lOO VERTEBRATE REPRODUCTIVE CYCLES 



2. EXTERNAL FACTORS AND BREEDING CYCLES 



New observations have recently been made on fishes, 

 reptiles, and birds. In the fishes it has been shown that 

 variations in temperature and light, separately or in 

 combination, may have a critical effect according to 

 species (see review by Harrington^ ^°), and although 

 information on the reptiles continues to be very in- 

 adequate, a similar conclusion may perhaps also be 

 drawn in this group. In his review Bartholomew ^^^ has 

 also drawn attention to the critical importance of con- 

 sidering the behaviour of reptiles in the wild. He points 

 out that, because these animals are cold blooded, each 

 species tends to seek out the environmental temperature 

 that suits it best. To take a simple illustration, a lizard 

 may appear above ground only for an hour or two in the 

 early morning and late evening. By night it may burrow 

 away from the cold and by day it may burrow away from 

 the heat. Thus it avoids any great temperature variation 

 and its exposure to daylight may vary inversely with the 

 temperatures of the day. Consequently, as Bartholomew 

 concludes, 'the complications imposed on the analysis 

 of reptilian photoperiodism by behavioural temperature 

 regulation are formidable'. 



However, most of the more recent work has related to 

 the birds of temperate regions in which reproductive 

 cycles are mainly, if not entirely, controlled by variations 

 in daylength (Wolfson;^^! Farner^^®, 2°^). It has been 

 confirmed that there are normally at least two phases in 

 the typical avian reproductive cycle. The first has been 

 variously called the recovery period (which may be con- 

 sidered as the final period of the previous cycle or the 

 preparatory period (which may be considered as the 

 first period of the next cycle) or the refractory period 

 (see p. 33) (because at this time the system cannot be 

 stimulated to further activity). It is perhaps best to use 

 the last name so as to emphasise the distinction between 

 this phase and the second phase of the cycle, the so-called 

 progressive phase, during which the bird can respond 



