Variation in Viability Selection Among Cohorts of Juvenile Red Squirrels (Tamiasciurus Hudsonicus)

Abstract

Selection will result in observable changes in traits only if it acts consistently in space and time, but few estimates of selection in natural populations have been temporally replicated. Here we estimate viability selection on nestling growth rates for 13 cohorts (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) of red squirrels (Tamiasciurus hudsonicus) from a natural population located in southwestern Yukon, Canada. Directional selection on nestling growth rates varied in magnitude and direction from one cohort to the next. The magnitude of directional selection was relatively weak in most years (median ␤Ј ϭ 0.24), but there were episodes of very strong viability selection (␤Ј Ͼ 0.5) in some cohorts. We found no evidence of significant stabilizing or disruptive selection on this trait. Examination of viability selection episodes over shorter time periods suggested that the strength of selection on juveniles in this population was positively related to the time scale over which selection was measured. Viability selection from birth to emergence from the natal nest (50 days of age) and from emergence to successful recruitment (100 days of age) were positively correlated, but were both independent of selection on nestling growth rates from recruitment to potential breeding age (one year). The strength of directional selection on growth rates prior to recruitment was negatively correlated with spring temperature whereas selection from recruitment to breeding was positively correlated with the abundance of spruce cones produced in the previous fall. Episodes of strong directional selection from birth to breeding age appear to be due to potentially rare combinations of environmental conditions. As a result, predicting the occurrence of very strong episodes of selection will be extremely difficult, but predicting the microevolutionary responses to observed selection on individual cohorts remains feasible.

Adopting kin enhances inclusive fitness in asocial red squirrels

“…Th e mechanism by which female red squirrels assess their relatedness to juveniles or their dams is unknown and is a current focus of research at our study site. Although not specifi cally designed to study kin discrimination, previous cross-fostering experiments found that genetic and foster off spring grew at the same rate 9,19 , suggesting that female red squirrels do not diff erentially invest in Cases A1 -A5 were adopted, whereas cases NA1 and NA2 were not adopted even though there were lactating kin ( r ≥ 0.125) nearby. The number of lactating females nearby was estimated relative to the orphaned juvenile and not the surrogate dam.…”

“…Using these natal relationships, we constructed a complete maternal pedigree spanning multiple generations. 19 and R é ale et al 21 ). Parturition date and juvenile growth rate were standardized within grid -year combinations, and the intercept of the model was forced through zero to correspond to the biologically relevant model in which females who did not produce any juveniles had zero ARS.…”

Eco-mechanics of lamellar autotomy in larval damselflies

“…Arnqvist and Johansson, 1998). Rearing damselfly larvae collected from a fishless site with versus without dragonfly predator cues could evaluate the degree to which breaking forces, cuticle width and joint size may express adaptive plastic responses to local predation risk, while longitudinal (cohort) methods in the wild could address selection (McAdam and Boutin, 2003).…”

The nature of nurture in a wild mammal's fitness

“…We found no effect of maternal age or litter size, but parturition date was associated with offspring LRS. Parturition date is a heritable maternal trait in red squirrels [54] that has been found to affect offspring recruitment [55]. Parturition date, therefore, represents a MGE on offspring fitness.…”