208 



JACKSON, VESSEY, AND BASTIAN 



Z .55 



U 



-I .50 



45 



Q 

 O 

 GQ 

 s -40 



■ 



^ .35 



OC .30 



O 



< .25 



.20 



f 

 m 



.m 



m 



f f 

 m 

 m 



12 3 4 



RATS/ TRAP HOUR 



6 



Fig. 3 Rats per trap hour (X 100) and adrenal weight/body length ratios for roof rats plotted 

 separately by islet (1977 and 1978); f = females, m = mjiles. 



TABLE 2 



Summary of Population Density Estimates 



and Adrenal Gland Weights for Roof 



Rats at Enewetak Atoll 



•Rats/trap hour X 100. 



Parasites and Adrenal Weight 



Two parasites were examined in the rats. The most 

 common was a stomach worm of the genus Protospiura, 

 which was present :n both species. The incidence or bur- 

 den of this parasite was not related to adrenal weight. The 

 other parasite was eui intestinal tapeworm, Hi/menolepis 

 diminuta, which was common only on Aomon. Rats of 



both sexes which had tapeworms had heavier adrenals 

 than did those without tapeworms (t-test, P < .05; 

 Table 4). 



Discussion 



The results generally support the idea that as popula- 

 tion density increases so does the incidence of fighting, an 

 observation made many times in simulated free-living 

 laboratory colonies and in field observations under red 

 lights and with starlight scope. As a result of fighting, cer- 

 tain individuals — the losers of these fights — undergo a 

 stress response leading to increased production of glu- 

 cocorticoid hormones and hypertrophy of the cortex of the 

 adrenal gland. Although these hormones prepare the body 

 for fight or flight, they also are associated with a decrease 

 in reproductive hormones and the suppression of the 

 body's defense mechanisms. We have not examined the 

 relationship between adrenal hormones and reproduction; 

 but rats that were under stress were more likely to have 

 tapeworms, suggesting the predicted inhibition of defense 

 mechanisms against disease. The failure to find an effect 

 with the stomach nematode may have been because this 

 parasite, while residing in the stomach, does not attach to 

 the host or otherwise stimulate an inflammatory or 

 immune response. 



The failure of the Polynesian rat to show a consistent 

 adrenal response with islet population density is difficult to 

 explain. However, this species typically lives at much 

 higher densities and seems more tolerant of crowding than 

 the roof rat. Although fighting is common among Polyne- 

 sian rats, as evidenced by simulated free-living laboratory 



