Camouflage In The Indochinese Box Turtle(Cuora Galbinifrons) And Keeled Box Turtle(C.mouhotii)

Camouflage is a common strategy adopted by prey to reduce the risk of detection and recognition by predators.The study of camouflage is crucial to the interpretation of animal morphological adaptation and evolution.Background matching,disruptive coloration and masquerade are three important camouflage strategies.With background matching,the color of animals tends to match with the background,while the disruptive coloration strategy has obvious patterns,disruptive edges,or formation of an internal false edge.These two strategies work by impeding the detection or recognition by predators.Masquerade is defined as the strategy wherein animals resemble objects(such as stones,twigs,deciduous leaves,and droppings)in the background to enable error recognition as inedible or innocuous objects by their predators.Although these three kinds of camouflage have received widespread attention,there are few studies on the camouflage strategies of animals with sympatric distribution but obvious coloration differences,few studies on the influence of shape on disruptive camouflage,and the shape with prominent patterns on camouflage.Small terrestrial turtles move slowly;hence,it is important to improve their fitness by camouflage to anti-predation.The sympatric Indochinese box turtle(Cuora galbinifrons)and keeled box turtle(C.mouhotii)live in terrestrial habitats in tropical forests,but their substrate preferences are different.The former prefers broad-leaved and bamboo-leaved substrates,while the latter prefers broad-leaved and bare ground substrates.Whether the preferred substrate can enhance their camouflage effeciency has not been studied.Their coloration also exhibits an evident difference.The carapace of the Indochinese box turtles exhibits a yellow stripe on each side to divide it into two parts of different colors,namely the middle stripe(vertebral shield),and side stripe(rib shield).The carapace of the keeled box turtle is uniform in coloration,has flatter scutes,and is divided into middle and side parts;the different shape of the two parts results in the formation of distinct middle and side stripes.The study of camouflage of both box turtle species is not only helpful to explain the ecological adaptation of their different coloration,but also helpful to understand the influence of animal shape on disruptive camouflage,and the disruptive and masquerade function of color and shape of prominent stripes.This study was conducted from April 2019 to March 2021 in the Diaoluoshan National Nature Reserve,Hainan Island,China.Digital photography was used to acquire images of the substrates and turtles.Images were standardized and quantified using the Image J software.The just noticeable difference in chroma and luminance,and edge disruption ratio between turtles and substrates were quantified to study background matching and disruptive coloration strategies.The shape and color similarity between turtles and leaves in the substrates were quantified to study masquerade.The camouflage strategies of both box turtle species were compared for different substrate types,and the effects of these strategies was evaluated by human vision.Additionally,the microhabitat preference and camouflage effect of intraspecific individuals with different coloration were also analyzed in the enclosure in the field.The detailed results have been described in the following sections.Background matching: The matching between the entire carapace of the both species was great in the preferred substrates,but the prominent stripes(side stripe of Indochinese box turtle and middle stripe of keeled box turtle)reduced the entire matching.The middle stripe of the carapace of the Indochinese box turtle matched the preferred substrates more than the non-preferred substrates.The side stripes of the Indochinese box turtle matched the bamboo-leaved substrate.The middle stripe of the keeled box turtle matched the preferred substrates less than the non-preferred substrates.The side stripe of the keeled box turtle matched the preferred substrates more than the non-preferred substrates.The head and limbs of the Indochinese box turtle matched the preferred substrates more than the non-preferred substrates.The limbs of the keeled box turtle matched the preferred substrate more than the non-preferred substrates,but the head less matched the preferred substrates.Disruptive coloration: The edge disruption ratio of both box turtle species in the preferred substrates was better than that in the non-preferred substrates,and both species had the highest edge disruption ratio in the broad-leaved substrate.The higher the edge disruption ratio is,the better the disruptive effect.In the Indochinese box turtle,the middle stripe matched the preferred substrate more than that of the non-preferred substrate,and the side stripe was conspicuous in the preferred substrate,which enhanced the disruptive effects in the preferred substrate.The chroma difference between the middle and side stripe of the keeled box turtle was less remarkable,but the lightness difference was considerable,which caused the surface disruption of the carapace.The side stripe of the keeled box turtle matched the preferred substrates,while the middle stripe was conspicuous,which enhanced the disruptive effects in the preferred substrate.The disruptive effects caused by shape disruption of the keeled box turtle were better than the color disruption of the Indochinese box turtle in the grass,broad-leaved and bare ground substrates.Masquerade: The area with the highest shape similarity between the box turtle species and the leaves in the substrate,and the perimeter,aspect ratio,solidity,circularity,and roundness were highly similar among the four substrate types,which ensured the shape similarity between the carapace of turtles and leaves.The middle stripe of the Indochinese box turtle exhibited the highest shape and color similarity with the leaves in broad-leaved substrate,while the middle stripe of the keeled box turtle showed the highest shape similarity with the leaves in the broad-leaved substrate.Although the color similarity was poor,it was below the threshold value of discrimination.The shape and color of the side stripes were most similar to those of the leaves in the bamboo-leaved substrate.The high similarity of stripes to deciduous leaves in the preferred substrate enhanced the partial masquerade effect;the more similar the stripes,the better the partial masquerade;predators thus error recognition the prominent stripes as deciduous leaves.When partially buried in leaf litter,the exposed middle stripe also has a partial masquerading function.Evaluation of camouflage effects by human vision: Total of 594 high school students were presented with images,including trichromatic images to evaluate the predation effects of trichromatic visual predators and dichromatic images to simulate the predation effects of dichromatic visual predators.The results of the detection experiment showed that the entire carapace of both box turtle species was more easily detected by observers in the dichromatic and trichromatic images than the middle and side stripes,and the detection probability of the entire carapace in the preferred substrates was lower than that in the non-preferred substrates.The middle stripe was less detected by observers in the dichromatic and trichromatic images in the preferred broad-leaved substrate,indicating that the exposed middle stripe in the substrate had a partial matching effect when both species were partially buried in leaf litter.The side stripes of both box turtle species had the lowest probability of being detected by observers in the dichromatic and trichromatic images.The side stripe of the keeled box turtles matched the substrate well,while the side stripe of the Indochinese box turtles did not match the substrate,which might be attributable to the presence of a smaller side stripe,apart from the color effect.The probability of being detected of both box turtle species in the dichromatic image is lower than that in the trichromatic image.The major predators of both box turtle species in the wild are large mammals with dichromatic vision.The better camouflage effeciency in the preferred substrate gain an advantage in anti-predation.The results of the recognition experiment showed that the inexperienced observers had higher error recognition rates for both box turtle species in the dichromatic and trichromatic images,and the inexperienced observers had a higher error recognition rate in the trichromatic image than dichromatic vision in the preferred substrate.In both dichromatic and trichromatic images,the probability of error recognition of inexperienced observers was higher than that of experienced observers.Predators tend to feed on a variety of prey in the wild,and do not exclusively feed on both box turtle species,which increases the advantage of masquerading.The high error recognition rate of the both species in the broad-leaved substrate is due to the high shape similarity between the turtles and the leaves.In addition to the high similarity,the increase of leaf density also increases the error recognition rate.The middle and side stripes of both box turtle species showed a high probability of error recognition,which played a role in partial camouflage.The micro-habitat preference and camouflage of light and dark individuals of both box turtle species: The dark individuals of both box turtle species preferred the substrate with more canopies,while the light individuals preferred the substrate with a lower canopy.Exchanging substrates for the preferences of dark and light individuals,there was no significant color difference between the dark and light individuals in the preferred substrates and the exchange substrates for the Indochinese box turtle,the values were all below the discriminating threshold,indicating all individuals were well matched.The chromatic difference between the dark and light individuals of the keeled box turtle in the preferred substrates matched that in the exchange substrates,indicating that substrate preference enhanced background matching.The preferred micro-habitats of both box turtle species are relatively fixed,and their range of activity is small,which reduces the predation risk caused by the occurrence of less matched substrates.In conclusion,the coloration and the shape of both box turtle species were adapted to the preferred substrate,and the camouflage effect in the preferred substrate was significantly better than that in the non-preferred substrate.Matching the background color to the substrate of both box turtle species is the basis of disruptive camouflage and partial masquerade,which compensate for the visual exposure caused by the mismatching of their prominent stripes to the substrate.The camouflage effect of background matching was poor when the entire carapace of both box turtle species was completely exposed.However,the prominent stripes did not match the substrate but were similar to the leaves in the preferred substrate,which enhanced the partial masquerade and disruptive effect.When they are partially buried in leaf litter,the exposed middle stripe also served as a partial match and masquerade.Additionally,the differences in micro-habitat preferences between light and dark individuals in both box turtle species enhanced the camouflage effeciency of background matching between individuals with different coloration.Evaluated the camouflage effects of both box turtles species by human eyes,which provided a new perspective for the protection of dependent on microhabitat camouflage‘s endangered turtles,and also provided theoretical reference for wild release and ex situ conservation of turtles.The results of this study are of reference value for future studies on camouflage and protection of turtles.Innovation:1.To the best of our knowledge,this study is the first to investigate disruptive coloration in turtles;in vertebrates,in addition to the stripes or patterns,the body shape also play a key role in disruptive camouflage.2.Partial background matching and masquerade have been studied for the first time.3.This study comprehensively explores the effects of multiple camouflage strategies on anti-predation of turtles.