NEWLY MATURE FEMALES 423 



In most terrestrial mammals the migratory movements, if any, are restricted by geographical or 

 physical barriers. The scale of migration is greatest in those animals, such as fish, birds, bats, seals 

 and whales, which live in a continuous medium and are less restricted in their movements by such 

 barriers. Whales appear to undertake longer migrations than other aquatic mammals. Some seals 

 migrate over a latitude range of about 25 : the migratory range of the northern fur seal (Callorhinns 

 ursinus) is from about 35 N. to about 6o° N. (Kenyon and Wilke, 1953), and the harp seal (Phoca 

 groenlandica) travels between 45 N. and about 70 N. (Nansen, 1925), although some individuals 

 may cover a greater distance. On the other hand, the humpback whale regularly migrates between 

 the breeding area in about 15 S. latitude and the antarctic feeding zone at about 66° S. (Dawbin, 

 1956, p. 193). The grey whale (Eschrichtius glaucas) migrates between a feeding area in the North 

 Pacific at about 6o° N. and a very restricted breeding area centred on lower California in latitude 

 25 N. (Gilmore, 1955). The fin whale undertakes similar annual migrations, though the evidence is 



Text-fig. 42. Above, monthly changes in day length (sunrise to sunset) at latitudes 20 S. and 6o° S. Below, estimated 

 monthly changes in day length experienced by whales migrating between 20° S. and 6o° S. 



not so complete (see above, p. 339), and in its breeding range the fin whale is probably more widely 

 dispersed than the humpback. One result of these migrations is that the fin whale, and other whales, 

 are subjected each year to two periods of increasing daylight hours and two periods when day lengths 

 are decreasing. Animals which do not migrate experience only one period of increase, and one of 

 decrease, in day lengths. 



Let us assume for present purposes that the fin whale migrates between a breeding area, centred on 

 about 20 S. latitude, and a feeding area which is for the most part south of 6o° S. Let us also assume 

 that the migration between these latitudes takes about a month. The change in the length of day which 

 would be experienced by animals migrating north or south in each month is shown in Text-fig. 42. 

 From this it is evident that animals migrating north in April, May, June, July, August and September 

 should experience increasing lengths of day with the maximum rate of increase influencing animals 

 migrating between the beginning and end of July and amounting to some 5 hr. Animals migrating 

 southwards in September, October, November, December, January and February should also 

 experience increasing daylight hours with a maximum rate of increase in November and December. 

 It can be shown that two cycles of increasing photo-periods would also be experienced if we assume 

 alternative latitudinal movements, such as between 15 S. and 65 S., 30 S. and 6o° S., 40 S. and 



DL 

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