For mammalian predators, chemical cues are
particularly problematic in terms of avoiding detection (Stoddart 1980).
Olfactory cues are especially important to mammalian predators as they
are frequently used to communicate information about sexual condition,
group membership, social status and territory ownership (Ralls 1971; Belet al. 1999; King & Gurnell 2007; Gelperin 2008; Sankar &
Archunan 2008). Scent cues are even utilised by marine mammals despite
them often being categorised as anosmic or microsmatic (no/reduced
olfactory processing centres) (Lowell & Flanigan 1980; Tyack & Sayigh
1997), such as female Australian sea lions using scent cues in offspring
recognition (Neophoca cinerea ) (Pitcher et al .
2010).
Chemoreception also plays a role in optimising the use of food caches,
the defence of shelter and resources, and for active defence (Henry
1977; Medill et al. 2011; Piñeiro & Barja 2015, Appleton &
Palmer 1988). In these contexts, scent-marks are used actively. In the
case of territory ownership, scent marks confer one significant
advantage; they enable territorial boundaries to be delineated without
the need for direct contact between conspecifics, therefore reducing the
risk of injury. Scent cues are, however, also left inadvertently by
individuals in the form of dander (usually comprised of skin cells or
fur) which is dispersed while traveling (Ferrero et al. 2011).
These “accidental” scents, as well as those associated with urine,
faeces and the excretions from scent glands, contain a complex mixture
of compounds that vary in their molecular weight, such that their
composition varies over time as different compounds aerosolise and
disappear or reduce in concentration (Seamans et al. 2002; Burger
2005). Consequently, conspecifics use the composition of these chemical
cues to indicate the length of time since their deposition and hence the
likeliness of the presence or absence of an individual (Wyatt 2003).
While a conspecific may incorporate chemical indications into its
assessment of social status and mating opportunity, prey species may use
chemical signals to assess the presence of a predator and likelihood of
predation. In predator-prey relationships, chemosensory cues are often
invaluable and the process of communication via chemicals has been
widely studied (Amo et al. 2007). Olfactory cues can provide
information on both direct and prospective predatory threats and this
can be particularly important during times of limited visibility (Kats
& Dill 1998). However, since aerosolised molecules are distributed by
prevailing wind currents, which themselves are susceptible to variation
in landscape topography and weather conditions, olfactory cues are also
less directional than visual cues (Burghardt 1966; Halpern & Frumin
1979; Redmond et al. 1982; Roth & Hobson 2000; Conover 2007;
Fogarty et al. 2018). Olfactory cues pose additional problems for
predators (especially mammalian) where intra-specific chemical
communication is frequent, and scent is inherently difficult to mask
from prey (Venuleo et al. 2017).
The detection of kairomones, specific to mammalian carnivores, often
initiates defensive behaviours, reduced foraging, and increased refugia
in mammalian prey (Mathis 2003; Foam et al. 2005; Preisseret al . 2005; Šmejkal et al. 2018). Similarly, rodents have
been observed to increase evasive behaviours in response to the
detection of 2-phenylethylamine which, though found in all mammalian
urine, exists at significantly higher concentrations in carnivore
excretions (Ferrero et al. 2011).