Directional sensitivity in the thermal response of the facial pit in western diamondback rattlesnakes (Crotalus atrox)

Abstract

SUMMARYRecent work published in the accompanying paper used a combination of 3D morphological reconstruction to define optical spread functions and heat transfer physics to study how external heat energy would reach the sensory membrane within the facial pit of pitvipers. The results from all of the species examined indicated asymmetric directional sensitivity, e.g. the pit would preferentially respond to stimuli located below and behind the snake. The present study was intended as a test of these findings through a quantitative neurophysiological analysis of directional sensitivity in the facial pit of the western diamondback rattlesnake, Crotalus atrox. An infrared emitter was positioned through a coordinate system (with varying angular orientations and distances) and the response it evoked measured through neurophysiological recordings of a trigeminal nerve branch composed of the afferents from the sensory membrane of the facial pit. Significant differences were found in the strength of the membraneʼs neural response to a constant stimulus presented at different orientations (relative to the facial pit opening) and over different distances. The peak sensitivity (at 12deg above and 20deg in front of the facial pit opening) was in good agreement with the predicted directional sensitivities based on optical spread functions and 3D topography. These findings support the hypothesis that the topography, and functional performance, of the facial pit has undergone an adaptive radiation within the pit vipers, and that differences in the behavioral ecology of the pit vipers (i.e. terrestrial versus arboreal) are reflected within the facial pits.

Eye and pit size are inversely correlated in crotalinae: Implications for selection pressure relaxation

“…Eye and pit sizes and ecological patterns were not consistently associated with specific clades, suggesting that these variations in eye and pit size in Crotalinae species result from selection pressures acting on each species and do not reflect characters associated with phylogeny. Interspecific variations in the exact position and orientation of the external opening of the facial pit might reflect ecological demands (Kohl et al, ). For instance, the three‐dimensional geometry of the facial pit determines its orientation sensitivity and imaging properties (Bakken and Krochmal, ) and different snout shapes result in different pit orientations (Bakken et al, ).…”

“…The correlations between the residuals of pit and eye areas in arboreal species of Crotalinae. ecological demands (Kohl et al, 2012). For instance, the three-dimensional geometry of the facial pit determines its orientation sensitivity and imaging properties (Bakken and Krochmal, 2007) and different snout shapes result in different pit orientations .…”

Organotopic organization of the primary Infrared Sensitive Nucleus (LTTD) in the western diamondback rattlesnake (Crotalus atrox)

“…In pit vipers, in which the pit organ is located in the upper jaw, the membrane is innervated by the V1, V2d, and V2s but not the V3 trigeminal nerve branch (Lynn, ; Bullock and Fox, ; Kohl et al, ; Figs. A,B, E).…”

Cooler snakes respond more strongly to infrared stimuli, but we have no idea why

“…The membrane is innervated by three branches of the trigeminal nerve that terminate in palmate warm receptors packed with mitochondria. These receptors sense the temperature distribution image on the pit membrane created by thermal radiation from environmental surfaces (Otto, 1972;Stanford and Hartline, 1980;de Cock Buning, 1984;Kohl et al, 2012). This image is merged with visual information in the optic tectum, and transmitted to the telencephalon, where it informs behavior (Goris and Terashima, 1973;Hartline et al, 1978;Gruberg et al, 1979;Terashima and Goris, 1979;Berson and Hartline, 1988).…”