Encounter-dilution Effects Research Articles

Variation in helminth infection prevalence, abundance, and co-infection in an intermediate host across a large spatial scale

Parasites are often distributed heterogeneously across host populations, but the controls of this heterogeneity across regional scales often remain unclear. Here, we test the relative importance of natural and anthropogenic environmental factors and biological attributes of host populations on the large-scale variability in infection probability and parasite abundance. We quantified a larval acanthocephalan parasite (Profilicollis altmani), as well as the trematodeMaritremasp. and the nematodeProleptussp., infection of mole crabsEmerita analogausing a hierarchical design at 8 sites spanning 500 km of the south-central shoreline of Chile. At each site, we measured the variables of beach size, morphodynamics, distance to nearest port, distance to tributaries, mole crab density, seagull density, and host body size. Using mixed-effects models, we analysed the associations of acanthocephalan infection probability and parasite abundance in mole crabs as functions of body size and the environmental site-level factors. Models accounted for 20 and 61% of variation in acanthocephalan infection probability and abundance, respectively, with mole crab body size exclusively accounting for nearly all of the model fits. Also, acanthocephalan abundance decreased with increasing mole crab density, suggesting a possible encounter-dilution effect. Co-infection was strong, with the 2 other larval parasite species correlated positively with acanthocephalan parasites. The low influence of environmental variables on acanthocephalan infection could be because spatial variability in these factors is relatively low or their influences are quick to saturate. Therefore, in this system, parasite infection is apparently more strongly related to the duration of host exposure than spatially variable environmental factors, even across large spatial scales.

Negative density-dependent parasitism in a group-living carnivore.

Animals living at high population densities commonly experience greater exposure to disease, leading to increased parasite burdens. However, social animals can benefit immunologically and hygienically from cooperation, and individuals may alter their socio-spatial behaviour in response to infection, both of which could counteract density-related increases in exposure. Consequently, the costs and benefits of sociality for disease are often uncertain. Here, we use a long-term study of a wild European badger population (Meles meles) to investigate how within-population variation in host density determines infection with multiple parasites. Four out of five parasite taxa exhibited consistent spatial hotspots of infection, which peaked among badgers living in areas of low local population density. Combined movement, survival, spatial and social network analyses revealed that parasite avoidance was the likely cause of this negative density dependence, with possible roles for localized mortality, encounter-dilution effects, and micronutrient-enhanced immunity. These findings demonstrate that animals can organize their societies in space to minimize parasite infection, with important implications for badger behavioural ecology and for the control of badger-associated diseases.

Parasite prevalence in intermediate hosts increases with waterbody age and abundance of final hosts.

Prevalence of a parasite may be influenced by age of the habitat (= time available for hosts and parasites to colonize habitats), assemblage composition of host and non-host species, or biotic and abiotic habitat features. For a trophically transmitted parasite, the intermediate host may be consumed by both final hosts and 'dead-end' predators that are incompetent to host the parasite. We tested biotic and abiotic factors that might influence parasite prevalence in a freshwater host-parasite system using a dataset collected from 36 water bodies in the vicinity of Edmonton, Alberta, Canada. In this system, eggs of thorny-headed worms (Acanthocephala: Polymorphus spp.) are eaten by intermediate-host amphipods (Gammarus lacustris Sars), which are then consumed by final vertebrate hosts (certain aquatic birds and muskrats) and various non-host waterbird species. We found that acanthocephalan prevalence in amphipods was positively correlated with waterbody age and with abundance of final-host species. In contrast, abundance of the intermediate host G. lacustris was less important and was negatively correlated with parasite prevalence ('encounter-dilution effect'). Similarly, parasite prevalence showed a marginally significant and negative correlation with abundance of 'dead-end' Gammarus-eating birds. We conclude that in our study system, time available for colonization and abundance of final hosts are more important for parasite prevalence in intermediate hosts than is abundance of intermediate and dead-end hosts.

Chimpanzee (Pan troglodytes schweinfurthii) Group Sleep and Pathogen-Vector Avoidance: Experimental Support for the Encounter-Dilution Effect

Sleep is essential for survival, yet it represents a time of extreme vulnerability, including through exposure to parasites and pathogens transmitted by biting insects. To reduce the risks of exposure to vector-borne disease, the encounter-dilution hypothesis proposes that the formation of groups at sleep sites is influenced by a “selfish herd” behavior, where individuals dilute risk by sleeping with other group members. To investigate this hypothesis in the context of chimpanzee (Pan troglodytes schweinfurthii) sleep site selection, we employed four light traps that we also baited with nontoxic chemical attractants to capture insects throughout the night. Across 74 nights with 294 traps set, we collected 66,545 individual insects. Consistent with the encounter-dilution hypothesis, we found that insect exposure, inferred by absolute numbers of insects caught in nighttime traps, was strongly influenced by the grouping of traps. Specifically, single traps caught more insects—including vector transmitting female mosquitoes—than grouped traps, and the number of insects caught increased with increasing distance between grouped traps. Moreover, ground sleep sites caught fewer insects than arboreal sleep sites. In addition, traps associated with Cynometra alexandri trees resulted in significantly lower catch rates than Pseudospondias microcarpa–associated traps. Our results suggest wild chimpanzees use group sleep as a strategy to avoid biting insects that serve as hosts for vector-borne diseases.

Correction to: Chimpanzee (Pan troglodytes schweinfurthii) Group Sleep and Pathogen-Vector Avoidance: Experimental Support for the Encounter-Dilution Effect

Correction to: Chimpanzee (Pan troglodytes schweinfurthii) Group Sleep and Pathogen-Vector Avoidance: Experimental Support for the Encounter-Dilution Effect

Avian phenotypic traits related to feeding preferences in two Culex mosquitoes.

Host choice by mosquitoes affects the transmission dynamics of vector-borne infectious diseases. Although asymmetries in mosquito attraction to vertebrate species have been reported, the relative importance of host characteristics in mosquito blood-feeding behavior is still poorly studied. Here, we investigate the relationship between avian phenotypic traits-in particular, morphometry, plumage coloration, and nesting and roosting behavior-and the blood-feeding patterns in two common Culex mosquito species on a North American avian community. Forage ratios of the mosquito species were unrelated to the phylogenetic relationships among bird species. Culex pipiens fed preferably on birds with lighter-colored plumage and longer tarsi; furthermore, solitary roosting avian species were both bitten by Cx. pipiens and Cx. restuans more often than expected. These associations may be explained by greater mosquito attraction towards larger birds with a greater color contrast against the background. Although communally roosting birds may release more cues and attract more mosquitoes, individuals may in fact receive fewer bites due to the encounter-dilution effect. Mosquito feeding behavior is a highly complex phenomenon, and our results may improve understanding of the non-random interaction between birds and mosquitoes in natural communities.

Parasite abundance decreases with host density: evidence of the encounter-dilution effect for a parasite with a complex life cycle

The relationship between host density and parasitism depends on a parasite’s life history. The abundance of a directly transmitted contagious parasite should increase with host density, whereas the abundance of a directly transmitted parasite that seeks its host might decrease due to the encounter-dilution effect. For parasites with complex life cycles, previous studies have found no association between parasite abundance and host density. We tested the relationship between host density and metacercarial abundance of a trematode parasite (Posthodiplostomum minimum) in two species of centrarchid fishes (Lepomis macrochirus and L. auritus) from eight small creeks. We found that host density was negatively associated with parasite abundance. Thus, our study represents the first evidence of the encounter-dilution effect for a parasite with complex life cycle in a natural system. We also report a positive association between total P. minimum population abundance and Lepomis spp. density, indicating that at low host density, cercarial mortality could moderate the encounter-dilution effect.

Human Activity Dampens the Benefits of Group Size on Vigilance in Khulan (Equus hemionus) in Western China.

Animals receive anti-predator benefits from social behavior. As part of a group, individuals spend less time being vigilant, and vigilance decreases with increasing group size. This phenomenon, called “the many-eyes effect”, together with the “encounter dilution effect”, is considered among the most important factors determining individual vigilance behavior. However, in addition to group size, other social and environmental factors also influence the degree of vigilance, including disturbance from human activities. In our study, we examined vigilance behavior of Khulans (Equus hemionus) in the Xinjiang Province in western China to test whether and how human disturbance and group size affect vigilance. According to our results, Khulan showed a negative correlation between group size and the percentage time spent vigilant, although this negative correlation depended on the groups’ disturbance level. Khulan in the more disturbed area had a dampened benefit from increases in group size, compared to those in the undisturbed core areas. Provision of continuous areas of high-quality habitat for Khulans will allow them to form larger undisturbed aggregations and to gain foraging benefits through reduced individual vigilance, as well as anti-predator benefits through increased probability of predator detection.

Density-dependent immunity and parasitism risk in experimental populations of lizards naturally infested by ixodid ticks.

When effective immune defenses against parasites are costly and resources limited, individuals are expected to alter their investment in immunity in response to the risk of infection. As an ecological factor that can affect both food abundance and parasite exposure, host density can play an important role in host immunity and host-parasite interactions. High levels of intraspecific competition for food and social stress at high host density may diminish immune defenses and increase host susceptibility to parasites. At the same time, for contagious and environmentally transmitted parasites, parasite exposure often increases with host density, whereas in mobile parasites that actively search for hosts, parasite exposure can decrease with host density due to the "encounter-dilution effect." To unravel these multiple and potentially opposing effects of host density on immunity, we manipulated density of the common lizard Zootoca vivipara and measured local inflammation in response to PHA injection and levels of infestation by the tick Ixodes ricinus, a mobile ectoparasite for which we expected an encounter-dilution effect to occur. Local inflammation strongly decreased with lizard density in adults, but not in yearlings. Tick infestation (abundance and prevalence) was negatively correlated with lizard density in both age classes. Using path analyses, we found independent, direct negative density feedbacks on immunity and parasite exposure in adults, supporting the hypothesis of energy constraints and/or physiological stress acting on immunity at high density. In contrast, for yearlings, the best path model showed that density diluted exposure to parasites, which themselves down-regulated immune defenses in lizards. These results highlight the importance of investigating the pathways among host density, host immunity, and parasite infestation, while accounting for relevant individual traits such as age.

Host group formation decreases exposure to vector-borne disease: a field experiment in a 'hotspot' of West Nile virus transmission.

Animals can decrease their individual risk of predation by forming groups. The encounter-dilution hypothesis extends the potential benefits of gregariousness to biting insects and vector-borne disease by predicting that the per capita number of insect bites should decrease within larger host groups. Although vector-borne diseases are common and can exert strong selective pressures on hosts, there have been few tests of the encounter-dilution effect in natural systems. We conducted an experimental test of the encounter-dilution hypothesis using the American robin (Turdus migratorius), a common host species for the West Nile virus (WNV), a mosquito-borne pathogen. By using sentinel hosts (house sparrows, Passer domesticus) caged in naturally occurring communal roosts in the suburbs of Chicago, we assessed sentinel host risk of WNV exposure inside and outside of roosts. We also estimated per capita host exposure to infected vectors inside roosts and outside of roosts. Sentinel birds caged inside roosts seroconverted to WNV more slowly than those outside of roosts, suggesting that social groups decrease per capita exposure to infected mosquitoes. These results therefore support the encounter-dilution hypothesis in a vector-borne disease system. Our results suggest that disease-related selective pressures on sociality may depend on the mode of disease transmission.

Do Animals Living in Larger Groups Experience Greater Parasitism? A Meta-Analysis

Parasitism is widely viewed as the primary cost of sociality and a constraint on group size, yet studies report varied associations between group size and parasitism. Using the largest database of its kind, we performed a meta-analysis of 69 studies of the relationship between group size and parasite risk, as measured by parasitism and immune defenses. We predicted a positive correlation between group size and parasitism with organisms that show contagious and environmental transmission and a negative correlation for searching parasites, parasitoids, and possibly vector-borne parasites (on the basis of the encounter-dilution effect). Overall, we found a positive effect of group size (r = 0.187) that varied in magnitude across transmission modes and measures of parasite risk, with only weak indications of publication bias. Among different groups of hosts, we found a stronger relationship between group size and parasite risk in birds than in mammals, which may be driven by ecological and social factors. A metaregression showed that effect sizes increased with maximum group size. Phylogenetic meta-analyses revealed no evidence for phylogenetic signal in the strength of the group size-parasitism relationship. We conclude that group size is a weak predictor of parasite risk except in species that live in large aggregations, such as colonial birds, in which effect sizes are larger.

Bad housekeeping: why do aphids leave their exuviae inside the colony?

BackgroundAnimals can gain protection against predators and parasites by living in groups. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to group size (i.e. encounter effect), so that predators do not offset the encounter effect by attacking more members of the group (i.e. dilution effect). In this paper, we propose a novel mechanism by which prey insects could gain by producing decoys that act as multiple targets for predators or parasitoids if these decoys are recognised as preys or hosts and negatively affect the patch foraging strategy of these predators and parasitoids. Such a decoy mechanism could be present in aphid colonies in which aphid exuviae are recognised and attacked by Aphidiine wasps.ResultsWe conducted a behavioural study to evaluate the effect of exuviae on parasitoid patch residence time and egg allocation in experimental aphid patches with or without exuviae. We showed that exuviae are recognised and attacked at the same level as aphids when both are present in the patch. While parasitism rate was not significantly lower in patches with exuviae when the parasitoid left the patch, the time wasted by parasitoids to handle exuviae did influence the patch residence time. As a consequence, the attack rate on the live aphids was lower in patches that contain exuviae.ConclusionAphids had more time available to flee and thus each individual might gain protection against parasitoids by leaving their exuviae near and within the colony. These results demonstrate that the encounter-dilution effect provided by living in a group can be enhanced by extra-materials that act as decoy for natural enemies.

Escaping parasitism in the selfish herd: age, size and density‐dependent warble fly infestation in reindeer

It has been suggested that animals may escape attack from mobile parasites by aggregating in selfish herds. A selfish herd disperses the risk of being attacked among its members and the per individual risk of parasite infection should therefore decrease with increasing animal density through the encounter–dilution effect. Moreover, in a selfish herd, dominant and agile animals should occupy the best positions and thereby receive fewer attacks compared to lower ranked animals at the periphery. We tested these predictions on reindeer (Rangifer tarandus tarandus) parasitized by warble flies (Hypoderma tarandi). Warble flies oviposit their eggs on reindeer during summer and induce strong anti‐parasitic behavioural responses in the herds. In this period, reindeer are sexually segregated; females and calves form large and dense herds while males are more solitary. After hatching, the warble fly larvae migrate under the skin of their host where they encyst. In the present study encysted larvae were counted on newly slaughtered hides of male calves and 1.5 year old males from 18 different reindeer herds in Finnmark, northern Norway with large contrasts in reindeer density. In reindeer, body mass is correlated with fitness and social status and we hypothesized that individual carcass mass reflected the animal's ability to occupy the best positions within the herd. Larval abundance was higher among the 1.5 year old males than among the calves. For calves we found in accordance with the selfish herd hypothesis a negative relationship between larval abundance and animal density and between larval abundance and body mass. These relationships were absent for the 1.5 year old males. We suggest that these differences were due to different grouping behaviour where calves and females, but not males, aggregated in selfish herds where they escaped parasitism.

Malaria infection and host behavior: a comparative study of Neotropical primates

Parasites are ubiquitous in populations of free-ranging animals and impact host fitness, but virtually nothing is known about the factors that influence patterns of disease risk across species and the effectiveness of behavioral defenses to reduce this risk. We investigated the correlates of malaria infection (prevalence) in Neotropical primates using data from the literature, focusing on host traits involving group size, body mass, and sleeping behavior. Malaria is spread to these monkeys through anopheline mosquitoes that search for hosts at night using olfactory cues. In comparative tests that used two different phylogenetic trees, we confirmed that malaria prevalence increases with group size in Neotropical primates, as suggested by a previous non-phylogenetic analysis. These results are consistent with the hypothesis that larger groups experience increased risk of attack by mosquitoes, and counter to the hypothesis that primates benefit from the encounter-dilution effect of avoiding actively-seeking insects by living in larger groups. In contrast to non-phylogenetic tests, body mass was significant in fewer phylogeny-based analyses, and primarily when group size was included as a covariate. We also found statistical support for the hypothesis that sleeping in closed microhabitats, such as tree holes or tangles of vegetation, reduces the risk of malaria infection by containing the host cues used by mosquitoes to locate hosts. Due to the small number of evolutionary transitions in sleeping behavior in this group of primates, however, this result is considered preliminary until repeated with a larger sample size. In summary, risk of infection with malaria and other vector-borne diseases are likely to act as a cost of living in groups, rather than a benefit, and sleeping site selection may provide benefits by reducing rates of attack by malaria vectors.

The effects of group size, shape and composition on ease of detection of cryptic prey

[The role of aggregation in prey defence has been the subject of much debate. For example, the commonly cited assumption of encounter-dilution effects requires that attack rates increase asymptotically with increasing group size. One key parameter that is still poorly understood is how ease of detection of cryptic prey aggregations is affected by group size, shape, density and composition. We have developed a computer based visual test to elucidate these effects using human subjects. We show that ease of detection of cryptic prey increases asymptotically and quickly saturates with group size: this is in accordance with a previously reported laboratory test of birds. We also discovered that horizontally-orientated groups were more easily detected than either circular or vertically arranged ones. No effect of the degree of spatial clustering of prey occurring in groups was found, and nor did heterogeneity or homogeneity in the visual appearance of group members affect ease of detection — although future work on these aspects is warranted., The role of aggregation in prey defence has been the subject of much debate. For example, the commonly cited assumption of encounter-dilution effects requires that attack rates increase asymptotically with increasing group size. One key parameter that is still poorly understood is how ease of detection of cryptic prey aggregations is affected by group size, shape, density and composition. We have developed a computer based visual test to elucidate these effects using human subjects. We show that ease of detection of cryptic prey increases asymptotically and quickly saturates with group size: this is in accordance with a previously reported laboratory test of birds. We also discovered that horizontally-orientated groups were more easily detected than either circular or vertically arranged ones. No effect of the degree of spatial clustering of prey occurring in groups was found, and nor did heterogeneity or homogeneity in the visual appearance of group members affect ease of detection — although future work on these aspects is warranted.]

The Role of Passive Defense (Selfish Herd and Dilution Effect) in the Gregarious Nesting of Liostenogaster Wasps (Vespidae, Hymenoptera, Stenogastrinae)

The advantages of the selfish herd and encounter-dilution effects were studied in two congeneric gregarious-nesting species of Stenogastrine wasps, Liostenogaster vechti Turillazzi and L. flavolineata (Cameron), in Malaysia. Results show that nesting in clusters affords these wasps some degree of protection against their main invertebrate predators, parasites and parasitoids. This advantage is reflected in the geometry of the aggregation as well as in the behavior of the individuals. The costs and benefits of group living and the implications of the selection of this nesting strategy in the evolution of sociality in these wasps are discussed.

Differential fitness returns in relation to spatial position in groups.

1. A general overview of position-related fitness differences in group-living animals is given for the first time. 2. Differential fitness returns in groups are often related to competition for resources among group members due to resource limitation. If resources are very scarce, then competition may finally lead to the disbandment of groups. The encounter-dilution effect predicts that grouping confers a benefit if the conspicuousness of large groups does not outweigh the dilution effect and each predator or parasite attacks only one (or a few) prey per encounter (Uetz & Hieber, 1993). While the encounter-dilution effect limits the number of predators and parasites a group is faced with at a given time, marginal predation and communal defence are probably the most important factors for position-related differences in predation risk. 3. Limitation of resources increases competition between groups members and should widen the gap between fitness returns of dominant and subordinate group members. In some group-living animals such as schooling fish, however, predator pressure has apparently selected for uniformity in morphology and behaviour which presumably restricts the potential for resource competition (Magurran & Seghers, 1991). 4. A general problem of assessing individual fitness returns in that different short-term strategies may achieve the same long term goals (Magurran, 1993). The use of feeding rates and predation risks as currencies for fitness returns over short time periods is therefore questionable and can only be a first step. So far, hardly any information is available on long-term effects of differences in positioning behaviour. 5. Food availability and food intake rates tend to be higher in edge positions, whereas energy expenditure does not differ significantly with group position in non-roosting mobile groups. This suggests that edge positions achieve a higher net-energy pay-off and should therefore be preferred by individuals with low energy reserves. 6. Differential predation risks are well documented for stationary and colony-breeding species, but there is a lack of data for mobile, non-breeding species. The reason for the latter is that predation is rarely observed (in the field) and often difficult to relate to group position in fast-moving species such as flocks of birds. Further studies, especially in the field, are needed. 7. Vigilance rates and attack rates do not necessarily give a true reflection of mortality risk. Vigilance is not exclusively related to predation and can be confounded by competition effects and hunger. Attack rates are difficult to translate into per capita risks if hardly any mortality is observed and therefore little information is available about the ratio of successful attacks/total attacks. 8. Most of the literature up to date consists of descriptive studies. There is a need for experimental work that investigate (a) the influence of predator attack mode on differential predation risks; (b) the influence of internal states (such as hunger) on the tradeoff between predation risk and foraging behaviour and its consequences for the positioning behaviour in groups; (c) the 'egalitarian' nature of groups with respect to group position. Do all individuals continuously rotate positions? Or do dominant individuals monopolize certain positions?, and (d) that separate the effects of nutritional state, vigilance and food availability on feeding rates in different group positions. 9. Reproductive success is often related to group position in breeding colonies and lekking animal species. Females probably prefer the safer territories in the group centre, but more empirical data are required to test this hypothesis. 10. Position-related differences in parasitism rate between group members are well documented with per capita rate of parasitism being lower in the centre of groups than at the periphery.(ABSTRACT TRUNCATED AT 400 WORDS)

On Chimpanzee Dormitories and Early Hominid Home Sites

Previous articleNext article No AccessDiscussion and CriticismOn Chimpanzee Dormitories and Early Hominid Home SitesAdriaan KortlandtAdriaan Kortlandt Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by Current Anthropology Volume 33, Number 4Aug. - Oct., 1992 Sponsored by the Wenner-Gren Foundation for Anthropological Research Article DOIhttps://doi.org/10.1086/204087 Views: 7Total views on this site Citations: 11Citations are reported from Crossref Copyright 1992 The Wenner-Gren Foundation for Anthropological ResearchPDF download Crossref reports the following articles citing this article:David R. Samson, Luke A. Louden, Katie Gerstner, Samantha Wylie, Ben Lake, Bradley J. White, Charles L. Nunn, Kevin D. Hunt Chimpanzee (Pan troglodytes schweinfurthii) Group Sleep and Pathogen-Vector Avoidance: Experimental Support for the Encounter-Dilution Effect, International Journal of Primatology 40, no.66 (Dec 2019): 647–659.https://doi.org/10.1007/s10764-019-00111-zL. Badji, P. I. Ndiaye, S. M. Lindshield, C. T. Ba, J. D. Pruetz Savanna chimpanzee (Pan troglodytes verus) nesting ecology at Bagnomba (Kedougou, Senegal), Primates 14 (Dec 2017).https://doi.org/10.1007/s10329-017-0647-2David R. Samson, Michael P. Muehlenbein, Kevin D. Hunt Do chimpanzees (Pan troglodytes schweinfurthii) exhibit sleep related behaviors that minimize exposure to parasitic arthropods? A preliminary report on the possible anti-vector function of chimpanzee sleeping platforms, Primates 54, no.11 (Sep 2012): 73–80.https://doi.org/10.1007/s10329-012-0329-zDavid R. Samson The chimpanzee nest quantified: morphology and ecology of arboreal sleeping platforms within the dry habitat site of Toro-Semliki Wildlife Reserve, Uganda, Primates 53, no.44 (May 2012): 357–364.https://doi.org/10.1007/s10329-012-0310-xDAVID R. SAMSON, KEVIN D. HUNT A Thermodynamic Comparison of Arboreal and Terrestrial Sleeping Sites for Dry-Habitat Chimpanzees (Pan troglodytes schweinfurthii) at the Toro-Semliki Wildlife Reserve, Uganda, American Journal of Primatology 74, no.99 (May 2012): 811–818.https://doi.org/10.1002/ajp.22031Kathelijne Koops, William C. McGrew, Han de Vries, Tetsuro Matsuzawa Nest-Building by Chimpanzees (Pan troglodytes verus) at Seringbara, Nimba Mountains: Antipredation, Thermoregulation, and Antivector Hypotheses, International Journal of Primatology 33, no.22 (Feb 2012): 356–380.https://doi.org/10.1007/s10764-012-9585-4Joana Sousa, André V. Barata, Cláudia Sousa, Catarina C. N. Casanova, Luís Vicente Chimpanzee oil-palm use in southern Cantanhez National Park, Guinea-Bissau, American Journal of Primatology 73, no.55 (Jan 2011): 485–497.https://doi.org/10.1002/ajp.20926R. Adriana Hernandez-Aguilar Chimpanzee nest distribution and site reuse in a dry habitat: implications for early hominin ranging, Journal of Human Evolution 57, no.44 (Oct 2009): 350–364.https://doi.org/10.1016/j.jhevol.2009.03.007J.D. Pruetz, S.J. Fulton, L.F. Marchant, W.C. McGrew, M. Schiel, M. Waller Arboreal nesting as anti-predator adaptation by savanna chimpanzees (Pan troglodytes verus) in southeastern Senegal, American Journal of Primatology 70, no.44 (Jan 2008): 393–401.https://doi.org/10.1002/ajp.20508Peter A. Corning Holistic Darwinism: The new evolutionary paradigm and some implications for political science, Politics and the Life Sciences 27, no.11 (May 2016): 22–54.https://doi.org/10.2990/27_1_22Peter A. Corning Synergy Goes to War: A Bioeconomic Theory of Collective Violence, Journal of Bioeconomics 9, no.22 (Aug 2007): 109–144.https://doi.org/10.1007/s10818-007-9017-2

Animal Grouping for Protection From Parasites: Selfish Herd and Encounter-Dilution Effects

AbstractGrouping has been widely accepted as a mechanism for protection from predation. Just as has been apparent with predation, there is now ample evidence that parasites (biting flies, warble flies and parasitoids) can impact an animal's individual fitness. Some aspects of grouping, namely an encounter-dilution effect and the selfish herd effect, appear to apply as much to protection of animals from flying parasites as protection from predators. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to an increase in group size (the encounter effect), provided that the parasites do not offset the encounter effect by attacking more members of the group (the dilution effect). The selfish herd effect provides protection from flying parasites to animals that are in the center of a group or more closely placed to other animals. Most of the quantitative evidence for the protection from flying parasites from grouping comes from studies on ungulates. Further investigation of these effects among a variety of taxa is needed for a full appreciation of the role of parasites in animal grouping and sociality.