Reproduction 4 1 



The rate of reproduction varies "greatly in different species and 

 is dependent chiefly upon three factors ; the age of puberty and 

 duration of sexual competency, the number of young brought 

 forth at a given birth and the frequency of parturition. Uni- 

 parity, or single births, is the rule in the larger animals ; the 

 smaller ruminants are to a great extent bi-parous, while the 

 smaller classes of animals are quite uniformly multiparous and 

 bring forth from three or four to twelve of more young at a given 

 time. 



The frequency of parturition varies greatly and the minimum 

 between two births is fixed by the duration of pregnancy, which, 

 among our domestic animals, finds its extremes between the four 

 weeks of the rabbit and the 21 months of the elephant. The fre- 

 quency of partuition is further influenced by a dormant period 

 in reproductive activity between the giving of birth to young and 

 readine.ss to again conceive. In some species the nursing of 

 young tends to inhibit the power of breeding, as is sometimes 

 seen in the mare and is said to be yet more marked in the ele- 

 phant. In all our larger animals there is usually an interval be- 

 tween the birth of a fetus and the power to conceive, pending 

 the recurrence of estrum and ovulation. In the mare this inter- 

 val is very brief, frequently but eight or nine days, while in the 

 cow it is longer. In these larger animals there is a tendency 

 toward one parturition each year and in the mare the resumption 

 of the power of conception after foaling needs be very prompt or 

 annual breeding becomes impossible, since the duration of preg- 

 nancy is about eleven and one third months, exceeding 12 months 

 in some cases, leaving an average of but about three weeks, in 

 which pregnancy may recur, and a second foal be born within a 

 year. Under such conditions it is natural that the mare does not 

 usually produce a foal each year over an extended period. On 

 the other hand, in the rabbit, conception normally recurs within a 

 few hours after giving birth to a litter of young, so that she may 

 breed each month. In nature, and still more in the domesticated 

 state, conceptions fall far short of the maximum possibilities and 

 the births are relatively much below the assumed number. Mares 

 used especially for breeding purposes produce ordinarily but two 

 foals in three years or even less, and elephants are said to pro- 

 duce young but once in three to four years. Exceptionally, we 



