206 



JACKSON, VESSEY, AND BASTIAN 



suspected Polynesian rat depredations. However, this was 

 an isolated, not a repeated, observation. 



Reproductive Patterns 



Necropsy data, assembled over 14 years and available 

 from all months except May and December, provide a 

 composite picture of breeding patterns. For Polynesian 

 rats, a bimodal increase in prevalence of pregnancy is sig- 

 nificantly correlated (r^ = 0.82) with spring and fall rainy 

 seasons. The fall period is longer, and the prevalence of 

 pregnancy reaches nearly 25 in October (Temme, 1981). 

 We hypothesize that the increased food supply (and 

 perhaps cover) associated with increased precipitation is 

 the basis for greater reproductive activity (Fig. 2). 



Despite the general data correlations, some individual 

 islet variations were observed. If coconut trees were 

 present, pregnant rats were more likely to be found. How- 

 ever, sufficient data were not collected to delineate the 

 extent of these intra-atoll variations. 



On the basis of embryo counts, average litter size for 

 Polynesian rats was determined to be 3.3 (Table 1). About 



a third (36%) of the pregnant females were primiparous. 

 These reproductive data are similar to those obtained else- 

 where in the tropics for this species. 



Most males (84%) had scrotal testes, and few young 

 males were caught early in the breeding seasons. Acces- 

 sory organs (e.g., seminal vesicles) regressed in size during 

 the nonbreeding January period and recrudesced in March. 

 However, the variations in seminal vesicle length and 

 prevalence of pregnancy were only weakly correlated (r = 

 0.42). Some effects of population density and stress are 

 suspected (Temme, 1981). 



Data from roof rat populations followed very similar 

 patterns, showing the same summer and fall reproductive 

 peak (Table 1); however, litter sizes were larger, averaging 

 4.2 young. 



Behavior and Population Regulation 



An atoll provides an unusual opportunity to study 

 natural rat populations because of the number of similar 

 islets in isolated proximity. Population densities vary from 

 islet to islet but are typically higher than those on the 





Enewetak Ato 



mean sum: 1^70 mm 



Fig. 2 Relationship between monthly mean rainfctll and previdence of 

 pregnancy of Polynesian rats (means 1964 to 1978). Sample sizes (tops 

 of bars) are based on mature fem<iles (perforate vaginal orifice) 

 (Temme, 1979). 



