THE REPRODUCTIVE CYCLE 4°5 



for 192=5-30 are averaged and there is a gap between 1940 and 1945 corresponding to the reduced 

 whaling activity during the war. Only 4 months (December-March) are considered, because the 

 samples from other months are small, and no mean value is given for December 1932 because there 

 were only eight individuals in this sample. From 1934 onwards the samples are large (usually from 

 a hundred to over a thousand). 



It will be seen that in the three decades covered by the material there has been no significant 

 progressive change in the mean monthly foetal lengths for December and January. The values for 

 March and perhaps February suggest that the rate of foetal growth may have increased slightly. There 

 have, however, been variations from year to year, although the pattern of the curves for the different 

 months is not always the same, indicating that some of these variations are not significant. There are 

 seasons when monthly mean foetal lengths deviate significantly from the monthly mean for the whole 



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z 



UJ 



2 



O 



z 

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4- 



3 - 



2 - 



I - 



OA — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — i — 1 

 1925 1930 1935 1940 1945 



YEARS 



Text-fig. 32. Variation in the mean monthly foetal lengths over the period 1925-58 (from International 



Whaling Statistics, Sandefjord). 



period. In January 1935 and February 1953 for instance, where there is a large deviation from the 

 overall mean for the period, the calculation of the mean ±2 S.E. shows that these particular values 

 deviate significantly by more than 0-25 m. and about 0-5 m. respectively, from the means for the full 

 period. 



Deviations of this order could represent displacements of the mean date of conception of about 

 i-2| weeks (from Text-fig. 30), but a number of complicating factors need to be considered in 

 assessing this temporal deviation. Have there, for instance, been any changes during this 30-year 

 period in the rates of foetal growth ? Have changes in the age structure of the populations of mature 

 females been sufficient to produce an apparent change in the pairing season? It is not possible to 

 provide conclusive answers to these questions, but such changes would probably be progressive and 

 would not account for variations from year to year. It may be that a slight increase in the rate of foetal 

 growth, and a shift towards lower ages in the age structure of the mature females, could produce an 

 apparent advancement or retardation of the pairing season. 



On the evidence available it seems probable that there has been no progressive change in the pairing 



