothed whales), while the data for the second part agreed quite well with an exponen- 

 tial growth rate. 



The humpback whale is the species in which the duration of pregnancy has been fixed with most 

 precision (Chittleborough, 1954, 1958). In Text-fig. 6 a curve of foetal growth in length has been 

 constructed for this species. The method has been to take the mean dates of conception and calving 

 as early August, so that the gestation period is 12 months, and the neonatal length given by Chittle- 

 borough (1954, 1958) has been plotted accordingly. The mean monthly foetal lengths given by 

 Matthews (1937) have been used and also the mean foetal length for a sample taken in the first week 

 in February, given by Symons and Weston (1958). The monthly means from March onwards are 

 based on very small samples and have not been plotted. A linear plot has been fitted to the foetal 

 length values for September to January and has been continued as an exponential curve up to the 

 neonatal point. The linear part of the curve intersects the abscissa at the end of the first week in 

 September, giving a value for Lt^^ of approximately 38 days. The slope of the linear segment of the 



