Risk management in optimal foragers: the effect of sightlines and predator type on patch use, time allocation, and vigilance in gerbils

Abstract

In the foraging game between gerbils and their predators, gerbils manage risk of predation using the tools of time allocation (where, when and for how long to forage) and vigilance. Th e optimal level of a forager ' s vigilance should be aff ected by its encounter rate with predators and the eff ectiveness of its vigilance in reducing mortality risk. Th e physical structure of the environment can alter the eff ectiveness of its vigilance and therefore alter its foraging behaviour. We tested this for gerbils at risk of predation from barn owls or foxes in a large vivarium. In particular, we reduced the eff ectiveness of vigilance by placing obstructions around feeding trays that blocked sight lines along either the vertical (vigilance directed against owls) or horizontal axis (vigilance directed against foxes), thereby changing the physical structure of the environment. In addition, we manipulated the presence of foxes and owls. In general, gerbils harvested fewer seeds, allocated less time to foraging in dangerous patches, and used more vigilance while foraging where and when risks were higher (i.e. in the presence of predators and in bright moonlight). Vertical and horizontal sightline treatments interacted synergistically to further raise perceived risk.Th ese results imply that blocking sight lines reduces the eff ectiveness of vigilance, causing gerbils to use it less. Moreover, in the presence of a predator, the gerbils ' response to the blocked sightlines was more severe -harvesting less food and spending less time and vigilance -in the patches with the increased risk. Th is was especially so in the presence of the predator that was expected to most benefi t from blocking that particular type of sight line: cover that blocked vertical sight lines was riskiest in the presence of owls, and cover that blocked horizontal sight lines was riskiest in the presence of foxes. Th ese results strongly indicate the importance of sightlines and landscape features such as bushes in the risk management and forging decisions of gerbils, demonstrating that bush cover provides mixed blessing to gerbils by providing cover, but making vigilance ineff ective.Th e clever forager reacts to the risk of predation by using behavioral tools such as time allocation and vigilance to reduce and manage the risk to which it is exposed (Lima 1987a).Apprehension and vigilance are behavioral tools used by animals to detect risks when foraging, including those from predators (Kotler et al. 2004a) and potential competitors for food and territory (Lima 1987b). Apprehension is defi ned as any reduction in attention to other activities (e.g. foraging) as a result of increasing the allocation of attention given to detecting and/or responding to potential predator activity . Kotler et al. (2004b) showed that gerbils foraged less effi ciently under complex conditions when risk of predation was high, as their attention was not wholly on the task at hand. Vigilance is defi ned as a special case where apprehension is total (i.e. all attention is direct...

Survival and causes of mortality for pygmy rabbits (Brachylagus idahoensis) in Oregon and Nevada

“…Previous research using olfactory cues to induce predation risk found that a cue might be unreliable if applied with a lack of spatial or temporal association with predators (Powell and Banks 2004). Certainly, the strength, timing, and type of predator cue used can influence responses by prey (Embar et al 2011). However, our results showing that pygmy rabbits respond to visual predation cues are consistent with observations of free-ranging pygmy rabbits that had lower perceptions of risk with higher levels of concealment from shrubs (Camp et al 2012).…”

“…other PSMs, fiber or crude protein, Au et al 2013, Bedoya-Pérez et al 2014b, Camp et al 2015 and predator (e.g. type of refuge or predator, Embar et al 2011) risks that we did not manipulate can influence foraging responses by herbivores. However, our relatively simple trials that isolated one measure of potential toxicity (a monoterpene) and one perceived predation risk (visual predator cues) demonstrated that tradeoff scenarios for a specialist mammalian herbivore are complex, even under controlled conditions.…”

Behaviour of American Crows (<em>Corvus brachyrhynchos</em>) when encountering an oncoming vehicle

“…One likely explanation is that the American Crows chose to walk away because they preferred having a clear sightline from which to detect a threat from the approaching vehicle. multiple studies suggest that prey organisms prefer having a clear sightline that allows them to better detect an approaching predator and consequently manage their risk of predation (Vijayan et al 2007;Embar et al 2011). Since the landscape beyond the curbs on both sides of the road was grass (approximately 20 cm high), the sightline of an American Crow moving onto the curb would have been blocked, thus increasing its perceived risk of predation.…”

Examining functional components of cover: the relationship between concealment and visibility in shrub‐steppe habitat

“…However, cover is not simply present or absent in, but occurs at a range of values in the landscape. Likewise, visibility, commonly termed ''sightlines'', often has been characterized as ''open'' or ''obscured'' (Whittingham et al 2004, Hannon et al 2006, Jones et al 2006, Embar et al 2011. In many cases, concealment has been measured (and called ''cover''), and visibility was assumed to be the lack of cover.…”

“…Both of these properties can influence the perceived and realized risk of predation for a variety of prey species (e.g., Longland and Price 1991, Tabor and Wutsbaugh 1991, Bowyer et al 1999, Budnik et al 2002, Ripple and Beschta 2004, Pietrek et al 2009, Embar et al 2011. Prey species might alter habitat use to increase visibility or concealment as their perception of safety changes (Whittingham and Evans 2004), which could depend on the characteristics of the animal and its potential predators, the habitat, and environmental factors, such as weather and moonlight (Stankowich andBlumstein 2005, Kotler et al 2010).…”