284 DISCOVERY REPORTS 



PREVIOUS WORK 



Guldberg (1886), from an examination of the lengths of foetuses of baleen whales in the North Atlantic, 

 was able to show that there is a definite pairing season. Subsequently, more data were collected and 

 reported on by Cocks (a series of papers 1886-90 referred to by Mackintosh and Wheeler, 1929), 

 Grieg (1889), Guldberg (1894), True (1904), CoUett (191 1), Hamilton (1915), Barrett-Hamilton 

 (Hinton, 1925), and others. An important paper by Risting (1928) was the first attempt at statistical 

 treatment of a large body of data ; he gives average growth curves and calculates the standard deviations 

 of the monthly samples. 



Hinton (1925) believed that in the southern fin whale, after an initial 61-day period of slow develop- 

 ment, growth was probably linear from 30 cm. to birth at 610 cm. and that this regular growth occu- 

 pied a period of 9 months. By a simple calculation he obtained the dates of pairing from individual 

 foetuses, and made similar calculations for other species. Risting (1928), Zenkovic (1935) and 

 Zemskiy (19500) also adopted this method. The main points of interest are, first, that initial growth 

 was assumed to be very slow, and secondly, that subsequent growth was assumed to proceed at a 

 uniform rate. 



Mackintosh and Wheeler (1929) reviewed the position at that time and came to some important 

 conclusions about the breeding season and foetal growth in blue and fin whales. They first considered 

 the more direct evidence concerning the time of the breeding season. This included information 

 about the seasonal change in the condition of the reproductive organs of both sexes and suggested 

 that the breeding season begins in the earlier part of the southern winter, i.e. in April and May. 

 Examination of foetal length records confirmed this and they concluded that the frequency of pairings 

 is at a maximum in June-July, that gestation lasts for a little less than a year and that 2 years is 

 probably the usual interval between successive pregnancies. 



In view of the fact that most later workers on foetal growth of whales have adopted the methods 

 used by Mackintosh and Wheeler (1929), a further brief explanation is called for here. Their own 

 material consisted of eighty-one fin whale and fifty blue whale foetuses, which they plotted graphically 

 according to lengths and the dates on which they were found. The average monthly lengths were also 

 plotted and a ' mean curve of growth ' was drawn in by eye ' to represent as well as possible the general 

 trend in the mass of plotted points, regard also being paid to the monthly average lengths ' (p. 422). 

 This curve shows a rate of growth which is slow initially and gradually increases throughout pregnancy. 

 The main weakness in this method lies in the uncertainty of the rate of grovvlh in the earliest stages, 

 but this was partly overcome by these authors by fixing the time of the pairing season, as described 

 above, from other sources. They concluded that the gestation periods of blue and fin whales are about 

 105 and ii| months. The former is in fairly close agreement with the conclusions reached in the 

 present paper and the latter estimate is almost identical with them. These methods were applied by 

 Matthews to the humpback whale (1937), the sperm whale, Physeter catodon Linn. (19380) and the 

 sei whale, Balaenoptera borealis Lesson (1938&). His conclusions about the length of gestation and 

 the pairing season in the humpback whale are very close to those of Chittleborough (1958) based on 

 more direct evidence. Matthews's figures (figs. 53, 54) in fact show a gestation period of almost 

 12 months, though he states in the text that gestation takes about 11 months (p. 58). For the sperm 

 whale Matthews shows that growth is linear, which suggests a fundamental diff'erence between this 

 species and the baleen whales. Later authors appear to have overlooked, or ignored, this (Mizue and 

 Jimbo, 1950; Clarke, 1956), and have drawn in a freehand curve. 



Paulsen (1939) reviewed Risting's (1928) work and collected information about the occurrence of 

 twinning and multiple foetuses. The next important paper is that by Brinkmann (1948) which 



