SUMMARY 30s 



and calving take place in winter in low latitudes and the annual feeding migrations occur in mid- 

 pregnancy and at the end of lactation or beginning of the anoestrus period. 



If the initial linear phase of growth were to persist it can be shown by extrapolation that the gestation 

 periods would have to be approximately doubled in order to attain the present neonatal sizes. One 

 effect of this would be to reduce the reproductive rate very greatly. Conversely, if gestation were 

 limited to 12 months and the present linear growth persisted throughout, the birth sizes would be 

 only half the present sizes. If the gestation periods, or the neonatal sizes, were intermediate between 

 these extremes, then it seems probable that the reproductive cycle would be out of step with the 

 feeding cycle. For instance, with a gestation period of 18 months, other things being equal, parturition 

 would occur in the Antarctic and the chances of survival of the calf might well be affected. 



With regard to feeding habits, the difference in the nature of the food is also perhaps significant 

 and it is interesting that in the blue, fin, sei and humpback whales the beginning of the great burst of 

 growth is associated with the arrival of these species on the polar feeding grounds. Little or nothing 

 is known of foetal growth in a temperate zone species like B. brydei, which is presumably subject to 

 rather different feeding conditions. 



The evidence for a period of exponential growth is by no means conclusive and it may be that gro\vth 

 could be described by a series of successive growth cycles. It should be noted that if exponential 

 growth is adopted as a means of increasing neonatal size, the duration of the growth period must be 

 kept relatively short, because a small increase in the length of the growth period will entail a dis- 

 proportionately great increase in the neonatal size, which is of course limited by the size of the mother. 

 Similarly, an increase in the slope of the curve of exponential growth may cause a very large increase 

 in the final size. In the blue whale, for instance, it is estimated that an increase of half a month in 

 the length of the gestation period to bring it into line with that of the fin whale would, if growth is 

 exponential, result in a neonatal length of 8-4 m., corresponding to a weight of about 4000 kg. or 

 3-93 tons; that is an increase of 60% in the birth weight for a gestation period only half a month 

 longer. In the larger baleen whales a shortening of the gestation period is seen to be essential if the 

 slope of the growth curve is further increased and on the evidence at present available this appears 

 to be the general trend in the large Balaenopterids. 



SUMMARY 



1 . The literature on foetal growth in whales is reviewed. 



2. It is shown that foetal growth in three species of toothed whales can be described by a linear 

 plot of length. The method of Huggett and Widdas (1951) is used to estimate the duration of the non- 

 linear early part of pregnancy and so to estimate mean conception dates and gestation periods. 



3. In the toothed whales studied larger neonatal sizes are attained by increased rates of growth 

 and by extension of the gestation period. 



4. In the baleen whales the first half of pregnancy follows this law, but thereafter growth can best 

 be described by a logarithmic plot of length or of the cube root of weight. So far as is known this is 

 unique among mammals. 



5. Original foetal length data for 956 southern fin whales and 1022 blue whales are presented, 

 examined and discussed. Mean foetal growth curves (length) are given for southern hemisphere fin 

 and blue whales. 



6. The average duration of gestation is estimated to be 11 J months (early June to mid-May) in 

 the southern fin whale; lof months (end of May to mid- April) in the southern blue whale. 



7. The length /weight relationship of foetal baleen whales is examined, the data comprising 

 243 records of foetal length and weight. The mean lengths at birth for southern hemisphere blue. 



