Why is the tongue of blue-tongued skinks blue? Reflectance of lingual surface and its consequences for visual perception by conspecifics and predators

Abstract

Blue-tongued skinks of the genus Tiliqua (Scincidae) are characterized by their large blue melanin-pigmented tongues, often displayed during open-mouth threats, when the animal feels endangered. It is not clear whether this unusual coloration is a direct anti-predation adaptation or it may rather serve intraspecific communication, as ultraviolet-blue color is a frequent visual signal in a number of lizard species. We used spectrophotometry and visual modeling to compare blue tongues of Tiliqua gigas with tongues and skin coloration of other lizard species, and to examine their appearance through the eyes of both the conspecifics and avian predators. Our results show that (1) the tongue coloration is probably not substantially influenced by the amount of melanin in the skin, (2) lingual and oral tissues are UV-reflective in general, with blue colored tongues having chromatic qualities similar to UV-blue skin patches of other lizard species, (3) UV-blue tongues are more conspicuous than pink tongues, especially in the visual model of conspecifics. We hypothesize that blue tongues may possibly serve as a semantic (honest) signal analogous to UV-blue skin patches of other lizard species due to greater UV-bias in the vision of diurnal lizards. Regarding the social behavior and high aggressiveness in Tiliqua and their relatives, such signal might serve, e.g., in intraspecific long-distance communication between conspecifics in order to avoid aggression, and its anti-predation effect may only be a secondary function (exaptation).

The functional morphology of lingual prey capture in a scincid lizard, Tiliqua scincoides (Reptilia: Squamata)

“…A larger number of circular muscle fibers, in turn, suggests a more effective ability to compress the m. hyoglossus bundles to decrease their diameter and increase their length. It is worth noting that T. scincoides and congeners evince other evidence of extensive hydrostatic tongue deformation in another context, that is, during their well‐known lingual display (Abramjan, Bauerová, Somerová, & Frynta, 2015; Badiane, Carazo, Price‐Rees, Ferrando‐Bernal, & Whiting, 2018; Carpenter & Murphy, 1978). The entire tongue is protruded beyond the mandibular margin, extended and widened as part of an anti‐predator startle (deimatic) display (Badiane et al, 2018).…”

Acquired Asymptomatic Blue Tongue: A Report of Exogenous Agent-associated Tongue Dyschromia and Review of Blue Tongue Etiologies

“…The tongues of several animals are also blue or blue-black, e.g., blue-tongued skink (Figure 2), chow-chow dog, giraffe, impala, Jersey cattle, okapis, polar bear, and Sha-Pei dog. Some investigators postulate that the skink’s tongue color is not substantially influenced by the amount of melanin in its skin, but that it has chromatic qualities similar to ultraviolet-blue skin patches of other lizard species and therefore the tongue tissue is ultraviolet-reflective [18].…”

The tongue of Leopard Gecko (Eublepharis macularius): LM, SEM and confocal laser study

“…As usual in reptiles, the way of feeding is characterised by the typical movements of the tongue for capturing the preys. Nevertheless, the role of the tongue was widely demonstrated in reptiles as chameleons, agamids and iguanids, as in numerous fish and birds too (Abbate et al, , , , ; Abbate, Guerrera, Montalbano, Ciriaco, & Germanà, ; Abbate, Guerrera, Montalbano, De Carlos, et al, ; Abramjan, Bauerová, Somerová, & Frynta, ; Bels & Baltus, ; Bels & Delheusy, ; Bels & Goose, ; Erdoğan & Iwasaki, ; Guerrera et al, ; Herrel, Cleuren, & Vree, ; Herrel, Deban, Schaerlaeken, Timmermans, & Adriaens, ; Herrel, Redding, Meyers, & Nishikawa, ; Jackowiak, Skieresz‐Szewczyk, Godynicki, Iwasaki, & Meyer, ; Jackowiak, Skieresz‐Szewczyk, Kwieciński, Trzcielińska‐Lorych, & Godynicki, ; Kraklau, ; Montalbano et al, ; Schwenk, ; Schwenk & Bell, ; Schwenk & Throckmorton, ; Skieresz‐Szewczyk & Jackowiak, ; Wainwright & Bennett, ; Wainwright, Kraklau, & Bennett, ; Yang & Wang, ). In reptiles, the tongue morphology was studied in several species (Baeckens et al, ; Beisser, Lemell, & Weisgram, ; Heiss et al, ; Jamniczky, Russell, Johnson, Montuelle, & Bels, ; Koca, Oğuz, & Osanç, ; Lemell, Beisser, & Weisgram, ; Mouton, Flemming, & Broeckhoven, ; Putterill & Soley, ; Rehorek et al, ), and in some cases, as happens in snakes, its role is important only for the olfaction (Filoramo & Schwenk, ; Nishida, Yoshie, & Fujita, ).…”