DOI: 10.5061/dryad.8kprr4xr5
title: Selfish herd effects depend on prey crypsis

description: Determining why some animals form groups while others remain solitary is a
 longstanding goal in behavioural ecology. Group formation can help
 mitigate predation risk through a variety of mechanisms, including risk
 dilution and group vigilance. The ‘selfish herd hypothesis’ proposes that
 prey can reduce their risk by minimizing the area around which all points
 in that area are closer to them than to another conspecific (i.e., by
 minimising their ‘domain of danger’). This hypothesis assumes that an
 individual’s predation risk is proportional to the size of its domain of
 danger, however, the relationship between risk and proximity to
 conspecifics may depend on additional factors. Specifically, approaching
 conspecifics may be costly for prey that rely on crypsis because group
 formation increases detectability. Using model prey, we experimentally
 manipulated prey colouration as well as the domain of danger, then tracked
 their ‘survival’ under natural field conditions. We found that an
 individual’s predation risk increased with their domain of danger for
 conspicuous (red) prey, but decreased with the domain of danger in cryptic
 (green) prey. Our results are consistent with patterns in natural systems
 and indicate that the relationship between predation risk and domain of
 danger depends on additional factors like prey colouration.
description: <strong>Study
 Site</strong>:<em> </em>Our experiment took place in
 Peterborough, Ontario, Canada, in a mixed deciduous forest. Potential
 predators of our models included several species of insectivorous birds
 observed during our experiment (e.g., <em style="text-indent:0.5in;">Poecile
 atricapillus</em>,
 <em style="text-indent:0.5in;">Sitta
 carolinesis, Picoides pubescens</em>; full list in Supplementary
 Material)<em style="text-indent:0.5in;">.</em> Caterpillars similar to our
 models which are active when we conducted our study include members of
 Saturnidae (e.g., <em style="text-indent:0.5in;">Actias luna</em>, <em style="text-indent:0.5in;">Antheraea
 polyphemus,</em>
 <em style="text-indent:0.5in;">Callosamia
 promethea</em>),
 Sphingidae (e.g., <em style="text-indent:0.5in;">Sphinx chersis</em>, <em style="text-indent:0.5in;">Eumorpha
 pandorus</em>),
 Noctuidae (e.g., <em style="text-indent:0.5in;">Acronicta
 clarescens</em>,
 <em style="text-indent:0.5in;">Noctua
 pronuba</em>) among
 others. Air temperature during trials one and two and averaged
 17<sup style="text-indent:0.5in;">˚</sup>C and 14<sup style="text-indent:0.5in;">˚</sup>C,
 respectively. <strong>Field
 Methods</strong>:<strong> </strong>Model caterpillars made from
 plasticine (40 mm × 10 mm) were created by hand and deployed along tree
 branches to test our hypotheses. Our experiment followed a 4 × 2 factorial
 design. Specifically, each tree received a single group of four
 caterpillars, following one of four prey spacing treatments (2, 4, 8 or 16
 cm between individuals) and one of two prey colour treatments (Green or
 Red). Differences in prey spacing allowed us to establish prey with a
 variety of DODs. The colour treatments were selected to examine the effect
 of prey conspicuousness (Green = cryptic, Red = conspicuous), as has been
 done previously (e.g., 23-25). Our characterization of green plasticine
 models as cryptic and red plasticine models as conspicuous from the
 viewpoint of insectivorous birds is supported by previous work [e.g.,
 22,26]. Further, human vision detects most of the relevant variation in
 colour within the visible range and is a valid proxy for avian colour
 perception [27,28]. While the DOD can be quantified in
 several ways, we arranged prey along tree branches so that the DOD could
 be manipulated and calculated along a single dimension. Specifically, we
 calculated the DOD as the linear distance within which the target prey is
 closer than any of the other prey items, up to a maximum of 2 prey body
 lengths on either side of the prey (i.e., a limited DOD). Each prey,
 therefore, had one of seven different DODs (6, 8, 12, 13, 14, 16 and 20
 cm). Prey were deployed as two 10-day
 trials (September 27 - October 7, 2022, and October 14 - October 24,
 2022), each with 5 replicates of each treatment (i.e., <em style="text-indent:0.5in;">n </em>= 320 prey total). Between the two
 trials, we left a 10-day rest period to avoid possible acclimation effects
 in the predator community. Treatment was assigned randomly to each tree,
 and trees with groups of prey were separated by no less than 1 m. The tree
 species we used were <em style="text-indent:0.5in;">Fagus
 grandifolia</em>,
 <em style="text-indent:0.5in;">Carpinus
 caroliniana</em>,
 and <em style="text-indent:0.5in;">Ulumus
 sp.</em> Models
 were deployed at least 15 m away from hiking trails on branches that were
 at least 64 cm long and between 1-2 m from the ground. Models were checked
 daily between 12:00h and 14:00h and considered attacked when birds left
 beak imprints on the model. Attacked models were photographed and
 immediately replaced with another model of the same treatment to maintain
 a constant DOD for all prey throughout the
 experiment.  <strong>Data
 Analysis</strong>: We used Cox proportional hazards
 regression to analyze patterns of survival. This approach uses a
 continuous measure of survival as the response variable and has been
 widely used to analyze survival of model prey deployed in the field [e.g.,
 23,29-30]. Separate models were created to evaluate ‘survival’ at the
 group level (i.e., time until a group member is attacked) and ‘survival’
 of individual prey. Each analysis was stratified by trial to account for
 possible differences in baseline hazard. We included survival data from
 prey that were added to replace attacked prey. Prey not attacked on the
 final day of each trial were censored at that time. The group-level
 analysis tests whether prey colour, spacing, or their interaction,
 influenced the probability that a group was attacked (Hypothesis 1). The
 individual-level analysis tests whether the prey colour, DOD, or their
 interaction influenced the survival of individual prey (Hypothesis 2).
 Spacing and DOD were treated as continuous variables. All analyses were
 conducted in R version 4.0.1 [31], using the survival package
 [32]. The assumption that hazard functions are proportional over time was
 tested using the cox.zph function.
description: Code is provided in R. Data files are in *.csv format which can
 be opened with Excel, Notebook, and other equivalent programs.