Peter A. Bednekoff

TEMPORAL VARIATION IN DANGER DRIVES ANTIPREDATOR BEHAVIOR: THE PREDATION RISK ALLOCATION HYPOTHESIS

The rapid response of animals to changes in predation risk has allowed behavioral ecologists to learn much about antipredator decision making. A largely unappreciated aspect of such decision making, however, is that it may be fundamentally driven by the very thing that allows it to be so readily studied: temporal variation in risk. We show theoretically that temporal variability in risk leaves animals with the problem of allocating feeding and antipredator efforts across different risk situations. Our analysis suggests that an animal should exhibit its greatest antipredator behavior in high‐risk situations that are brief and infrequent. An animal should also allocate more antipredator effort to high‐risk situations and more feeding to low‐risk situations, with an increase in the relative degree of risk in high‐risk situations. However, the need to feed leaves an animal with little choice but to decrease its allocation of antipredator effort to high‐risk situations as they become more frequent or lengthy; here, antipredator effort in low‐risk situations may drop to low levels as an animal allocates as much feeding as possible to brief periods of low risk. These conclusions hold under various scenarios of interrupted feeding, state‐dependent behavior, and stochastic variation in risk situations. Our analysis also suggests that a common experimental protocol, in which prey animals are maintained under low risk and then exposed to a brief “pulse” of high risk, is likely to overestimate the intensity of antipredator behavior expected under field situations or chronic exposure to high risk.

Predation, scramble competition, and the vigilance group size effect in dark-eyed juncos (Junco hyemalis)

Abstract In socially feeding birds and mammals, as group size increases, individuals devote less time to scanning their environment and more time to feeding. This vigilance “group size effect” has long been attributed to the anti-predatory benefits of group living, but many investigators have suggested that this effect may be driven by scramble competition for limited food. We addressed this issue of causation by focusing on the way in which the scan durations of free-living dark-eyed juncos (Junco hyemalis) decrease with group size. We were particularly interested in vigilance scanning concomitant with the handling of food items, since a decrease in food handling times (i.e. scan durations) with increasing group size could theoretically be driven by scramble competition for limited food resources. However, we showed that food-handling scan durations decrease with group size in an environment with an effectively unlimited food supply. Furthermore, this food-handling effect was qualitatively similar to that observed in the duration of standard vigilance scans (scanning exclusive of food ingestion), and both responded to changes in the risk of predation (proximity of a refuge) as one might expect based upon anti-predator considerations. The group size effects in both food-handling and standard scan durations may reflect a lesser need for personal information about risk as group size increases. Scramble competition may influence vigilance in some circumstances, but demonstrating an effect of competition beyond that of predation may prove challenging.

Great tit fat reserves under unpredictable temperatures

We tested the effects of unpredictable temperatures on fat reserves in Great Tits Parus major. During one treatment, the temperature was constant at 8.50C. In the other, temperatures fluctuated between 1.5 and 15.50 on a 24-h basis, with changes occurring just after lights-out. Residual evening weights were higher during the period of unpredictable temperatures. At the end of the period with unpredictable temperatures, more weight was gained on cold than on warm days. During the unpredictable temperature treatment, birds defecated less while eating the same amount. Nightly weight loss depended upon evening weight level, but not upon overnight temperature. Our results suggest that Great Tits use daily temperatures to predict conditions for the following night and that they regulate overnight expenditures to match reserve levels.

Testing for peripheral vigilance: do birds value what they see when not overtly vigilant?

Previous studies showed that dark-eyed juncos, Junco hyemalis, gain only partial information when their flockmates detect threats, and that they can detect threats peripherally even when they are feeding intensely. Here, we tested how this ‘peripheral (head-down) vigilance’ interacts with overt (head-up) vigilance and the collective detection of threats. Low barriers were used to prevent juncos from seeing to the side while pecking for food on the ground; a bird with its head raised could easily see over the barriers. Juncos and other ground-feeding birds preferred to feed without the barriers. When the barriers were present, birds generally took longer scans but spent similar periods between scans. Also, scanning patterns were less variable when the barriers were present. Following the threat-induced departure of a single bird, nondetectors were more likely to initiate scans when feeding among barriers. Barriers prevented feeding juncos from seeing the early stages of threat-induced departures of flockmates and thus impeded the collective detection of attack. The results suggest that juncos value peripheral vigilance and that scanning patterns are affected by interactions between peripheral vigilance and overt scanning behaviour.

Risk allocation and competition in foraging groups: reversed effects of competition if group size varies under risk of predation

Animals often feed more quickly when in larger groups. This group–size effect is often explained by safety advantages for groups but an alternative explanation is that animals feed faster in larger groups because of greater scramble competition for limited food. We show that predation risk enhances the group–size effect if groups vary in size. By contrast, competition leads to the group–size effect only when individuals feed in groups of constant size. When individuals feed in groups that vary in size, the best strategy for dealing with competition is to feed intensely when in smaller groups and feed little when in larger (more competitive) groups. In all situations, the effects of competition interact with the effects of predation risk in a simple multiplicative way. Our results suggest that scramble competition is not a general explanation for the group–size effect on vigilance in situations where group size changes relatively rapidly.

Evolution of antipredator behavior in an island lizard species, Podarcis erhardii (Reptilia: Lacertidae): The sum of all fears?

Organisms generally have many defenses against predation, yet may lack effective defenses if from populations without predators. Evolutionary theory predicts that “costly” antipredator behaviors will be selected against when predation risk diminishes. We examined antipredator behaviors in Aegean wall lizards, Podarcis erhardii, across an archipelago of land‐bridge islands that vary in predator diversity and period of isolation. We examined two defenses, flight initiation distance and tail autotomy. Flight initiation distance generally decreased with declining predator diversity. All predator types had distinctive effects on flight initiation distance with mammals and birds having the largest estimated effects. Rates of autotomy observed in the field were highest on predator‐free islands, yet laboratory‐induced autotomy increased linearly with overall predator diversity. Against expectation from previous work, tail autotomy was not explained solely by the presence of vipers. Analyses of populations directly isolated from rich predator communities revealed that flight initiation distance decreased with increased duration of isolation in addition to the effects of current predator diversity, whereas tail autotomy could be explained simply by current predator diversity. Although selection against costly defenses should depend on time with reduced threats, different defenses may diminish along different trajectories even within the same predator–prey system.

Effects of feral cats on the evolution of anti-predator behaviours in island reptiles: insights from an ancient introduction.

Exotic predators have driven the extinction of many island species. We examined impacts of feral cats on the abundance and anti-predator behaviours of Aegean wall lizards in the Cyclades (Greece), where cats were introduced thousands of years ago. We compared populations with high and low cat density on Naxos Island and populations on surrounding islets with no cats. Cats reduced wall lizard populations by half. Lizards facing greater risk from cats stayed closer to refuges, were more likely to shed their tails in a standardized assay, and fled at greater distances when approached by either a person in the field or a mounted cat decoy in the laboratory. All populations showed phenotypic plasticity in flight initiation distance, suggesting that this feature is ancient and could have helped wall lizards survive the initial introduction of cats to the region. Lizards from islets sought shelter less frequently and often initially approached the cat decoy. These differences reflect changes since islet isolation and could render islet lizards strongly susceptible to cat predation.

Animal Behavior in Introductory Textbooks: Consensus on Topics, Confusion over Terms

Abstract I surveyed chapters on animal behavior in 11 introductory textbooks to see how well textbooks introduce current research in the field. Chapters on animal behavior were placed in or near the sections on ecology and often near sections on animals. Within chapters, the introductory textbooks tended to present the same topics in a standard sequence. This sequence generally agrees with the sequence of chapters in an advanced textbook. Textbooks showed little consistency, however, in the terms they presented in boldface type. Different textbooks presented different terms in boldface type, so most “essential terms” were featured in only one textbook. Terms in boldface from introductory textbooks were not often used in an advanced textbook or research articles in animal behavior. Textbooks rarely showed alternative hypotheses or data from control groups when presenting animal behavior. Textbooks seem to present an abundance of unnecessary terms and miss the opportunity to illustrate the process of science using observations of animals.

Model Based Inference in the Life Sciences: A Primer on Evidence

CHAPTER 2 Question 2. One worry might be that the algorithm did not converge to the maximum. This could be because the log-likelihood was very flat near the maximum point or that sparce data cause a lack of estimability. A second concern might be that mixture distributions are not in the exponential family and the log-likelihood function might have more than a single mode. Both issues might be helped by using different starting positions for the parameters. A more sophisticated approach would be to use simulated annealing.