CHAPTER II 



CYCLES OF REPRODUCTION 



I. INTRODUCTION 



BEFORE an animal can breed it must first grow to 

 maturity, and between species there is the widest 

 variation in the time taken for this to happen. Among 

 fishes it seems probable that the majority spawn for the 

 first time when they are one or two years old, but the 

 extremes extend from only two or three months in some 

 small tropical species to more than 20 years. Common 

 food fishes like the cod and haddock are mature at about 

 4 years of age,ii6 the hake at about 10 years, los and the 

 sturgeon at from 10 to 15 years. ^^ The extreme case is 

 that of the European eel, which spawns in the western 

 Atlantic. The young take 3 years to return to Europe 

 where they spend a further 10 to 19 years in fresh- 

 water.^ 2 So this fish has a total period of immaturity of 

 up to 22 years, which is the longest known with certainty 

 in any animal. 



The amphibians on the other hand do not appear to 

 live to great ages, and there is evidence that the common 

 frog and toad spawn for the first time when they are 

 between 2 and 4 years old. The reptiles and birds also 

 tend to avoid extremes. Many common species breed 

 for the first time when about i year old. The adder for 

 instance does not do so until it is about 5 years old^^^ and 

 some tortoises and turtles may even approach the record 

 of over 20 years held by the eel. Among birds the longest 

 recorded period of immaturity seems to be about 4 years 

 as in the golden eagle, gannet and herring gull. In 

 mammals the range again widens, and no general state- 

 ment is possible. Extremes are set by the house mouse, 

 which may become pregnant when 6 weeks old, and the 



