Cohort effects in red squirrels: the influence of density, food abundance and temperature on future survival and reproductive success

1. Environmental conditions experienced early in life may have long-lasting effects on individual performance, thereby creating 'silver-spoon effects'. 2. We used 15 years of data from a North American red squirrel (Tamiasciurus hudsonicus Erxleben) population to investigate influences of food availability, density and spring temperature experienced early in life on reproduction and survival of female squirrels during adulthood. 3. We found that spring temperature and food availability did not affect female survival after 1 year of age, whereas higher squirrel densities led to lower survival, thereby affecting longevity and lifetime fitness. 4. In addition, both food availability experienced between birth and weaning, and spring temperature in the year of birth, had long-lasting positive effects on female reproductive success. These results emphasize the critical effect environmental conditions during the early life stages can have on the lifetime performance of small mammals. 5. These long-term effects of early food and temperature were apparent only once we controlled for conditions experienced during adulthood. This suggests that silver-spoon effects can be masked when conditions experienced early in life are correlated to some environmental conditions experienced later in life. 6. The general importance of silver-spoon effects for adult demographic performance might therefore be underestimated, and taking adult environment into account appears to be necessary when studying long-term cohort effects.     

Territorial bequeathal by red squirrel mothers

Red squirrels (Tamiasciurus hudsonicus) defend individual, food-basedterritories year round. These territories are crucial for overwintersurvival, yet some female red squirrels in northern populationsbequeath their territories to offspring and search for a newone. Bequeathal involves active, strategic dispersal by breedingfemales. Our study documents this unusual behavior and investigatesits correlates in two red squirrel populations in northern Canada.Thirty percent of breeding females dispersed. Bequeathal wasrelated to breeding date, with late-breeding females more oftendispersing, but was not related detectably to female conditionor territory quality. As an underlying trade-off, early-bornjuveniles more likely acquired a territory independently, butearly-breeding females lost mass and may have increased theirrisk of territory loss. We suggest that bequeathal has evolvedas a consequence of condition dependence in breeding date coupledwith limited resources.     

Post-breeding dispersal by female red squirrels (Tamiasciurus hudsonicus): the effect of local vacancies

Red squirrels (Tamiasciurus hudsonicus) defend food-based territoriesyear round, and juveniles must acquire a territory before winterto survive. We experimentally removed territory owners duringthe time that juveniles were becoming independent to examinethe effect of local vacancies on dispersal patterns. Juvenilesattempted to take over removal territories most frequently.However, females with offspring still on the natal territoryactually took over twice as many territories as juveniles. Thesefemales did not appear to move because of low reproductive potentialor to increase territory quality. Instead, moving to a removalterritory allowed more of their offspring to remain on the natalterritory, which appeared to increase juvenile survival.     

Social constraints limit dispersal and settlement decisions in a group-living bird species

Dispersal is a fundamental process affecting the genetic structureof populations, speciation, and extinction. Nevertheless, ourunderstanding of the evolution of dispersal is limited by ourpaucity of knowledge on dispersal decisions at the individuallevel. We investigated the effect of interactions between residentsand juvenile dispersers on individual dispersal and settlementdecisions in Siberian jays (Perisoreus infaustus). In this group-livingbird species, some offspring remain on the parental territoryfor up to 3 years (retained juveniles) whereas other offspringdisperse within 2 months of fledging (dispersers). We foundthat retained juveniles constrained settlement decisions ofdispersers by aggressively chasing dispersers off their territory,resulting in dispersers continuing to disperse and settlingin groups without retained juveniles. Experimental removal ofmale breeders during the dispersal period also demonstratedthat dispersers were unable to settle in high-quality breedingopenings, which were instead filled by older nonbreeding residents.Rather, dispersers immigrated into groups without retained offspringwhere they became subordinate group members, queuing for a breedingopening. Also, they preferably settled in groups with shortqueues where no same-sex juveniles were present. Dispersal didnot inflict a cost to dispersers through increased mortality.However, the presence of immigrants was costly for breedersbecause it increased the rate of conflicts during the breedingseason which negatively affected nestling condition. These resultsdemonstrate that resident individuals constrain both dispersaland settlement decisions of dispersers. Social interactionsbetween residents and dispersers can thus be a key factor tounderstand the evolution of dispersal.     

Ecologically determined natal philopatry within a colony of great cormorants

Dispersal patterns of individuals within populations have implicationsfor the social and genetic structure of local populations.Knowing what factors determine individual dispersal behavioris essential for predicting how the population structure willbe influenced by environmental and demographic changes. Inthis study, I investigated whether the settling pattern of individuals breeding for the first time within a colony of greatcormorants was determined by ecological or genetic factors.Furthermore, I examined the possible effects of age and gender.First-time breeders that came back to breed within their natalcolony showed strong philopatry toward their natal breedingsites. Because of the simultaneous strong fidelity of breederstoward their former breeding sites, this caused kin to clusterto some extent around the natal site. However, genetic factors(attraction to close kin) are less likely to explain natalphilopatry than ecological ones (attraction to the natal siteitself). Younger first-time breeders were more philopatric than older ones, in accordance with a decrease in the predictabilityof the quality of breeding sites with increasing time lags.Furthermore, males dispersed farther from the natal breedingsite than females. This result is contrary to what is generallyexpected for a breeding system where the male is dependent on a breeding territory for mate acquisition. I suggest thatthis sex difference could arise because first-time breedingmales are constrained from settling in the natal site by interferencecompetition with older males or because males are better informedabout alternative breeding sites of high quality within thecolony.     

Dispersing brush mice prefer habitat like home

During natal dispersal, young animals leave their natal area and search for a new area to live. In species in which individuals inhabit different types of habitat, experience with a natal habitat may increase the probability that a disperser will select the same type of habitat post-dispersal (natal habitat preference induction or NHPI). Despite considerable interest in the ecological and the evolutionary implications of NHPI, we lack empirical evidence that it occurs in nature. Here we show that dispersing brush mice (Peromyscus boylii) are more likely to search and settle within their natal habitat type than expected based on habitat availability. These results document the occurrence of NHPI in nature and highlight the relevance of experience-generated habitat preferences for ecological and evolutionary processes.