GESTATION 



959 



iiiatcly 69 days, whereas in the iH-iniate, 

 with a gestation period of 6 to 9 months, 

 depending on the species, the young are 

 helpless at birth. A partial summary of 

 gestation length and tlie litter size of a 

 representative but not inclusive list of 

 mammals is presented in Table 16.1. 



The length of gestation appears to be 

 rather constant for each species or at least 

 within a strain. Even where the phenom- 

 enon of delayed implantation is a natural 

 event the length of pregnancy remains con- 

 stant, although the quiescent period may 

 vary. It is, however, possible for delayed 

 implantation to occur in a species where 

 this does not ordinarily appear, which 

 could lead to a marked increase in the 

 duration of gestation. Thus, an increase of 

 1 to 7 days has been reported in the rat 

 or mouse if mated while lactating (Pincus, 

 1936). Recently, Bruce and East (1956) 

 examined the effect of concurrent lactation 

 on the number and viability of the young 

 and the length of pregnancy in the mouse. 

 They observed a wide variation in the 

 delay of implantation for every size of 

 litter studied, but, in general, the delay 

 tended to be longer for the larger suckling 

 litters. 



Smith, Albert and Wilson (1951) re- 

 ported a 310-day pregnancy period in a 

 human female. Gestation was confirmed 

 early by pregnancy tests and a normal 

 child with respect to body weight was born 

 at 30 days after the expected parturition. 

 Such phenomena seem to be rare in pri- 

 mates and no explanation is possible at the 

 present time. 



Although the lengths of the gestation 

 periods are quite constant for a given 

 strain, the length of gestation is inversely 

 related to the litter size. This has been dem- 

 onstrated in both a genetically pure strain 

 and a heterogeneous strain of quinea pigs 

 (Goy, Hoar and Young, 1957). An average 

 gestation length of 69.9 days was obtained 

 in the pure strain of guinea pigs with a 

 litter size of 1, as compared with a gesta- 

 tion length of 65.3 days for a litter size of 

 6. 



A sex difference has also been postulated 

 in length of gestation. Although the dif- 

 ference is very small, e.g., only a fraction 

 ■of a day in man, the difference is signifi- 



cant. Recently, McKeown and MacMahon 

 (1956) concluded that pregnancy is longer 

 in the cow, horse, and possibly the sheep 

 and camel when the offspring are male, and 

 longer in man and possibly the guinea pig 

 when the offspring are female. 



III. Normal Reproductive Potential 



The reproductive potential in the pri- 

 mate is limited to the period from the 

 menarche to the menopause. Hence, it is 

 much shorter than the total life span of the 

 female. Fertility studies as a function of 

 age have l)ecn rather sparse for different 

 species although it is generally agreed that 

 fertility declines with age. A reproductive 

 period considerably shorter than the life 

 span of the animal has also been reported 

 in certain strains of mice (Thung, Boot and 

 Miihlbock, 1956) and in the rat (Ingram, 

 Mandl and Zuckerman, 1958). 



Although Slonaker (1928) showed that 

 the rat may remain fertile for 22 months, 

 it is known that the average size of suc- 

 cessive litters in both rats and mice first 

 rises to a maximum and then falls (King, 

 1924; Ingram, Mandl and Zuckerman, 

 1958). The latter have shown both a de- 

 crease in the number of fertile female rats 

 with each successive litter and hence with 

 age (Fig. 16.1) and a decline in the number 

 of young with each successive litter (Fig. 

 16.2). 



These results indicate that the reproduc- 

 tive potential of both the colony of rats and 

 of the individual rat declines with age. 

 Many factors may obviously be at work 

 here, such as nutrition, size, and part 

 played by the male. Ingram, Mandl and 

 Zuckerman ( 1958 ) feel that none of the 

 above factors is responsible for the decline 

 in litter size and offer the following four 

 possibilities: (1) the number of follicles 

 which mature and ovulate declines with 

 age, (2) the capacity of the ovum to be 

 fertilized declines with age, (3) the number 

 of fertilized ova that develop to term de- 

 clines with age, and (4) the total number of 

 available oocytes declines with age. 



Evidence from the pig (Perry, 1954) 

 and rat indicates that factors 2 and 3 are 

 certainly involved. Inasmuch as the num- 

 ber of corpora lutea rises with age in the 

 pig, the decline in size of litters can be 



