Behavioral Ecology - current issue

Programmed versus stimulus-driven antiparasitic grooming in a desert rodent

Hawlena, H., Bashary, D., Abramsky, Z., Khokhlova, I. S., Krasnov, B. R. — 2008-09-12

We tested 2 hypotheses concerning regulation of grooming in flea-infested rodents and examined if 2 grooming components, scan and scratch grooming, are controlled by programmed and stimulus-driven regulation, respectively. The programmed grooming hypothesis proposes central programming that periodically evokes a bout of grooming to remove ectoparasites before they are attached and predicts that juvenile rodents 1) regardless of infestation status will invest more time in grooming (the body size principle) and 2) sustain lower flea densities than adults. The stimulus-driven grooming hypothesis postulates a direct response to irritation from ectoparasite bites and predicts that under flea-free conditions, 1) the stimulus-driven grooming regulation will not be activated, thereby neither juveniles nor adults will engage in grooming, but under flea infestation, 2) adults will invest more time in grooming than juveniles and sustain similar flea densities. We recorded the behavior of adult and juvenile flea-parasitized and nonparasitized rodents and quantified the frequency and duration of the 2 grooming components. Flea infestation increased the time devoted to grooming, supporting the existence of a regulation mechanism. However, the results did not support the dominance of neither hypothesis. Both forms of grooming were affected similarly by flea infestation and host age, hence may not necessarily be linked to a given regulation mechanism. Regardless of infestation status, time devoted to grooming was lower in juveniles, and both age groups sustained fleas at similar densities. We suggest that the assumptions and predictions of the 2 hypotheses should take into account the morphology and natural history of the host organism.

Trapline foraging by bumble bees: V. Effects of experience and priority on competitive performance

Ohashi, K., Leslie, A., Thomson, J. D. — 2008-09-12

Animals collecting resources that are fixed in space but replenish over time, such as floral nectar and pollen, often establish small foraging areas to which they return faithfully. Some repeatedly visit a set of patches in a significantly predictable sequence (so-called "trapline foraging"), which may allow them to focus on more profitable patches in their foraging areas. The functional significance of trapline foraging itself, however, has not been empirically demonstrated, especially in competitive situations. We conducted laboratory experiments with artificial flowers to test whether and how accumulated foraging experience in bumble bees affects their movement patterns and foraging performance in the presence of competition. Experienced bees with prior access to flowers achieved higher rates of nectar intake than did later arrivals because they traveled faster between flowers and returned to flowers at more regular intervals. These behavioral skills improved foraging performance in competitive situations in 2 ways: nectar that accumulated in flowers could be harvested before its replenishment rate slowed down, and nectar could be taken before the arrival of a competitor. In each foraging trip, however, bees traveled more slowly as they followed more repeatable routes. Despite this trade-off between speed and accuracy in traplining, bees constantly upgraded both skills as they gained experience from trip to trip. This upgrading still occurred in the absence of a competitor. Foraging area fidelity thus allowed bumble bees to establish long-term spatial memory required for fast movements and accurate traplining and, in turn, increased their foraging performance in competition with less experienced individuals.

Effects of experimentally increased costs of activity during reproduction on parental investment and self-maintenance in tropical house wrens

Tieleman, B. I., Dijkstra, T. H., Klasing, K. C., Visser, G. H., Williams, J. B. — 2008-09-12

Life-history theory assumes that organisms trade-off current against future reproduction to maximize fitness. Experimental explorations of the costs of reproduction have not yielded a clear understanding of the nature of these costs but rather point to a complex set of allocation possibilities among several physiological functions and behaviors. We investigated how experimentally increased flight costs affected the trade-off between parental investment and self-maintenance in tropical house wrens, which have relatively high annual survival and multiple breeding opportunities per year. We predicted that handicapped wrens would not increase their energy expenditure but instead decrease their effort to rear young in order to maintain their own body condition. Our results largely supported these predictions: handicapped parents decreased their nestling feeding frequency but did neither alter their field metabolic rate (FMR) nor compromise their body condition as measured by basal metabolic rate (BMR) and several measures of innate immune function. Reduced feeding rates did not affect nestling body mass growth but resulted in decreased structural growth (length of tarsus). The latter result can be explained if parents shifted the type of prey brought to offspring or altered the amount of food brought per trip. The experiment-wide positive correlations among FMR, BMR, and feeding frequency are in agreement with the hypothesis that hard work requires elevated levels of BMR. These correlations, in combination with the absence of a handicap treatment effect on FMR or BMR, do not lend support for predictions from studies in the laboratory that birds compensate hard work during the day by lowering their BMR at night. Considering a complex set of allocation possibilities among several physiological functions and behaviors, we conclude that tropical wrens take out the costs of a handicap largely on their offspring quality not on self-maintenance processes.

Love will tear you apart: different components of female choice exert contrasting selection pressures on male field crickets

Bailey, N. W. — 2008-09-12

Female mate choice is a driving force in the evolution of male secondary sexual characters. It can be dissected into several components: discrimination describes the degree to which females distinguish male trait variation, responsiveness indicates the speed or likelihood of females' reactions to a mate, and preference functions illustrate how the probability of mating relates to male trait variation. Relatively little is known about how these components interact to produce final mating decisions and influence the strength and direction of sexual selection, so I used female field crickets (Teleogryllus oceanicus) to measure interactions between individual preference functions, discrimination, and 2 measures of responsiveness (number of responses and response effort). Preference function shape varied considerably between individual females. Highly discriminating females showed greater numbers of responses to playbacks and were more likely to have stabilizing preference functions. When I constructed 2 population-level preference functions using either response number or response effort, the first yielded a linear, directional function, whereas the second implied a stabilizing function. The clear differences between the 2 imply that different components of female choice can exert contrasting selection pressure on a single male trait. Overall, both response number and discrimination were mutually reinforcing and likely govern the strength of sexual selection in this population. The direction of selection in a wild setting ultimately depends on the relative importance of response number versus response effort, where exogenous factors such as predation risk or density will determine which component of female choice predominates mating decisions.

Availability of nonpigmentary antioxidant affects red coloration in gulls

Perez, C., Lores, M., Velando, A. — 2008-09-12

Red, orange, and yellow carotenoid-based colorations displayed by fishes and birds may function as honest sexual signals of the bearer's quality. However, the mechanisms underlying the expression of these traits and the information they convey are still controversial. Because carotenoids are antioxidants and carotenoid-based pigmentation is bleached as a consequence of oxidative processes, it has been suggested that the pigmentation may signal antioxidant status. We tested this hypothesis in the yellow-legged gull (Larus michahellis), a seabird that exhibits a carotenoid-based red spot on the lower mandible. The availability of a nonpigmentary antioxidant (i.e., vitamin E) to the gulls was modified before egg laying by means of a supplementary feeding experiment. During the incubation period, breeding pairs were captured to assess the intensity of the color and the size of the red bill spots. We measured the plasma level of lipid peroxidation, total antioxidant capacity, and carotenoids. We found that males that received vitamin E supplements had larger red spot than control birds but that color intensity was not affected by the supplements. Moreover, we found that only those plasma carotenoids involved in the red coloration were affected by the antioxidant supplementation, suggesting an active mechanism to increase red coloration. Overall, our results provide experimental evidence for the hypothesis that carotenoid-based coloration reflects the bearer's antioxidant status in male gulls.

Evolutionarily costly courtship displays in a wolf spider: a test of viability indicator theory

Hoefler, C. D., Persons, M. H., Rypstra, A. L. — 2008-09-12

The costs of secondary sexual traits are crucial to our understanding of sexual selection. Although it is broadly accepted that sexual traits are indirectly or directly costly to express, few studies have quantified such costs. Thus, it is unclear if costs are evolutionarily meaningful and to what degree. Costs play a key role in viability indicator models, which assume that 1) the expression of sexual traits reduces the fitness of the trait bearer, 2) sexual trait expression is dependent on condition, and 3) the costs of sexual trait expression are borne differentially, that is, they are less for individuals in good condition. Using 2 syntopic species of wolf spiders, we addressed the importance of direct predation costs on the viability indicator mechanism. Pardosa milvina is a small wolf spider that has conspicuous male courtship behaviors in the form of front leg raises. Hogna helluo is a large species that preys on P. milvina. In laboratory experiments, we discovered that predation risk from H. helluo was higher for courting P. milvina males than noncourting males, male P. milvina manipulated to be in good condition courted at higher rates than males manipulated to be in poor condition, and males in good condition survived predation risk better than males in poor condition. Our study suggests that predation is a significant, evolutionary cost that can satisfy viability indicator mechanism assumptions.

Larval amphibians learn to match antipredator response intensity to temporal patterns of risk

Ferrari, M. C.O., Messier, F., Chivers, D. P. — 2008-09-12

The importance of temporal variability in risk has recently come to the forefront of research examining the behavioral ecology of predator–prey relationships. Temporal variability has been known to drive patterns of behavioral responses associated with foraging, reproduction, and territorial defense of prey animals. However, it is unknown if such behavioral responses are a result of selective depredation, which leads to innate temporal patterns of behavior, or, alternatively, are a result of learning by the prey. Here, we investigated whether larval wood frog (Rana sylvatica) tadpoles can learn to adjust the intensity of their antipredator responses to match the temporal patterns of risk they experience. Tadpoles were exposed to the odor of a predatory salamander paired with injured conspecific cues (salamander present and feeding) during the morning and received the salamander odor alone in the evening (salamander present but not feeding—morning risk treatment), whereas another group received the opposite treatments (evening risk treatment). The 2 groups were treated for 9 days. When subsequently exposed to salamander alone in the evenings, the tadpoles from the evening risk treatment responded with greater antipredator response intensity than the tadpoles from the morning risk treatment. This indicates that tadpoles have the ability to learn the change in predation risk they experience throughout the day and respond to such threats with an intensity reflecting their vulnerability to the predators.

Sector fidelity--an advantageous foraging behavior resulting from a heuristic search strategy

Buchkremer, E. M., Reinhold, K. — 2008-09-12

The desert ant Cataglyphis exhibits a strong tendency toward returning to its previous foraging direction when the last foraging run was successful. This behavior is called sector fidelity. A very simple behavioral rule, the -rule, has previously been identified as the possible underlying mechanism. Up to now, sector fidelity has been considered a means of facilitating navigation by exploiting familiar landmark information. We propose that sector fidelity enhances the foraging success of ants or other animals in an environment with a heterogeneous resource distribution. We tested the -rule and another promising behavioral rule in a very simple environment and modeled the foraging success of these strategies. For each condition, the parameters of the 2 heuristics were optimized using an evolutionary algorithm. The results of our simulations show that the -rule performs very well under different resource distributions and availabilities. It can therefore function as a very general adaptive foraging strategy for finding prey in landscapes with heterogeneous resource distributions.

Why be diurnal? Shifts in activity time enable young cane toads to evade cannibalistic conspecifics

Pizzatto, L., Child, T., Shine, R. — 2008-09-12

Why are some animals active by day and others by night? The selective forces that favor diurnal versus nocturnal activity may be evaluated by comparing age classes within a species that exhibits intraspecific (ontogenetic) variation in activity times. In many species of toads, adults are nocturnally active but postmetamorphic animals are primarily diurnal. The small body sizes of these animals render them vulnerable to desiccation and overheating—so why are they active by day? To answer this question, we studied an invasive population of cane toads (Bufo marinus) in tropical Australia. In the field, these small toads often encounter cannibalistic conspecifics because desiccation risk concentrates toads around the moist margins of the natal pond. We manipulated factors that differ between day and night (time of day, illumination, presence of cannibalistic conspecifics, scent, or visual cues from cannibalistic conspecifics) to identify the proximate cues and fitness advantages associated with diurnal versus nocturnal activity. Activity levels, response to disturbance, and feeding rates of metamorph toads were enhanced by light but suppressed by the presence of a larger conspecific. Metamorphs used both visual and scent cues to detect larger toads. An endogenous diel rhythm in activity was present also but weaker in metamorph toads than in larger (cannibal sized) individuals. The risk of cannibalism was high only at night and only in dark conditions. Thus, the diurnal activity of metamorph toads enables these vulnerable animals to avoid conspecific predators.

Context-dependent mate choice in relation to social composition in green swordtails Xiphophorus helleri

Royle, N. J., Lindstrom, J., Metcalfe, N. B. — 2008-09-12

Comparative evaluation mechanisms of mate choice in relation to social composition of potential mates have not been considered in nonhuman animals. Models of rational decision making suggest that when choice is based on absolute values, the addition of a third option should take choices from the original 2 options in proportion to their original shares and not result in an increase in the absolute preference for any of the 3 options in the set. However, studies of foraging behavior have shown that choice alternatives are often not irrelevant, specifically when preference is based on 2 or more dimensions (multiple cues) and when the third option is an asymmetrically dominated "decoy" (i.e., it has a lower value than both original options on one dimension but is only lower than one of the 2 original options on the other dimension). Asymmetrically dominated decoys are predicted to increase preference for the option that dominates it on both dimensions if mate choice is context dependent. We tested whether mate choice is dependent on or independent of social context in green swordtails, a species where females commonly use multiple cues in mate choice decisions. Addition of a third, decoy, male to the set of options resulted in females shifting preference away from the phenotype of male that each preferred in binary comparisons. Consequently, although mate choice was context dependent, the asymmetrically dominated decoy effect was not observed. Instead, females showed negative frequency-dependent preference for the rare-male phenotype, which may act to maintain genetic variation under sexual selection.

The influence of sex and sociality on parasite loads in an African ground squirrel

Hillegass, M. A., Waterman, J. M., Roth, J. D. — 2008-09-12

Male-biased parasite loads, which are common in vertebrates, could be a consequence of sexual selection, and for species that group, the costs of parasites could vary with group size or social structure. We examined sex-biased parasitism and the influence of group size on parasite loads in Cape ground squirrels (Xerus inauris), a highly social species that occurs in the arid regions of southern Africa. Group size did not affect numbers of ectoparasites or endoparasites. Males carried 3 times as many ectoparasites as females, but females harbored nearly 3 times as many endoparasites as males. Age class did not affect parasite loads in females, but in males, adults carried more ectoparasites than juveniles. Allogrooming was performed primarily by females, but no sex difference was found in autogrooming. Males in the subadult age class are becoming scrotal (indicating an increase in sex hormones) but typically remain in the natal group until adulthood, maintaining home range sizes comparable to adult females. Our results suggest that sexual selection does influence parasite loads in this species; increased androgen levels may reduce ectoparasite resistance in males, and smaller home ranges may increase females’ exposure to endoparasites. Allogrooming could reduce ectoparasite loads of the group and mitigate the costs of grouping.

Being conspicuous and defended: selective benefits for the individual

Halpin, C. G., Skelhorn, J., Rowe, C. — 2008-09-12

Aposematic insects conspicuously advertise their unprofitability to potential predators. However, when these prey initially evolved, they were likely to have been rare and presumably at a greater risk of being detected and killed by naive predators. Both kin and individual selection theories have been used in attempts to explain this apparent paradox, with much of the empirical research supporting kin selection–based theories. Here, we experimentally test how chemical defence levels in prey and avian color biases influence the probability of a rare conspicuous morph having an initial survival advantage. We used newly hatched domestic chicks (Gallus gallus domesticus) foraging on green and purple prey, on a green or purple background, to model the evolutionary scenario of a rare conspicuous morph arising in a population of already defended cryptic prey. Defended prey were produced by spraying them with quinine solution, which the birds readily detect and can learn to avoid. Although attack rates were initially similar for both defended prey types, the chicks only learned to avoid defended prey when they were conspicuous, not when they were cryptic. In addition, defended conspicuous prey were more likely to be rejected on attack than defended cryptic prey, even when first encountered by a predator. These data suggest that there could be a selective advantage for a rare conspicuous morph to arise in a population of cryptic defended prey due to increased avoidance learning and taste-rejection in naive predators. Our findings also suggest that being a non-preferred color and/or highly defended will increase the probability of this evolutionary scenario.

Sperm investment in relation to weapon size in a male trimorphic insect?

Kelly, C. D. — 2008-09-12

In the harem polygynous Wellington tree weta (Hemideina crassidens), early maturation at the eighth instar by males is associated with smaller mandibular weaponry. Because these males, compared with larger males (i.e., matured at 10th instar), are less successful at holding harems, they appear to have decreased mating success. Therefore, smaller males may acquire mates using tactics different than those of males with larger weaponry. A source of empirical support for this hypothesis is to show that smaller males offset their mating disadvantage by investing more in spermatogenesis to achieve fertilization success. Contrary to expectation, relative investment in testes (controlling for body size) is similar for smaller and larger males. I suggest that this lack of difference in testes size may be due to both morphs experiencing similar risks of sperm competition on Maud Island. Despite having testes similar in size to larger males, smaller males produce significantly larger ejaculates than males with bigger weaponry. This difference in ejaculate size may stem from larger males investing submaximally in current ejaculates in anticipation of their higher future mating success.

Male and female golden whistlers respond differently to static and dynamic signals of male intruders

van Dongen, W. F. D., Mulder, R. A. — 2008-09-12

Bird displays vary widely in their sensitivity to changes in signaler condition. Plumage ornaments are generally static and undergo minimal change. Behavioral ornaments, by contrast, are dynamic and may track signaler condition or motivation more closely. The relative importance of each trait type in signaling individual quality has been examined during female choice, but rarely in territorial defense. Male golden whistlers (Pachycephala pectoralis) display both a static plumage signal (throat patch) and a dynamic signal (song) during territorial disputes. We examined sex differences in the responses of territorial pairs to these traits during simulated territory intrusions by decoy males that had either normal or experimentally reduced throat patch sizes, in combination with low or high singing rates. Males paid attention to both categories of signals when estimating rival threat, responding for longer toward males with normal throat patches, and spending more time close to the intruder in high song rate trials. In contrast, females responded differentially only to dynamic signals. Patch size may reflect a male's long-term viability and status, whereas singing rates may correlate with willingness to escalate contests. As females participate in joint territorial defense and interactions between neighbors are sex specific, the song rates of intruding males may provide clues to their partners' motivation to escalate contests. By contrast, knowledge of an intruding male's intrasexual competitive abilities (signaled via patch size) may be unimportant to females. Differences in the signaling properties of static and dynamic signals may maintain the existence of multiple antagonistic signals in this species.

The ecology of vocal signaling: male spacing and communication distance of different song traits in nightingales

Naguib, M., Schmidt, R., Sprau, P., Roth, T., Florcke, C., Amrhein, V. — 2008-09-12

The ecology of a species and its communication system require mutual adaptations. Specifically, information transfer between individuals needs to be adapted to the social and ecological requirements and constraints. As a consequence, constraints on signal transmission are likely to play a role in the evolution of spatiotemporal behavior and settlement and vice versa. In long-distance signaling, such as in bird song, adaptations can be reflected in specific signal structures that transmit to a socially relevant distance without degradation masking the coded information. Here, we studied sound transmission properties of 2 different song components in male nightingale (Luscinia megarhynchos) territorial song, i.e., transmission of whistle songs and songs with rapid broadband trills. We also determined spacing of nocturnal song posts using a global positioning system. The results revealed highly significant differences in transmission of the 2 functionally different song traits, with information in whistles traveling well beyond the typical spacing between neighboring individuals. Information coded in trills bandwidth did not even travel the average distance toward the nearest neighbor, showing that information coded in trill bandwidth is not available for receivers at typical spacing distances. The results emphasize that for a better understanding of evolutionary processes in communication, ecological components such as spatial distance as well as signal structure and signal degradation have to be taken into account.

Heterospecific eavesdropping in a nonsocial species

Lea, A. J., Barrera, J. P., Tom, L. M., Blumstein, D. T. — 2008-09-12

Many species respond to heterospecific alarm calls, and the majority are social taxa and possess complex alarm calls themselves. Thus, the ability to respond to heterospecific alarm calls may be facilitated by possessing both these traits. Gunther's dik-dik (Madoqua guentheri) is a monogamous, territorial, and nonsocial miniature antelope with a simple vocal repertoire. Dik-diks are highly vulnerable to predation and could benefit from eavesdropping on heterospecific alarm calls. We broadcast recordings of white-bellied go-away bird (Corythaixoides leucogaster) alarm calls to dik-diks to test whether they could respond to heterospecific alarm calls. On hearing a go-away bird alarm call, dik-diks increased their likelihood to run to cover, decreased their foraging activity, increased their rate of head turning, and increased their period of vigilance compared with a nonthreatening bird song. Thus, predation risk alone, in the absence of complex sociality or complex communicative abilities, may be sufficient to drive the evolution of heterospecific eavesdropping.

Helping as a signal: does removal of potential audiences alter helper behavior in the bell miner?

McDonald, P. G., Kazem, A. J.N., Clarke, M. F., Wright, J. — 2008-09-12

Despite many studies on the evolution of cooperative breeding and helping at the nest, relatively few have explored the possibility that helping functions as a signal to gain social advantage within groups (the "pay to stay" and "social prestige" hypotheses). One of the most promising candidates for such a signaling system is the cooperatively breeding bell miner, Manorina melanophrys. Large numbers of unrelated helpers attend multiple nests while giving individually identifiable vocalizations, breeding females usually remain within monitoring distance of the nest area, and females often re-pair with the hardest working male helper after the death/removal of their breeding partner. We examined the possibility that helping operates as a signal by temporarily removing the potential audience: the breeding male or the breeding female. However, there was no discernable change in provisioning behavior of helpers, relative to control periods. We also simulated the presence of the removed birds through playbacks of their individual-specific calls and again found no effect on others’ visit rates, prey types, load size, and a variety of other behaviors at the nest. If either signaling hypothesis explained helping in this system, we might have expected facultative decreases in conspicuous provisioning behaviors when one or other potential audience was absent. Thus, despite possessing many of the prerequisites of a signal-based helping system, there is no evidence for such a phenomenon in bell miners. Cooperation in these groups of mixed relatedness may instead be driven by a combination of kin selection and direct benefits via group augmentation and/or pseudoreciprocity.

Predator-induced reductions in nest visitation rates are modified by forest cover and food availability

Eggers, S., Griesser, M., Ekman, J. — 2008-09-12

Bird parents can alert predators to the location of their nest. One mitigating option is that parents reduce their nest visitation rate in exchange for a lower predation risk. Here, using field data and experiments, we show that Siberian jay Perisoreus infaustus parents adjust feeding visit rates depending on an interaction of 3 factors: predator activity, nest concealment, and food availability. The rate of nest visits increased with the degree of nest concealment; yet, this relationship was modified by the presence of corvid predators. As the vegetation became more dense, parents at sites with high corvid activity disproportionately increased their feeding visit rates when compared with birds at sites with low corvid activity. We experimentally assessed how nesting cover affects this response of parents to the presence of corvids by using an Eurasian jay Garrulus glandarius model. Parents nesting at open sites ceased nest visits, whereas those nesting in dense forest continued feeding, albeit at a lower rate. Cover may thus not fully compensate for the effect of predator activity on feeding visit rates. However, offspring exposed to high predator activity might still receive the same amount of food because parents may adjust load sizes to compensate. This idea was confirmed by an experiment showing that in areas of high predator activity, food-supplemented birds significantly decreased nest visits when compared with nonsupplemented birds. These results indicate that some bird species can employ multiple nest-defense strategies to reduce predator-attracting nest visits; yet, these strategies may carry fitness consequences through reduced offspring quality.

Cleaner fish cause predators to reduce aggression toward bystanders at cleaning stations

Cheney, K. L., Bshary, R., Grutter, A. S. — 2008-09-12

Mutualisms, in which both participants gain a net benefit, are ubiquitous in all ecosystems, and the importance of understanding their broader ecological context has been demonstrated many times. Indirect effects of mutualisms may have important implications for surrounding ecosystems through changes in density, species composition, or behavior; however, the latter has been difficult to quantify. In fish cleaning mutualisms, cleaners benefit by removing and consuming ectoparasites from clients, whereas clients benefit from a reduction in parasite load. Cleaner fish are also thought to benefit from immunity to predation and use tactile stimulation as a preconflict management strategy to manipulate partners' decisions and to avoid being eaten by piscivorous client fish. Here we show, using a laboratory experiment, that the presence of cleaner fish resulted in nearby fish not involved in the cleaner–client mutualism experiencing less aggression (chases) from predatory clients. In addition, the rate that piscivorous clients chased prey was negatively correlated with the amount of tactile stimulation given to the predator by the cleaner. These data suggest that, in the laboratory, the risk of aggression from predators toward nearby prey fish was greatly reduced as a by-product of cleaner fish presence and tactile stimulation of predators by cleaner fish. These results raise the question of whether cleaning stations act as safe havens from predator aggression.