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).