Safety in numbers: the dilution effect and other drivers of group life in the face of danger

Lehtonen, Jussi; Jaatinen, Kim

doi:10.1007/s00265-016-2075-5

Cited by 145 publications

(108 citation statements)

References 82 publications

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“…Nonetheless, the per-person attack rate is at an all-time low because an increase in beachgoers has diluted individual risk (Ferretti et al 2015). This benefit, termed "safety in numbers," occurs when prey aggregation reduces per capita predation risk for all group members (Turner andPitcher 1986, Lehtonen andJaatinen 2016), and requires that predator reproduction and prey detectability do not track prey patchiness. In contrast, parasite reproduction is often assumed to track host density, leading to a positive association between local host density and infection risk.…”

Section: Introductionmentioning

confidence: 99%

Host density increases parasite recruitment but decreases host risk in a snail–trematode system

Buck

Hechinger

Wood

et al. 2017

Ecology

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Most species aggregate in local patches. High host density in patches increases contact rate between hosts and parasites, increasing parasite transmission success. At the same time, for environmentally transmitted parasites, high host density can decrease infection risk to individual hosts, because infective stages are divided among all hosts in a patch, leading to safety in numbers. We tested these predictions using the California horn snail, Cerithideopsis californica (=Cerithidea californica), which is the first intermediate host for at least 19 digenean trematode species in California estuaries. Snails become infected by ingesting trematode eggs or through penetration by free-swimming miracidia that hatch from trematode eggs deposited with final-host (bird or mammal) feces. This complex life cycle decouples infective-stage production from transmission, raising the possibility of an inverse relationship between host density and infection risk at local scales. In a field survey, higher snail density was associated with increased trematode (infected snail) density, but decreased trematode prevalence, consistent with either safety in numbers, parasitic castration, or both. To determine the extent to which safety in numbers drove the negative snail-density-trematode-prevalence association, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (California, USA). At each site, we quantified snail density and used data on final-host (bird and raccoon) distributions to control for between-site variation in infective-stage supply. After three months, overall trematode infections per cage increased with snail biomass density. For egg-transmitted trematodes, per-snail infection risk decreased with snail biomass density in the cage and surrounding area, whereas per-snail infection risk did not decrease for miracidium-transmitted trematodes. Furthermore, both trematode recruitment and infection risk increased with infective-stage input, but this was significant only for miracidium-transmitted species. A model parameterized with our experimental results and snail densities from 524 field transects estimated that safety in numbers, when combined with patchy host density, halved per capita infection risk in this snail population. We conclude that, depending on transmission mode, host density can enhance parasite recruitment and reduce per capita infection risk.

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Section: Introductionmentioning

confidence: 99%

Host density increases parasite recruitment but decreases host risk in a snail–trematode system

Buck

Hechinger

Wood

et al. 2017

Ecology

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“…Variables other than the quality of social ties and rank can also influence survival in group-living animals. Large group size may enhance survival by providing benefits similar to those of maintaining particularly affiliative relationships, such as more effective vigilance for predators (Elgar, 1989;Lehtonen & Jaatinen, 2016;Roberts, 1996;van Schaik & Van Noordwijk, 1986), defense of young offspring (Grinnell & McComb, 1996;Wolff & Peterson, 1998), or defense of feeding territories (Radford & du Plessis, 2004;Roth & Cords, 2016). Nevertheless, living in larger groups may also exact costs by increasing within-group competition for food (Roberts & Cords, 2013;VanderWaal, Mosser, & Packer, 2009), or by increasing the risk of male takeovers and subsequent infanticide (Steenbeek & van Schaik, 2001).…”

mentioning

confidence: 99%

Stronger social bonds do not always predict greater longevity in a gregarious primate

Thompson

Cords

2018

Ecology and Evolution

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In group‐living species, individuals often have preferred affiliative social partners, with whom ties or bonds can confer advantages that correspond with greater fitness. For example, in adult female baboons and juvenile horses, individuals with stronger or more social ties experience greater survival. We used detailed behavioral and life history records to explore the relationship between tie quality and survival in a gregarious monkey (Cercopithecus mitis stuhlmanni), while controlling for dominance rank, group size, and life history strategy. We used Cox proportional hazards regressions to model the cumulative (multi‐year) and current (single‐year) relationships of social ties and the hazard of mortality in 83 wild adult females of known age, observed 2–8 years each (437 subject‐years) in eight social groups. The strength of bonds with close partners was associated with increased mortality risk under certain conditions: Females that had strong bonds with close partners that were inconsistent over multiple years had a higher risk of mortality than females adopting any other social strategy. Within a given year, females had a higher risk of death if they were strongly bonded with partners that changed from the previous year versus with partners that remained consistent. Dominance rank, number of adult female groupmates, and age at first reproduction did not predict the risk of death. This study demonstrates that costs and benefits of strong social bonds can be context‐dependent, relating to the consistency of social partners over time.

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“…Heterospecific presence would also indicate breeding habitat suitability if resource requirements were similar between the species (Mönkkönen, Härdling, Forsman, & Tuomi, ; Seppänen, Forsman, Mönkkönen, & Thomson, ), but potentially unsuitable if the costs of settling near dominant competitors outweigh the benefits of social cue use for habitat selection (Fletcher, , ; Seppänen et al., ). Similarly, certain positive density‐dependent effects on breeding success, such as increased defenses against shared enemies, only emerge at higher conspecific and/or heterospecific densities (reduced vigilance: Beauchamp, ; Sharpe, Joustra, & Cherry, ; dilution effects: Lehtonen & Jaatinen, ; and offspring defense: Krama et al., ; Schadelin, Fischer, & Wagner, ). Studies suggest both conspecific and heterospecific attraction should be strongest at moderate population densities (Fletcher, ; Forsman, Hjernquist, Taipale, & Gustafsson, ; Mönkkönen et al., ).…”

Section: Introductionmentioning

confidence: 99%

Who is in the neighborhood? Conspecific and heterospecific responses to perceived density for breeding habitat selection

et al. 2018

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Theoretical models of habitat selection often incorporate negative density dependence. Despite strong negative density‐dependent effects on habitat selection, more recent studies indicate that animals settle near members of their own (conspecific) and other species (heterospecific) when selecting habitat with social cues. Social cue use for habitat selection is particularly common among songbirds, but few studies have investigated if songbirds use social cues to assess conspecific or heterospecific density (as opposed to just presence/absence) when making settlement decisions. We conducted a playback experiment to evaluate if yellow warblers (Setophaga petechia) and willow flycatchers (Empidonax traillii), two potential competitors for breeding habitat, use social cues to assess density (conspecific for warblers and heterospecific for flycatchers) when selecting breeding locations at two spatial scales. We simulated yellow warbler density to be high or low at multiple treatment plots (3.14 ha) with song playback and then evaluated settlement decisions by comparing yellow warbler and willow flycatcher abundances across plots (broad‐scale habitat selection) and individual space use within plots (fine‐scale territory establishment). Yellow warbler density treatments did not affect habitat selection by yellow warblers at the broad scale, but caused individuals to cluster territories at high‐density treatments. Willow flycatchers were most abundant at high‐density treatment plots, but yellow warbler density treatments did not affect territory locations. The results indicate that perceived density affects the habitat selection process for both conspecifics and heterospecifics.

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Safety in numbers: the dilution effect and other drivers of group life in the face of danger

Cited by 145 publications

References 82 publications

Host density increases parasite recruitment but decreases host risk in a snail–trematode system

Host density increases parasite recruitment but decreases host risk in a snail–trematode system

Stronger social bonds do not always predict greater longevity in a gregarious primate

Who is in the neighborhood? Conspecific and heterospecific responses to perceived density for breeding habitat selection

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