Can information sharing explain recruitment to food from communal roosts?

The debate over whether communal nests and roosts function primarilyas information centers (they facilitate the sharing of foraginginformation) remains unresolved. Here I use evolutionary gametheory to investigate the relative importance of this influentialhypothesis and an alternative: that roosts, in particular,function as recruitment centers (they facilitate aggregationat food patches). Basing my model on juvenile common raven (Corvus corax) behavior, I assume there is no net cost to beingat food patches in groups, and foragers roost communally. Moreover,one strategic outcome is the observed raven behavior: individualssearch independently and recruit from the roost once a patchis found (they play Search-and-Recruit, or SR). I investigatethe stability of this in two scenarios that differ in the magnitudeof the lost opportunity costs to mutants playing SR in populations of other strategies. When these costs only involve a chanceof not being in a group at a located carcass, SR is the onlyevolutionarily stable strategy under all conditions. However,when these costs also include missing opportunities to be sociallydominant, SR no longer enjoys exclusive dominance in the strategyset. Nevertheless, in both cases, there are conditions where group foraging benefits have no effect on the evolutionary stabilityof SR. Thus, contrary to assertions in the literature, theopportunity to share foraging information can be sufficientto drive the evolution and maintenance of recruitment to foodfrom communal roosts. However, I conclude that both informationand grouping benefits are likely to underlie communal roostingbehavior in my focal system.     

The benefit of being a social butterfly: communal roosting deters predation

Aposematic passion-vine butterflies from the genus Heliconius form communal roosts on a nightly basis. This behaviour has been hypothesized to be beneficial in terms of information sharing and/or anti-predator defence. To better understand the adaptive value of communal roosting, we tested these two hypotheses in field studies. The information-sharing hypothesis was addressed by examining following behaviour of butterflies departing from natural roosts. We found no evidence of roost mates following one another to resources, thus providing no support for this hypothesis. The anti-predator defence hypothesis was tested using avian-indiscriminable Heliconius erato models placed singly and in aggregations at field sites. A significantly higher number of predation attempts were observed on solitary models versus aggregations of models. This relationship between aggregation size and attack rate suggests that communally roosting butterflies enjoy the benefits of both overall decreased attack frequency as well as a prey dilution effect. Communal roosts probably deter predators through collective aposematism in which aggregations of conspicuous, unpalatable prey communicate a more effective repel signal to predators. On the basis of our results, we propose that predation by birds is a key selective pressure maintaining Heliconius communal roosting behaviour.     

Common Myna Roosts Are Not Recruitment Centres

We studied communal roosting in the Common Myna (Acridotheres tristis) in the light of the recruitment centre hypothesis and predation at the roost. The number and sizes of flocks departing from and arriving at focal roosts were recorded over a two year period. We also recorded the sizes and behaviour of foraging flocks. We found that flock sizes of birds departing from roosts at sunrise were larger than those at the feeding site, suggesting that there was no recruitment from the roosts. Flocks entering the roosts during sunset were larger on average than those leaving the following sunrise, suggesting no consolidation of flocks in the morning. Flocks entering the roosts at sunset were also larger on average than those that had left that sunrise, although there was no recruitment at the feeding site. There was no effect of group size on the proportion of time spent feeding. Contrary to expectation, single birds showed lower apparent vigilance than birds that foraged in pairs or groups, possibly due to scrounging tactics being used in the presence of feeding companions. Thus, the recruitment centre hypothesis did not hold in our study population of mynas. Predation at dawn and dusk were also not important to communal roosting: predators near the roosts did not result in larger flocks, and resulted in larger durations of arrival/departure contrary to expectation. Since flock sizes were smallest at the feeding site and larger in the evening than in the morning, but did not coincide with predator activity, information transfer unrelated to food (such as breeding opportunities) may possibly give rise to the evening aggregations.     

The selfish nature of generosity: harassment and food sharing in primates

Animals may share food to gain immediate or delayed fitness benefits. Previous studies of sharing have concentrated on delayed benefits such as reciprocity, trade and punishment. This study tests an alternative model (the harassment or sharing-under-pressure hypothesis) in which a food owner immediately benefits because sharing avoids costly harassment from a beggar. I present an experiment that varies the potential ability of the beggar to harass, and of the owner to defend the food, to examine the effects of harassment on food sharing in two primate species: chimpanzees (Pan troglodytes) and squirrel monkeys (Saimiri boliviensis). For both species, high levels of harassment potential significantly increased both beggar harassment and sharing by the owner. Food defensibility did not affect harassment or sharing. Interestingly, squirrel monkeys and chimpanzees shared equally frequently with conspecifics despite a much higher natural sharing rate in chimpanzees. These results suggest that harassment can play a significant role in primate food sharing, providing a simple alternative to reciprocity. The selfish nature of harassment has implications for economic, psychological and evolutionary studies of cooperative systems.     

A conceptual framework for nonkin food sharing: timing and currency of benefits

Many animal species, from arthropods to apes, share food. This paper presents a new framework that categorizes nonkin food sharing according to two axes: (1) the interval between sharing and receiving the benefits of sharing, and (2) the currency units in which benefits accrue to the sharer (especially food versus nonfood). Sharers can obtain immediate benefits from increased foraging efficiency, predation avoidance, mate provisioning, or manipulative mutualism. Reciprocity, trade, status enhancement and group augmentation can delay benefits. When benefits are delayed or when food is exchanged for nonfood benefits, maintaining sharing can become more difficult because animals face discounting and currency conversion problems. Explanations that involve delayed or nonfood benefits may require specialized adaptations to account for timing and currency-exchange problems. The immediate, selfish fitness benefits that a sharer may gain through by-product or manipulative mutualism, however, apply to various food-sharing situations across many species and may provide a simpler, more general explanation of sharing.     

High surface temperatures select for individual foraging in ants

Natural selection favors signals, receptors, and signaling behaviorthat maximize the received signal relative to background noiseand that minimize signal degradation. The physical propertiesof the environment affect rates of attenuation and degradationof the signal, and thus temperature may influence the evolutionand maintenance of volatile chemical signals. We tested this hypothesis in ants, where nest mate recruitment to a food sourceby laying trail pheromones on a surface is a common phenomenon.We collected data on maximal soil surface temperatures duringthe ants' activity and mode of foraging (recruitment or solitary).By using two different comparative methodologies, we demonstrateda relationship between maximal soil temperature at which speciesare active and recruitment behavior (which is hypothesized to be related to the presence or absence of chemical signals).The species that were active at lower temperatures proved tobe those that used chemical signals to recruit nest mates duringforaging. This is also the case when comparing sympatric speciesand thereby controlling for other environmental factors. Moreover,all seven nonrecruiter species developed from recruiter ancestries,which is consistent with our hypothesis because ample evidence suggests a forest and tropical origin for ants. Thus, contraryto previous hypotheses, species that forage individually cannotbe categorically considered primitive, but rather appear tobe derived from recruiter species. Therefore, we conclude thattemperature influences the evolution and/or stability of chemicalsignals in ants by determining the recruitment of nest mates.     

Information transfer about roosts in female Bechstein's bats: an experimental field study

Information transfer among group members is believed to play an important part in the evolution of coloniality in both birds and bats. Although information transfer has received much scientific interest, field studies using experiments to test the underlying hypotheses are rare. We used a field experiment to test if communally breeding female Bechstein's bats (Myotis bechsteinii) exchange information regarding novel roosts. We supplied a wild colony, comprising 17 adult females of known relatedness, with pairs of suitable and unsuitable roosts and monitored the arrival of individuals marked with transponders (PIT-tags) over 2 years. As expected with information transfer, significantly more naive females were recruited towards suitable than towards unsuitable roosts. We conclude that information transfer about roosts has two functions: (i) it generates communal knowledge of a large set of roosts; and (ii) it aids avoidance of colony fission during roost switching. Both functions seem important in Bechstein's bats, in which colonies depend on many day roosts and where colony members live together for many years.     

Selection on body size in a raptor with pronounced reversed sexual size dimorphism: are bigger females better?

An overabundance of hypotheses have been proposed to accountfor reversed sexual size dimorphism (RSD; females the largersex) in raptors. Previous research principally focused on examininginterspecific patterns of RSD, rarely testing predictions ofvarious hypotheses within populations. To redress this, we useddata from both sexes of a large brown falcon, Falco berigora,population to evaluate the importance of size and body conditionindices on the hunting prowess of males and the reproductivesuccess, recruitment, and survival probabilities of both sexes.Female-female competition for territorial vacancies was likelyto be intense as the floating population was female-biased andintrasexual agonistic interactions were frequently observed.In this competitive population, larger adult females were morelikely to be recruited, indicating directional selection favoringincreased female body size. Furthermore, after recruitment largerfemales were more likely to successfully fledge offspring, providinga mechanism by which RSD is maintained in the population. Incontrast, male recruitment was unrelated to either body sizeor condition indices. Smaller immature males more often heldtheir territories (survived) over two breeding seasons thandid their larger counterparts; however, they also took smallprey more frequently, a diet related to poor reproductive success.We argue that, together, these results are indicative of selectionfavoring an increase in female body size and a reduction ormaintenance in male body size. Of all the hypotheses proposedto account for the maintenance and evolution of RSD in raptors,this scenario is consistent only with the predictions of theintrasexual competition hypothesis.     

Recruitment, Search Behavior, and Flight Ranges of Honey Bees

During the past three decades, considerable evidence has beengathered in attempts to understand more fully honey bee recruitmentto food sources. Those efforts also apply directly to two long-standingand competing recruitment hypotheses: odor search vs. "dancelanguage" communication. However, whereas most researchers havefocused on individual interactions and behavior, the colonycan also be viewed as a unit. A review of evidence from a colonyperspective reveals that colony members range an average distancefrom their home base, whether while foraging on food sources,while collecting water, or while relocating as swarms. Thoseaverages, based on the logarithm of the distance from the colony,vary with the type of resource exploited and size of the odorfield. Such a mathematical correspondence between distancestravelled from parent colonies may well agree with an odorsearchrecruitment model, but is hardly reconcilable with the "dancelanguage" hypothesis.     

Genetic similarity in long-eared owl communal winter roosts: a DNA fingerprinting study

The long-eared owl is a nocturnal predator which winters communally and breeds in the same areas in loose colonies during the spring. We tested the hypothesis that roosts, particularly stable roosts, are formed by close relatives, a condition under which group-related behaviours such as information sharing or helping at nest are more likely to be developed. DNA fingerprinting analysis was used to examine genetic similarity within and between two long-eared owl populations, one wintering in a traditional roost and the other in an unstable roost, and both breeding around their roosting sites. Although genetic similarity within roosts was higher than that between roosts, the difference was not significant. Observed genetic similarity within roosts was smaller than that reported in the bird species whose roosts work as information centres. On the other hand, the presence of some closely related individuals in the roost and behavioural observations suggest that co-operation between kin might have occurred, at least in one of the two study sites.