304 DISCOVERY REPORTS 



CONCLUSIONS 

 Previous papers dealing with foetal growth of whales and with the breeding season and the gestation 

 period have attacked the problem by plotting the mean monthly foetal lengths and then drawing in 

 a curve by eye. The method developed by Huggett and Widdas (i 951) for dealing with foetal length 

 and weight data appears to hold good for the three species of toothed whales considered in this paper 

 and to a limited extent for the five Balaenopterid species which have been studied. In particular this 

 method provides for an objective estimate of the length of the initial period of very slow growth 

 before the placental circulation is fully established. This is impossible by means of freehand 

 extrapolation. 



Huggett and Widdas (1951, p. 314) remark that in '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 will offer an explanation of Rubner's finding in 1908 

 that in all species except man the birth weight is proportional to the gestation time.' 



It will be apparent that this is not true of the marine mammals which have been studied. In most 

 species of seals the period of foetal growth is of more or less equal duration, but the birth weight of 

 the largest species is ten times that of the smallest species. Increased size at birth is attained by 

 increasing the growth rate, rather than by extending the gestation period. In the toothed whales which 

 have been studied, increased size at birth appears to be attained both by increasing the rate of growth 

 and by extending the period of gestation, as in the sperm whale. 



The growth velocities of the species of toothed whales studied fall within the ranges a ^0-05 and 

 a ^0-02 (for weight) as do all the species of mammals considered by Huggett and Widdas (1951, 

 fig. 8), with the conspicuous exception of the fin and blue whales. In the Balaenopterids, the evidence 

 discussed in this paper shows that for the first part of gestation the growth velocity is within this range, 

 but the growth rate (for L or W-') probably increases exponentially after the fifth month of pregnancy. 

 In all Balaenopterid species the gestation period is 12 months or less and the very large neonatal sizes 

 are attained by this phenomenal burst of growth in the second half of pregnancy, not as in the sperm 

 whale by extending the gestation period. In fact, it appears that in the genus Balaeiioptera there is 

 a tendency to shorten the gestation period in association with this increased growth. 



Huggett and Widdas (1951) were unable from their data to suggest any biochemical or physiological 

 reason why growth should conform to a cube root law, and it is even more difficult to see why baleen 

 whale foetuses should follow an exponential cube root law. 



For some fundamental reason the gestation period of the rorquals does not exceed 12 months 

 whereas in the sperm whale it is possible for gestation to occupy 16 months. One of the most con- 

 spicuous differences in the behaviour of baleen whales as compared with the toothed whales is the 

 very discontinuous feeding cycle of the former. Almost all baleen whales must enter polar waters to 

 feed at a time when their planktonic food is available in suitable quantity; when not feeding it is 

 probable that they must seek warmer waters where energy loss due to heat radiation is less. The period 

 when food is available in amounts large enough to make its collection economical for the whales is 

 limited in the Southern Ocean to a period of 3 or 4 months (Marr, 1956) although some animals 

 move south before this period and others stay in polar waters later (see Mackintosh and Brown, 1956, 

 fig. 2). During the feeding period the baleen whales lay down thick reserves of blubber (Ash, 1956) 

 on which they draw for the remainder of the year. 



Most Balaenopterid species have a 2-year sexual cycle, comprising a gestation period of less than a 

 year, lactation occupying about 6 months and the remaining 6 or 7 months in anoestrus. Conception 



