60 THE PHYSIOLOGY OF REPRODUCTION 



Farther north, however, at Jan Mayen, they are not born until 

 about March 23rd or 24th. Turner's notes ^ on the breeding 

 habits of seals also point to the conclusion that the sexual 

 season with these animals is restricted to regular periods of com- 

 paratively short duration, so that it may probably be assumed 

 that seals are moncestrous. The males of seals, as already 

 remarked, experience rut at the time of the female sexual 

 season. Whether the male generative organs are functional {e.g. 

 whether the testes produce sperms) at other times does not 

 appear to be known. It is of interest to note that in many 

 species the rut is experienced during a period of complete fasting. 

 Thus it is stated that the male fur seal, after coming to land, 

 may Uve for over a hundred days without taking food, and that 

 during this period he is constantly engaged in struggles with 

 other males, finally leaving the shore in a state of extreme 

 emaciation. 



The walrus affords an example of a Mammal which bears 

 young only once in three years. Parturition takes place about 

 May or June, and the sexual season recurs about two years 

 subsequently. Thus the nursing or lactation period extends 

 for nearly two years, while gestation lasts about one year.^ 



Insectivora 



The majority of the animals in this order are almost 

 certainly polycestrous, but comparatively little is known con- 

 cerning their breeding habits. The shrew in this country may 

 be found breeding in any month from April until November, so 

 that it is practically certain that this animal is polycestrous, and 

 may have two Utters, if not three htters in a year. It is ex- 

 tremely probable also that the water-shrew breeds twice a year. 

 In the hedgehog in this country, Ktters are born at the end of 

 May or June, and in August or September.^ In Germany it is 

 said that the breeding season extends from March until July.* 



1 Turner, "On the Placentatiou of Seals," Trans. Soy. Soc. Edin., vol. 

 xxvii., 1875. 



^ Millais, loc. cit. 



^ Millais, The Mammals of Great Britain and Ireland, vol. i., London, 

 1904. 



* Van Herwerden, "Beitrag zur Kenntniss des menstruellen Cyklus," 

 Monatsschr. f. Geburtshulfe und Oyndk., vol. xxiv., 1906. 



