Microgeographic genetic structure in the yellow-pine chipmunk (Tamias amoenus)

While there is evidence for broad-scale genetic structure in small mammals, few studies have used variable DNA-based genetic markers to examine genetic differentiation at microgeographic (tens of kilometres) scales. Yellow-pine chipmunks (Tamias amoenus) live in the heterogeneous landscape of the Rockies in southwest Alberta and are generally restricted to areas of low elevation. We used seven microsatellite loci to determine whether chipmunks show evidence of population genetic structure among three closely situated sites (< 15 km) in the Kananaskis Valley, Alberta. We found evidence for genetic structure in the form of significant differences in allele frequencies among populations and significantly nonzero values of FST for both overall and pairwise population comparisons. However, FIS values for each population were not significantly different from zero, suggesting little evidence for inbreeding within populations. Genetic differentiation probably occurs as a result of the strong effect of drift in very small (N(e) approximately 25) populations of these animals even in the face of substantial immigration rates.     

Linking summer foraging to winter survival in yellow pine chipmunks (Tamias amoenus)

This study links summer foraging and scatter-hoarding to winter larder-hoarding and winter survival in yellow pine chipmunks (Tamias amoenus) by comparing patterns of time allocation and winter larder contents in 2 years with very different levels of resource availability. In 2003, seed production and the number of trees and shrubs producing seeds were high. In 2004 seed crops were small. Chipmunks allocated more time to foraging when food resources were scarce (66% in 2004) compared to when they were abundant (39% in 2003). Increased time allocated to foraging in 2004 corresponded to significant decreases in time allocated to vigilance, resting, and social interactions. When seeds were scarce (i.e., in autumn 2004), chipmunks spent more time searching for cached food items than gathering seeds from plants or the ground surface. Despite the increase in foraging effort, the edible mass and caloric contents of larders were significantly smaller in 2004. In the year with low seed production, the diversity of seed species found in larders increased, and many of these seeds were of species that ripened in summer. When autumnal seed production by Jeffrey pine seeds was high, Jeffrey pine seeds were nearly the exclusive food item found in larders. Larder contents would have provisioned chipmunks for an estimated 116-257 days in 2003 and but only 6-111 days in 2004. It is likely that all chipmunks would have survived the winter of 2003 (duration ~110-120 days). However, none of the larders recovered in 2004 contained enough food to have provisioned the inhabitant for the ~148-158 days of winter.     

Effects of experimentally elevated testosterone on plasma glucocorticoids, body mass, and recapture rates in yellow-pine chipmunks, Tamias amoenus

In male yellow-pine chipmunks plasma levels of glucocorticoids (GCs) are low while plasma testosterone (T) levels peak during the mating season, suggesting that T suppresses GC levels. To test this hypothesis, free-living, post-reproductive males were implanted during summer with either a T-filled (T-males) or an empty silastic implant (controls or C-males). Body mass and plasma levels of corticosterone, cortisol, and T were measured immediately before and 1 month after implantation. Exogenous testosterone increased T to high physiological levels typical of reproductively active males. By 1 month after implantation, T-males decreased their mean body mass and plasma GC levels, while C-males maintained their mean body mass and GC levels. Even though T-males lost mass, recapture success 1 month after implantation for T-males (71%) was equal to that of C-males (71%). However, the overwinter recapture rate of C-males (83%) was significantly greater than that of T-males (20%). The results support the hypothesis that high plasma T of males during mating has a suppressive effect on plasma GC levels. Additionally, experimentally elevated T significantly reduced the rate of recapture during the following spring, and this may reflect a reduction in local overwinter survival. The suppression of adrenocortical activity by T may contribute to the reductions in prehibernation body mass and post-emergence recapture success. J. Exp. Zool. 287:378-383, 2000.     

Extensive mtDNA variation within the yellow-pine chipmunk,Tamias amoenus (Rodentia: Sciuridae), and phylogeographic inferences for northwest North America

The yellow-pine chipmunk, Tamias amoenus, is common in xerophytic forests throughout much of northwest North America. We analyzed cytochrome b sequence variation from 155 individuals representing 57 localities across the distribution of T. amoenus including 10 additional species of Tamias. Maximum likelihood and parsimony tree estimation methods were used in conjunction with nested clade analysis to infer both deep and population-level processes. Our results indicate that two currently recognized subspecies of T. amoenus (T. a. canicaudus and T. a. cratericus) are not nested within other samples of T. amoenus. Maximum uncorrected levels of intraspecific sequence divergence within remaining samples of T. amoenus are >7%. Substantial geographic variation is characterized by 12 well-supported clades that correspond to distinct mountain ranges, but do not necessarily follow existing subspecific taxonomy. Significant association between geography and genealogy was detected within many of these clades and can be attributed to different population-level processes including past fragmentation, recent range expansion, and isolation by distance.     

Female-biased sexual size dimorphism in the yellow-pine chipmunk (Tamias amoenus): sex-specific patterns of annual reproductive success and survival

Sexual size dimorphism is ultimately the result of independent, sex-specific selection on body size. In mammals, male-biased sexual size dimorphism is the predominant pattern, and it is usually attributed to the polygynous mating system prevalent in most mammals. This sole explanation is unsatisfying because selection acts on both sexes simultaneously, therefore any explanation of sexual size dimorphism should explain why one sex is relatively large and the other is small. Using mark-recapture techniques and DNA microsatellite loci to assign parentage, we examined sex-specific patterns of annual reproductive success and survival in the yellow-pine chipmunk (Tamias amoenus), a small mammal with female-biased sexual size dimorphism, to test the hypothesis that the dimorphism was related to sex differences in the relationship between body size and fitness. Chipmunks were monitored and body size components measured over three years in the Kananaskis Valley, Alberta, Canada. Male reproductive success was independent of body size perhaps due to trade-offs in body size associated with behavioral components of male mating success: dominance and running speed. Male survival was consistent with stabilizing selection for overall body size and body size components. The relationship between reproductive success and female body size fluctuated. In two of three years the relationship was positive, whereas in one year the relationship was negative. This may have been the result of differences in environmental conditions among years. Large females require more energy to maintain their soma than small females and may be unable to maintain lactation in the face of challenging environmental conditions. Female survival was positively related to body size, with little evidence for stabilizing selection. Sex differences in the relationship between body size and fitness (reproductive success and survival) were the result of different processes, but were ultimately consistent with female-biased sexual size dimorphism evident in this species.     

Reproduction of Asian chipmunks (Tamias sibiricus) in captivity

Factors affecting reproduction in captive Asian chipmunks, Tamias sibiricus, were examined by means of a survey of chipmunk breeders in Great Britain. Sixteen breeders were asked about the conditions under which their chipmunks were kept and their success in breeding them. Results covered 205 female-years of pairing. Breeding was promoted by large cage size and early weaning of young, and inhibited by the presence of other rodent species nearby and by extended photoperiod. A diet of seeds and nuts, with some fresh fruits and vegetables, appeared to be adequate for breeding. Breeding occurred in 80–91& of female-years when neither extra lighting nor other rodents were present, irrespective of cage size. Weaning age affected the occurrence of second litters in a year. Second litters were born less often (0–13&) of breeding (female-years) if young of the first litter remained with the mother 8 weeks or longer. Second litters were more frequent (14–45& of breeding female-years) if first litters were removed by 6.5 weeks of age, and this frequency increased with cage size. Litter size also increased with cage size (means ranged from 3 to 7). Overall breeding success increased with cage size and decreased with the presence of other rodents. In most colonies without other rodents near, mean success rate ranged from 3.1 to 8.5 young/female-year. In colonies with other rodents present mean success rate varied from 0 to 3.3 young/female-year. It is suggested that the mechanisms that control population size in the wild also act to inhibit or promote breeding in captivity.     

The influence of environmental conditions on cache recovery and cache pilferage by yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus)

I conducted a field experiment in 10 x 10 m enclosures to explorehow seed and soil moisture levels influence the ability ofknowledgeable and naive rodents to find natural caches of Jeffreypine (Pinus jeffreyi) seeds. Subjects were yellow pine chipmunks(Tamias amoenus) and deer mice (Peromyscus maniculatus) searchingfor caches that they had made, caches made by other individualsof the same species, or caches made by individuals of the otherspecies. Subjects that made caches (knowledgeable subjects)relied on spatial memory to find many of their own caches during recovery sessions, and their ability to locate caches was notaffected by water content of seeds or soil. Naive subjectsfound few caches under dry conditions, but under wet conditions,they located as many caches as did the rodents that made them.Naive subjects apparently relied on olfaction to find caches,a sensory modality that works more effectively under moist conditions. Subjects had as much success foraging for caches made by membersof their own species as for caches made by the other species.I present a hypothesis that predicts how foragers could modifypredominately memory-based search to predominately olfactory-basedsearch as the weather changes from dry to wet. When foragersrely on spatial memory, those foragers find only their own caches, but when they can also use olfaction, they pilfer cachesmade by other individuals. Consequently, the nature of competitiveinteractions among members of the seed-caching guild may changeas the weather changes.     

Positional behavior of Siberian chipmunks (Tamias sibiricus) in captivity

Arboreal and semi-arboreal mammals have remarkably diverse positional behavior and associated morpho-functional adaptations related to the three-dimensional nature of their arboreal habitat. In this context, we investigated the positional behavior of captive Siberian chipmunks (Tamias sibiricus), small bodied semi-arboreal sciurids, in an aviary-type wire-mesh cage containing both terrestrial and arboreal supports. We sampled four adult individuals during a five-month period using focal animal sampling every 30 s. Results showed that animals preferred 8–10 cm horizontal supports and always avoided vertical supports. Locomotion occurred on both terrestrial and 8–10 cm arboreal supports whereas postural behavior occurred primarily on 8–10 cm arboreal supports. Quadrupedal walk dominated during locomotion, and occurred primarily on terrestrial horizontal supports, as is observed for other squirrels. The predominance of quadrupedal locomotion is consistent with the postcranial morphology of chipmunks. In contrast, clawed locomotion occurred on wire mesh and on >13 cm arboreal vertical supports. Finally, pronograde and orthograde sitting, both on 8–10 cm arboreal supports and on terrestrial supports, were the predominant postures, implying general predisposition to selection of stable postures on stable supports for food item manipulation and ingestion.     

Characterization of 12 polymorphic microsatellite loci for eastern chipmunks (Tamias striatus)

Eastern chipmunks are small forest rodents for which ecological data at multiple spatial scales exist, but no molecular markers have been developed that provide adequate resolution to examine genetic structure at fine scales in this species. Thus, we developed 12 variable microsatellite loci, which amplified in three multiplexed reactions, and screened them in 27 individuals from northern Indiana, USA. Observed heterozygosities across loci ranged from 0.26 to 0.91 and number of alleles per locus ranged from three to 10. We will use these markers along with ecological data to understand population dynamics of eastern chipmunks at small spatial scales.     

Phylogeography and introgressive hybridization: chipmunks (genus Tamias) in the northern Rocky Mountains

Abstract. If phylogeographic studies are to be broadly used for assessing population-level processes relevant to speciation and systematics, the ability to identify and incorporate instances of hybridization into the analytical framework is essential. Here, we examine the evolutionary history of two chipmunk species, Tamias ruficaudus and Tamias amoenus , in the northern Rocky Mountains by integrating multivariate morphometrics of bacular (os penis) variation, phylogenetic estimation, and nested clade analysis with regional biogeography. Our results indicate multiple examples of mitochondrial DNA introgression layered within the evolutionary history of these nonsister species. Three of these events are most consistent with recent and/or ongoing asymmetric introgression of mitochondrial DNA across morphologically defined secondary contact zones. In addition, we find preliminary evidence where a fourth instance of nonconcordant characters may represent complete fixation of introgressed mitochondrial DNA via a more ancient hybridization event, although alternative explanations of convergence or incomplete sorting of ancestral polymorphisms cannot be dismissed with these data. The demonstration of hybridization among chipmunks with strongly differentiated bacular morphology contradicts long-standing assumptions that variation within this character is diagnostic of complete reproductive isolation within Tamias . Our results illustrate the utility of phylogeographic analyses for detecting instances of reticulate evolution and for incorporating this and other information in the inference of the evolutionary history of species.     

Milk composition of a free-ranging African elephant (Loxodonta africana) cow during early lactation

Only one study previously reported comprehensively on the composition of African elephant's (Loxodonta africana) milk that was collected from 30 dead animals. In the current study milk was obtained from a tame but free-ranging African elephant cow without immobilization during the period when she was 4-47 days postpartum. At the respective collection times the nutrient content was 21.8 and 25.0 g protein; 56.0 and 76.0 g fat; 71.1 and 26.0 g sugars per kilogram of milk. The protein fraction, respectively, consisted of 10.0 and 14.0 g caseins/kg milk and of 11.8 and 11 g whey proteins/kg milk. During lactation the lactose content dropped from 52.5 to 11.8 g/kg milk, while the oligosaccharide content increased from 11.8 to 15.2 g/kg milk. The oligosaccharide was characterized as a galactosyllactose, which is digestible by cellulase. Electrophoresis and identification of protein bands showed a similar migrating sequence of proteins as seen in cow's milk, but some of the corresponding proteins were less negatively charged. The lipid fraction contains a high content of capric and lauric acids, approximately 60% of the total fatty acids, and low content of myristic, palmitic and oleic acids.     

Milk composition of three free-ranging African elephant (Loxodonta africana africana) cows during mid lactation

Data are presented that indicate the dynamic changes of nutrients in milk from three free ranging African elephant (Loxodonta africana africana) cows during lactation. At the respective collection times of 12, 14 and 18 months of lactation the nutrient content was 47.3, 52.0 and 68.6 g protein; 60.7, 87.4 and 170.8 g fat; 1.6, 2.1 0.5 g lactose and 20.9, 21.5 and 8.6 g oligosaccharides per kg milk. The protein fraction respectively consisted of 18.0, 31.7 and 45.9 g caseins/kg milk and of 29.3, 20.3 and 22.7 g whey proteins/kg milk. Electrophoresis and identification of protein bands showed that polymorphs of one whey protein may be present in elephant's milk similar to polymorphs of alpha-lactalbumin found in cow's milk. From the middle of the lactation time lactose was replaced by oligosaccharides as major carbohydrate, and the major compound of these was identified as isoglobotriose by 1H NMR spectroscopy. The lipid fraction contains a high content, of capric and lauric acids, approximately 70% of the total fatty acids, and low content of myristic, palmitic and oleic acids. During these lactation times the content of short chain fatty acids, capric and caprylic acids increased, while fatty acids lauric acid and longer decreased.     

Assessment of gene flow across a hybrid zone in red-tailed chipmunks (Tamias ruficaudus)

The role of hybridization in animal speciation is controversial and recent research has challenged the long-standing criterion of complete reproductive isolation to attain species status. The speciation-with-gene-flow model posits that the genome is semi-permeable and hybridization may be a phase in the process of divergence. Here, we apply these concepts to a previously identified zone of mtDNA introgression between the two strongly morphologically differentiated subspecies of red-tailed chipmunk ( Tamias ruficaudus ) in the US Inland Northwest. Using multilocus genotype data from the southern, older contact zone, we demonstrate that neutral gene flow is unusually low between the subspecies across the Lochsa River. This is geographically congruent with the discontinuity in bacular morphology, indicating that the cline of mitochondrial DNA (mtDNA) haplotypes is displaced. Furthermore, we elucidate the evolutionary forces responsible by testing hypotheses of lineage sorting and hybridization. We determined that introgressive hybridization is the cause of mtDNA/morphology incongruence because there are non-zero levels of migration and gene flow. Although our estimate of the age of the hybrid zone has wide credibility intervals, the hybridization events occurred in the Late Pleistocene and the divergence occurred in the Middle Pleistocene. Finally, we assessed substructure within and adjacent to the hybrid zone and found that the hybrid zone constitutes a set of populations that are genetically differentiated from parental sets of populations; therefore, hybridization in this system is not likely an evolutionary sink, but has generated novel combinations of genotypes.     

Evolutionary processes and its environmental correlates in the cranial morphology of western chipmunks (Tamias)

The importance of the environment in shaping phenotypic evolution lies at the core of evolutionary biology. Chipmunks of the genus Tamias (subgenus Neotamias) are part of a very recent radiation, occupying a wide range of environments with marked niche partitioning among species. One open question is if and how those differences in environments affected phenotypic evolution in this lineage. Herein we examine the relative importance of genetic drift versus natural selection in the origin of cranial diversity exhibited by clade members. We also explore the degree to which variation in potential selective agents (environmental variables) are correlated with the patterns of morphological variation presented. We found that genetic drift cannot explain morphological diversification in the group, thus supporting the potential role of natural selection as the predominant evolutionary force during Neotamias cranial diversification, although the strength of selection varied greatly among species. This morphological diversification, in turn, was correlated with environmental conditions, suggesting a possible causal relationship. These results underscore that extant Neotamias represent a radiation in which aspects of the environment might have acted as the selective force driving species’ divergence.     

`Little Chipmunk' Syndrome? Male Body Size and Dominance in Captive Yellow‐Pine Chipmunks (Tamias amoenus)

In mammals, large males are often assumed to have higher mating success because they have greater success at contest competition. This relationship is often used to explain the prevalence of male-biased sexual size dimorphism in mammals. However, in many small vertebrates, large individuals are not always dominant. Using staged dyadic encounters, we examined the relationship between male body size and social dominance in captive male yellow-pine chipmunks ( Tamias amoenus ), a species with female-biased sexual size dimorphism. The yellow-pine chipmunk has a mating system in which males participate in mating chases and dominant males may have an advantage in acquiring matings with oestrous females. Captive male chipmunks were aggressive in only 28% of 144 paired encounters; however, several lines of evidence indicated that smaller chipmunks were dominant over large chipmunks: (1) small males were dominant in more dyads than large males; (2) within dyads, dominant males were smaller than subordinate males; and (3) small males performed more aggressive behaviour than large males. These results are not consistent with the prediction that large males are typically dominant. If large chipmunks are able to gain matings with females because of qualities other than dominance (such as the ability to successfully find and/or chase receptive females), then the costs of aggression to large chipmunks may outweigh any potential benefits. Small males, but not large males, may improve their mating success by being aggressive.     

Water flux and mass gain during lactation in free-living ringed seal (Phoca hispida) pups

Three ringed seal ( Phoca hispida ) pups were used to calculate water flux and milk intake, based on mass increase and dilution of injected tritiated water. Biological half life of tritium in ringed seal pups was 130 ± 17 h (mean ± S.D.). The plateau level of the isotope indicating equilibrium with the body water was, in all cases (N = 8), reached before the first blood sample was collected after 0·5 h. Daily water flux in the pups was 62·9 ± 21·5 ml kg^-1. Estimated daily milk intake of 1103 ± 388 ml milk resulted in a mass increase of 386 ± 104 g. Neonates had a fat content of 4·75%, and a water content of 70·1% (N = 3). Corresponding figures for a pup close to weaning were 36·5% and 46·3%. Isotope dilution calculations of body water gave an overestimate of 1·6% compared with body water content from desiccation. Ringed seal milk was found to contain 38·1% fat, 9·9% protein, 2·3% lactose and 1% ash.