Reproduction 43 



tion of young is very great in all mammalian animals but differs 

 widely according to species. In a mare weighing 1,500 pounds, 

 the new-born foal may weigh about 125 pounds, or 8 % of her 

 body weight, to which must be added an additional demand of 

 the young, in the form of milk as food, covering a period of five, 

 six or even more months after birth. 



The nutritive demands of the fetus of the mare extend over a 

 long period, comprised of eleven and one-third months of preg- 

 nanc}' and five, six or more months of nursing, or a total period of 

 about one and one-half ^ears. And her average rate of producing 

 young is reduced to approximately one in two, or two in three 

 years. In the cow there is a greater nutritive excess or reserve and, 

 while the young is .somewhat larger as related to the size of the 

 mother than obtains in the case of the mare, or about ib %; the 

 intra-uterine term of existence is shortened from about eleven and 

 one-third to nine and one-third months; and the period of sucking 

 is also reduced .somewhat, thus materially decreasing the demands 

 upon the maternal system, with a corresponding increa.se in repro- 

 ductive power and the rate of increase of the species. In the 

 natural state, fewer of the young reach maturity. In multipara 

 the young are relatively smaller, but parturition occurs more 

 frequenth'. In the sow, parturition occurs about twice annually 

 and the number of young may reach ten or more at each birth. 

 A sow weighing 300 pounds may thus give birth to twenty pigs 

 in a year, each of which may weigh two, or a total of forty 

 pounds, equal to 13 % of the maternal body weight. 



The character and abundance of food exerts a well defined 

 influence upon the number of young produced, a highly nutri- 

 tive and well balanced ration with other favorable environment 

 tending to greatly increase fecundity. 



The completeness of development of the fetus at birth varies 

 widely and the demands upon the nutritive reserve of the 

 mother correspond to the degree of embryonic evolution at- 

 tained by the young prior to birth. The i.ew-born rabbit, after 

 four weeks of intra-uterine life, is a very innnature animal, inca- 

 pable of locomotion, its eyes not open and its body almost naked 

 of hair ; the young Guinea pig, after the same duration of 

 intra-uterine existence, is born with a den.se coat of hair, with 

 eyes open and its locomotor}- apparatus .so completely developed 

 that it can move about with almost the same celerity as its dam. 



