Camera traps are electrical instruments that emit sounds and light. In recent decades they have become a tool of choice in wildlife research and monitoring. The variability between camera trap models and the methods used are considerable, and little is known about how animals respond to camera trap emissions. It has been reported that some animals show a response to camera traps, and in research this is often undesirable so it is important to understand why the animals are disturbed. We conducted laboratory based investigations to test the audio and infrared optical outputs of 12 camera trap models. Camera traps were measured for audio outputs in an anechoic chamber; we also measured ultrasonic (n = 5) and infrared illumination outputs (n = 7) of a subset of the camera trap models. We then compared the perceptive hearing range (n = 21) and assessed the vision ranges (n = 3) of mammals species (where data existed) to determine if animals can see and hear camera traps. We report that camera traps produce sounds that are well within the perceptive range of most mammals’ hearing and produce illumination that can be seen by many species.

Copyright: © 2014 Meek et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

Camera traps are being used widely throughout the world although the limitations and constraints of these devices are rarely considered. The study of animal ecology, biology and behaviour requires thorough planning, robust analysis and an element of good luck. Irrespective of the tools being used, there will always be expected errors, variability, unknowns or biases, described as being similar to the “Observer Effect” or “Heisenberg’s Uncertainty Principle” [1]. The study of animals can only provide an insight into their life history; nothing is absolute and understanding the variability is an important component of research investigations. Camera trapping is a survey tool that has improved our capacity to infer the life history of animals, especially where minimising observer effects on animal behaviour is critical [2]–[4]. Some consider that camera traps are a non-intrusive method of studying animals [5]. However, there is increasing evidence throughout the world that animal behaviour is affected by the presence of camera traps [6]–[9]. In some circumstances this ‘effect’ may have little impact on the investigation. In other studies, for example those using indices and mark-recapture estimators (e.g., [1], [10], [11]), it is paramount that the technology used does not alter animal behaviour during or between monitoring sessions to ensure constancy of detectability [8]. Where bias occurs, it is crucial that this effect is understood and measured when interpreting the results of the observations; “the accuracy of an index is irrelevant; precision is paramount” [11]. Irrespective of the hypothesis being tested, the effect on behaviour can scarcely be considered non-intrusive [8] if animals display behavioural responses to sampling tools.

Observations of responses to mensurative devices strongly imply that learning can occur as a consequence of exposure to the devices. For examples, camera traps could be detected by animals for the following reasons:

Auditory – by the emission of sounds from the electronic and mechanical components of the device: these could be in the infra, audible and ultra-sound ranges. Olfactory – metal, plastic and human scents on the device [6], [9], Learned association – avoidance of the camera trap through wariness of human presence at a site [6] or attraction to the camera trap through lures and food baits, Visual (day) – neophobia towards foreign objects introduced into their environment; regular-shaped objects (essentially rectangular prisms) attached to trees or posts [12], [13], Visual (night) – the flash of xenon light, white LED or infrared LED illumination [7].

The hearing and vision [22] of animals varies depending on their life history, hunting modis operandi, body size [23], [24] and favoured prey [25]. It is commonly accepted that the combination of hearing and vision is important for animal localisation acuity [22], for hunting and social interactions and to avoid predators [26].

Auditory ranges Hearing ranges are broad in mammals, as an example; mice (Mus domesticus) have a range from 2.3–92 kHz [29], horses (Equus cabalus) hear up to 33.5 kHz, cows (Bos taurus) to 35 kHz [32], kangaroo rat (Dipodomys merriami) to 74 kHz, while the rabbit (Oryctologus cuniculus) can only hear to 49 kHz., cotton rat (Sugmondon hispidus) to 72 kHz [29], wood rat (Neotoma floridana) to 56 kHz, grasshopper mouse (Onychomys leucogaster) 69 KhZ [33], and fox squirrel (Sciurus niger) 49 kHz [34]. A small Australian predator, the northern quoll (Dasyurus hallucatus) hear best from 8–10 kHz although their hearing range is 0.5–40 kHz [31]. Six Australian Brush-tailed possum (Trichosurus vulpecula) were trained to respond to frequencies of 88 kHz [35]. Only bats, dolphins and shrews have been reported to recognise and detect high frequency signals [36], although the authors propose that “it is not impossible that all primitive mammals are capable of echolocation”. Our associated research primarily focuses on the management of introduced predators [1], wild dogs (Canis lupus ssp) and European red foxes (Vulpes vulpes) and to a lesser extent on feral cats (Felis catus). Feral and domestic cats have one of the broadest hearing ranges of all mammals [27], ranging from 48 Hz to 85 kHz, although responses have been reported up to 100 kHz [28]. Dogs show variability in sensitivity to sound depending on breed (6–45 kHz) (https://www.lsu.edu/deafness/HearingRange.html accessed 3 July 2013) and as high as 65 kHz [28], although this has been disputed [30]. Foxes have evolved with a wide ranging hearing capacity (0.9–34 kHz) with optimal hearing at 10–14 kHz and an upper limit of 34 kHz [25] and 65 kHz [28].

Visual ranges Dogs are known to have dichromatic colour vision with an upper limit of detection around 555 nm [16], while Mustelids have been reported to have the capacity to detect infrared light up to 870 nm [17]. In the case of Australian marsupials there is clear evidence of colour vision [18]–[20] with taxa variability in regards to spectral sensitivity (dichromatic vs trichromatic) [21]. Camera traps that use xenon white flash to illuminate animals have been widely used in hunting and wildlife research [7] even though there is concern that the bright flash affects the short and long term behaviour of target animals. In a study of Kinkajous (Potos flavus) behavioural avoidance of ‘canopy-highway’ branches where white flash camera traps were placed has been reported [8]. Tiger (Panthera tigris tigris) capture rates in Nepal decreased by 50% over 5 nights of camera trapping using xenon flash devices [7] and similar concerns have been raised in studies of grey wolves (Canis lupus) [14]. Technological advances have resulted in infrared camera traps dominating the market based on claims that animals can’t see the infrared flash [15]. Most of the mammal species being studied using camera traps are nocturnal-crepuscular animals, although not always [19], with some showing a slight preponderance for diurnal activity; so their eye physiology reflects this behaviour. It would not be accurate to state that animals can “see in the dark”; a more accurate description may be that they are able to “see what is in the dark” [37]. Knowledge on the vision capabilities of animals continues to improve despite limitations in fully understanding how they view the world because of the challenges of measuring what they perceive [38]. In fact some believe that the perception of colour vision requires some form of learning, association and consciousness [39]. Moreover, there is uncertainty as to whether animals perceive brightness and hue [39] or if colour vision is in fact important to cats and dogs [40]. Interestingly, apart from Mustela spp. [17] very little is known about the detection of infrared signals by animals. In the three main species of interest to us (dogs, cats and foxes), their night visual acuity as primarily nocturnal predators is high; in the case of the cat, and more than likely foxes and dogs, their superior night vision is adapted for low visual stimuli [41]. Of most interest is the animal’s ability to detect near infrared (700–3000 nm) illumination: the part of the light spectrum used in infra-red camera traps.