THE OVARY AS TIMEPIECE 



has freed from dependence upon the wild crops and has im- 

 proved for more rapid breeding; others have acquired their 

 cycles for no obvious reason. The shortest cycle is that of 

 the domestic fowl, which lays an egg once a day ; the longest, 

 that of the locusts that come swarming from the ground at 

 intervals of seventeen years, in obedience to some obscure 

 signal, to deposit their eggs and then to die. 



In mammals, and in the human race, the ovary is no less 

 cyclical than in lower animals. Many wild mammals have an 

 annual cycle so timed that they may bring forth their infants 

 when food is plentiful for the mothers. Other species have 

 estrous cycles at intervals throughout the year, or a sexual 

 season of several cycles at a favorable time of the year. Rats 

 and mice have very short cycles, ovulating every 4^ to 5 

 days, except when the cycles are interrupted by pregnancy. 

 The guinea pig has a 15-day cycle. Cows, mares, and swine 

 have estrous periods at 21-day intervals throughout the year. 

 Sheep have several cycles in the late summer; during the 

 winter they are anestrous. Dogs and cats have two or three 

 estrous periods each year ; so, apparently, have lions. Many 

 other carnivores are monestrous (having one period each 

 year). Not only do the time intervals vary in different species, 

 but fhere are all sorts of different behavior patterns at estrus 

 and a good many differences in the details of internal phy- 

 siology. 



The cycle of the sow. To make this matter clear, let us 

 follow through the cycle of one particular species and then 

 discuss some of the variations. That valuable creature, the 

 domestic sow, will serve us admirably for this purpose. She 

 has an estrous cycle of 3 weeks' duration. During 2% weeks 

 of each cyclic interval she goes about her usual program of 

 eating and sleeping and if there is a boar in the herd she shows 

 no interest in him. Then there is a change of mood and be- 

 havior. For 3 days she undergoes a well-marked phase of 

 estrous excitability, marked by restlessness, diminished appe- 



{ 65 ) 



