Vol. 23, Ho. S 
Page 2 
Ecology and Management of Squirrels - W-66-R C. M. Mixon, 
L. P. Hansen 
Subadult fox squirrels frequently disperse from the area where they were 
born, due either to an innate (genetic) dispersal tendency or to harassment by 
older squirrels (social stress), Which of these 2 influences is a greater cause 
of fox squirrel dispersal is not known. We do know that some subadults remain 
close to their birth area, which suggests that an innate dispersal tendency, 
although perhaps present in all subadult fox squirrels, is associated with a 
variable response to agonistic adult behavior. Murray (1967- Ecol. *t3:975~978) 
felt that the only difference between possible dispersing animals in distance 
moved lies in their ability to procure a breeding site; e.g. , how effectively 
they resist adult harassment. 
On our 112-acre study area near Danville, Illinois, we compared the number 
of male and female subadults captured within the home range of an adult male or 
female during spring (breeding period) and fall (postbreeding) 1ivetrapping. We 
used trap captures of subadults captured in the same live traps as an adult 
during the same trapping period to indicate home range overlap. 
In the spring there was no difference in the abundance of subadult males 
and females in the home ranges of adult males (93 male:97 female subadults) or 
in home ranges of adult females (81 male:30 female subadults). During fall 
1ivetrapping, we found a nearly significant difference (0.10 > P > 0.05) between 
the abundance of subadult males (N = 65) and subadult females (N = 91) in the 
home ranges of adult males but not in the ranges of adult females. 
Adult males may be less tolerant of subadult males than of subadult females 
during the fall, but it is also possible that subadult females are easier to 
livetrap during the fall. We assume that the sex ratio at birth is 50:50 and 
that mortality rates are similar for both sexes. We have no evidence that these 
assumptions are false, although most published reports show increased numbers of 
males shot or trapped during the fall. However, the increased number of subadult 
males shot compared with subadult females may be the result of hostile adult 
males forcing these subadult males to disperse. Dispersing animals are con¬ 
sidered to be more prone to accidents and predation than residents, although 
there is little hard evidence to show that they are. Studies in Michigan have 
demonstrated that the age class most vulnerable to shooting is subadult males 
and that adult males are the least vulnerable. Most exploited (hunted) popu¬ 
lations seem to contain an excess of males during the fall. Such exploited 
populations may function as "dispersal sinks" where, in the absence of adults 
shot by hunters, dispersing subadults settle, many of them destined to be shot 
in turn. If most of these dispersing subadults are males driven out by adult 
males, we would expect fall-shot samples to contain more males than females. 
During an earlier study we found that hunter-shot samples of subadult fox 
squirrels in Illinois averaged 59% males (r! = 2,431 ) in the south zone and 52% 
males (N = 1,366) in the north zone. 
