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

Abstract

Mate, prey, and predator recognition often depend on the integration of information from multiple sensory modalities including visual, auditory, and/or olfactory inputs. In Crotalinae, the eyes sense visible light while the pit organs detect infrared (IR) radiation. Previous studies indicate that there is significant overlap between the eye and pit sensory fields and that both senses are involved in recognition processes. This study investigated the relationships between eye and pit sizes in this taxonomic group as a function of phylogeny and habitat. In view of the fact that pit orientation depends largely on snout shape, pit vipers were grouped as follows: 1) arboreal, 2) terrestrial with rounded snout, and 3) terrestrial with pointed snout. The pit orientations and habitant patterns were fully independent of the Crotalinae phylogenetic tree. The phylogenetic generalized least squares model showed that both eye and pit areas were not of significantly phylogenetic relatedness, implying alternatively a strong effect of adaptation on eye and pit sizes. Negative correlations between relative eye and pit areas in terrestrial (both pointed and rounded snouts) and arboreal species were statistically significant. Our results suggest that the eyes and pits function in a complementary fashion such that selection for IR-perception relaxes selection pressures on the visual system and selection for visual discrimination relaxes selection pressures acting on the IR-system. J.

Evolution of the eyes of vipers with and without infrared-sensing pit organs

“…We find no evidence of a trade-off in terms of diminution or other alteration of vision (at least for visual media, visual opsins, and photoreceptor complement) coincident with the acquisition of IR sensing in vipers. Despite the reported inverse relationship between eye and pit size in pit vipers (Liu et al, 2016), pit vipers retain the same visual opsins as non-pit vipers and there is no compelling evidence for differences in patterns of visual opsin gene selection in the two groups. The one previously supposed major difference, the lack of small single cones in crotalines, was found to at least not be universal within pit vipers (present in Crotalus durissus).…”

“…The impact of the evolution of infrared (IR) detection in pit vipers may be a good case study, because these snakes use their apomorphic IR detection for several important functions including prey (and possibly predator) detection and thermoregulation (Krochmal, 2004), and their IR imaging sensors are the most sensitive and efficient known (e.g., Grace & Matshita, 2007). Additionally, an inverse relationship has been found between the size of the eyes and the pits in pit vipers (Liu et al, 2016).…”

A current perspective on snake venom composition and constituent protein families

“…Pit vipers are distinguished by the presence of infra-red sensitive facial pits (Fig. 1B), giving pit vipers the ability to see in the infra-red spectrum (Gower et al 2019;Liu et al 2016).…”

Infrared Imaging: A Motion Detection Circuit for Rattlesnake Thermal Vision

“…Therefore, it seems much more likely that the images generated by the two sensory modalities -the thermal image and the visual image -are overlaid as two separate spatial representations of the snake's prey. Which of the two is more important seems to differ between different species: snakes with larger eyes have smaller pit organs and vice versa [15], but the combination of both sensory modalities improves the hunting success [16].…”