Mammalian lures monitored with time-lapse cameras increase detection of pythons and other snakes

Background

Enhancing detection of cryptic snakes is critical for the development of conservation and management strategies; yet, finding methods that provide adequate detection remains challenging. Issues with detecting snakes can be particularly problematic for some species, like the invasive Burmese python (Python bivittatus) in the Florida Everglades.

Methods

Using multiple survey methods, we predicted that our ability to detect pythons, larger snakes and all other snakes would be enhanced with the use of live mammalian lures (domesticated rabbits; Oryctolagus cuniculus). Specifically, we used visual surveys, python detection dogs, and time-lapse game cameras to determine if domesticated rabbits were an effective lure.

Results

Time-lapse game cameras detected almost 40 times more snakes (n = 375, treatment = 245, control = 130) than visual surveys (n = 10). We recorded 21 independent detections of pythons at treatment pens (with lures) and one detection at a control pen (without lures). In addition, we found larger snakes, and all other snakes were 165% and 74% more likely to be detected at treatment pens compared to control pens, respectively. Time-lapse cameras detected almost 40 times more snakes than visual surveys; we did not detect any pythons with python detection dogs.

Conclusions

Our study presents compelling evidence that the detection of snakes is improved by coupling live mammalian lures with time-lapse game cameras. Although the identification of smaller snake species was limited, this was due to pixel resolution, which could be improved by changing the camera focal length. For larger snakes with individually distinctive patterns, this method could potentially be used to identify unique individuals and thus allow researchers to estimate population dynamics.

Pit viper thermography: the pit organ used by crotaline snakes to detect thermal contrast has poor spatial resolution

Pit vipers detect infrared radiation by means of temperature contrasts created on their pit organ membranes. Signals from pit organs integrate with visual signals in the optic tectum, leading to the conjecture that the facial pits operate as an extension of the visual system. Because similar mechanisms underlie thermal imaging technology, imagery from thermal cameras is often used to infer how pit vipers perceive their environment. However, pit organs lack a focusing mechanism, and biophysical models predict that pit organs should have poor spatial resolution compared with thermal imaging cameras. Nevertheless, behavioral studies occasionally suggest pits may have better resolution than predicted by biophysical models, indicating that processing in the central nervous system may improve imaging. To estimate the spatial resolution of the neural image informing behavior, we recorded snake responses evoked by targets moving across backgrounds composed of two contrasting temperatures with an average temperature equal to the target temperature. An unresolved background would appear uniform; thus, the target would be detectable only if the background pattern were resolved. Western rattlesnakes (Crotalus oreganus) displayed no statistically significant responses to targets presented in front of patterned backgrounds, regardless of the temperature contrasts or spatial frequencies within the background, but responded strongly to targets presented in front of homogeneous backgrounds. We found no evidence that the pit organ system can resolve spatial details subtending an angle of 9 deg or less. We discuss the implications of these results for understanding pit organ function in ecologically relevant habitats with thermal heterogeneity.

Dogs can sense weak thermal radiation

The dog rhinarium (naked and often moist skin on the nose-tip) is prominent and richly innervated, suggesting a sensory function. Compared to nose-tips of herbivorous artio- and perissodactyla, carnivoran rhinaria are considerably colder. We hypothesized that this coldness makes the dog rhinarium particularly sensitive to radiating heat. We trained three dogs to distinguish between two distant objects based on radiating heat; the neutral object was about ambient temperature, the warm object was about the same surface temperature as a furry mammal. In addition, we employed functional magnetic resonance imaging on 13 awake dogs, comparing the responses to heat stimuli of about the same temperatures as in the behavioural experiment. The warm stimulus elicited increased neural response in the left somatosensory association cortex. Our results demonstrate a hitherto undiscovered sensory modality in a carnivoran species.

Infrared-sensing snakes select ambush orientation based on thermal backgrounds

Sensory information drives the ecology and behaviour of animals, and some animals are able to detect environmental cues unavailable to us. For example, rattlesnakes use infrared (IR) radiation to detect warm prey at night when visual cues are reduced. Until recently these sensory worlds have been inaccessible to human observers; now technology can allow us to "eavesdrop" on these species and understand how sensory perception drives ecology and behaviour. We used thermography and computer simulations to examine how prey-background temperature contrast and areas of temperature transitions influence the angular orientation of free-ranging rattlesnakes once they have selected an ambush site. We tracked free-ranging sidewinder rattlesnakes Crotalus cerastes to their selected ambush sites and recorded 360° near-ground thermographic panoramas from the centre of the ambush site. A computer simulation then moved a simulated prey item across the panorama and computed a contrast index for all directions. Rattlesnakes did not face ambush directions that offered stronger contrast than average, but they demonstrated a striking tendency to face directions with strong thermal transitions. Background transitions likely create a readily detected, rapidly changing stimulus when a prey animal passes. Quantifications of sensory environments like this one can boost our comprehension of how sensory function impacts the ecology, behaviour, and evolution of animals.