F. Stephen Dobson

Population genetics meets behavioral ecology.

Populations are often composed of more than just randomly mating subpopulations - many organisms from social groups with distinct patterns of mating and dispersal. Such patterns have recieved much attention in behavioral ecology, yet theories of population genetics rarely take social structures into account. Consequently, population geneticists often report high levels of apparent in breeding and concomitantly low efective sizes, even for species that avoid mating between close kin. Recently, a view of gene dynamics has been introduced that takes dispersal and social structure into account. Accounting for social structure in population genetics leads to a different perspective on how genetic variation is partitoned and the rate at which genic diversity is lost in natural populations - a view that is more consistent with observed behaviors for the minimization of inbreeding.

Senescence rates are determined by ranking on the fast-slow life-history continuum

Comparative analyses of survival senescence by using life tables have identified generalizations including the observation that mammals senesce faster than similar-sized birds. These generalizations have been challenged because of limitations of life-table approaches and the growing appreciation that senescence is more than an increasing probability of death. Without using life tables, we examine senescence rates in annual individual fitness using 20 individual-based data sets of terrestrial vertebrates with contrasting life histories and body size. We find that senescence is widespread in the wild and equally likely to occur in survival and reproduction. Additionally, mammals senesce faster than birds because they have a faster life history for a given body size. By allowing us to disentangle the effects of two major fitness components our methods allow an assessment of the robustness of the prevalent life-table approach. Focusing on one aspect of life history - survival or recruitment - can provide reliable information on overall senescence.

Maternal Traits and Reproduction in Richardson's Ground Squirrels

Differences among conspecifics in body mass result from underlying differ- ences in structural size and physiological condition. To determine whether the structural or physiological component of body mass has a stronger influence on reproductive traits at parturition, we studied the body composition (lean dry mass and fat content), structural size (1st principal component scores computed from 10 skeletal measurements), and body condition (residuals from regression of body mass on structural size) of yearling and older female Richardsons ground squirrels (Spermophilus richardsonii). At parturition, differ- ences among yearlings in body mass primarily reflected differences in structural size as- sociated with continuing structural growth. Older females appeared to reach a deterministic adult structural size, and body mass of older females was most strongly associated with body condition. Structural size of yearling females and body condition of older females had significant positive effects on litter mass. For older females, date of parturition had a significant negative influence on litter size. Both yearling and older females exhibited significant positive effects of parturition date and body condition on neonate mass. In older females, a trade-off resulted in smaller litters of heavier neonates as the breeding season progressed, perhaps reflecting the need for rapid growth of offspring before hibernation. Maternal size, maternal condition, and seasonal timing were important aspects of repro- duction in Richardsons ground squirrels, but these characteristics are seldom considered concurrently in studies of life histories.

Delayed phenology and reduced fitness associated with climate change in a wild hibernator

The most commonly reported ecological effects of climate change are shifts in phenologies, in particular of warmer spring temperatures leading to earlier timing of key events. Among animals, however, these reports have been heavily biased towards avian phenologies, whereas we still know comparatively little about other seasonal adaptations, such as mammalian hibernation. Here we show a significant delay (0.47 days per year, over a 20-year period) in the hibernation emergence date of adult females in a wild population of Columbian ground squirrels in Alberta, Canada. This finding was related to the climatic conditions at our study location: owing to within-individual phenotypic plasticity, females emerged later during years of lower spring temperature and delayed snowmelt. Although there has not been a significant annual trend in spring temperature, the date of snowmelt has become progressively later owing to an increasing prevalence of late-season snowstorms. Importantly, years of later emergence were also associated with decreased individual fitness. There has consequently been a decline in mean fitness (that is, population growth rate) across the past two decades. Our results show that plastic responses to climate change may be driven by climatic trends other than increasing temperature, and may be associated with declines in individual fitness and, hence, population viability.

The Demographic Basis of Population Regulation in Columbian Ground Squirrels

Environmental factors influence the dynamics and regulation of biological populations through their influences on demographic variables, but demographic mechanisms of population regulation have received little attention. We investigated the demographic basis of regulation of Columbian ground squirrel (Spermophilus columbianus) populations under natural and experimentally food‐supplemented conditions. Food supplementation caused substantial increases in population density, and population densities returned to pretreatment levels when the supplementation ended. Control (untreated) populations remained relatively stable throughout the study period (1981–1986). Because food resources regulated the size of the ground squirrel populations, we used life‐table response experiment (LTRE) analyses to examine the demographic basis of changes in population growth rate and thus also demographic influences on population regulation. LTRE analyses of two foodmanipulated populations revealed that changes in age at maturity and fertility rate of females generally made the largest contributions to observed changes in population growth rate. Thus, our results suggested that abundance of food resources regulated the size of our study populations through the effects of food resources on age at maturity and fertility rates. Our results also indicated that different demographic mechanisms can underlie population regulation under different environmental conditions, because lower juvenile survival substantially contributed to population decline, but in only one of the populations. Demographic analyses of experimental data, such as those presented here, offer a rigorous and unambiguous means to elucidate the demographic basis of population regulation and to help identify environmental factors that underlie dynamics and regulation of biological populations.

Interpretation of Intraspecific Life History Patterns: Evidence from Columbian Ground Squirrels

In populations of Columbian ground squirrels in the Rocky Mountains of southwestern Alberta, life history patterns consistently differ at different elevations. At present, these differences are not appropriately explained by the traditional models of life history evolution, r- and K-selection and bet-hedging. Nor do more-recent models based on the interspecific scaling of body weight appear to be appropriate. To explain different life history patterns of the ground squirrels, we present a simple qualitative hypothesis that invokes phenotypically plastic responses to the limitation of demonstrably influential environmental resources (food, and its availability) and to other environmental factors that may influence the acquisition and expenditure of resources. The principal strength of this resource-limitation hypothesis (and other hypotheses based on environmental factors influencing patterns of life history within species) is that it does not invoke genic differences among populations, though it need not preclude the influence of such differences. Because evidence indicates substantial plasticity in the life history patterns of ground squirrels and other vertebrates, different life history patterns within species might result from different phenotypic expressions of similar genotypes, either with or without different genotypic adaptations to different environments.

YOU CAN'T JUDGE A PIGMENT BY ITS COLOR: CAROTENOID AND MELANIN CONTENT OF YELLOW AND BROWN FEATHERS IN SWALLOWS, BLUEBIRDS, PENGUINS, AND DOMESTIC CHICKENS

Abstract The two main pigment types in bird feathers are the red, orange, and yellow carotenoids and the black, gray, and brown melanins. Reports conflict, however, regarding the potential for melanins to produce yellow colors or for carotenoids to produce brown plumages. We used high-performance liquid chromatography to analyze carotenoids and melanins present in the yellow and brown feathers of five avian species: Eastern Bluebirds (Sialia sialis), Barn Swallows (Hirundo rustica), King Penguins (Aptenodytes patagonicus), Macaroni Penguins (Eudyptes chrysolophus), and neonatal chickens (Gallus domesticus). In none of these species did we detect carotenoid pigments in feathers. Although carotenoids are reportedly contained in the ventral plumage of European Barn Swallows (Hirundo rustica rustica), we instead found high concentrations of both eumelanins and phaeomelanins in North American Barn Swallows (H. r. erythrogaster). We believe we have detected a new form of plumage pigment that gives penguin and domestic- chick feathers their yellow appearance. No Puedes Juzgar un Pigmento por su Color: Contenido de Carotenoide y Melanina de Plumas Amarillas y Marrones en Golondrinas, Azulejos, Pingüinos y Gallinas Domésticas Resumen. Los dos tipos principales de pigmentos que las aves incorporan en sus plumas son carotenoides, para desarrollar plumajes rojo, naranja o amarillo, y melaninas, para adquirir coloración negra, marrón, gris o tonalidades color tierra. Sin embargo, existe información conflictiva sobre la potencial coloración de plumas amarillas basadas en melanina y la presencia de caroteniodes en el plumaje marrón de ciertas especies. En este estudio, usamos cromatografía líquida de alto rendimiento para analizar los tipos y cantidades de carotenoides y melaninas presentes en las plumas amarillas y marrones de cinco especies de aves: el azulejo Sialia sialis y la golondrina Hirundo rustica, los pingüinos Aptenodytes patagonicus y Eudyptes chrysolophus y el plumón natal amarillo de la gallina doméstica Gallus domesticus. En ninguna de estas especies detectamos pigmentos carotenoides en las plumas. A pesar de que los carotenoides han sido encontrados en el plumaje ventral de la golondrina Hirundo rustica rustica, nosotros en cambio encontramos altas concentraciones de eumelaninas y feomelaninas en H. r. erythrogaster y en azulejos que variaron entre individuos y regiones de plumaje. Creemos que hemos detectado una nueva forma de pigmento de plumaje que le da a las plumas de pingüinos y pollos domésticos su apariencia amarilla.

DO BLACK-TAILED PRAIRIE DOGS MINIMIZE INBREEDING ?

Considerable controversy surrounds the importance of inbreeding in natural populations. The rate of natural inbreeding and the influences of behavioral mechanisms that serve to promote or minimize inbreeding (e.g., philopatry vs. dispersal) are poorly understood. We studied inbreeding and social structuring of a population of black‐tailed prairie dogs (Cynomys ludovicianus) to assess the influence of dispersal and mating behavior on patterns of genetic variation. We examined 15 years of data on prairie dogs, including survival and reproduction, social behavior, pedigrees, and allozyme alleles. Pedigrees revealed mean inbreeding coefficients (F) of 1–2%. A breeding‐group model that incorporated details of prairie dog behavior and demography was used to estimate values of fixation indices (F‐statistics). Model predictions were consistent with the minimization of inbreeding within breeding groups (“coteries,” asymptotic FIL = –0.18) and random mating within the subpopulation (“colony,” asymptotic FIS = 0.00). Estimates from pedigrees (mean FIL = –0.23, mean FIS = 0.00) and allozyme data (mean FIL = –0.21, mean FIS = –0.01) were consistent with predictions of the model. The breeding‐group model, pedigrees, and allozyme data showed remarkably congruent results, and indicated strong genetic structuring within the colony (FLS = 0.16, 0.19, and 0.17, respectively). We concluded that although inbreeding occurred in the colony, the rate of inbreeding was strongly minimized at the level of breeding groups, but not at the subpopulation level. The behavioral mechanisms most important to the minimization of inbreeding appeared to be patterns of male‐biased dispersal of both subadults and adults, associated with strong philopatry of females. Incest avoidance also occurred, associated with recognition of close kin via direct social learning within the breeding groups.

How Life History Influences Population Dynamics in Fluctuating Environments

A major question in ecology is how age-specific variation in demographic parameters influences population dynamics. Based on long-term studies of growing populations of birds and mammals, we analyze population dynamics by using fluctuations in the total reproductive value of the population. This enables us to account for random fluctuations in age distribution. The influence of demographic and environmental stochasticity on the population dynamics of a species decreased with generation time. Variation in age-specific contributions to total reproductive value and to stochastic components of population dynamics was correlated with the position of the species along the slow-fast continuum of life-history variation. Younger age classes relative to the generation time accounted for larger contributions to the total reproductive value and to demographic stochasticity in “slow” than in “fast” species, in which many age classes contributed more equally. In contrast, fluctuations in population growth rate attributable to stochastic environmental variation involved a larger proportion of all age classes independent of life history. Thus, changes in population growth rates can be surprisingly well explained by basic species-specific life-history characteristics.

An Experimental Study of Dispersal in the California Ground Squirrel

Previous studies suggest three hypotheses to explain the importance of dispersal in small mammal populations: (1) regulation of density, (2) promotion of outcrossing, and (3) avoidance of competition for mates. These hypotheses were examined in an experimental study of the California ground squirrel (Spermophilus beecheyi). One ground squirrel colony was supplied with supplemental food for 3 mo, while two other colonies were monitored for reference. More adult ground squirrels moved to the supplemented colony than to the reference colonies. All young males disappeared from their natal colonies, even on two areas with different adult sex ratios (.09 and .56 male per female). Most young females remained in their natal colonies through the last month of the study. More young male immigrants were trapped at a reference colony than at the supplemented colony. More young females moved to the supplented colony than to the reference colonies; both via exploratory visits from nearby colonies and by immigration. Al...