962 



SPERM, OVA, AND PREGNANCY 



attributed only to failure of fertilization 

 or to fetal death. A similar increased in- 

 cidence of embryonic death or failure of 

 fertilization has been described in the aged 

 rat, although it should be noted that the 

 number of corpora lutea present at parturi- 

 tion in the rat is not an index of the 

 number of ova released before conception 

 because the corpora lutea in old age may 

 persist for longer periods. Finally, a marked 

 reduction in the number of oocytes with 

 age has been shown in the rat, a drop from 

 approximately 20,000 oocytes at age day 

 1 to approximately 2000 oocytes at age 250 

 to 300 days (Mandl and Zuckerman, 1951). 

 In addition, Ingram (1958) showed that 

 the litter size in rats declined markedly with 

 the reduction in number of oocytes follow- 

 ing graded doses of x-ray. This experiment 

 tends to confirm the concept that the de- 

 cline in fertility with age is due to a de- 

 cline in the number of oocytes. 



IV. Environment 



A. CROWDING 



The factors concerned in the growth and 

 survival of a population under natural con- 

 ditions may obviously involve reproduction. 

 Variations in population level have been of 

 great interest to the mammalogist and 

 student of wildlife for many years. De- 

 creased productivity in mammalian popu- 

 lations associated with increased density 

 of the population has been considered a 

 controlling factor in the regulation of wild- 

 life population. 



Experimental analysis by Christian and 

 Lemunyan (1958) indicated that a number 

 of factors are involved. These authors ex- 

 l)osed mice to excessive crowding and noted 

 the number of implantation sites, embryos. 



and births. All the females became preg- 

 nant but only 3 of the 10 bore litters during 

 the period of crowding or later (Table 

 16.2). It would seem that the crowded 

 females were unable to maintain normal 

 pregnancies and that the environmental 

 situation interfered with the endocrine bal- 

 ance and resulted in a marked pregnancy 

 wastage. The data reveal that in addition 

 to a postimplantation loss, there was also 

 a prc-implantation loss because the number 

 of implantation scars in the uterus was 

 markedly less in the crowded females. This 

 could be due to a failure of the fertilized 

 egg to implant or to a decrease in the 

 number of available ova. Although direct 

 data are not available, it is of interest to 

 speculate as to whether this effect of crowd- 

 ing is mediated by way of the pituitary- 

 adrenocortical axis. 



B. BODY TEMPERATURE AND HYPOXIA 



Disturbances in reproduction have been 

 noted in mammals exposed to high tem- 

 peratures or chronic hypoxia. It has been 

 known for some time that women moving 

 to the tropics show a high rate of abortion 

 (Castellani and Chalmers, 1919). Recently 

 Macfarlane, Pennyamt and Thrifte (1957) 

 reported a 30 per cent reduction in human 

 conception rates in the summer as com- 

 pared with the winter in Australia. The 

 same authors reported a marked degree 

 of fetal resorption in rats exposed to a tem- 

 perature of 35°C. This confirmed the pre- 

 vious observations of Sundstroem (1927) in 

 rats and Oegle (1934) in mice where ex- 

 posure to 31 °C. caused a reduction in litter 

 size. 



In a similar manner, disturbances have 

 also been reported in reproduction follow- 

 ing exposure to decreased oxygen tension. 



TABLE 16.2 

 Productivity of mice crowded 10 pairs to a cage compared to their 10 control pairs 

 Note that all of the females became pregnant but that the crowded females exhibited a marked 

 intrauterine loss of young, reduction of implantation sites, reduction of litter size, and significant de- 

 lay until the birth of first litters compared to the controls. Crowding produced a 75 per cent loss in the 



