Predicted fitness consequences of threat-sensitive hiding behavior

In studies of refuge use as a form of antipredator behavior,where prey hide in response to a predator's approach, factorssuch as foraging costs and the perceived risk in a predator'sapproach have been shown to influence the hiding behavior ofprey. Because few studies of waiting games have focused on mammals,we studied the hiding behavior of the yellow-bellied marmot(Marmota flaviventris), a ground-dwelling rodent. We testedthe prediction that marmots vary hiding time as a function ofpredator approach speed and presence and absence of food outsidetheir refuge and that marmots hide differently depending ontheir relative condition. We conducted "fast approaches" and"slow approaches" in the presence and absence of extra foodand evaluated hiding times. Multiple regression analyses demonstratedthat the interaction between the approach speed and the presenceand absence of food influenced hiding behavior; body conditionhad a smaller, but nonsignificant effect. We then developeda state-dependent dynamic model to explore potential fitnessconsequences of these decisions. The model suggested that theoverall survival of a population is substantially reduced whenindividuals make suboptimal decisions. Our research builds onprevious studies, indicating that animals integrate both costsand benefits of hiding when determining their hiding times.     

The influence of habitat on travel speed, intermittent locomotion, and vigilance in a diurnal rodent

We studied effects of habitat structure on routine travel velocities, intermittent locomotion, and vigilance by the degu (Octodondegus), a diurnal rodent of central Chile. We predicted thattravel speed, pauses during locomotion, and vigilance wouldbe greater in open (riskier) than in shrub (safer) habitats.Video recordings of marked individuals in the wild were used to measure speed and other variables of spontaneous locomotionnot triggered by predatory attack or any other noticeable stimulusduring nonforaging periods. Time spent vigilant while foragingwas also measured. Because degus use bare-ground runways fordistant movements (e.g., between burrow openings and/or foodpatches), data on locomotion decisions were not confounded by effects of obstructive vegetation cover and/or resource abundance.When moving across the habitat between different feeding places,degus showed an intermittent pattern of locomotion, interruptingrunning events with short pauses. As predicted, travel speedand the duration of pauses between locomotion bursts were significantlygreater in open habitats. Further, the duration of locomotionbursts between feeding sites or between feeding sites and burrowswas significantly longer in open habitats. Our assumption that pauses and velocities are independent decisions was supportedby the lack of correlation between pauses and speeds duringlocomotion events. During foraging, degus devoted more timeto vigilance in open than in shrub habitats. The static positionadopted by degus during pauses, the speeds attained during movements, and the concordance between pausing behavior andvigilance across habitats suggest that pausing has an antipredatoryrole and is not limited to orientation and/or physiologicalrecovery. Our results support the view that degus perceivehigher predation risk in open areas and that flexible movement behavior reflects an adaptive antipredator response.     

Native species vulnerability to introduced predators: testing an inducible defense and a refuge from predation

To manage the impacts of biological invasions, it is important to determine the mechanisms responsible for the effects invasive species have on native populations. When predation by an invader is the mechanism causing declines in a native population, protecting the native species will involve elucidating the factors that affect native vulnerability. To examine those factors, this study measured how a native species responded to an introduced predator, and whether the native response could result in a refuge from predation. Predation by the green crab, Carcinus maenas, has contributed to the decline in numbers of native soft-shell clams, Mya arenaria, and efforts to eradicate crabs have proven futile. We tested how crab foraging affected clam burrowing, and how depth in the sediment affected clam survival. Clams responded to crab foraging by burrowing deeper in the sediment. Clams at shallow depths were more vulnerable to predation by crabs. Results suggest soft-shell clam burrowing is an inducible defense in response to green crab predation because burrowing deeper results in a potential refuge from predation by crabs. For restoring the native clam populations, tents could exclude crabs and protect clams, but when tents must be removed, exposing the clams to cues from foraging crabs should induce the clams to burrow deeper and decrease vulnerability. In general, by exposing potential native prey to cues from introduced predators, we can test how the natives respond, identify whether the response results in a potential refuge, and evaluate the risks to native species survival in invaded communities.     

Asset protection in juvenile salmon: how adding biological realism changes a dynamic foraging model

The "asset-protection principle" created by Clark is based ona dynamic programming model and states that individuals should(1) become more averse to predation risk as they accumulatefitness assets but (2) generally be more willing to acceptpredation risk later in the foraging season. To test whetherthese predictions hold under biologically meaningful foraging parameters, I constructed a dynamic model of the optimal trade-offbetween foraging and predator avoidance in juvenile salmon.The model incorporates temperature and body-size dependentbio-energetic constraints typical for juvenile fish, whichgrow by orders of magnitude over a season. In its simplestform using seasonally constant growth potential and a linear over-winter survival function, my results equal those of Clark'smodel. Adding a fitness function and environmental data fromfield studies accentuates the asset-protection effect and fundamentallychanges the seasonal pattern of optimal effort. Simulationof typical poor feeding conditions in mid-summer yields theprediction of increased foraging in the spring in anticipationof worsening conditions. Increasing overall predation riskresults in smaller fish at the end of the season with a trade-offbetween summer and winter survival. The model generates testablepredictions for juvenile salmon and provides insights for otherorganisms (particularly poikilotherms) that are subject tosize-dependent or seasonally changing foraging dynamics.     

Trading safety for food: evidence from gut contents in roach and bleak captured at different distances offshore from their daytime littoral refuge

1. Regular diel habitat shifts in roach were detected by hydro‐acoustics in five moderately eutrophic, stratifying (maximum depth 24–27 m) and approximately circular lakes (of surface area 15, 75, 125, 300 and 900 ha and diameters 250, 600, 1000, 1700 and 2600 m) in north‐eastern Poland in the years 1998–2000, when the lakes were free of smelt and other typical offshore planktivores, and their offshore areas were completely free of fish during the day. 2. The diel change in roach distribution was shown to assume a similar pattern in each lake: fish migrated from a daytime littoral refuge towards the centre of the lake at dusk, and returned to the littoral refuge at dawn. After sunset, fish gradually dispersed offshore until they covered the entire lake area in each of the three smaller lakes. In each of the two larger lakes, only small numbers of fish were seen in the central area at night, implying that the centre of the lake retained high food availability throughout the summer. 3. Inshore–offshore gradients in zooplankton prey density, body size, and numbers of eggs per clutch were weak or undetectable in the two smallest lakes, but strong and persistent in the three larger lakes, with Daphnia densities 5–30 times as high and body length 1.2–1.5 times as great in the central area as inshore. 4. The likely increase in the potential predation risk with distance from the littoral daytime refuge was found to be compensated by increased food gains in those fish which moved offshore at dusk to feed within a short time window, when light intensity was lower to make the risk reduced, but still high enough to see zooplankton prey. The benefit because of increased prey acquisition was greatest in the centre of the largest lake (at 1300 m from the shore), as revealed from gut inspections of roach and bleak trawl‐sampled at different distances from the edge of the reed belt, and seen as a gradual, order‐of‐magnitude increase in the volume of food in the foregut, The food volume against distance‐from‐shore regression was highly significant on each of the four sampling dates in the largest lake, in spite of the wide variability of food volume in individual fish.     

The benefits of stealing from a predator: foraging rates, predation risk, and intraspecific aggression in the kleptoparasitic spider Argyrodes antipodiana

In the trade-off between food and safety, the role of aggressiveintraspecific interactions has not been extensively examined.Here I present information on this system using a kleptoparasiticspider, Argyrodts antipodiana, and its host spider and potentialpredator, Eriophora pustulosa. A. antipodiana can feed eitherat a potentially dangerous site (the hub of its host's web withthe host), or at a relatively safe site (on food bundles aroundthe edge of the host's web). I found that A. antipodiana cangain food very quickly when feeding with the host, apparentlyby exploiting the host's ability to digest the prey. Thus A.antipodiana follows predictions based on foraging models inthat it accepts a higher predation risk at the hub because ofthe higher food payoff. A. antipodiana also aggressively competesfor access to more food. However, aggressive competition increasesthe predation risk from the host, especially at tile hub wherethe host is very close. Consequently, A. antipodiana modifiesits level of intraspecific aggressiveness in accordance withits position on the web: at the hub, where the cost of aggressionis high (due to predation risk), A. antipodiana reduces itsaggressiveness, but it is aggressive away from the hub whencompeting for food bundles. The ability of A. antipodiana tochange interaction intensity as a function of its position onthe web enables it to exploit a rich, but risky, food sourceand provides a new angle for examining food and safety tradeoffs in light of intraspecific competition for food