Foraging Behaviour of Subordinate Great Tits (Parus major). Can Morphology Reduce the Cost of Subordination?

This paper studies the magnitude of the behavioural shift, from forage standing to forage hanging, of subordinate great tits ( Parus major ) in two different social contexts: feeding solitarily vs. feeding with a dominant conspecific. The aim is to test the hypothesis that differences in morphological design provide subordinates with varying abilities to reduce the presumed costs of subordination. We find that different subordinate individuals change the foraging behaviour, occupying a different niche when an intra-specific competitor is present. Morphology linked to sexual dimorphism, specifically body mass, is the factor responsible for the different magnitudes of change. Lighter subordinates can remain longer than heavier ones at the feeding patch without interrupting their foraging. Thereby, the former reduce the costs of being subordinate more than the latter. Among subordinates, females are lighter than males; they also spend more time feeding in the presence of a dominant conspecific than males do. No differences are found between age categories. We find no relationship between tarsus length and individual ecological plasticity. Our results support the idea that the ecological plasticity due to morphological differences is a mechanism that allows subordinate individuals to overcome costs associated with subordination.     

The effect of depletion and predictability of distinct food patches on the timing of aggression in red deer stags

For group-foraging ungulates, forage is generally widely and relatively evenly dispersed. However, for free-ranging red deer Cervus elaphus supplementary winter feeding provides distinct patches of predictable food. These patches differ in size, but also in temporal distribution and depletion rate. Interference competition is known to increase with increasing spatial clumping (decreasing patch size), but the influence of temporal clumping and the predictability of food occurrence has received much less attention. Therefore in this study we investigated the effects of different degrees of spatial and temporal clumping of food on interference competition during feeding. Patch size was the main parameter influencing participation in feeding as well as interference competition during feeding on the respective patch. Temporal dispersion and the predictability of food occurrence were however, important parameters for the timing of aggressive interactions. Generally, aggression occurs during feeding and increases with decreasing patch size. But when depletion rate was high food availability was predictably short and the patch occurred predictably (such as hay), middle ranking stags increased aggression already prior to feeding at the respective patch. We suggest that in this way they confirmed hierarchy outside feeding on the quickly depleted patch and as a result gained actual feeding time when feeding on the respective patch. With the patch occurring predictably but varying in size the number of participating subordinates varied concomitantly with variation in patch size. Subordinates assessed patch profitability and left without having fed when patch size was too small for an efficient participation. When patch size was predictably small enough to be defended exclusively (feed blocks), subordinate stags did not assess profitability each time but did not participate at all in feeding at the respective patch. The relative importance of these various food-related parameters (patch size depletion rate, predictability) influencing feeding competition and the timing of aggression will vary with group size, rank, alternative food sources, physical characteristics of the food as well as different hierarchy systems of the feeding animals.     

Ecological correlates to social structure in two lemur species in Madagascar

In this study, I tested two hypotheses regarding the relationship of ecological variables (size, density, and distribution of patches) and infant developmental patterns to lemur social structure using two prosimian primates in Ranomafana, Madagascar: the rufous lemur (Eulemur fulvus rufus) and the red-bellied lemur (Eulemur rubriventer). Three predictions regarding the general effects of patch size and subgroup size on lemur feeding rates were supported: (1) Rufous lemurs used large patches; red-bellied lemurs used smaller patches; (2) larger subgroups of rufous lemurs used larger patches; and (3) rufous lemur feeding rates decreased significantly with increases in subgroup size and patch size, whereas size and patch size had no significant effect on red-bellied lemur feeding rates. However, food item size (fruit) had a more significant effect on rufous and red-bellied lemur feeding rates than either patch size or subgroup size. When similar-sized fruits were compared, rufous lemur feeding rates on small fruit were most affected by patch size, yet feeding rates on medium-sized fruit were most affected by subgroup size. Neither lemur species used patches in consistent ways seasonally. During periods of food abundance, rufous lemurs used many small, common, and clumped patches. In food scarcity periods, they used fewer, larger, rarer, and less clumped patches; groups migrated when food became most scarce. Red-bellied lemurs also used patches in variable ways, but these patterns were not linked with food availability. Finally, infant development patterns differed between lemur species; red-bellied lemur males cared for offspring and infants reached developmental landmarks faster than rufous lemur infants. Therefore, red-bellied lemur group size may be constrained by the need for additional infant care by other group members. In contrast, rufous lemur group size may be constrained by patch availability during the most critical period of food scarcity. © 1996 Wiley-Liss, Inc.     

The effect of dominance on food hoarding: a game theoretical model

Many food hoarding animals live in small groups structured by rank. The presence of conspecifics in the hoarding area increases the risk of losing stored supplies. The possibility of stealing from others depends on a forager's rank in the group. Highly ranked individuals can steal from subordinates and also protect their own caches. Since storing incurs both costs and benefits, the optimal hoarding investment will differ between individuals of different rank. In a game theoretical model, we investigate how dominant and subordinate individuals should optimize their hoarding effort. Our model imagines animals that are large-scale hoarders in autumn and dependent on stored supplies for winter survival. Many examples can be found in the bird families Paridae and Corvidae, but the model can be used for any hoarding species that forage in groups. Predictions from the model are as follows: First, subordinates should store more than dominants, but in a predictable environment, this difference will decrease as the environment gets harsher. Under harsh conditions, dominants should store almost as much as subordinates and, later, spend almost as much time retrieving their own caches as subordinates. Second, if on the other hand, bad winter conditions were not expected when storing, dominants should spend more time pilfering caches from subordinates. Third, in populations that are highly dependent on stored supplies, dominants should store relatively more than in populations that are less dependent on stored supplies. Fourth, harsher environments will favor hoarding. And finally, if dominant individuals store, it implies that hoarders have a selfish recovery advantage over conspecific pilferers.     

Group-Living Herbivores Weigh Up Food Availability and Dominance Status when Making Patch-Joining Decisions

Two key factors that influence the foraging behaviour of group-living herbivores are food availability and individual dominance status. Yet, how the combination of these factors influences the patch-joining decisions of individuals foraging within groups has scarcely been explored. To address this, we focused on the patch-joining decisions of group-living domestic goats (Capra hircus). When individuals were tested against the top four ranked goats of the herd, we found that at patches with low food availability they avoided these dominant patch-holders and only joined subordinates (i.e. costs outweighed benefits). However, as the amount of food increased, the avoidance of the top ranked individuals declined. Specifically, goats shifted and joined the patch of an individual one dominance rank higher than the previous dominant patch holder when the initial quantity of food in the new patch was twice that of the lower ranking individual’s patch (i.e. benefits outweighed costs). In contrast, when individuals chose between patches held by dominant goats, other than the top four ranked goats, and subordinate individuals, we found that they equally joined the dominant and subordinate patch-holders. This joining was irrespective of the dominance gap, absolute rank of the dominant patch-holder, sex or food availability (i.e. benefits outweighed costs). Ultimately, our results highlight that herbivores weigh up the costs and benefits of both food availability and patch-holder dominance status when making patch-joining decisions. Furthermore, as the initial quantity of food increases, food availability becomes more important than dominance with regard to influencing patch-joining decisions.     

Strong and weak cross-sex correlations govern the quantitative-genetic architecture of social group choice in Drosophila melanogaster

When genotypes differ in niche-constructing traits, genotypes are expected to differ in which environments they experience, providing a novel causal relationship between genotypes, environments, and behavior. Such genetic variation in niche construction (or, more precisely, environment construction) is predicted to be especially important for social environments, yet the quantitative-genetic parameters governing such variation are still poorly understood. Here, we examine genetic variation and cross-sex genetic correlations for social environment-constructing behaviors. We focus on whether genetic variation in patch use—the tendency to spend time near food patches where conspecifics may be present—and group-size preference—the specific group size chosen when individuals are affiliating—is correlated or decoupled across sexes in the fruit fly, Drosophila melanogaster. Across three choice treatments, we find genotype and sex differences in how much time individuals spend near patches, and which group sizes they prefer. We find that the genetic basis of patch use is strongly coupled across sexes, whereas the genetic basis of group-size preference is completely decoupled across sexes. We discuss how these findings augment and complicate our understanding of the evolutionary genetics of social behaviors.     

Social status, access to food, and compensatory growth in juvenile Atlantic salmon

Juvenile Atlantic salmon Salmo salar subjected to three weeks of cooler temperatures were 8·5% smaller than controls at the end of the temperature manipulation, but had caught up in size 20 weeks later. The behavioural means is examined by which this catch-up or compensatory growth is achieved. While on average compensating fish did not spend more time feeding, dominant fish within each group gained more exclusive access to the feeding area during periods of catch-up growth. Therefore the extent to which compensatory growth could be achieved was dependent on both the social status of the individual and the dominants' ability to monopolize the food patch.     

Does an opportunistic predator preferentially attack nonvigilant prey?

The dilution effect as an antipredation behaviour is the main theoretical reason for grouping in animals and states that all individuals in a group have an equal risk of being predated if equally spaced from each other and the predator. Stalking predators, however, increase their chance of attack success by preferentially targeting nonvigilant individuals, potentially making relative vigilance rates in a group relatively important in determining predation compared with the dilution effect. Many predators, however, attack opportunistically without stalking, when targeting of nonvigilant individuals may be less likely, so that the dilution effect will then be a relatively more important antipredation reason for grouping. We tested whether an opportunistically hunting predator, the sparrowhawk, Accipiter nisus, preferentially attacked vigilant or feeding prey models presented in pairs. We found that sparrowhawks attacked vigilant and feeding mounts at similar frequencies. Our results suggest that individuals should prioritize maximizing group size or individual vigilance dependent on the type of predator from which they are at risk. When the most likely predator is a stalker, individuals should aim to have the highest vigilance levels in a group, and there may be relatively little selective advantage to being in the largest group. In contrast, if the most likely predator is an opportunist, then individuals should simply aim to be in the largest group and can also spend more time foraging without compromising predation risk. For most natural systems this will mean a trade-off between the two strategies dependent on the frequency of attack of each predator type. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.     

The Effect of Food Size and Dispersion Pattern on Retrieval Rate by the Argentine Ant, Linepithema humile (Hymenoptera: Formicidae)

Food acquisition in central-place foraging animals demands efficient detection and retrieval of resources. Most ant species rely on a mass recruitment foraging strategy, which requires that some potential foragers remain at the nest where they can be recruited to food once resources are found. Because this strategy reduces the number of workers initially looking for food, it may reduce the food detection rate while increasing the postdiscovery food retrieval rate. In previous studies this tradeoff has been analyzed by computer simulation and mathematical models. Both kinds of models show that food acquisition rate is greatly influenced by food distribution and resource patch size: as food is condensed into fewer patches, the maximal acquisition rate is achieved by a shift to fewer initial searchers and more potential recruits. In general, these models show that a mass recruitment strategy is most effective when resources are clumped. We tested this prediction in two experiments by letting laboratory colonies of the Argentine ant (Linepithema humile) forage for resources placed in different distributions. When all prey were small, retrieval rate increased with increasing resource patch size, in support of foraging models. When prey were large, however, the mass of prey returned to the colony over time was much lower than when prey were small and widely distributed. As more ants reached a large prey item, the distance the prey item was transported decreased due to a greater emphasis on feeding rather than transport. Because Argentine ants can transport more biomass externally than they can ingest, food retrieval that depends only on ingestion can depress the biomass retrieval rate. Thus, our results generally support theoretical foraging models, but we show how prey size, through differential prey-handling behavior, can produce an outcome greatly different from that predicted only on the distribution of resources.     

Unequal food distribution among great egret Ardea alba nestlings: parental choice or sibling aggression?

In broods of great egrets Ardea alba and other birds with siblicidal nestlings, the first-hatched brood members generally secure far more food than do their juniors. This feeding advantage could be caused by parental favoritism, or by seniors attacking and thereby dominating their juniors. We investigated these possibilities by comparing how fathers and mothers allocated food among their offspring when chicks were free to fight versus when they were physically separated by a Plexiglas barrier. When free to fight, dominant nestlings received significantly more food than did their subordinates. When nestlings were separated, mothers, but not fathers, delivered significantly more food per meal to the β (second-ranked) chick than to other nestlings. This is the first experimental evidence of differential feeding by parents in a species with aggressive nestlings.     

Antipredator vigilance in birds: modelling the 'edge' effect

Many animals spend a large proportion of their time either foraging for food or watching out for predators (antipredator vigilance). There have been many theoretical and empirical studies investigating the trade-off between these two activities, especially in birds. Previous models of antipredator vigilance assume that all birds within the group spend the same amount of time feeding. However, many empirical studies have shown that individuals on the edge of flocks are more vigilant. Here we describe a vigilance model which investigates the effect of position on the birds' strategies by dividing the feeding area into an inner and outer region. The model examines how various parameters such as food availability and predation risk affect the strategies of individual birds according to whether they are in the inner or outer region. Our model predicts that birds in the outer group are always more vigilant than those in the inner region. Birds in the centre receive a higher payoff in each of the wide range of scenarios that we have considered, and so our model also indicates why dominant birds would choose to feed in the centre of the group; a prediction in accord with several empirical studies.     

Mallards Feed Longer to Maintain Intake Rate under Competition on a Natural Food Distribution

Animals foraging in groups may benefit from a faster detection of food and predators, but competition by conspecifics may reduce intake rate. Competition may also alter the foraging behaviour of individuals, which can be influenced by dominance status and the way food is distributed over the environment. Many studies measuring the effects of competition and dominance status have been conducted on a uniform or highly clumped food distribution, while in reality prey distributions are often in‐between these two extremes. The few studies that used a more natural food distribution only detected subtle effects of interference and dominance. We therefore conducted an experiment on a natural food distribution with focal mallards Anas platyrhynchos foraging alone and in a group of three, having a dominant, intermediate or subordinate dominance status. In this way, the foraging behaviour of the same individual in different treatments could be compared, and the effect of dominance was tested independently of individual identity. The experiment was balanced using a 4 × 4 Latin square design, with four focal and six non‐focal birds. Individuals in a group achieved a similar intake rate (i.e. number of consumed seeds divided by trial length) as when foraging alone, because of an increase in the proportion of time feeding (albeit not significant for subordinate birds). Patch residence time and the number of different patches visited did not differ when birds were foraging alone or in a group. Besides some agonistic interactions, no differences in foraging behaviour between dominant, intermediate and subordinate birds were measured in group trials. Possibly group‐foraging birds increased their feeding time because there was less need for vigilance or because they increased foraging intensity to compensate for competition. This study underlines that a higher competitor density does not necessarily lead to a lower intake rate, irrespective of dominance status.     

Anticipation of conflict by chimpanzees

Captive chimpanzees appear to anticipate the occurrence of conflict during feeding by grooming and being in proximity at increased rates during the hour prior to feeding. The effect is more marked when food is clumped than when it is dispersed, suggesting that the proximate cause is the anticipation of increased levels of competition. Chimpanzees did not choose high ranking individuals more often as prefeed grooming partners; rather, they preferred to associate with their normal grooming partners (as reflected in post-feed grooming preferences) and close kin. A strong correlation between prefeed association patterns and spatial proximity during clumped feeding sessions suggests that their main concern is to be allowed to feed near individuals who are able to monopolize food sources.     

Social behavior of juvenile chum salmon, Oncorhynchus keta, under risk of predation: the influence of food distribution

Synopsis Social interactions can influence both foraging reward and vulnerability to predators. We examined social interactions in groups of juvenile chum salmon, Oncorhynchus keta, receiving food that was either spatially dispersed, with many food items appearing synchronously, or spatially clumped, with individual food items appearing asynchronously. These experiments were conducted both in the presence and absence of predators. when food was dispersed and predators were absent, juvenile chum formed schools and all individuals had access to food, despite frequent agonistic interactions. When predators were present, schooling and feeding continued, but agonistic interactions ceased. In contrast, when food was clumped, dominant fish utilized aggression to monopolize food regardless of whether predators were present or absent, resulting in decreased group cohesion. These results illustrate that food distribution and social interaction may play a role in determining how fish balance predation risk against foraging reward.     

Effect of Group Size on Feeding Rate when Patches are Exhaustible

One benefit of group foraging is that individual foragers can join the food discoveries of companions and thus increase encounter rate with food patches. When food patches are exhaustible, however, individual shares of each patch will decrease with group size negating the effect of increased encounter rate. Mean feeding rate may actually decrease with group size as a result of aggression or time wasted joining already depleted patches, or when searching to join the food discoveries of others, which is referred to as scrounging, precludes finding food. I examined the relationship between mean feeding rate and group size in captive flocks of zebra finches (Taenopygia guttata) foraging for small clumps of seeds. Finches in groups of two or four fared better than solitary birds in terms of mean feeding rate despite the fact that birds in groups scrounged a large proportion of their food. Solitary birds initiated feeding activity after a longer delay, which led to their lower success. Early departures by food finders from food patches joined by others may have lessened the impact of scrounging on mean feeding rate. As a result of benefits from the presence of companions, group foraging in zebra finches appears a viable alternative to foraging alone despite the cost of sharing resources.     

Feeding Competition in <Emphasis Type="Italic">Macaca fascicularis</Emphasis>: An Assessment of the Early Arrival Tactic

In primate species with unidirectional dominance relationships, rank order restricts the access of nondominant females to clumped resources. However, females might attempt to bypass the rank order by reaching feeding sites before the highest ranking individuals (early arrival tactic) when there are net benefits. We therefore analyzed the order of arrival to the feeding site of the adult members of a captive group of long-tailed macaques. We used 2 experimental conditions that differed in the spatial distribution of a fixed amount of food (large vs. small patch). Though each condition induced contest competition, it was stronger in the small-patch condition. Arrival order does not correlate with dominance rank in either experimental condition. The α-male and α-female reached the feeding site 10–30 s after the beginning of the test. Some females seized on opportunities to reach the feeding site before them, especially in the large-patch condition. They used the early arrival tactic when the risks of aggression were relatively low, which subjects accomplished either by being dominant or by being nondominant but tolerated by the α-male. Social tolerance may provide individuals with an alternative means to obtain resources. In sum, variation in food abundance and distribution may affect the extent to which rank order determines order of arrival to feeding sites. A higher rank may confer priority in the choice of tactics, but not necessarily priority of access to the resources themselves.     

捕食风险对不同饥饿状态下艾虎取食行为的影响

在室内条件下,将大鵟作为艾虎的天敌动物,通过双通道选择实验确定6 只成体艾虎在3 个捕食风险水平和4 种饥饿状态条件下的取食行为,探讨艾虎在取食过程中对饥饿风险与捕食风险的权衡策略。研究结果表明:在无捕食风险存在时,艾虎被剥夺食物0 d 和1 d 后对食物量不同的两个斑块中的取食量和利用频次均无明显不同(P > 0. 05),但对高食物量斑块的利用时间均明显高于低食物量斑块的(P <0.05),而艾虎被剥夺食物2 d和3 d后对高食物量斑块中的取食量和利用时间均明显高于低食物量斑块中的(P < 0.05),但在利用频次上均无明显差异(P > 0.05)。在面临低风险时,艾虎在4 种饥饿状态下均只利用无天敌动物存在的低食物量斑块,而基本不利用有天敌动物存在的高食物量斑块。在面临高风险时,艾虎不得不利用有天敌动物存在的食物斑块,被剥夺食物0 d 时艾虎对无风险、无食物量斑块的利用时间基本相同于对高风险、有食物量斑块的利用时间(P>0.05),而被剥夺食物1d、2 d 和3 d 后艾虎对高风险、有食物量斑块的利用时间明显高于无风险、无食物量斑块的(P< 0. 05)。在相同风险条件下,随着饥饿程度增加,艾虎在斑块中的取食量均明显增加(P< 0.05),而对斑块的利用时间和利用频次明显降低(P<0.05)。在相同的饥饿状态下,不同风险水平时,艾虎在斑块中的取食量无明显的差异(P>0.05),但在低风险和高风险时对斑块的利用时间和频次均明显低于无风险时的(P <0.05)。以上结果说明艾虎能够根据食物摄取率和自身的能量需求在捕食风险和饥饿风险之间做出权衡,当饥饿风险小于捕食风险时,艾虎趋于躲避捕食风险,当饥饿风险大于捕食风险时,艾虎趋于面对捕食风险,所采用的取食策略是减少活动时间和能量消耗,最大程度地提高单位时间内获得的能量。     

Resource distribution mediates synchronization of physiological rhythms in locust groups

Synchronized behaviour is common in animal groups. In ant colonies, synchronization occurs because active ants stimulate their neighbours to activity. We use oscillator theory to explain how stimulation from active neighbours synchronizes activity in groups of solitarious locusts via entrainment of internal physiological rhythms. We also show that the spatial distribution of food resources controls coupling between individual locusts and the emergence of synchronized activity. In locusts (Schistocerca gregaria), individual schedules of activity and quiescence arise from an irregular physiological oscillation in feeding excitation (i.e. hunger). We show that contact with an active neighbour increases the probability that a locust becomes active. This entrained activity decreases the time until the locust feeds, shifting the phase of its hunger oscillation. The locusts' internal physiological rhythms are thus brought into alignment and their activity becomes synchronized. When food resources are clumped, contact with active locusts increases, and this increase in the strength of coupling between individuals leads to greater synchronization of behaviour. Activity synchronization might have functional significance in inhibiting swarming when resources are dispersed and accelerating it in more favourable clumped environments.     

Variations in competitive mechanisms of captive male hamadryas-like baboons in two feeding situations

Experiments were carried out to determine the influence of food distribution (clumped vs dispersed) on processes of competition among seven captive male hamadryas-like baboons. Spatial cohesion, cohesive behaviours that males performed and received from their females, and aggressive and feeding behaviours of the males were collected during 35 min after food supply. Median values and variability of these variables were compared in both the feeding situations. Food distribution influenced both asymmetries in access and the form of competition. When food was clumped, those with less access fed by means of compensatory or alternative strategies (away from the main source and/or by feeding supplants), and they were more aggressive towards other males. When food was dispersed, all individuals could feed simultaneously, males that were more aggressive were those with less cohesive OMUs, despite the fact that cohesive behaviours remained constant. Females appeared to contribute to variation in the OMU's cohesion through their responses to both food distribution and males' competitive ability.     

Dominance and feeding interference in small groups of blackbirds

Dominance and/or interference parameters play a pivotal rolein most ideal free distribution models, but there has beenscant empirical study of the exact manner in which they jointlyoperate. We investigate how foraging effort and success variedamongst individuals of different dominance rankings in groupsof 1-3 wild blackbirds (Turdus merula) attracted to patchesof hidden food. Foraging effort (number of feeding movementsper unit time), as opposed to vigilance tradeoffs, was greaterwhen an individual fed with a subordinate conspecific thanwhen it fed alone, but tended to be less when it fed with adominant individual. Within dyads, changes in foraging effortwere associated with the direction of the dominance relationship,but not the relative difference in dominance rank between thetwo individuals. Similarly, amongst threesomes, top-rankedbirds (but not the lowest-ranked individual) showed higherforaging effort compared to when foraging alone. Top-ranked birds also profited from a greater increase in foraging success(food items per unit effort) than bottom-ranked birds whenfeeding in threesomes than when feeding alone. Dominant birdsshowed increased foraging success, but not effort, after displacinga subordinate. Our results suggest that an individual's foragingeffort is determined by the interplay of group vigilance benefitsand interference costs, the latter being more expensive for subordinate individuals. The foraging success of dominant birdsmay further increase if they use subordinates as food-finders.We discuss the implications of our findings for interferenceparameters in current Ideal Free Distribution models.