Foraging behavior of the midday gerbil (Meriones meridianus): combined effects of distance and microhabitat

We used the giving-up density (GUD) method and direct observation to study the combined effects of travel distance and microhabitat on foraging behavior of the midday gerbil (Meriones meridianus), which often acts as a central place forager. We provided animals with artificial seed trays in which dry and unhusked pumpkin seeds were mixed with fine sand. Gerbils practiced an eat-and-carry strategy in patches of bush microhabitat that were far from central places (BF patches), and tended to carry all seeds back in the other three treatments. Resource protection, predation risk avoidance and the balance between future and present value of food items may contribute to the eat-and-carry strategy. When distance was held constant, GUDs in open patches were higher than in bush patches, which was consistent with most studies. When microhabitat was held constant, GUDs in nearer patches were normally lower than in farther patches. In most cases, gerbils preferred to carry more seeds back rather than consume them immediately. We concluded that this tendency was due to the gerbils balancing the factors of future value and present value, and individual fitness and inclusive fitness.     

The effect of barn owls (Tyto alba) on the activity and microhabitat selection of Gerbillus allenbyi and G. pyramidum

Predation plays an important role in ecological communities by affecting prey behavior such as foraging and by physical removal of individual prey. In regard to foraging, animals such as desert rodents often balance conflicting demands for food and safety. This has been studied in the field by indirectly manipulating predatory risk through the alteration of cues associated with increased risk such as cover or illumination. It has also been studied by directly manipulating the presence of predators in aviaries. Here, we report on experiments in which we directly manipulated actual predatory risk to desert rodents in the field. We conducted a series of experiments in the field using a trained barn owl (Tyto alba) to investigate how two species of coexisting gerbils (Gerbillus allenbyi and G. pyramidum) respond to various cues of predatory risk in their natural environment. The gerbils responded to risk of predation, in the form of owl flights and owl hunger calls, by reducing their activity in the risky plot relative to the control plot. The strongest response was to owl flights and the weakest to recorded hunger calls of owls. Furthermore, when risk of predation was relatively high, as in the case with barn owl flights, both gerbil species mostly limited their activity to the safer bush microhabitat. The response of the gerbils to risk of predation disappeared very quickly following removal of the treatment, and the gerbils returned to normal levels of activity within the same night. The gerbils did not respond to experimental cues (alarm clock), the presence of the investigators, the presence of a quiet owl, and recorded white noise. Using trained barn owls, we were able to effectively manipulate actual risk of predation to gerbils in natural habitats and to quantify how gerbils alter their behavior in order to balance conflicting demands of food and safety. The method allows assessment of aspects of behavior, population interactions, and community characteristics involving predation in natural habitats.     

The effects of owl predation on the foraging behavior of heteromyid rodents

Summary Researchers have documented microhabitat partitioning among the heteromyid rodents of the deserts of North America that may result from microhabitat specific predation rates; large/bipedal species predominate in the open/risky microhabitat and small/quadrupedal species predominate in the bush/safer microhabitat. Here, we provide direct experimental evidence on the role of predatory risk in affecting the foraging behavior of three species of heteromyid rodents: Arizona pocket mouse (Perognathus amplus; small/quadrupedal), Bailey's pocket mouse (P. baileyi; large/quadrupedal), and Merriam's kangaroo rat (Dipodomys merriami; large/bipedal). Both kangaroo rats and pocket mice are behaviorally flexible and able to adjust their foraging behavior to nightly changes in predatory risk. Under low levels of perceived predatory risk the kangaroo rat foraged relatively more in the open microhabitat than the two pocket mouse species. In response to the presence of barn owls, however, all three species shifted their habitat use towards the bush microhabitat. In response to direct measures of predatory risk, i.e. the actual presence of owls, all three species reduced foraging and left resource patches at higher giving up densities of seeds. In response to indirect indicators of predatory risk, i.e. illumination, there was a tendency for all three species to reduce foraging. The differences in morphology between pocket mice and kangaroo rats do appear to influence their behavioral responses to predatory risk.     

Driven to distraction: detecting the hidden costs of flea parasitism through foraging behaviour in gerbils

Gerbilline rodents such as Allenby's gerbils (Gerbillus andersoni allenbyi), when parasitized by fleas such as Synosternus cleopatrae pyramidis, devote long hours of grooming to remove the ectoparasites. Yet no detrimental energetic or immunological effects of the ectoparasites have been found in adult Allenby's gerbil. Why should gerbils go to such trouble? We tested for the various ways that fleas can negatively affect gerbils by manipulating flea infestation on gerbils and the presence of a fox. We demonstrate that gerbils responded to fleas by leaving resource patches at higher giving-up densities. Furthermore, they stayed in those resource patches less time and left them at higher quitting harvest rates so long as a fox was also present. When flea-ridden, gerbils also abandoned using vigilance to manage risk and relied mainly on time allocation. Thus, having fleas imposed a foraging cost similar in nature to that arising from the risk of predation from foxes and may be even larger in magnitude. More than that, the presence of fleas acted as a magnifier of foraging costs, especially those arising from the risk of predation. The fleas reduced the gerbils' foraging aptitude and altered how they went about managing risk of predation. We hypothesize that fleas reduce the attention that gerbils otherwise have for foraging and predator detection. We suggest that this is the major cost of ectoparasitism.     

The use of time and space by male and female gerbils exploiting a pulsed resource

Foraging theory postulates that interference is a foraging cost and affects patch exploitation and activity times. One such system contains two species of seed-eating gerbils inhabiting sandy habitats in the Negev Desert of Israel. Low population densities of the dominant species allowed us to examine the interaction between males and females of the subordinate species, Gerbillus andersoni allenbyi , as a function of interference and resource renewal. We used giving-up densities (GUDs; the amount of food left in a resource patch when a forager abandons the patch) in seed trays to quantify patch use by gerbils. By placing 6 trays at each foraging station and either presenting all 6 trays at the start of the night (pulse treatment) or presenting one tray at a station 6 times per night (renewal treatment), we were able to manipulate characteristics of resource renewal. We used radio telemetry to obtain an independent assessment of activity. Male and female G. a. allenbyi differed in their timing of activity, with males beginning earlier than females and remaining active later. This was most pronounced for the pulse treatment. For the renewal treatment, female activity in trays was more intense early in the night, but thereafter male activity was more intense. At the same time, telemetry showed that males and females did not differ in their total activity in or out of trays. This suggests that males begin their activity on the renewal treatment by exploiting the richest natural patches of seeds. Only later when these are depleted do they move to dominate the renewing seed trays. Finally, females exploited stabilized sand habitats more than did males, especially during the renewal treatment. Taken together, these findings suggest that male G. a. allenbyi interfere with foraging in females, causing temporal shifts in their use of space and resources.     

Competition between birds and mammals: A comparison of giving-up densities between crested larks and gerbils

We combined the concept of mechanisms of co-existence with the approach of giving-up densities to study inter-taxon competition between seed-eating birds and mammals. We measured feeding behaviour in food patches to define and study the guild of seed-eating vertebrates occupying sandy habitats at Bir Asluj, Negev Desert, Israel. Despite a large number of putatively granivorous rodents and birds at the site, two gerbil species (Allenbys gerbil, Gerbillus allenbyi, and the greater Egyptian gerbil, G. pyramidum) dominated nocturnal foraging, and a single bird species (crested lark, Galerida cristata) contributed all of the daytime foraging. We used giving-up densities to quantify foraging behaviour and foraging efficiencies. A low giving-up density demonstrates the ability of a forager to profitably harvest food at low abundances and to profitably utilize the foraging opportunities left behind by the less efficient forager. Gerbils had lower giving-up densities in the bush than open microhabitat, and lower giving-up densities in the semi-stabilized than stabilized sand habitats. Crested larks showed the opposite: lower giving-up densities in the open than bush, and on the stabilized than semi-stabilized sand habitats. Despite these patterns, gerbils had substantially lower giving-up densities than crested larks in both microhabitats, all sand habitats, and during each month. Several mechanisms may permit the crested lark to co-exist with the gerbils. Larks may be cream skimmers on the high spatial and temporal variability in seed abundances. Larks may rely on insects, fruit or smaller seeds. Or, larks may rely on adjacent rocky habitats.     

Predation hazard and seed removal by small mammals: microhabitat versus patch scale effects

Predator avoidance may involve response strategies of prey species that are time and space specific. Many studies have shown that foraging individuals avoid predators by altering microhabitat usage; alternatively, sites may be selected according to larger-scale features of the habitat mosaic. We measured seed removal by two small mammal species (Peromyscus leucopus, and Microtus pennsylvanicus) at 474 stations over an experimentally created landscape of 12 patches, and under conditions of relatively high (full moon) and low (new moon) predatory hazard. Our objective was to determine whether predator avoidance involved the selection of small-, medium-, or large-scale features of the landscape (i.e., at the scale of microhabitats, habitats, or habitat patches). We found rates of seed removal to vary more with features of whole patches than according to variation in structural microhabitats within patches. Specific responses included: under-utilization of patch edge habitats during full moon periods, and microhabitat effects that were only significant when considered in conjunction with larger-scale features of the landscape. Individuals residing on large patches altered use of microhabitats/habitats to a greater extent than those on smaller patches. Studies just focusing on patterns of microhabitat use will miss responses at the larger scales, and may underestimate the importance of predation to animal foraging behavior.     

Foraging strategies of an insectivorous marsupial,Sminthopsis youngsoni (Marsupialia: Dasyuridae), in Australian sandridge desert

This study investigated the effects of predation risk, dune position and microhabitat on foraging of the Lesser Hairy‐footed Dunnart Sminthopsis youngsoni, a small insectivorous marsupial, in the Simpson Desert of western Queensland. The intensity of foraging was assessed by establishing feeding stations (dishes containing mealworms) in open and bush microhabitats at three levels on sand dunes, and recording the numbers of prey taken by dunnarts from the stations after nightly bouts of foraging. Risk of predation was manipulated by provision of artificial illumination at alternate feeding stations on each of five occasions when trials were run. The numbers of mealworms left after feeding bouts varied inconsistently across trials, providing little evidence that dunnarts respond to habitat or predation risk while foraging. These results contrast sharply with studies of arid zone rodents, where foraging is usually sensitive to both predation risk and resource distribution. We suggest that S. youngsoni forages equally in all habitats of its sandridge environment, and experiences relatively low risk of predation whilst doing so.     

Risk management in optimal foragers: the effect of sightlines and predator type on patch use,time allocation,and vigilance in gerbils

In the foraging game between gerbils and their predators, gerbils manage risk of predation using the tools of time allocation (where, when and for how long to forage) and vigilance. The optimal level of a forager's vigilance should be affected by its encounter rate with predators and the effectiveness of its vigilance in reducing mortality risk. The physical structure of the environment can alter the effectiveness of its vigilance and therefore alter its foraging behaviour. We tested this for gerbils at risk of predation from barn owls or foxes in a large vivarium. In particular, we reduced the effectiveness of vigilance by placing obstructions around feeding trays that blocked sight lines along either the vertical (vigilance directed against owls) or horizontal axis (vigilance directed against foxes), thereby changing the physical structure of the environment. In addition, we manipulated the presence of foxes and owls. In general, gerbils harvested fewer seeds, allocated less time to foraging in dangerous patches, and used more vigilance while foraging where and when risks were higher (i.e. in the presence of predators and in bright moonlight). Vertical and horizontal sightline treatments interacted synergistically to further raise perceived risk. These results imply that blocking sight lines reduces the effectiveness of vigilance, causing gerbils to use it less. Moreover, in the presence of a predator, the gerbils’ response to the blocked sightlines was more severe – harvesting less food and spending less time and vigilance – in the patches with the increased risk. This was especially so in the presence of the predator that was expected to most benefit from blocking that particular type of sight line: cover that blocked vertical sight lines was riskiest in the presence of owls, and cover that blocked horizontal sight lines was riskiest in the presence of foxes. These results strongly indicate the importance of sightlines and landscape features such as bushes in the risk management and forging decisions of gerbils, demonstrating that bush cover provides mixed blessing to gerbils by providing cover, but making vigilance ineffective.     

The fear diet: Risk,refuge, and biological control by omnivorous weed seed predators

Weed seed biocontrol by omnivorous mice and insects can limit weed seedbanks, but this ecosystem service can be difficult to predict given the broad diet breadth of seed predators and their potential for intraguild predation. Seed foraging behavior is further modified by fluctuating cues of predation risk from higher trophic levels and the availability of refuge habitat. Uncertainty about whether co-occurring insects and mice additively contribute to weed biocontrol or interfere with each other via intraguild predation limits our ability to recommend habitat management strategies that reliably promote seed destruction. Using seed removal assays, fluorescent powder tracking, and stable isotope analyses, we assessed effects of a predation risk cue (moonlight) on mouse foraging patterns in a patchwork of vegetated and exposed plots in a cultivated field. Mouse foraging activity decreased on exposed ground during the full moon, compared to dark nights, yet foraging movements were unaffected by moon cycle within refuge patches. Weed seed consumption was more than three times higher in cover than exposed soil, and 78% of that difference was attributable to invertebrate granivores. Mice and invertebrate granivores both exhibited higher foraging activity in cover, indicating co-occurrence of intraguild predators and prey. However, stable isotope analyses of fecal samples revealed that mice captured in refuge habitats fed at slightly lower trophic levels than those in exposed habitats (suggesting minimal intraguild predation in refuge habitat), and mouse diet was unaffected by moonlight. Despite increased availability of invertebrate prey in cover patches, mice do not appear to preferentially exploit prey when avoiding their own predators or interfere with weed seed predation. Therefore, functional redundancy of mice and invertebrate seed predators in cover crops and other refuge habitats may strengthen and stabilize weed seed biocontrol.     

Patch use in time and space for a meso-predator in a risky world

Predator–prey studies often assume a three trophic level system where predators forage free from any risk of predation. Since meso-predators themselves are also prospective prey, they too need to trade-off between food and safety. We applied foraging theory to study patch use and habitat selection by a meso-predator, the red fox. We present evidence that foxes use a quitting harvest rate rule when deciding whether or not to abandon a foraging patch, and experience diminishing returns when foraging from a depletable food patch. Furthermore, our data suggest that patch use decisions of red foxes are influenced not just by the availability of food, but also by their perceived risk of predation. Fox behavior was affected by moonlight, with foxes depleting food resources more thoroughly (lower giving-up density) on darker nights compared to moonlit nights. Foxes reduced risk from hyenas by being more active where and when hyena activity was low. While hyenas were least active during moon, and most active during full moon nights, the reverse was true for foxes. Foxes showed twice as much activity during new moon compared to full moon nights, suggesting different costs of predation. Interestingly, resources in patches with cues of another predator (scat of wolf) were depleted to significantly lower levels compared to patches without. Our results emphasize the need for considering risk of predation for intermediate predators, and also shows how patch use theory and experimental food patches can be used for a predator. Taken together, these results may help us better understand trophic interactions.     

Agricultural farming alters predator–prey interactions in nearby natural habitats

Agricultural farming is a major consumer of global arable lands and has a direct effect on species decline through habitat destruction. However, agricultural endeavours can also evoke indirect threats that will result in behavioural modifications of indigenous species. In a desert ecosystem, where a political border led to a farming dichotomy between intensive cultivates in Israel and intact lands in Jordan, we compared the foraging behaviours and abundances of the red fox and two species of gerbils, close to and distant from farms, and during two moon phases. We estimated fox and gerbil foraging levels by track counts, and measured gerbil time allocation, vigilance and apprehension by the giving-up density method. While foxes were significantly more abundant and active at locations close to farms, gerbils were significantly more abundant and active at locations distant from farms. Moreover, the typical reduction in food consumption during full-moon nights was exhibited only at locations close to farms. These results could suggest that indicators of predation risk, such as illumination intensity or distance to cover, are not universal, and their effectiveness may depend indirectly on anthropogenic activities, such as agricultural farming. The results could also suggest that although intensive agricultural endeavours benefit foxes, they might increase the predatory pressure on gerbils in addition to the already known effects of habitat loss. Therefore, agriculture acts as a double-edged sword by reducing natural habitats, while at the same time changing the predator–prey natural balance.     

Rodent foraging is affected by indirect, but not by direct, cues of predation risk

We used foraging trays to determine whether oldfield mice, Peromyscuspolionotus, altered foraging in response to direct cues of predationrisk (urine of native and nonnative predators) and indirectcues of predation risk (foraging microhabitat, precipitation,and moon illumination). The proportion of seeds remaining ineach tray (a measure of the giving-up density [GUD]) was usedto measure risk perceived by mice. Mice did not alter theirGUD when presented with cues of native predators (bobcats, Lynxrufus, and red foxes, Vulpes vulpes), recently introduced predators(coyotes, Canis latrans), nonnative predators (ocelots, Leoparduspardalis), a native herbivore (white-tailed deer, Odocoileusvirginianus), or a water control. Rather, GUD was related tomicrohabitat: rodents removed more seeds from foraging trayssheltered beneath vegetative cover compared with exposed traysoutside of cover. Rodents also removed more seeds during nightswith precipitation and when moon illumination was low. Our resultssuggest that P. polionotus used indirect cues rather than directcues to assess risk of vertebrate predation. Indirect cues maybe more reliable than are direct scent cues for estimating riskfrom multiple vertebrate predators that present the most riskin open environments.     

Giving-up densities of foraging gerbils: the effect of interspecific competition on patch use

We studied the effect of a dominant species, Gerbillus pyramidum (Egyptian sand gerbil), on the patch use of its subordinate competitor, G. andersoni allenbyi (Allenby's gerbil), to better understand interspecific competition between the two species. We used manipulated resource patches (seed trays) covered with cages with two adjustable species-specific gates (either opened or closed to the bigger-dominant species, but always opened to the subordinate one). We recorded species tracks around and on the seed trays and giving-up densities (GUDs) of seeds in the trays after each night of foraging. G. a. allenbyi depleted seed patches to a lower level whenever G. pyramidum was given the opportunity to forage on the seed trays (i.e., present on the grid). This result held regardless of whether G. pyramidum was actually present at a particular station. We suggest that competition from G. pyramidum occurs both directly by interference, in which G. a. allenbyi is forced to be active in the late part of the night, and indirectly by exploitation via resource depletion by G. pyramidum in the early part of the night. The results suggest that interspecific competition from G. pyramidum reduces seed availability and the richness of the environment for G. a. allenbyi enough to affect the marginal value of energy for G. a. allenbyi individuals and cause them to experience lower costs of predation and manifest lower GUDs.     

Foraging Patch Selection in Winter: A Balance between Predation Risk and Thermoregulation Benefit

In winter, foraging activity is intended to optimize food search while minimizing both thermoregulation costs and predation risk. Here we quantify the relative importance of thermoregulation and predation in foraging patch selection of woodland birds wintering in a Mediterranean montane forest. Specifically, we account for thermoregulation benefits related to temperature, and predation risk associated with both illumination of the feeding patch and distance to the nearest refuge provided by vegetation. We measured the amount of time that 38 marked individual birds belonging to five small passerine species spent foraging at artificial feeders. Feeders were located in forest patches that vary in distance to protective cover and exposure to sun radiation; temperature and illumination were registered locally by data loggers. Our results support the influence of both thermoregulation benefits and predation costs on feeding patch choice. The influence of distance to refuge (negative relationship) was nearly three times higher than that of temperature (positive relationship) in determining total foraging time spent at a patch. Light intensity had a negligible and no significant effect. This pattern was generalizable among species and individuals within species, and highlights the preponderance of latent predation risk over thermoregulation benefits on foraging decisions of birds wintering in temperate Mediterranean forests.     

Microhabitat use, giving-up densities and temporal activity as short- and long-term anti-predator behaviors in common voles

We used depletable food patches to determine the effect of microhabitat (mowed versus unmowed adjacent grasslands) and time (day versus night) on the foraging behavior of common voles (Microtus arvalis). The food remaining after 12‐h periods (giving‐up density, GUD) measured the vole's habitat selection under predation risk. In accord with several other rodent species and the effects of avian predators, voles had significantly lower GUDs in the unmowed than mowed portion of the grassland. GUDs in patches along the border between adjacent habitats were more similar to the risky mowed grassland than the safe unmowed grass. Time interacted strongly with microhabitat. In the mowed grass, voles had significantly higher GUDs at night than day. Whereas in the unmowed grass, GUDs were significantly higher during the day than night. Vole GUDs did not vary with time along the boundary. This suggests that predators are more abundant or effective in the mowed grass at night (owls?), and in the unmowed grass during the day (weasels?). In terms of predation risk, the voles perceived the mowed grass at night as the riskiest and the unmowed grass at night as the safest. Voles may have difficulties assessing resources under high predation risk: GUDs among patches were well equalized in the unmowed microhabitat whereas in the mowed grass only day GUDs did not vary significantly among patches. We linked these results to the vole's day‐night‐activity and life span. For the 533 voles live‐trapped at the study area, the ratio of day versus night captures for each individual served as an activity index and the span between first and last capture measured minimum life span. In accord with higher GUDs at night, very few individuals behaved selectively towards the night, but individual life expectancy increased with temporal opportunism. Microhabitat differences in GUDs reflect short‐term strategies of predator avoidance and the trapping data reflect long‐term patterns of anti‐predator behavior.     

Exploration and exploitation of foraging patches by desert sparrows: environmental indicators and local evaluation of spatially correlated costs and benefits

Conventional evolutionary and behavioral reasoning expects foragers to show strong spatial preferences in environments with heterogeneous resource distribution. Moreover, consumers should benefit from exploiting the information embedded in environmental features that indicate resource abundance. In desert soils seed abundance associates strong and reliably with vegetation and litter cover at small spatial scales. However, other spatially correlated factors (substrate complexity, temperature, predation risk) may affect foraging costs, benefits and decisions by ground‐feeding granivores. We used a sequence of three semi‐controlled field experiments of binary spatial choice within a portable aviary to identify the main cause of foraging microhabitat selection by the most abundant postdispersal granivorous bird in the central Monte desert (Argentina). In the first experiment we placed the aviary at field to offer pairs of adjacent microhabitats of unmodified, naturally‐contrasting substrates and environmental conditions to single, untrained rufous‐collared sparrows Zonotrichia capensis. Birds selected covered microhabitats in winter and summer, ruling out substrate complexity or thermoregulation as main single causes of patch selection. The other two experiments dissociated seed abundance, tree cover and litter to reveal their effects on patch selection. The results indicate that 1) sparrows do not restrict microhabitat exploration relying on environmental indicators, 2) distance to tree cover influences the order of patch exploration, probably in association with apprehension or risk‐assessment behavior, and 3) patch exploitation is determined by short‐term local estimation of seed abundance. The integration of these with previous results obtained under variable degrees of realism and experimental control allows for a better explanation of the spatial component of postdispersal granivory and its consequences on plants. The unconstrained selective foraging strategy of these sparrows would allow them to detect sporadic or ephemeral rich patches with structural characteristics indicating ‘low‐quality’, should promote the spatial homogenization of the palatable seed bank, and would favor indirect interactions between plants.     

Foraging and community consequences of seed size for coexisting Negev Desert granivores

We examined the effects of seed size on patch use and diet selection for three co-existing Negev Desert granivores: Allenby's gerbil ( Gerbillus allenbyi ), greater Egyptian sand gerbil ( Gerbillus pyramidum ), and crested lark ( Galerida cristata ). We manipulated size and spatial distribution of seeds in experimental food patches and quantified foraging behavior by measuring giving-up densities (GUDs: the amount of food remaining in a resource patch following exploitation by a forager). In one experiment, we presented small (<1.4 mm in diameter cracked wheat), medium (2.0–3.3 mm), and large (>3.4 mm) seeds in separate trays; in a second, we presented small and medium seeds separately and mixed together. Gerbils had a higher handling time efficiency on smaller seeds, but a much higher encounter probability on larger seeds (20 times higher on large than medium seeds, and 2–5 times higher on medium than small seeds). This led gerbils to have significantly lower GUDs on larger seeds than smaller seeds and to harvest a higher proportion of the larger seeds. When presented with rich and poor patches, G. allenbyi tended to equalize GUDs in both patches, indicating a quitting harvest rate rule for patch exploitation. In contrast, larks appeared to use a fixed time rule for patch exploitation. For larks, seed size did not influence encounter probabilities, and they showed no seed-size selectivity. Still, larks had higher handling efficiencies on smaller than larger seeds, and consequently had a significantly lower GUD on small than medium seeds. Despite large differences between the gerbils and larks in their foraging, our results do not support species coexistence via seed-size partitioning: the larks had much higher GUDs than the gerbils on all seed sizes. Nonetheless, seed size, seed abundance, seed distribution and the animal's patch use behavior all played major roles in determining gerbils' and larks' diet selectivities and GUDs.     

Effect of predation risk on selectivity in heteromyid rodents

Variations in predation risk affect the costs of foraging and may therefore warrant different foraging decisions. One class of models ("higher requisite profit") predicts that foragers should become more selective when predation risk increases, as low-profitability items that do not cover the increased costs are dropped from the diet. An alternative class of models ("reduced finickiness") predicts that foragers should become less selective when predation risk increases, because selectivity requires more extensive assessment and/or search behaviour, prolonging exposure to risk. We assessed the selectivity of foraging heteromyid rodents (Merriam's kangaroo rats, Dipodomys merriami, and pocket mice, Chaetodipus spp.) by comparing differences in "giving up densities" (GUD: the quantity of cryptic food left in a patch by animals for whom the diminishing marginal gains from foraging have dropped below the threshold for continued search) for foods of different value as a measure of selectivity in patches varying in predation risk. Data collected over two field seasons revealed that heteromyids were more selective when predation risk was highest; away from the protective cover of shrubs during the full moon. These findings support the predictions of higher requisite profit models.