THE ENVIRONMENT AND REPRODUCTION 3I 



it is discussed in the following section. The question of 

 the tropics is considered on p. 34. 



6. AUTUMN-BREEDING ANIMALS 



The first experimental study of an autumn-breeding 

 species was that of Hoover and Hubbard^ who in 

 1937 published their observations on the brook trout of 

 New Hampshire, U.S.A. They subjected numbers of 

 these fish to an artificial summer length of day early in 

 the year, and then, as summer approached, the daily 

 light periods were progressively reduced. The result was 

 that the gonads matured and the fish spawned in an 

 artificial autumn considerably in advance of their normal 

 time. 



No similar experiments have yet been performed with 

 amphibians, reptiles, or birds, but as already pointed 

 out very few of these animals are autumn breeders. 

 However, it seems possible that such birds as the emperor 

 penguin of the Antarctic and the kea of New Zealand, ^^ 

 which both breed in the southern autumn, may some day 

 be found to be influenced in this way. 



With mammals an experiment similar to that of 

 Hoover and Hubbard was carried out by Bissonnette^s in 

 America, and as a result he found it possible to modify 

 the breeding cycle of the goat. The conclusions reached 

 were that temperature is unimportant to this animal, 

 and that the critical factor which induces, or permits, the 

 onset of oestrus is a shortened daylength. In England 

 Yeates^^® has confirmed these conclusions using the 

 sheep. He exposed animals to an artificial length of day 

 rising from 13 hours in mid-October to 21 hours at the 

 end of January, and because of this they passed out of 

 breeding condition two months in advance of the control 

 animals. Then their daylength was reduced until by the 

 end of June they were receiving only 5J hours of light 

 per day. The first of these sheep came into breeding 

 condition on 20 May, and within the following three 



