Glucocorticoid stress hormones and the effect of predation risk on elk reproduction

Predators affect prey demography through direct predation and through the costs of antipredator behavioral responses, or risk effects. Experiments have shown that risk effects can comprise a substantial proportion of a predator's total effect on prey dynamics, but we know little about their strength in wild populations, or the physiological mechanisms that mediate them. When wolves are present, elk alter their grouping patterns, vigilance, foraging behavior, habitat selection, and diet. These responses are associated with decreased progesterone levels, decreased calf production, and reduced population size [Creel S, Christianson D, Liley S, Winnie JA (2007) Science 315:960]. Two general mechanisms for the effect of predation risk on reproduction have been proposed: the predation stress hypothesis and the predator-sensitive-food hypothesis. Here, we used enzyme immunoassay to measure fecal glucocorticoid metabolite concentrations for 1,205 samples collected from 4 elk populations over 4 winters to test the hypothesis that the effect of predation risk on elk reproduction is mediated by chronic stress. Across populations and years, fecal glucocorticoid concentrations were not related to predator-prey ratios, progesterone concentrations or calf-cow ratios. Overall, the effect of wolf presence on elk reproduction is better explained by changes in foraging patterns that carry nutritional costs than by changes in glucocorticoid concentrations.

Does habitat type modify group size in roe deer and red deer under predation risk by Iberian wolves?

We examined whether group size in red deer ( Cervus elaphus L., 1758) and roe deer ( Capreolus capreolus (L., 1758)) under predation risk by Iberian wolves ( Canis lupus L., 1758) is affected by the type of habitat in which the deer reside. We hypothesized that group size (i) would be larger in open than in closed habitats, since it is an antipredator response, and (ii) would vary more with habitat type in the species that had higher wolf predation rates. In the study area, wolves were the only predator of wild ungulates, with roe deer being the main target prey. We performed monthly transects along paths to observe the group size of red and roe deer. In roe deer, the mean group size was significantly higher in open than in closed habitats, serving as an antipredator response. However, in red deer, habitat type did not affect group size. The results indicate that under predation risk by wolves the habitat type influences the grouping behavior of roe deer but not red deer. Furthermore, compared with forests, heaths offer less protection from predators and species in this habitat would benefit from larger group sizes.

The social structure and strategies of delphinids: predictions based on an ecological framework

Dolphins live in complex social groupings with a wide variety of social strategies. In this chapter we investigate the role that differing habitats and ecological conditions have played in the evolution of delphinid social strategies. We propose a conceptual framework for understanding natural patterns of delphinid social structure in which the spatial and temporal predictability of resources influences the ranging patterns of individuals and communities. The framework predicts that when resources are spatially and temporally predictable, dolphins should remain resident in relatively small areas. Predictable resources are often found in complex inshore environments where dolphins may hide from predators or avoid areas with high predator density. Additionally, available food resources may limit group size. Thus, we predict that there are few benefits to forming large groups and potentially many benefits to being solitary or in small groups. Males may be able to sequester solitary females, controlling mating opportunities. Observations of inshore populations of bottlenose dolphins (Tursiops sp.) and island-associated spinner dolphins (Stenella longirostris) seem to fit this pattern well, along with forest-dwelling African antelope and primates such as vervets (Cercopithicus aethiops), baboons (Papio sp.), macaques (Macaca sp.) and chimpanzees (Pan troglodytes). In contrast, the framework predicts that when resources such as food are unpredictable, individuals must range further to find the necessary resources. Forming groups may be the only strategy available to avoid predation, especially in the open ocean. Larger home ranges are likely to support a greater number of individuals; however, prey is often sparsely distributed, which may act to reduce foraging competition. Cooperative foraging and herding of prey schools may be advantageous, potentially facilitating the formation of long-term bonds. Alternately, individuals may display many short-term affiliations. These large groups make it difficult for a male or a small group of males to sequester a female, and polygynandry is the most likely mating strategy. While it is difficult to study wide-ranging delphinids to examine these predictions, this ranging and behavioural pattern has been suggested for dusky dolphins (Lagenorhynchus obscurus), coastal bottlenose dolphins (Tursiops sp.) and mixed species of dolphins in the Eastern Tropical Pacific. These patterns also resemble the ranging and social strategies of open savannah African antelopes and desert-dwelling macropods. Resource availability exists in a range of complex distributions and we predict that delphinid ranging patterns will also vary. At intermediate-ranging patterns, the framework predicts that individuals should form mid-sized groups balancing intra-group competition with predation protection. Humpback dolphins (Sousa sp.) appear to fit this pattern, with some site fidelity over relatively large ranges. They display fluid associations with other individuals. Predation pressure is not sufficiently high to cause large groups to form, and individuals probably reduce predation pressure more by hiding whenever possible. This pattern is likely to prevent the formation of long-term complex bonds. In contrast, killer whales (Orcinus orca) also display intermediate-ranging patterns, but have extremely strong social bonds within familial groups. Cooperative and altruistic behaviour in killer whales facilitate the formation of life-long bonds, similar to those observations in sperm whales (Physeter macrocephalus) and elephants (Loxodonta africana). This conceptual framework remains largely untested, and for many species it is not currently possible to describe ranging behaviours, anti-predator tactics or social behaviour in sufficient detail for appropriate examination of these ideas. Few studies on dolphins have been conducted to explicitly test this type of framework; however, existing observations of delphinid social strategies and communities are used throughout this chapter to examine this framework. Additionally, we anticipate that the present framework may provide a starting point to test hypotheses regarding the evolution of social strategies of delphinids.

Assessment of prey vulnerability through analysis of wolf movements and kill sites

Within predator-prey systems behavior can heavily influence spatial dynamics, and accordingly, the theoretical study of how spatial dynamics relate to stability within these systems has a rich history. However, our understanding of these behaviors in large mammalian systems is poorly developed. To address the relationship between predator selection patterns, prey density, and prey vulnerability, we quantified selection patterns for two fine-scale behaviors of a recovering wolf (Canis lupus) population in Yellowstone National Park, Wyoming, USA. Wolf spatial data were collected between November and May from 1998-1999 until 2001-2002. Over four winters, 244 aerial locations, 522 ground-based telemetry locations, 1287 km of movement data from snow tracking, and the locations of 279 wolf kill sites were recorded. There was evidence that elk (Cervus elaphus) and bison (Bison bison) densities had a weak effect on the sites where wolves traveled and made kills. Wolf movements showed a strong selection for geothermal areas, meadows, and areas near various types of habitat edges. Proximity to edge and habitat class also had a strong influence on the locations where elk were most vulnerable to predation. There was little evidence that wolf kill sites differed from the places where wolves traveled, indicating that elk vulnerability influenced where wolves selected to travel. Our results indicate that elk are more vulnerable to wolves under certain conditions and that wolves are capable of selecting for these conditions. As such, vulnerability plays a central role in predator-prey behavioral games and can potentially impact the systems to which they relate.

Gregarious behaviour of evasive prey

Gregarious behavior of potential prey was explained by Hamilton (1971) on the basis of risk-sharing: The probability of being picked up by a predator is small when one makes part of a large aggregate of prey. This argument holds only if the predator chooses its victims at random. It is not the case for herds of evasive prey in the open, where prey's gregarious behavior, favorable for the fast group members, makes it easier for the predator to home in on the slowest ones. We show conditions under which gregarious behavior of the relatively fast prey individuals leaves slowest prey with no other choice but to join the group. Failing to do so would signal their vulnerability, making them a preferred target for the predator. Analysis of an n + 1 player game of a predator and n unequal prey individuals clarifies conditions for fully gregarious, partially gregarious, or solitary behavior of the prey.

Modelling roe deer (Capreolus capreolus) in a gradient of forest fragmentation: behavioural plasticity and choice of cover

The ability of a species to exhibit behavioural plasticity to environmental conditions has consequences for its success in fragmented landscapes. The roe deer, Capreolus capreolus (L., 1758), is one of the foremost examples of behavioural flexibility among ungulates. This species has increased rapidly in range from its original forest-mosaic habitat into open agricultural plains. Open-land roe deer populations show distinct differences in spatial and social behaviour, including larger group sizes, compared with forest-living roe deer populations. This is traditionally viewed as an antipredator strategy. The presence of strong behavioural plasticity in species response to landscape structure suggests that this should also be a concern in models attempting to describe effects of landscape change on species distribution. To date the implications of behavioural plasticity for modelling species' response to environmental conditions has received little attention. We used an individual-based model of roe deer to evaluate the consequences of behavioural plasticity for predictions made regarding population response to woodland fragmentation. The inclusion of a flexible behavioural strategy, where increased group size could buffer lack of woodland cover, resulted in significantly higher estimates of population size, population persistence, and the ability of the population to cope with fragmentation. This clearly demonstrates that behavioural plasticity in species response to landscape structure may affect our ability to accurately predict the effects of landscape change and should be a concern to modellers.

A community-level evaluation of the impact of prey behavioural and ecological characteristics on predator diet composition

Although predation avoidance is the most commonly invoked explanation for vertebrate social evolution, there is little evidence that individuals in larger groups experience lower predation rates than those in small groups. We compare the morphological and behavioural traits of mammal prey species in the Taï forest, Ivory Coast, with the diet preferences of three of their non-human predators: leopards, chimpanzees and African crowned eagles. Individual predators show marked differences in their predation rates on prey species of different body sizes, but clear patterns with prey behaviour were apparent only when differences in prey habitat use were incorporated into the analyses. Leopard predation rates are highest for terrestrial species living in smaller groups, whereas eagle predation rates are negatively correlated with group size only among arboreal prey. When prey predation rates are summed over all three predators, terrestrial species incur higher predation rates than arboreal species and, within both categories, predation rates decline with increasing prey group size and decreasing density of groups in the habitat. These results reveal that it is necessary to consider anti-predator strategies in the context of a dynamic behavioural interaction between predators and prey.

Group composition and cohesion in sympatric white-tailed deer and mule deer

White-tailed deer (Odocoileus virginianus) have been reported to live in smaller groups than mule deer (Odocoileus hemionus). Group size, however, generally varies with habitat conditions, and no comparison has been made between the social structures of the two species living in the same ecological conditions. I compared the size, composition, and stability of groups formed by sympatric whitetails and mule deer living in a prairie habitat in southern Alberta. Seasonal trends were similar for the two species. Females and fawns usually formed small groups during summer. Larger mixed-sex groups became increasingly common during winter, well after the breeding season. Despite the similar seasonal trend, mule deer were significantly more likely than whitetails to occur in relatively large groups composed of both sexes during winter, and whitetails were more likely to occur in small female groups. Mule deer groups were more stable than whitetail groups, and marked mule deer fawns developed strong associations with other known fawns. Habitat variation was limited and was not related to group size. These results show that the composition and cohesion of whitetail and mule deer groups differ, even when the species live in similar circumstances. The differences in grouping behaviour, larger more cohesive groups formed by mule deer than by whitetails, are consistent with those expected to result from the selection pressure of predation.

Elk population dynamics in areas with and without predation by recolonizing wolves in Banff National Park, Alberta

Gray wolves (Canis lupus) recolonized the Bow Valley of Banff National Park in the mid-1980s after a 30-year absence. Wolves recolonized one zone of the Bow Valley in 1985 and another in 1991, but human activity excluded wolves from a third zone throughout the study. Elk (Cervus elaphus) are the primary prey of wolves in Banff National Park. We studied the effects of wolf predation, snow depth, elk density, and human-caused mortality on the elk population growth rate in the three different wolf recolonization treatments from 1985 to 2000. We constructed a set of generalized linear models of factors affecting population growth, and used Akaike Information Criteria to guide model selection and inference. In the low wolf predation zone, elk population growth was density-dependent and limited by human-caused mortality. In the zone that wolves recolonized in 1991, elk population growth was limited by the combined effects of snow depth and wolf predation after wolf recolonization, in addition to preexisting mortality caused by humans and other predators. Our correlative approach failed to yield insights into population dynamics in the zone where wolves were present throughout the study. However, by comparing zones we demonstrate important differences in ungulate population dynamics in the presence and absence of wolf predation.