Predation is a strong evolutionary force acting
as a selective pressure on various traits within a prey species
including, but not limited to, size, morphology, coloration, development
of sexual maturity, and patterns of behaviour (Werner & Anholt 1993;
Maynard-Smith & Harper 2003; Scott-Phillips 2008). Ultimately, the
distribution, abundance, community composition, size, diversity,
breeding success and ecosystem functioning of one or more species are
impacted (Hall et al. 1976; Berger et al. 2001; Kneitel &
Chase 2004; Beauchamp et al. 2007; Hawlena & Schmitz 2010;
Letnic et al. 2011). Interactions between predators and prey can
over time make communities vulnerable to biotic and/or abiotic
perturbations (Mooney et al. 2010), including the invasion of
non-native species as a consequence of human activity (Snyder et
al. 2004). At the most general level, predation results in the reduced
survival of prey (consumptive effects), but the (real or perceived) risk
of predation can also affect prey animals in a variety of ways (Zanetteet al. 2011), including stinted growth, poor reproductive output
and reduced foraging efficiency (Sih 1980; Lima & Dill 1990; Peckarskyet al. 1993; Abrams et al. 1996; Peacor & Werner 1997).
Individuals most capable of evading predators have an increased
potential for producing offspring, and therefore should achieve higher
reproductive success than less capable conspecifics; female cheetahs
(Acinonyx jubatus ) with higher fecundity avoided habitats with
indications of elevated spotted hyaena (Crocuta crocuta ) or lion
(Panthera leo ) densities more than females with lower fecundity
(Durant 2000).
In response to the severity of the risks posed by predators, prey
species have evolved a wide range of anatomical, physiological,
neurobiological and behavioural mechanisms to increase their chances of
survival. Adaptations related to morphology and physiology are
particularly energetically expensive, and thus prey may display
different responses to a threat depending on how severe they perceive it
to be, known as the threat-sensitive predator avoidance hypothesis
(Chivers et al. 2001; Monclús et al. 2009).
Some adaptations reduce the chance of initial detection by a predator
including: camouflage (Cott 1940; Ruxton et al. 2004; Stevens &
Merilaita 2008) which includes crypsis where an individual resembles its
surroundings and masquerading, when an individual resembles a single
inanimate object (Ruxton et al. 2004; Quicke 2017); and Batesian
mimicry in non-toxic species that have evolved to imitate the appearance
of toxic species (Lindström et al. 1997).
Others do not decrease initial detection, and in some cases may increase
the chance of detection, but instead reduce the chance of the predator
successfully carrying out the job and serve as a means of protection
from a predator after detection including: defensive structures such as
spines, shells or scales (Ruxton et al. 2004; Yadun & Halpern
2008); autotomy (loss of a body part) and regeneration (Maginnis 2006);
warning colouration and associated toxins (Fisher 1930; Harvey et
al. 1982), both of which are also associated with mimetic species
(Huheey 1961; Brower & Brower 1963; Brower et al. 1963; Poughet al. 1973); venoms (Holding et al. 2016); construction
of appropriate refugia e.g. bolt holes (Baugh & Deacon 1988); and
plastic growth and development (Edmunds 1974; Lima & Dill 1990;
Tollrian & Harvell 1999; Sokolowska et al. 2000). For example,
the wide variation in shell structure in marine invertebrates is
believed to be a response against shell-destroying predators (Vermeij
1997). Similarly, the freshwater three-spine stickleback
(Gasterosteus aculeatus ) exhibits a complex of distinct
morphological forms, varying in the degree of spinal and lateral plates
as a consequence of predation risk (Colosimo et al. 2005).
Group aggregations are another example of an adaption in response to
predator threat, leading to benefits including increased vigilance
(Treves et al. 2001), selfish-herd effects (King et al.2012), and active defence e.g. musk ox Ovibos moschatus (Dixon
1998). Furthermore, such group behaviours have given rise to the
development of group warning/alarm calls in some species (Smith 1986),
and sometimes leads to sentinel behaviour in which one individual scans
for predators whilst the other group members forage, observed in vervet
monkeys (Chlorocebus pygerythrus ) (Horrocks & Hunte 1986). In
Gunnison’s prairie dogs (Cynomys gunnisoni) this has led to
species-specific warning calls, allowing the group to react in
accordance to the threat (Slobodchikoff et al. 1991; Kiriazis &
Slobodchikoff 2006;
Slobodchikoff et al. 2009).
In addition to the mechanisms outlined above, prey animals use basic
sensory modalities (visual, aural, olfactory) to detect the presence of
predators in the same way that predators use both direct and indirect
cues to identify and locate the position of the prey (Dwernychuk & Boag
1972; Sugden & Beyersbergen 1986; Nams 1997; Santisteban et al.2002; Carthey et al. 2011). In these contexts, visual, aural and
olfactory cues each offer advantages and disadvantages to predator and
prey in terms of being detected and/or avoiding detection. Vision is a
directional cue and relies upon direct “line of sight” between two
individuals, so can be used by prey to indicate the exact position and
identity of the predator, and vice versa (Hemmi 2005). Predators may
reduce the risk of detection while residing amongst vegetation, for
instance. Often, prey may only observe their immediate environment at
the expense of feeding, resulting in a trade-off between foraging and
vigilance. Though social species may deploy sentinels to detect
approaching predators, such individuals in turn experience a foraging
cost by foregoing feeding to protect other group members (Doolan &
Macdonald 1996; Whittingham et al. 2004). The slender-tailed
meerkat (Suricata suricatta ), vervet monkey (Chlorocebus
pygerythrus ), and black-tailed prairie dog (Cynomys
ludovicianus ) exhibit such complexes within their respective social
groups (Magle et al. 2005). Furthermore, visual cues are of
limited use for nocturnal species and those living in dense vegetation.
Auditory cues are also directional, although generally to a lesser
extent than visual cues due to sound resonating in a non-linear fashion
from a point source. Predators, therefore, may inadvertently reveal
their proximity when stalking prey, but such signals may not disclose
exact locations and are less valuable to prey than direct sight.
Furthermore, auditory signals are attenuated and reflected by vegetation
and other surfaces, reducing the distance they travel and obscuring
their origin (Goerlitz et al. 2008). Similar effects may arise in
poor weather conditions, where sounds are refracted in strong winds
(Clinchy et al. 2013). Consequently, predators that stalk prey
must approach conspicuously and, preferably, upwind. Sound, however, is
not likely a significant concern for ambush (sit-and-wait) predators
(Boonstra et al. 2013).