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Some terrestrial predators "stalk" their prey: they sneak up on it slowly, maintaining a low profile, while keeping as close to silent as possible. This makes sense from an evolutionary perspective: silent predators catch more prey.
However, the prey near where I live are hopelessly noisy. Deer will freeze if they suspect the presence of a predator, then bolt when they see one for sure. Squirrels and other small mammals run around at top speed all the time - you can hear them dozens of feet away, rustling the dead leaves around. Hedgehogs just wander, making a similar noise. It seems like the only "stealth" option they have, as far as sound is concerned, is to remain still.
Are there any examples of terrestrial herbivores evolving to move more quietly in order to avoid predation? I'm not looking for a comprehensive list, but rather simply individual examples.
Does soundproofing count?
Most predators locate prey using visual or olfactory cues; the only predators I can think of that use auditory cues are nocturnal ones - owls and bats. Bats, of course, use echolocation, so even completely silent prey are still detectable. It has been suggested that the soft, fur-like body coverings of some owlet moths (Noctuidae) and tiger moths (within the Arctiinae) help hide them from echolocating predators by passively absorbing sound, although subsequent studies have suggested that they may instead be mimicking the acoustic signature of unpalatable prey.
(public domain image: source)
I have a suspicion this isn't what you meant by 'land-dwelling herbivore', but insects are people too…
Edit: just discovered this paper from May that provides evidence supporting the hypothesis that the twisted tails of luna moths (Actias luna) disrupt tracking by echolocating bats.
Image owned by Shawn Hanrahan, licensed under CC BY-SA 2.5
I think that there are too many examples of animals being deliberately quiet when grazing to count. I'll discuss specifically the exceptions you raise and how they are exceptional. Hopefully you'll see my point: A lot of grazers are quiet and cautious or failing that have other adaptive strategies for not being eaten.
Deer are incredibly quiet and cautious when grazing, and fast and agile once they decide it's not safe. Squirrels use trees as an defense - not many predators of theirs can even climb trees let alone keep up with a squirrel darting through them, (related question about bushy tails of squirrels being counter intuitive). And hedgehogs… well… how do you eat one without getting a mouthful of pain? On a bigger scale, here is a related video of a juvenile lion trying to attack a porcupine. Being unsubtle helps them.
Herbivore diet breadth mediates the cascading effects of carnivores in food webs
This study shows the far-reaching effects of herbivore dietary specialization on the ecological and evolutionary dynamics of carnivore–herbivore–plant interactions. First, we test the long-standing hypothesis that dietary specialization of insect herbivores mediates the strength of bird predation on herbivores. Accounting for phylogenetic nonindependence of herbivores and plants, we show for the first time (to our knowledge) that dietary specialization of herbivore species is associated with reduced bird predation across an herbivore phylogeny, and that dietary specialization of herbivores increases the antipredator effects of camouflage and aposematism. Second, this study develops and finds support for the novel hypothesis that the proportion of dietary specialist species in a plant’s herbivore community predicts the degree of antiherbivore protection birds provide to plants.
Ecological and evolutionary outcomes of species interactions can only be fully understood after considering the multi-trophic setting in which species are embedded. For example, phytophagous insects in terrestrial ecosystems go through periodic outbreaks in North America and Europe, destroying millions of hectares of forest each year (McManus et al. 1992 Li et al. 2015 ). These outbreaks are often driven by both the loss of natural enemies (parasitoids, predators or pathogens), which would otherwise keep herbivore populations in check (Turchin et al. 1999 ), as well as by changes in host plant resistance and nutritional quality (Turchin et al. 1991 ). Similarly, highly damaging algal blooms in aquatic systems worldwide are driven both by increases in algal resources from eutrophication, as well as by natural enemy suppression of herbivorous zooplankton that otherwise would regulate algal density (Carpenter et al. 1985 Micheli 1999 ). But critically, the effects of these multiple drivers can also interact, resulting in emergent properties across multiple trophic levels that cannot be predicted from separately analysing each component pairwise interaction for example, tree defences can alter predation or parasitism of herbivores by reducing herbivore performance (Elderd et al. 2013 ), whereas nutrient-driven algal blooms can lengthen food chains that feedback to increase predator top-down control (Oksanen et al. 1981 Power 1990 ). Accordingly, these and numerous other so-called tri-trophic interactions (TTIs) determine the population biology and evolutionary dynamics of species at all trophic levels and drive fundamental aspects of community structure and ecosystem dynamics. By improving our understanding of ecological and evolutionary processes, tri-trophic research provides opportunities to establish linkages across levels of biological organisation.
Despite its unifying potential, research on TTIs has been fragmented, following two distinct paths. On the one hand, researchers have adopted a ‘species interactions perspective’, focusing on the evolutionary ecology and population biology of simple food chains of interacting species consisting of herbivore and natural enemy species or guilds associated with one or a few plant species (e.g. Price et al. 1980 Mooney & Singer 2012 ). Concurrently, a separate ‘ecosystem perspective’ on TTIs has focused on the bottom-up (resource) and top-down (consumer) controls over the distribution of biomass across trophic levels and other ecosystem-level properties (e.g. Polis 1999 Borer et al. 2005 Hillebrand et al. 2007 ). Bridging these levels of organisation is not only a grand challenge but also a fundamental requirement for developing a full and predictive understanding of community and ecosystem functioning.
We argue that an expanded theory on TTIs can unify our understanding of biological processes across levels of organisation, from species interactions and evolution within simple food chains, to community structure and ecosystem function. Although tri-trophic research has been previously summarised within distinct subfields (see Box 1), no broad synthesis across all facets of research on TTIs has been offered. Here, we first review the history of research within the species interactions and ecosystem perspectives. Second, we point at gaps in tri-trophic research within each level of organisation and identify promising opportunities to bridge focal interactions and ecosystem processes, including the application of new technologies and data sources. A common challenge to all of biology (and science) is to link processes across scales and levels of organisation. We hereby argue that a tri-trophic framework is necessary to address such a challenge in ecology and evolutionary biology.
Box 1. Previously reviewed sub-fields of research on tri-trophic interactions
The study of TTIs reaches back more than four decades and has been synthesised by reviews focusing on two separate perspectives, one studying population dynamics and evolutionary ecology of species interactions in simple linear food chains and the other on ecosystem-level processes. A comprehensive synthesis across topics within each perspective or across perspectives has not yet been offered. Below, we provide representative examples of syntheses within sub-fields of each perspective.
- Plant effects on herbivore development time influencing susceptibility to predation (i.e. ‘Slow-Growth, High-Mortality Hypothesis’ e.g. Williams 1999 ).
- Dual effects of plants and natural enemies on herbivore behaviour and evolution (i.e. ‘Enemy Free Space Hypothesis’ and ‘Physiological Efficiency Hypothesis’ e.g. Singer & Stireman 2005 Mooney et al.2012 Vidal & Murphy 2018 ).
- Plant indirect defences from traits attracting natural enemies that reduce herbivory (e.g. Kessler & Heil 2011 Turlings & Erb 2018 Pearse et al. in review ).
- Trophic cascades involving natural enemy-induced changes in herbivore behaviour (Preisser et al.2005 ).
- Indirect evolutionary effects of natural enemies on lower trophic levels, including natural enemy indirect effects on plant fitness (Romero & Koricheva 2011 ) and non-additive selection in tri-trophic systems (Estes et al.2013 Abdala-Roberts & Mooney 2015 ).
- Elevational gradients in natural enemy effects and plant indirect defences (Moreira et al.2018b ).
- Plant diversity effects on natural enemy abundance and diversity and its feedback on lower trophic levels (e.g. Letourneau et al.2011 Moreira et al.2016 ).
- Ecosystem-level trophic cascades in aquatic or terrestrial systems involving vertebrate predators (e.g. Strong 1992 Pace et al.1999 Shurin et al.2002 Mooney et al.2010 Estes et al.2011 Dirzo et al.2014 Sydeman et al.2015 ).
- Trophic cascades involving vertebrate and/or invertebrate natural enemies in terrestrial communities (e.g. Schmitz et al.2000 Halaj & Wise 2001 ).
- Ecosystem consequences of trophic cascades involving predator effects on herbivore behaviour (e.g. Ripple & Beschta 2004 Schmitz et al.2004 ).
- Effects of consumers and resources on biomass distribution across trophic levels (e.g. Borer et al.2006 Gruner et al.2008 ).
Linking the evolution and form of warning coloration in nature
Many animals are toxic or unpalatable and signal this to predators with warning signals (aposematism). Aposematic appearance has long been a classical system to study predator–prey interactions, communication and signalling, and animal behaviour and learning. The area has received considerable empirical and theoretical investigation. However, most research has centred on understanding the initial evolution of aposematism, despite the fact that these studies often tell us little about the form and diversity of real warning signals in nature. In contrast, less attention has been given to the mechanistic basis of aposematic markings that is, ‘what makes an effective warning signal?’, and the efficacy of warning signals has been neglected. Furthermore, unlike other areas of adaptive coloration research (such as camouflage and mate choice), studies of warning coloration have often been slow to address predator vision and psychology. Here, we review the current understanding of warning signal form, with an aim to comprehend the diversity of warning signals in nature. We present hypotheses and suggestions for future work regarding our current understanding of several inter-related questions covering the form of warning signals and their relationship with predator vision, learning, and links to broader issues in evolutionary ecology such as mate choice and speciation.
Many animals are toxic, unpalatable, or otherwise unprofitable, and advertise this to predators with conspicuous and/or distinctive warning signals (aposematism [1,2]). Aposematic appearance has been a classical system to study evolution and adaptation for over 150 years, and has received considerable empirical and theoretical investigation. However, research has centred on understanding the initial evolution of aposematism within an originally cryptic population (for which there are a wide range of non-mutually exclusive explanations [1,2]), but these studies generally tell us little about the form and diversity of real warning signals. In contrast, less attention has been given to the mechanistic basis of aposematic markings that is, ‘what makes an effective warning signal?’. As Rowe & Skelhorn  argue, studies of animal communication have been dominated by issues of signal cost and reliability, and have largely overlooked psychological mechanisms, which can explain the specific make-up of the vast range of signals that exist . Furthermore, unlike some other areas of adaptive coloration research (such as camouflage and mate choice), studies of warning coloration have been slow to explicitly consider the role of predator vision (and how it will often differ substantially from human vision).
Here, we review current understanding of warning signal form, with an aim to understand the diversity of warning signals in nature. We focus on the following inter-related key issues: