THE FOETAL GROWTH RATES OF WHALES 



WITH SPECIAL REFERENCE TO THE FIN 

 WHALE, BALAENOPTERA PHYSALUS LINN, 



By R. M. Laws 



(National Institute of Oceanography) 

 (Plate XIII, text-figures 1-13) 



INTRODUCTION 



IN the course of an investigation of the reproductive cycle of the southern hemisphere fin whale, 

 Balaenoptera physalus Linn., it was necessary to obtain an accurate mean curve of foetal growth 

 in length. It is usual to describe foetal growth in terms of weight, but there are relatively few records 

 of foetal weights of whales owing to the practical difliculties. For present purposes length is therefore 

 much more suitable than weight and as there is a constant relation between length and weight (Text- 

 fig- 13). the curve of growth in length may later be converted to a weight-growth curve by reference 

 to the relatively small number of foetal weight records. 



This is an interesting problem quite apart from any indirect use which may be made of the results. 

 The main stimulus for this study was the need to obtain accurate information about the mating 

 season, length of pregnancy and the calving season in the fin whale. It is not possible in this species 

 to study any of these problems directly and in fact the humpback whale, Megaptera novaeangliae 

 Borowski, is the only species in which the length of the gestation period and progress of the pairing 

 and calving seasons have been fixed accurately (Chittleborough, 1954, 1958) because this is the only 

 species in which the breeding population has been studied directly in the breeding season. It can 

 therefore be used as a comparative check on the conclusions about other species. 



In the fin whale and most other species an indirect approach is necessary because it has not been 

 possible to study the animals in the breeding area. 



The most profitable approach to this problem in the fin whale was thought to be a comparison with 

 other species of Mysticetes and with Odontocetes, together with a detailed examination of the data 

 accumulated by 'Discovery' Investigations since 1925 and in recent years by the National Institute 

 of Oceanography. These comprise records of foetal length (snout— notch of flukes) of 956 fin whales 

 and 1 1 12 blue whales {Balaenoptera muscidus Linn.), together with some records of other species. 

 In addition there are 115 records of foetal weight. These lengths and weights were all measured by 

 biologists or specially trained observers; length records accumulated since 1954 have not been in- 

 cluded, but the weight data include records up to the 1957/58 whaling season. 



The'records of foetal lengths of a number of species given in the International Whaling Statistics 

 from 1932 onwards have not been used. It is well known that these measurements are not accurate, 

 though this in itself will probably not affect an average growth curve. More serious is the fact that 

 small foetuses are almost invariably missed, and as pointed out by Brinkmann (1948), this has the 

 effect of altering the slope of the growth curve. 



I am indebted to Dr N. A. Mackintosh, C.B.E. and Mr S. G. Brown for their helpful suggestions 

 and to the biologists, too numerous to name here, who collected the original data for the 'Discovery' 

 Committee and the National Institute of Oceanography. Mr A. Style drew most of the figures. 



