Jeffrey R. Lucas

The Role of Foraging Time Constraints and Variable Prey Encounter in Optimal Diet Choice

Predictions generated from optimality models are inescapably based on a number of assumptions. The predictive value of these models is often determined by the degree to which the behavior of an organism fits the underlying assumptions of the model. I analyzed optimal diet choice by relaxing two sets of assumptions made in previous optimality models. (1) Foraging-bout length (the uninterrupted time devoted just to foraging), generally treated as infinitely long, was shown to affect optimal diet choice. For many foragers, foraging-bout length may be considerably shortened by the presence of predators, or by physical or social features of the foragers environment. A model was derived which incorporates a short bout length into the decision of diet choice. The model predicts that animals should become more catholic in their diet choice as the amount of uninterrupted foraging time decreases. This prediction appears to be supported by three studies from the literature. Jaeger et al. (1981) showed that salamanders incorporated more lower ranked prey (small flies) when they were either on the territory of a conspecific or on no territory as compared with prey choice when they were on their own territory. In this case, foraging time was uninterrupted when the salamanders were feeding selectively, but continuously interrupted by submissive behavior and marking behavior when no diet choice was exhibited. Freed (1981) showed that wrens foraging for nestlings spent less time per foraging bout when a predator was in the nesting area than when no predator was in sight. The reduction in foraging bout time correlated with a reduction in prey size fed to the young. The foraging time of some intertidal snails was shown to be confined by the length of the low tide cycle (Menge 1974). As the end of the low tide drew near, the snails decreased diet selectivity. Thus, as the remaining time available for foraging decreased, the predator exhibited a lower degree of prey selection. (2) Variance in prey encounter interval was shown to affect the utility of classical optimal diet models in predicting the optimal diet. Charnovs (1976) model is shown to overestimate the net rate of energy intake when mean encounter rate varies about some fixed level. Predictions from Charnovs model are incorrect over some ranges of prey encounter rates because of this overestimation. I show that as variance in prey encounter rate increases, the time over which the forager estimates prey encounter rate will have a strong effect on the ability of the forager to maximize the net rate of energy intake. Foragers that forage on patchily distributed prey should use a shorter amount of time to estimate prey density than foragers that prey on evenly dispersed prey. Thus, animals that are capable of reducing the time required to estimate prey density (for example, predators that hunt by sight in areas of high prey density) should alter their diet in response to local variation in prey density. For this type of forager, as variance in prey encounter rate increases, fluctuations in the number of prey types in the diet will increase. As a result, there should be an increase in the degree of partial prey preference exhibited by the forager with increasing variance in prey encounter rate.

Does hippocampal size correlate with the degree of caching specialization

A correlation between the degree of specialization for food hoarding and the volume of the hippocampal formation in passerine birds has been accepted for over a decade. The relationship was first demonstrated in family–level comparisons, and subsequently in species comparisons within two families containing a large number of hoarding species, the Corvidae and the Paridae. Recently, this approach has been criticized as invalid and excessively adaptationist. A recent test of the predicted trends with data pooled from previous studies found no evidence for such a correlation in either of these two families. This result has been interpreted as support for the critique. Here we reanalyse the original dataset and also include additional new data on several parid species. Our results show a surprising difference between continents, with North American species possessing significantly smaller hippocampi than Eurasian ones. Controlling for the continent effect makes the hoarding capacity/hippocampal formation correlation clearly significant in both families. We discuss possible reasons for the continent effect.

Heat or insulation: behavioral titration of mouse preference for warmth or access to a nest.

In laboratories, mice are housed at 20–24°C, which is below their lower critical temperature (≈30°C). This increased thermal stress has the potential to alter scientific outcomes. Nesting material should allow for improved behavioral thermoregulation and thus alleviate this thermal stress. Nesting behavior should change with temperature and material, and the choice between nesting or thermotaxis (movement in response to temperature) should also depend on the balance of these factors, such that mice titrate nesting material against temperature. Naïve CD-1, BALB/c, and C57BL/6 mice (36 male and 36 female/strain in groups of 3) were housed in a set of 2 connected cages, each maintained at a different temperature using a water bath. One cage in each set was 20°C (Nesting cage; NC) while the other was one of 6 temperatures (Temperature cage; TC: 20, 23, 26, 29, 32, or 35°C). The NC contained one of 6 nesting provisions (0, 2, 4, 6, 8, or 10g), changed daily. Food intake and nest scores were measured in both cages. As the difference in temperature between paired cages increased, feed consumption in NC increased. Nesting provision altered differences in nest scores between the 2 paired temperatures. Nest scores in NC increased with increasing provision. In addition, temperature pairings altered the difference in nest scores with the smallest difference between locations at 26°C and 29°C. Mice transferred material from NC to TC but the likelihood of transfer decreased with increasing provision. Overall, mice of different strains and sexes prefer temperatures between 26–29°C and the shift from thermotaxis to nest building is seen between 6 and 10 g of material. Our results suggest that under normal laboratory temperatures, mice should be provided with no less than 6 grams of nesting material, but up to 10 grams may be needed to alleviate thermal distress under typical temperatures.

Receivers respond differently to chick-a-dee calls varying in note composition in Carolina chickadees, Poecile carolinensis

The chick-a-dee call of the avian genus Poecile is a structurally complex vocal system because it possesses a set of simple rules that governs how the notes of the call are ordered, and variable numbers of each of the note types strung together can generate an extraordinary number of unique calls. Whereas it has been hypothesized that chick-a-dee calls with different notes may convey different information, no experimental evidence has been offered in support of the hypothesis. Previously published studies suggested that flock members use chick-a-dee calls in the context of moving to or from a feeding site. Here, we tested Carolina chickadees’ responses to playbacks of chick-a-dee calls that differed in note composition. Playbacks were conducted in the field in the context of a novel food source. Our pilot data had indicated that chick-a-dee calls with relatively large numbers of ‘C’ notes were given by birds on their first contact with a novel seed stand. In the present study, we found that chickadees flew in close to the playback speaker and subsequently took seed from a seed stand more often during playbacks of chick-a-dee calls containing C notes than chick-a-dee calls not containing C notes or than control playbacks. Vocal responses of chickadees to the playbacks also differed in relation to the particular vocal signal being played back. These results indicate that receivers respond differently to chick-a-dee calls containing different compositions of note types and represent a first step to link variation in note composition and ordering in these calls to possible meanings.

A comparative study of avian auditory brainstem responses: correlations with phylogeny and vocal complexity, and seasonal effects

Abstract. We conducted a comparative study of the peripheral auditory system in six avian species (downy woodpeckers, Carolina chickadees, tufted titmice, white-breasted nuthatches, house sparrows, and European starlings). These species differ in the complexity and frequency characteristics of their vocal repertoires. Physiological measures of hearing were collected on anesthetized birds using the auditory brainstem response to broadband click stimuli. If auditory brainstem response patterns are phylogenetically conserved, we predicted woodpeckers, sparrows, and starlings to be outliers relative to the other species, because woodpeckers are in a different Order (Piciformes) and, within the Order Passeriformes, sparrows and starlings are in different Superfamilies than the nuthatches, chickadees, and titmice. However, nuthatches and woodpeckers have the simplest vocal repertoires at the lowest frequencies of these six species. If auditory brainstem responses correlate with vocal complexity, therefore, we would predict nuthatches and woodpeckers to be outliers relative to the other four species. Our results indicate that auditory brainstem responses measures in the spring broadly correlated with both vocal complexity and, in some cases, phylogeny. However, these auditory brainstem response patterns shift from spring to winter due to species-specific seasonal changes. These seasonal changes suggest plasticity at the auditory periphery in adult birds.

Time Constraints and Diet Choice: Different Predictions from Different Constraints

The time that an animal spends foraging may often be constrained by simultaneous requirements for the expression of other behaviors. For example, the need for mating, vigilance, or antipredator escape responses may significantly alter the time that an animal allocates to foraging behavior. I show here that the time allocated to foraging behavior may, in turn, significantly alter predictions concerning diet choice. I review predictions from two classes of models that specifically address the influence of time constraints on prey selection. Bout length for each class of model is treated either as a known constant or as a random variable with either of two frequency distributions (normal or negatively exponential). Two major predictions are derived from the models. (1) If foraging-bout length is such that only a single prey is taken per bout (class 1), the forager should decrease prey selectivity as the time left in the bout decreases. If more than one prey may be taken (class 2) and if the bout length is known, then selectivity exhibited within the bout depends on the relative value of available prey. If prey are similar in rank, then the forager should generalize. If prey differ markedly in rank, then selectivity should decrease as the time left in the bout decreases. It is predicted that intermediate differences generate oscillations in selectivity toward the end of the bout from a generalist policy to a specialist policy, then back to a generalist policy. Thus, both classes of model generally predict an end-of-bout change in preference. This end-of-bout effect, however, declines as the variance in bout length increases, and vanishes altogether when the time of the end of the bout is unpredictable. (2) In addition to the end-of-bout effect, mean selectivity exhibited throughout the bout should differ between bouts that differ in length; mean selectivity should generally decline with a decrease in bout length. This between-bouts effect is predicted regardless of the variance in bout length. Several studies from the literature report either an end-of-bout effect or a between-bouts effect for animals foraging under time constraints.

The biophysics of pit construction by antlion larvae (Myrmeleon, Neuroptera)

Abstract An antlion pit is lined with fine particles during construction. This feature appears to increase the effectiveness of the pit in prey capture. Pit structure is influenced by physical properties of sand and the building behaviour of the antlion. Two physical properties of sand govern pit structure: the angle of repose and Stokes Law drag force. These two properties complement each other as follows: (a) Since larger particles have a lower angle of repose than smaller particles, fine sand grains tend to stay on the pit walls, whereas larger particles fall to the pits centre. (b) Large particles have a lower drag to momentum ratio than do small particles. Thus, larger particles are more likely to be thrown out of the pit than are smaller particles. Several behavioural modifications were demonstrated that increase the number of fine particles on the pit walls while reducing construction costs for the antlion. (a) A trajectory angle of 45° is used when the antlion throws particles out of the pit. This angle will maximize the distance to which larger particles are thrown. A trajectory angle of 60° is used at the end of pit construction when the antlion is throwing fine particles on the sides of the pit. This angle reduces the number of these fine particles leaving the pit. (b) Antlions can alter the velocity with which they throw particles. When discarding prey carcasses and debris that have accumulated during prey capture, they use a velocity that is approximately 39% higher than the velocity used during pit construction. (c) By vibrating their forelegs, antlions appear to sift out the finer particles before each throw. This increases the percentage of larger particles discarded from the pit.

When should chickadees hoard food? Theory and experimental results.

Abstract A dynamic programming model was developed to evaluate conditions that promote food caching. Long-term observations (3–4 months per bird) of caching behaviour by Carolina chickadees, Parus carolinensis , are discussed in the light of the predictions from the model. Two different fitness functions were modelled: net energy-rate maximization and survival-rate maximization. Under most simulated conditions, energy-rate maximizers are predicted to cache at uniformly high rates. Two tradeoffs are important in caching decisions of survival-rate maximizers. (1) When fat levels are high, caching decisions should reflect trade-offs between time allocated to foraging and non-foraging behaviour. At the highest fat levels, the expression of non-foraging behaviour should be relatively more valuable than foraging. As a result, caching is not predicted because it increases foraging time and thus reduces time available for alternative behaviour. As fat levels drop below the maximum, foraging requirements become more important and caching rates are predicted to increase. (2) At intermediate to low fat levels, caching decisions should reflect a response to trade-offs affecting starvation risk. At very low fat levels, the forager should eat any food at the site of discovery to reduce the risk of starvation in the near term. At intermediate fat levels, the forager is not at immediate risk of starvation, so it should cache to reduce starvation risk in the long-term. These mass-dependent trade-offs should also affect diurnal patterns in caching rates: caching rates should be low at dawn, when the foragers fat reserves are at their minimum, and at dusk, when fat reserves are at their peak; caching rates should be highest at midday. The experimental results suggest that chickadees do not maximize net energy intake rates; instead, their behaviour is in broad agreement with the predictions of a survival-rate maximizer. However, one additional prediction was clearly not supported: larger birds cached less than smaller birds. Observed seasonal patterns in caching, retrieval and recaching rates are also discussed.

Ecological correlates of explosive seed dispersal

SummaryBased on the constraints of the ballistic mechanism, we suggest that plants that utilize explosive dispersal are either maximizing ballistic distances or maximizing secondary dispersal (and thus are ballistically short-distance dispersers). Explosive seed dispersal of seven plant species was investigated in terms of factors contributing to the distance that seeds are thrown. As predicted, the long distance dispersers (4 species) showed more constancy in distance that seeds were thrown relative to the ballistically short-distance dispersers (3 species). The distribution of explosively dispersed seeds in terms of a resource for post-dispersal predators was evaluated by computer simulation. The results indicated that seed predation is unlikely to contribute to maximizing seed dispersal distance. A model for explosive seed dispersal was developed, based on the constraints of such dispersal. Long-distance dispersers were expected to use small, closely-spaced patches or to use large patches in which inbreeding depression or competition selects for maximal dispersal distances. Short-distance dispersers were expected to use small, widely-spaced patches where ballistic dispersal in itself is an inadequate means to disperse seeds, or to use large patches in which there is no premium for dispersal distance. Preliminary evidence supported the model.

ON ALTERNATIVE REPRODUCTIVE TACTICS IN ANURANS: DYNAMIC GAMES WITH DENSITY AND FREQUENCY DEPENDENCE

Intense mating competition and mate choice has favored the evolution of alternative mating tactics in many species. We developed a stochastic dynamic game to evaluate the choice among three alternative mating tactics by male anurans: call, satellite, or leave the chorus to forage or hide. The strategies were assumed to differ in mating success (call > satellite forage = hide), predation risk (call > satellite > forage > hide), and energetic expense (call > satellite = hide > forage). Six major predictions were derived. First, in relatively stable environments, males should enter a chorus in a synchronous pulsed pattern, with first-year males primarily choosing to be satellites on older calling males. The pulsed pattern is caused by the relatively high energetic cost of calling and the dependence of female arrival rates on chorus size (which promotes synchronization). In unstable environments, energetic constraints are reduced because favorable conditions are rare, so most males should call on favorable nights. Under all conditions, energy-constrained males should forage. Second, with an accelerating relationship between female arrival rate (measured per calling male) and chorus size, first-year males in a stable environment should drop out of choruses before the end of the breeding season because the payoff to attendance decreases as the population declines; with a decelerating function, first-year males should remain the entire year. In an unstable environment, males should enter a chorus on favorable days, irrespective of the shape of the female arrival function. Third, variation in the form of predation risk with respect to chorus size has little or no effect on mating patterns; however, high mean predation risk should increase the frequency of satellites in the chorus. Fourth, conflicts can occur between old males that are selected to call and young males that are selected to act as satellites. Under some conditions, the only resolution to this conflict is for all males to abandon the chorus. Fifth, if there are differences among first-year males in when they come into breeding condition, the few males that mature early should be satellites. Later in the season, first-year males should tend to call. This prediction is unaffected by density dependence in predation risk, but it does not hold when female arrival rate is density independent. Finally, first-year males should call more when their mating success decreases with an increase in the number of calling older males in comparison with conditions in which mating success is a fixed fraction of the mating success of older males. We show that the properties of a number of anuran species make them amenable to test these predictions.