ANNUAL REPRODUCTIVE SUCCESS IN 3 COHORTS OF COLUMBIAN GROUND-SQUIRRELS - FOUNDING IMMIGRANTS, SUBSEQUENT IMMIGRANTS, AND NATAL RESIDENTS

We compared, over an 8-year period, the annual reproductive success (R S) of male and female founding immigrants (FIs) with that of subsequen t immigrants (SIs), and the latter with that of natal residents (NRs) in two recently established colonies of Columbian ground squirrels (Sp ermophilus columbianus) in southwestern Alberta. For males, actual ann ual RS was unknown and so was estimated; for females, RS was measured directly. Among females, FIs had significantly more juveniles at litte r emergence, and more yearlings present to age of emigration annually than did SIs. Juvenile and yearling offspring of SI females suffered h igher annual rates of disappearance (presumed mortality) over the acti ve season than did those of FIs. Additionally, the matrilines of FIs p roduced significantly more juveniles, more yearlings to time of emigra tion, and more daughters to reproductive maturity annually than did th e matrilines of SI females. Male FIs had higher annual survival rates (and consequently lived and were territorial significantly longer), an d had more female neighbours per year than did SIs, which suggests tha t the former had higher annual RS. We suggest that the higher annual R S of male and female FIs resulted mainly from their monopolization of the best resources and a lower risk of predation as a result of their more central position in the colony. Comparison of the annual RS of SI s and NRs showed that most (approximately 70%) adult males present in the colonies were SIs. These males were significantly more likely to h old territories and to do so for significantly longer periods than the ir NR counterparts. Conversely, SI females annually brought above grou nd significantly fewer juveniles and had significantly fewer yearling offspring present to time of emigration than did NR females. Furthermo re, the young of SI females suffered significantly higher rates of dis appearance (presumed mortality) during the active season than did thos e of NR females, possibly because the former resided in more periphera l areas within the colony where predation rates may have been higher. We found no differences between these groups of females in the number of female offspring reaching reproductive maturity in the natal colony . These results provide preliminary support for the hypothesis that fe males, apart from FIs, benefit from philopatry; whereas males gain rep roductive advantage through dispersal.