Larry L. Wolf

Modulation of aggressive behaviour by fighting experience: mechanisms and contest outcomes

Experience in aggressive contests often affects behaviour during, and the outcome of, later contests. This review discusses evidence for, variations in, and consequences of such effects. Generally, prior winning experiences increase, and prior losing experiences decrease, the probability of winning in later contests, reflecting modifications of expected fighting ability. We examine differences in the methodologies used to study experience effects, and the relative importance and persistence of winning and losing experiences within and across taxa. We review the voluminous, but somewhat disconnected, literature on the neuroendocrine mechanisms that mediate experience effects. Most studies focus on only one of a number of possible mechanisms without providing a comprehensive view of how these mechanisms are integrated into overt behaviour. More carefully controlled work on the mechanisms underlying experience effects is needed before firm conclusions can be drawn.

Economics of Feeding Territoriality in the Golden-Winged Sunbird

Feeding territories of Golden-winged Sunbirds contain enough energy to sup- port an individuals daily energy requirements, and the amount of nectar per flower inside a territory tends to average higher than in adjacent undefended flowers. When undefended nectar levels are low (especially below 2 /ul per flower) the costs of territorial defense can easily be offset by energy saved from shortened foraging time budgets made possible by feeding at the higher average nectar levels. At higher undefended nectar levels the costs of territorial defense should not be recoverable. The balance between these costs and gains appears to define the conditions when territorial defense in this species is advantageous.

Ecological Determinants of Group Sizes of Foraging Lions

The literature concerning animal group size follows a basic dichotomy. One approach entails mathematical descriptions of numbers within groups and their rates of change (e.g., Cohen 1969, 1972; Kingman 1969). These authors developed Markov population processes where the rates of arrival, departure, and transfer of individuals among groups are linear (open systems) or quadratic (closed systems) functions of individual group sizes (Cohen 1972). Cohen has estimated that primate group size frequencies tend to follow the negative binomial or will approach a Poisson distribution, if the arrival rate is zero or independent of group size. Analyzing Struhsakers (1965) data on the number of individuals in vervet monkey sleeping groups, Cohen (1972) found a reasonable fit to the mathematically predicted distribution. While such an approach offers interesting descriptions of group size, it considers only some possible biological phenomena regulating these groups. Crook (1970) suggested that the numbers in vervet monkey sleeping groups were related to their ecology. He interpreted the dispersal of the troop into small nocturnal groups as an antipredator defense. The vervets selected sleeping sites for proper concealment, and each site provided protection for a limited number of monkeys, thereby regulating the size of individual groups. In this view, the troops resource-the size of concealed sites-generated Cohens (1972) negative binomial distribution. A number of authors have related ecological conditions to group size in birds and mammals. Avian flocking behavior and flock size have been correlated with increased foraging efficiency (Cody 1971; Horn 1968), hypothesized to reduce vulnerability to predation (Hamilton 1971; Vine 1971), and related to territorial defense (Brown 1963; Schoener 1971). Jarman (1974) suggested that ecological factors regulate both the minimum and maximum group sizes for African antelopes. Crook (1972) listed the ecological determinants of primate group size as predation pressure, food abundance, the distribution of other essential resources (e.g., water holes), and the optimization of time and energy budgets in response to resource distributions. Denham (1971) proposed a similar ecological model for primate sociality to predict not only group sizes but sex ratios, sexual dimorphism, and mate-selection systems. Each work hypothesized that group sizes are a response to environmental characteristics.

Time and Energy Budgets of Territorial Hummingbirds

From laboratory data of flying and resting metabolism and field data on time budgets of territorial male Eulampis jugularis hummingbirds we were able to estimate caloric energy budgets associated with territorial behavior. The relative time and energy expenditures for territorial defense, foraging, and sitting varied in territories with different species of flowers. For example, territories centered around banana flowers had lower foraging costs than those centered around Myrtaceae or Inga—Hibiscus. The costs for flycatching, hovering,and perch changes accounted for relatively little of the time and energy budgets at any of the territorial defense and foraging were expended in sitting on perches within the territory. Territorial defense was of relatively low cost in time and energy because of the inactive defense utilized by Eulampis. Selection will optimize time and/or energy budgets, but the type of budget optimized will depend in large part on the exploitation technique of the species. For birds that do not fly to forage, but exploit food items that require a long search time per calorie, time that can be released for other activities may be more important than major shifts in energy budgeting. For hummingbirds, however, optimizing energy may be more important to insure a positive energy budget.

Nonrandom Foraging by Sunbirds in a Patchy Environment

Sunbirds (Nectarinia spp.) feeding at the East African mint Leonotis nepetifolia en- counter great variations in the nectar contents of flowers blooming in dense fields. The dispersion patterns of nectar are attributable to nectar removal from some flowers by the sunbirds themselves in earlier foraging and also to intrinsic floral variations. The problem facing the foraging sunbird is to increase its foraging efficiency (net energy gain per unit time) by avoiding recently visited, empty flowers and by visiting flowers with greater than average nectar volumes. Sunbirds patterned their foraging in 3 major ways. First, they used initial flowers probed on an inflorescence as an assay of what the rest of the flowers in that inflorescence contained and they rejected inflorescences with little nectar. Such rejection increased nectar intake per flower by as much as 15%. Second, territorial sunbirds preferentially fed at unvisited inflorescences, increasing nectar intake 25% relative to random foraging. This was accomplished at least in part by foraging at different heights on successive foraging bouts. Third, flight distances to the next flower changed in response to immediate reward levels in some species but not in others. In general, sunbirds feeding at Leonotis responded less to reward levels by differential turning and movement than some other organisms, possibly reflecting different prey distributions or boundary constraints on their foraging.

Nectar Characteristics and food selection by hummingbirds

SummaryHummingbirds selected food in choice experiments based primarily on sugar concentration and secondarily on rate of intake and position. Sugar compositions had little effect on food choice, but the preferred sugar compositions appear to be the most common in nectars of plants visited by hummingbirds. Most amino acids in sugar water were not detected at concentrations found in nectars. Higher amino acid concentrations generally resulted in rejection. Hummingbirds did not necessarily select food in the laboratory to maximize feeding efficiency, but under natural circumstances similar choices could result in optimal feeding efficiencies. The determinants of food choice by hummingbirds provide a rationale for viewing factors important in plant competition for pollinator visits.

Temporal patterning of feeding by hummingbirds

Abstract For five species of hummingbirds in the laboratory, time between meals was related to energy intake on the first meal and rate of energy expenditure between meals. Field observations gave similar results. Average meal sizes were similar at one intake rate independent of food caloric density; females averaged longer bouts than males. When rate of intake was approximately halved, meal duration approximately doubled and volume intake remained similar. We postulate that feeding is initiated when crop contents reach a lower threshold and that feeding is terminated after ingestion of an optimal volume determined by the added weight of the meal.

Optimal Meal Size in Hummingbirds

Optimization theory is applied to hummingbird foraging to explain the observation that birds in the laboratory with access to functionally unlimited food supplies usually do not take as large a meal as they could. Foraging bouts are modeled in terms of time and energy; birds are constrained by the increased energetic costs associated with the added weight of their meal. Predictions of optimal meal size which maximize the rate of net energy gain or which maximize efficiency agree closely with laboratory and limited field data; we presently are unable to discriminate between these two optimality criteria. Predictions which assume optimal use of time or which maximize net energy gain or which neglect weight of the meal all proved unsatisfactory, generally by predicting optimal meal sizes in excess of capacity. Sensitivity analysis revealed that model predictions may be much influenced by parameter values potentially under physiological and/or behavioral control by a hummingbird. While we presently do not understand what governs the values of these parameters, we suggest that this control will eventually prove explicable by an optimality model of bird behavior.

POLLEN LIMITATION IN A MONOCARPIC SPECIES, IPOMOPSIS AGGREGATA

(1) We tested whether seed production in the perennial monocarpic plant Ipomopsis aggregata (Polemoniaceae) was pollen-limited by adding outcross pollen to open stigmas of plants on three occasions at weekly intervals. We compared fruit set and seed set per fruit with a set of adjacent, untreated plants of the same average height and flower number. (2) Experimental pollination doubled the number of flowers producing seed after each occasion and increased the average seed set per fruit for the first one. (3) Plants treated with pollen showed correlations between seed production and height and aborted seed production consistent with resource limitation, but untreated, naturally pollinated plants did not.

Crop volume, nectar concentration and hummingbird energetics

Abstract 1. 1. X-ray measurements of hummingbirds indicated that initially food passed only to the crop. 2. 2. Food intake experiments with nine tropical species of hummingbirds (2·7–10·1 g) indicated that crop volume is a linear function of body weight. 3. 3. Nectar concentrations of flowers visited by hummingbirds in the lowlands were equivalent to 0·61–1·23 M sucrose, while nectar from highland flowers was equivalent to 0·29–0·68 M sucrose. 4. 4. At a given ambient temperature, small hummingbirds have less storage capacity in their crops relative to their metabolic rates per g than larger hummingbirds. 5. 5. A major factor for the energetics of hummingbirds is the lower nectar concentration from flowers in the highlands where the hummingbirds are exposed to lower ambient temperatures.