Population Of Columbian Ground Squirrels Research Articles

Population density and vegetation resources influence demography in a hibernating herbivorous mammal.

Demography of herbivorous mammal populations may be affected by changes in predation, population density, harvesting, and climate. Whereas numerous studies have focused on the effect of single environmental variables on individual demographic processes, attempts to integrate the consequences of several environmental variables on numerous functional traits and demographic rates are rare. Over a 32-year period, we examined how forage availability (vegetation assessed through NDVI) and population density affected the functional traits and demographic rates of a population of Columbian ground squirrels (Urocitellus columbianus), a herbivorous hibernating rodent. We focused on mean population phenology, body mass, breeding success, and survival. We found a negative effect of population density on demographic rates, including on breeding success and pup and adult survival to the next year. We found diverging effects of vegetation phenology on demographic rates: positive effects of alater start of the growing season on adultand yearling femalesurvival, and juvenile survival, but no clear effect on male survival. Interestingly, neither population density nor vegetation affected population phenology or body condition in the following year. Vegetative growth rate had a positive influence on female mass gain (somatic investment) over a season, but bothvegetative growth rate and biomass, surprisingly, had negative effects on the survival of young through their first hibernation. Thus, ground squirrels appeared to benefit more from later timing of vegetation than increases in vegetative biomass per se. Our study provides evidence for complex ecological effects of vegetation and population density on functional traits and demographic rates of small mammal populations.

Subtle short-term physiological costs of an experimental augmentation of fleas in wild Columbian ground squirrels.

Parasites affect many aspects of host physiology and behavior, and thus are generally thought to negatively impact host fitness. However, changes in form of short-term parasite effects on host physiological markers have generally been overlooked in favor of fitness measures. Here, we studied flea (Oropsylla idahoensis and Oropsylla opisocroistis tuberculata) parasitism on a natural population of Columbian ground squirrels (Urocitellus columbianus) in Sheep River Provincial Park, AB, Canada. Fleas were experimentally added to adult female U. columbianus at physiologically demanding times, including birth, lactation and weaning of their young. The body mass of adult females, as well as their oxidative stress and immunity were recorded multiple times over the active season under flea-augmented and control conditions. We also measured the prevalence of an internal parasite (Trypanosoma otospermophili). Doubly labeled water (DLW) was intra-peritoneally injected at peak lactation to examine energy expenditure. Effects of parasites on oxidative stress were only observed after offspring were weaned. There was no direct effect of experimentally heightened flea prevalence on energy use. A short-term 24 h mass loss (-17 g) was detected briefly after parasite addition, likely due to U. columbianus preferentially allocating time for grooming. Our parasite augmentation did not strongly affect hosts and suggested that short-term physiological effects were unlikely to culminate in long-term fitness consequences. Columbian ground squirrels appear to rapidly manage parasite costs, probably through grooming.

Better bullets to shoot small mammals without poisoning scavengers

ABSTRACTRecreational shooters kill millions of small mammals each year and scavengers that ingest the carcasses can be poisoned if lead bullets are used. Less toxic bullets exist, but their performance and potential risk to deposit lead in carcasses is understudied. In May 2016, shooters controlled populations of Columbian ground squirrels (Urocitellus columbianus) at ranches in Idaho and Montana, USA, by shooting them with .17 HMR (Hornady Magnum Rimfire), .22 LR (long rifle), and .223 Rem (Remington) rifles with expanding and nonexpanding lead and nonlead bullets. We recorded whether bullets instantly incapacitated ground squirrels and then counted the number of bullet fragments in each carcass to estimate concentrations of residual lead. All types of lead bullets left lead in at least one‐third of the Columbian ground squirrels. Unexpectedly, estimated concentrations of lead in carcasses did not differ between expanding and nonexpanding bullets within the .17 HMR and .22 LR calibers, partially because of the high variability in fragmentation. The greatest estimated concentrations of lead were in Columbian ground squirrels shot with expanding ammunition in .17 HMR and .223 Rem, which had an average of 23.6 mg and 91.2 mg Pb/carcass, respectively. Nonlead bullets incapacitated similar to lead bullets. Our results indicate that nonlead bullets eliminate the risk of additional lead exposure to scavengers while maintaining the lethality of lead bullets. © 2017 The Wildlife Society.

Testing the predictions of energy allocation decisions in the evolution of life‐history trade‐offs

Summary Allocating a greater amount of limited resources, such as energy, to current reproduction can reduce the amount of energy available for somatic maintenance and can ultimately impair future breeding success or maternal survival (i.e. cost of reproduction hypothesis). Although there is some support for the cost of reproduction hypothesis in birds, few empirical studies of mammals have demonstrated a trade‐off between current and future reproduction. More importantly, most studies testing ultimate costs have neglected to confirm that the proximate costs of reproduction are high. We experimentally manipulated litter size in a wild population of Columbian ground squirrels for 2 years to examine both the proximate energetic and ultimate fitness (i.e. survival and breeding) costs of reproduction. We predicted that females raising augmented litters would have the highest rates of daily energy expenditure and as a result would experience lower survival rates or future fecundity. Females raising augmented litters weaned more pups, had the highest litter masses at weaning, and had field metabolic rates that were almost 1·5 times greater than females raising control or reduced litters. Contrary to our prediction, there were no negative impacts of greater maternal investment and higher energy expenditure on the probability of maternal survival or future reproduction. Pups from augmented litters grew more slowly during the lactation period were smaller at weaning and had a lower probability of survival over‐winter. Thus, although females were capable of raising more young than they gave birth to without short‐term costs of reduced survival or fecundity, our observations suggest that limitations to litter size are not due to a trade‐off in the allocation of energy, but rather due to the reduced survival of offspring from larger litters. Examining the proximate mechanisms hypothesized to underlie life‐history trade‐offs can be challenging but is critical for a comprehensive understanding of the evolution of life histories.

The Influence of Nonrandom Mating on Population Growth

When nonrandom mating alters offspring numbers or the distribution of offspring phenotypes, it has the potential to impact the population growth rate. Similarly, sex-specific demographic parameters that influence the availability of mating partners can leave a signature on the population growth rate. We develop a general framework to explore how mating patterns and sex differences influence the population growth rate. We do this by constructing a two-sex integral projection model to explore ways in which altering the mating behavior from random to nonrandom mating (assortative, disassortative, or selection for size) and altering demographic parameters in one or both sexes (growth, survival, and parental contribution to offspring phenotype) affect the population growth rate. We demonstrate our framework using data from a population of Columbian ground squirrels. Our results suggest that the population growth rate is substantially affected when nonrandom mating is linked to sex differences in demographic parameters or parental contributions to offspring phenotype, but interestingly, the effect of the mating pattern alone is rather small. Our results also suggest that the population growth rate of Columbian ground squirrels would increase with the degree of disassortative mating and with the degree of the mating advantage of large individuals.

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.

A quantitative genetic analysis of hibernation emergence date in a wild population of Columbian ground squirrels

The life history schedules of wild organisms have long attracted scientific interest, and, in light of ongoing climate change, an understanding of their genetic and environmental underpinnings is increasingly becoming of applied concern. We used a multi-generation pedigree and detailed phenotypic records, spanning 18 years, to estimate the quantitative genetic influences on the timing of hibernation emergence in a wild population of Columbian ground squirrels (Urocitellus columbianus). Emergence date was significantly heritable [h(2) = 0.22 ± 0.05 (in females) and 0.34 ± 0.14 (in males)], and there was a positive genetic correlation (r(G) = 0.76 ± 0.22) between male and female emergence dates. In adult females, the heritabilities of body mass at emergence and oestrous date were h(2) = 0.23 ± 0.09 and h(2) = 0.18 ± 0.12, respectively. The date of hibernation emergence has been hypothesized to have evolved so as to synchronize subsequent reproduction with upcoming peaks in vegetation abundance. In support of this hypothesis, although levels of phenotypic variance in emergence date were higher than oestrous date, there was a highly significant genetic correlation between the two (r(G) = 0.98 ± 0.01). Hibernation is a prominent feature in the annual cycle of many small mammals, but our understanding of its influences lags behind that for phenological traits in many other taxa. Our results provide the first insight into its quantitative genetic influences and thus help contribute to a more general understanding of its evolutionary significance.

Maternal influences on reproduction in two populations of Columbian ground squirrels

In this study, we examined influences of maternal traits on offspring birth mass, growth rate, and weaning mass for two populations of Columbian ground squirrels (Spermophilus columbianus). We tested relationships between maternal body condition, structural size, change in mass (during gestation, during lactation, and during the entire reproductive period), timing of reproduction, and litter size on offspring traits using path analyses. To assess whether maternal investment in offspring traits extended beyond the period of direct maternal care, we examined associations between offspring traits and overwinter survival of pups. In general, females in better condition raised pups that were heavier at weaning and that had faster growth rates during lactation. Litter size had a negative effect on mass and growth rate, and only litter size had a significant effect on birth mass. For both populations, the average weaning mass of pups within a litter had a positive effect on the number of pups that survived to yearling age. In a population for which birth masses and growth rates were available, pups with faster growth rates survived better to yearling age, whereas birth mass had no effect on the number of surviving offspring in litters. We found substantial maternal influences on offspring growth and size, and evidence that these influences may extend beyond the juvenile period and constitute influences on fitness. The key to arriving at these conclusions was to take the number of offspring into account before testing for maternal effects.

Mechanisms of the group-size effect on vigilance in Columbian ground squirrels: dilution versus detection

Antipredator vigilance decreases with group size in many vertebrate prey species. This pattern might be explained by collective detection (increased probability of observing a predator) or risk dilution (decreased likelihood of being captured with larger group size). If dilution is the main cause of the group-size effect, nearest-neighbour distances and distance to the edge of the group would affect vigilance, and alarm calling and factors that affect hearing alarm calls would not affect vigilance. The opposite outcomes are expected if detection is the primary influence. We used both model fitting and hypothesis testing to determine which mechanism had the greater effect, using 14 populations of Columbian ground squirrels, Spermophilus columbianus . Vigilance decreased significantly with increases in group size ( R 2 = 0.70, P = 0.0002). Model fitting revealed that the detection model was a statistically good fit to our data and the dilution model was not. We found no significant effect of nearest-neighbour distance on the proportion of time vigilant. Distance of individuals to the edge of the meadow habitat had a marginally significant effect on vigilance in one analysis, but no significant effect in three other analyses. Alarm calls occurred in 56.1% of observations and had a significant effect on vigilance. Wind speed did not show a significant effect on proportion of time vigilant during foraging bouts (mixed models: F = 2.41, P = 0.12), but squirrels were observed to forage less in high wind. All of these results support the detection hypothesis. Group type and means of information transfer about predators may be indicators of which mechanism, detection or dilution, has the greatest influence on the group-size effect on vigilance.

Age of primiparity and implications of early reproduction on life history in female Columbian ground squirrels

Summary Factors leading to differences in age of primiparity and possible life‐history implications arising from these differences were studied in a population of Columbian ground squirrels (Spermophilus columbianus; Ord, 1815). We used data collected from 1992 to 2000 on two neighbouring colonies of ground squirrels in south‐eastern Alberta, Canada. Earlier born females were more likely to wean successfully a litter at the age of 2. Among 2‐year‐old females, those who emerged earlier and heavier from hibernation were more likely to wean offspring than those females who emerged later and lighter from hibernation. The presence of the mother had a significant positive impact on reproductive output in 2‐year‐old females. Although there was a positive correlation between lifetime reproductive success and age at death, there was no difference in lifetime reproductive success or in age at death, depending on age of primiparity. For individual females, litter size at weaning at 2 years old did not differ from that at 3 years old. We conclude that body condition, presence or absence of the mother and environmental conditions are the main factors leading to differences in age of first successful reproduction in Columbian ground squirrels. Previous breeding experience does not seem to play a role in determining reproductive success. Finally, differences in age of first successful reproduction do not seem to have an effect on lifetime reproductive success or longevity.

Increasing returns in the life history of Columbian ground squirrels

1. We examined positive associations and trade‐offs of maternal and reproductive traits in a population of Columbian ground squirrels, Spermophilus columbianus. 2. Structural size, body condition, mother's personal allocation to body mass during reproduction, and timing of littering were estimated for live‐trapped reproductive females that were observed during an 8‐year period, and were compared to litter mass, litter size, and average pup mass using path analyses. 3. Mothers exhibited age‐structured traits that influenced reproductive patterns. Yearling mothers were significantly smaller, bred later, and had smaller litters than older females. Mothers that gained more body mass during reproduction and older mothers in good body condition that were structurally large had larger litters. 4. Early seasonal timing of littering was an important positive influence on successful reproduction by older mothers only in early breeding seasons and in years when conditions for reproduction were good for all females. 5. The number of offspring that survived to 1 year of age was most strongly associated with litter mass and litter size; date of breeding was of secondary influence, with earlier litters exhibiting greater success. 6. In general, mothers that gained the most in body mass during reproduction were concurrently more successful in weaning large litters (perhaps due to better quality of foraging habitat). 7. In addition to expected reproductive trade‐offs, reproduction by Columbian ground squirrels exhibited positive associations of life‐history traits that may reflect evolutionary increasing returns.

High incidence of infanticide by lactating females in a population of Columbian ground squirrels ( Spermophilus columbianus)

This is the first record of cercariae of Monascus filiformis (Rudolphi, 1819) and of Chaetognatha as a secondintermediate host in the southwest Atlantic Ocean. The morphology of the sporocyst and cercaria from Nucula obliqua (Bivalvia:Nuculidae) and a full description of the metacercaria from hydromedusae are given. The life cycle of M. filiformis involves threehosts. The bivalve N. obliqua is the first intermediate host, Chaetognatha and medusae are the second intermediate hosts, and thejurel Trachurus lathami (Pisces: Carangidae) is the final host. The life cycle of M. filiformis occurs in shallow waters in theArgentine Sea and differs from Køie’s experimental scheme for the North Sea in the addition of planktonic invertebrates assecond intermediate hosts. The life cycle proposed here follows the general pattern given for the family Fellodistomidae.

High incidence of infanticide by lactating females in a population of Columbian ground squirrels ( Spermophilus columbianus)

High incidence of infanticide by lactating females in a population of Columbian ground squirrels ( Spermophilus columbianus)

Is Mean Litter Size the Most Productive? A Test in Columbian Ground Squirrels

We studied the evolution of litter size in natural and experimentally manipulated populations of Columbian ground squirrels (Spermophilus columbianus) in eastern Washington state and southwest Alberta. Litter size at weaning in a large natural population (mean = 3.51 pups/litter, 248 litters) was significantly lower than the litter size (6 pups) that produced the most offspring surviving to yearling age class. This evidence contradicted Lack's "optimal litter size" hypothesis, which predicts that the most productive litter size should approximate the mean. Litter size had no significant effect on the subsequent survival or reproduction of mothers, contrary to the negative effects predicted by the "cost of reproduction" hypothesis. Litter success varied among years, and good and bad years for reproduction could be experimentally simulated with food supplementation. However, small samples of the largest litters rendered the "bad—years" hypothesis inappropriate for application to our data. Proportional survival of offspring was relatively constant among different litter sizes, contrary to the necessary condition of the "cliff edge" hypothesis of a dramatic decrease in survival of young from the largest litters. The data supported the "individual optimization" hypothesis most strongly. As predicted, the number of surviving offspring increased with litter size, although in one population the six largest litters (2.4% of 248 litters) suffered reduced success. In natural and food—supplemented populations, changes in maternal body mass were associated with changes in litter size (r = 0.205 to 0.926). Because survival of young remained relatively constant as litter size increased, these correlations resulted in greater offspring success for mothers in better body condition.

Measures of gene flow in the Columbian ground squirrel

From analyses of published data and a review of the literature, I studied indirect and direct measures of gene flow among populations of Columbian ground squirrels, Spermophilus columbianus. New analyses were used to examine an allozyme data set (seven polymorphic loci) that had been collected by Zammuto and Millar (1985a) from six populations of ground squirrels that were spread over 183 km. G-tests indicated significant variation in allele frequencies among populations, but F-statistics revealed relatively little population differentiation (average F ST=0.026). F ST values were used to estimate rates of gene flow indirectly and indicated fairly high rates of gene flow (average N e m=13.5). Recorded dispersal distances of individual ground squirrels were fairly short (most<4 km, maximum recorded distance was 8.5 km), and the minimum distance between populations used to create the allozyme data set was about 25 km. Thus, direct dispersal among the populations in the allozyme data set was highly unlikely. Small genetically effective populations may have experienced high rates of migration over short distances (about 43% of adults in local populations were immigrants), however, resulting in homogeneous allele frequencies over the geographic range. This explanation provides an alternative to invoking gene flow in the recent past to explain discrepancies between dispersal distances in the field and homogenization of allele frequencies over large ranges, Mammalian species that have virtually complete dispersal of subadult males from the natal area might be expected to exhibit relatively high rates of gene flow, regardless of actual dispersal distances. Genetically effective populations may be much smaller than more extensive ecological populations and experience higher rates of gene flow.

Effects of Litter Size and Population Dynamics on Juvenile and Maternal Survival in Columbian Ground Squirrels

(1) The relationships between litter size and juvenile and maternal survival were investigated for 9 years in a marked population of Columbian ground squirrels (Spermophilus columbianus Ord.) in south-western Alberta, Canada. (2) A population increase during the first 4 years was characterized by larger litter size, greater adult female body weight and greater survival than in the last 4 years, during which the population decreased. (3) During the increase phase, modal litter size at emergence from the natal burrow was four, and the number of juveniles surviving to the following spring increased with litter size. During the decrease, modal litter size was three, and juveniles in large litters experienced high mortality. (4) Maternal survival was independent of litter size during the increase phase, but declined for litters larger than three in the decrease phase. The fitness costs of different litter sizes seem to vary according to population dynamics, possibly as a result of changes in resource availability.

Sociality of Columbian ground squirrels in relation to their seasonal energy intake.

Seasonal energy intake was estimated for ten populations of Columbian ground squirrels (Spermophilus columbianus) in northwestern Montana. We calculated daily energy intake for an average ground squirrel in each population using measurements of feeding time, consumption rates of different vegetation types (monocots vs. dicots), and the proportion of monocots and dicots in the diet. These daily energy intakes were multiplied by the length of the plant growing season for each population to estimate seasonal energy intake, i.e. over the ground squirrel active season. Amicable interaction rates measured for each population varied with seasonal energy intake, but not with environmental heterogeneity, sex ratio, or the ratio of adults to juveniles. In particular, amicable interactions among adult-juvenile and juvenile-juvenile pairs increased as seasonal energy intake decreased. The proportion of females breeding as yearlings increased as seasonal energy intake increased. This suggests that "harsh" environments reduce the energy available for juvenile growth and development, leading to delayed dispersal and age at first reproduction. These responses may promote the formation of kin groups and increased amicable interactions within those groups. The length of the plant growing season may determine environmental "harshness" across elevational gradients, but at a particular elevation, "harshness" may depend on factors determining daily food intake.

Environmental Influences on Infanticide in Columbian Ground Squirrels

AbstractInfanticide of preweaned and weanling juvenile Columbian ground squirrels (Spermophilus columbianus) has been previously reported, but its prevalence in most populations is poorly known. I examined evidence of infanticide in a six‐year study of four populations in southwestern Alberta, two of which received an experimental supplementation of food for two years. Evidence of infanticide of weanling ground squirrels was uncommon, except for one population and year (previously supplemented population at 1580 m elevation in 1984) in which 9 dead young were found, suggesting an increased rate of infanticide. The rate of total loss of pre‐weaned litters also increased at 1580 m in 1984. At higher elevation sites (2100 m), however, no evidence of infanticide was found under experimental or unmanipulated conditions. One factor that might have influenced the rate of infanticide of weanlings was their degree of development when they first emerged from natal burrows. A review of studies of 11 populations of Columbian ground squirrels suggested that under natural conditions, infanticide may have differed among environments. Differences in density among populations appeared to have little influence on the rate of infanticide, but elevation was significantly negatively associated with infanticide.

Spacing of female kin in Columbian ground squirrels (Spermophilus columbianus)

The dispersion of matrilineally related females was examined within a high-density population of Columbian ground squirrels (Spermophilus columbianus) in southwestern Alberta. Females tended to nest near their natal site when no other female occupied it. Breeding females spaced themselves evenly so that close relatives formed intact groups but not dense clusters. The kin group was typically composed of a mother with one or two daughters that were nonlittermate sisters. Females appeared to be attracted to natal areas and the resources they contained, perhaps burrows.

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.