300 



DISCOVERY REPORTS 



The data for the blue whale have been examined in the same way as those for the fin whale, but it 

 is not possible to reach such firm conclusions. Although there are more records of blue whale foetal 

 lengths than there are for fin whales, they are spread over a shorter period (Tables 3, 5, Text-fig. 8). 



It will be seen from Text-fig. 8 that a straight line could be fitted to the monthly mean values of 

 foetal length for the blue whale which would intersect the abscissa in September, would attain 7 m. 

 in June, and would be very much steeper than any other linear plot of foetal growth in mammalia. 



Analysis of the monthly foetal length frequencies by the method used for the fin whale gives a 

 similar picture of foetal growth to that in the fin whale, showing that growth is not linear in the second 

 half of pregnancy. The length frequency curve for November is slightly skewed (corresponding to 

 the December frequency distribution in the fin whale) ; the curves for December and January show 



1/1 



UJ 



a 



U 



2 



I 

 (- 

 o 



z 



UJ 



p 



01 2 3 4 5 6 7 8 9 10 II (2 13 14 15 



' — 'o — *«- *g-'o (f^onths) ► 



Text-fig. 12. Diagram showing estimated mean foetal growth curves of five Balaenopterids and three Odontocetes. 

 For comparative purposes the origin of the curves is taken as i„. {Bm = Balaenoptera muscultis ; Bp = 5. physalus; 

 Bb = B. borealis; Ba = B. acutorostrata; Mn = Megaptera novaeangliae; Fc = Physeter catodon; D\ = Delphinapterus leucas; 

 Pp = Phocaena phocoena). 



increased skewness, and the distributions for February and March show a reversal of this trend. 

 The same inferences are drawn from the shapes of these monthly length-frequency distributions as 

 for the fin whale frequency distributions, and an exponential curve has been fitted to the mean foetal 

 lengths for November, December, January and February. This curve attains the average neonatal 

 length (7.0 m.) in mid-April. 



The slope of the linear segment of the blue whale growth curve is based on very sparse data. It has 

 been plotted with reference to six records for October, two for September, two for August and two 

 for July. It cuts the abscissa at the end of June and the period from conception to Lt^ is estimated 

 to be 30 days, slightly shorter than the similar period in the fin whale (33 days) because the slope of 

 the linear segment of the growth curve is steeper (Table i). This gives an estimated mean date of 

 conception in the fourth week in May, which fits quite well with inferences made from other evidence. 

 The estimated gestation period of the blue whale is therefore lof months and although less confidence 



