Stephen H. Jenkins

Individual Variation in Mammals

The study of individual variation offers an underexploited wealth of opportunities for mammalogists. This paper addresses recent developments in the study of both intra- and inter-individual variation. After reviewing several methods (e.g., intraclass correlation, product-moment correlation, and confirmatory factor analysis) for quantifying intra-individual consistency or repeatability, we discuss how these measures of repeatability can serve as guides for appropriately defining traits and how they may be helpful in ensuring that appropriate statistical models are used (e.g., in accounting for measurement errors in regression analyses). We discuss three aspects of inter-individual variation; phenotypic selection, alternative individual strategies and phenotypic integration, and quantitative genetic analyses. The value of these approaches for studying inter-individual variation is illustrated with recent examples from the literature. Finally, we discuss how many field studies of mammals may be well poised to exploit the unique insights that can be gained from studying individual variation.

A SIZE-DISTANCE RELATION IN FOOD SELECTION BY BEAVERS'

The relationship between sizes of trees cut by beavers and distances from the borders of their ponds was examined at three sites in central Massachusetts. For most tree genera, the beavers cut a smaller range of sizes far from shore than close to shore, and relatively more small trees and fewer large trees at greater distances. The second of these results differs from the pattern of preference found in other studies in which predators were much larger than their prey, unlike this case of beavers feeding on trees. Both kinds of results are consistent with an optimal foraging model of size—distance relations in which pursuit or provisioning time depends on size of prey as well as distance (Schoener 1979).

Review and Meta-Analysis of Camera Effects on Avian Nest Success

Abstract Identifying nest predators is critical to understanding predation pressures that birds face, and using surveillance cameras appears to be the most reliable method of nest predator identification. However, presence and methods of using camera equipment may introduce bias in predation rates. To summarize potential effects of cameras on nest success we reviewed published and unpublished studies that estimated daily nest predation for bird nests with and without surveillance cameras. We used meta-analyses to quantitatively synthesize the direction and magnitude of these effects from independent studies. We found evidence that, on average, use of camera equipment may reduce nest predation rates, although these differences were not always significant and varied relative to geographic regions, vegetation types, and study duration. Researchers using camera surveillance to monitor nests must be aware that the equipment may be affecting rates of predation and possibly biasing data collected on predator identity. Based on our review and analysis, we provide recommendations for researchers seeking to minimize or control for potential bias when using surveillance cameras to monitor nest predation.

Optimal central-place foraging by beavers: tree-size selection in relation to defensive chemicals of quaking aspen

SummaryAt a newly occupied pond, beavers preferentially felled aspen smaller than 7.5 cm in diameter and selected against larger size classes. After one year of cutting, 10% of the aspen had been cut and 14% of the living aspen exhibited the juvenile growth form. A phenolic compound which may act as a deterrent to beavers was found in low concentrations in aspen bark, and there was no significant regression of relative concentration of this compound on tree diameter. At a pond which had been intermittently occupied by beavers for over 20 years, beavers selected against aspen smaller than 4.5 cm in diameter, and selected in favor of aspen larger than 19.5 cm in diameter. After more than 28 years of cutting at this site, 51% of the aspen had been cut and 49% of the living aspen were juvenileform. The phenolic compound was found in significantly higher concentrations in aspen bark than at the newly occupied site, and there was a significant negative regression of relative concentration on tree diameter. The results of this study show that responses to browsing by trees place constraints on the predictive value of standard energy-based optimal foraging models, and limitations on the use of such models. Future models should attempt to account for inducible responses of plants to damage and increases in concentrations of secondary metabolites through time.

Behavioural syndromes in Merriam's kangaroo rats (Dipodomys merriami): a test of competing hypotheses

Behavioural syndromes, correlations of behaviours conceptually analogous to personalities, have been a topic of recent attention due to their potential to explain trade-offs in behavioural responses, apparently maladaptive behaviour and limits to plasticity. Using Merriams kangaroo rats (Dipodomys merriami), we assessed the explanatory power and generality of hypothesized syndrome structures derived from the literature and the natural history of the species. Several aspects of functionally distinct behavioural responses of D. merriami were quantified. Syndrome structures were compared using structural equation modelling and model selection procedures. A domain-general behavioural syndrome incorporating cross-functional relationships between measures of boldness, agonistic behaviour, flexibility and food hoarding best explained the data. This pattern suggests that D. merriami behaviours should not be viewed as discrete elements but as components of a multivariate landscape. Our results support arguments that a lack of independence between behaviours may be a general aspect of behavioural phenotypes and suggest that the ability of D. merriamis behaviour to respond to selection may be constrained by underlying connections.

Seasonal and year-to-year differences in food selection by beavers

SummarySelection of trees for food by a colony of beavers in central Massachusetts was studied from September 1972 through April 1974. The beavers exhibited both seasonal and year-to-year differences in preference for certain genera. Pine was selected against during fall but not spring, and there was a switch in preference from birch during fall 1972 to oak and witch hazel in fall 1973. These differences may partly reflect greater seasonal stability in concentrations of stored nutrients in coniferous tree bark than in deciduous tree bark, and greater year-to-year stability in bark concentrations of stored nutrients in non-mast-seeding species than in mast-seeding species (1972 was a mast year for oaks in central Massachusetts).

Forest rodents provide directed dispersal of Jeffrey pine seeds

Some species of animals provide directed dispersal of plant seeds by transporting them nonrandomly to microsites where their chances of producing healthy seedlings are enhanced. We investigated whether this mutualistic interaction occurs between granivorous rodents and Jeffrey pine (Pinus jeffreyi) in the eastern Sierra Nevada by comparing the effectiveness of random abiotic seed dispersal with the dispersal performed by four species of rodents: deer mice (Peromyscus maniculatus), yellow-pine and long-eared chipmunks (Tamias amoenus and T. quadrimaculatus), and golden-mantled ground squirrels (Spermophilus lateralis). We conducted two caching studies using radio-labeled seeds, the first with individual animals in field enclosures and the second with a community of rodents in open forest. We used artificial caches to compare the fates of seeds placed at the range of microsites and depths used by animals with the fates of seeds dispersed abiotically. Finally, we examined the distribution and survival of naturally establishing seedlings over an eight-year period. Several lines of evidence suggested that this community of rodents provided directed dispersal. Animals preferred to cache seeds in microsites that were favorable for emergence or survival of seedlings and avoided caching in microsites in which seedlings fared worst. Seeds buried at depths typical of animal caches (5-25 mm) produced at least five times more seedlings than did seeds on the forest floor. The four species of rodents differed in the quality of dispersal they provided. Small, shallow caches made by deer mice most resembled seeds dispersed by abiotic processes, whereas many of the large caches made by ground squirrels were buried too deeply for successful emergence of seedlings. Chipmunks made the greatest number of caches within the range of depths and microsites favorable for establishment of pine seedlings. Directed dispersal is an important element of the population dynamics of Jeffrey pine, a dominant tree species in the eastern Sierra Nevada. Quantifying the occurrence and dynamics of directed dispersal in this and other cases will contribute to better understanding of mutualistic coevolution of plants and animals and to more effective management of ecosystems in which directed dispersal is a keystone process.

Food Hoarding by Merriam's Kangaroo Rats: A Test of Alternative Hypotheses

Merriams kangaroo rats (Dipodomys merriami) store seeds in burrows (lar- derhoarding) and in small clumps in shallow holes dug in the soil (scatterhoarding). We used a large laboratory arena with an artificial burrow for larderhoarding and four sand- filled compartments for scatterhoarding to test several alternative hypotheses about spatial patterns of food caching. The hypotheses were that kangaroo rats prefer to (1) larderhoard seeds in burrows, (2) scatterhoard seeds near burrows, (3) scatterhoard seeds away from burrows, (4) scatterhoard seeds near food sources, or (5) make widely spaced scatterhoards. Three treatments that differed in the distance between the burrow and food source were used to discriminate among hypotheses (2) through (4). There was a substantial amount of variation among individuals in proportion of seeds that were larderhoarded. Subjects initially scatterhoarded seeds close to the food source, but distributed caches more evenly among caching compartments as trials progressed. Increased evenness of cache distribution resulted from harvesting and repositioning of extant caches as well as selective placement of new caches. Initial caching close to food may be adaptive by maximizing harvest rates during flushes of seed production and making seeds unavailable to non-digging competitors (birds and ants). Subsequent redistribution of caches may make them less available to other rodents that locate buried seeds by smell and use area-restricted search to find closely spaced scatterhoards.

Sex and Age Affect Vulnerability of Desert Rodents to Owl Predation

Pelvic morphology was examined to distinguish sexes of three prey species eaten by great horned owls, and age classes of one of these species. Comparisons of sex ratios in prey populations with frequencies of males and females in owl diets indicated that male Dipodomys ordii and female Peromyscus maniculatus were more vulnerable to owl predation than the opposite sexes of these species. Furthermore, juvenile Microtus montanus were more vulnerable than adults and, within the juveniles, females were more vulnerable than males. We examined morphological (body size) and behavioral (degree of activity) attributes of these prey species, and concluded that either morphology or behavior may facilitate differential vulnerability in Dipodomys , whereas behavioral differences between sex and age classes are more likely to account for differential vulnerability in Microtus . For Peromyscus there are no body size or activity differences between sexes that are consistent with differential vulnerability, but body weights of female Peromyscus are more variable than weights of males, which could promote female-biased vulnerability if predators select large prey.

Developing multiple hypotheses in behavioral ecology

Researchers in behavioral ecology are increasingly turning to research methods that allow the simultaneous evaluation of hypotheses. This approach has great potential to increase our scientific understanding, but researchers interested in the approach should be aware of its long and somewhat contentious history. Also, prior to implementing multiple hypothesis evaluation, researchers should be aware of the importance of clearly specifying a priori hypotheses. This is one of the more difficult aspects of research based on multiple hypothesis evaluation, and we outline and provide examples of three approaches for doing so. Finally, multiple hypothesis evaluation has some limitations important to behavioral ecologists; we discuss two practical issues behavioral ecologists are likely to face.