| The keeled box turtle(Cuora mouhotii,CM)and Indochinese box turtle(C.galbinifrons,CG)are sympatric,but they reside in different microhabitats.The former mainly lives in a rocky microhabitat within forests and often hides in rock crevices.The latter mainly lives in a microhabitat with deciduous leaves and often hides underneath these leaves.Their morphology also displays many differences;the former’s carapace is flat with a serrated posterior margin,whereas the latter has a high carapace and no serrated posterior margin.The morphological structure of an animal determines its behavioral function and how it adapts to its habitat.Therefore,there is a certain correlation among animal morphology,resource utilization,and function.Although the ecomorphology of turtles has been extensively studied,there have been few studies on its function in relation to closing force,friction force,and mechanical pressure resistance.To reveal the relationship between morphology and closing force,friction force,and mechanical pressure resistance,the keeled box turtle and the Indochinese box turtle were used to compare the relationship between the morphology and the function of these two species and to clarify how their morphology affect their functions to adapt to different microhabitats.The results were as follows:Morphological differences between the two species:a total of 48 morphological indicators were measured in the keeled box turtle(n=38)and Indochinese box turtle(n=38).These included head length,head width,head height,neck length,neck width,carapace length,carapace width,carapace height,carapace curve length,carapace curve width,width from the first to the fifth vertebral scute,carapace circumference,width,and angle of four serrations,plastron length,plastron curve length,gular scute width,humeral scute width,hinge length,femoral scute width,anal scute width,bridge length,forelimb length,forelimb width,hindlimb length,foot length,foot width,claw length,claw width,tail length and body weight.The results showed that the head and neck lengths of the keeled box turtle were significantly greater than those of the Indochinese box turtle.The former lives in a rocky habitat,often moving between uneven rocks,and is prone to capsize.Therefore,a large head and long neck are useful for turning the body over.In addition,it is mainly carnivorous,with a large head and strong bite force.The Indochinese box turtle mainly eats plants,so its head is smaller than that of the keeled box turtle because it does not require as strong of a bite force.The claw length of the keeled box turtle was significantly greater than that of the Indochinese box turtle,but the claw width was significantly lower.This is because the keeled box turtle lives in rocky habitats that require greater claw power,whereas the latter lives in a litter habitat.The carapace length and second,third,and fourth vertebral shields of the keeled box turtle were significantly greater than those of the Indochinese box turtle,whereas the forward profile roundness,side-view profile roundness,and carapace height to carapace width(RHW value)were significantly lower.The length and curve of the plastron of the keeled box turtle were significantly greater than those of the Indochinese box turtle,and the humeral scute width,femoral scute width,and bridge length were significantly lower than those of the Indochinese box turtle.Relationship between morphology and closing ability of the two species:The morphological characteristics of the keeled box turtle(n=38)and the Indochinese box turtle(n=38)as well as the closing force and the closing time of prefrontal and posterior plastron were measured.The results showed that the anterior and posterior closing force of the two species were positively correlated with their carapace length:the longer the carapace,the greater the closing force(anterior closing force:CG,r~2=0.89,P<0.0001;CM,r~2=0.889,P<0.0001;posterior closing force:CG,r~2=0.695,P<0.0001;CM,r~2=0.701,P<0.0001).This indicates that the closing force of the two species is positively correlated with growth and increases with the growth of the individual.An analysis of covariance using the carapace length as the oblique variable showed that there was no difference in the closing force between the larvae and the subadults of the two species,but there was a significant difference between that of the subadults and the adults.There were significant differences in the closing force between the two species.The anterior and posterior closing forces(55.37±7.11 N and 41.46±5.96 N,respectively)of the Indochinese box turtle were significantly higher than those of the keeled box turtle(14.12±1.73 N and 12.79±1.76 N,respectively).The former lives in the litter and leaves,whereas the latter lives in a steep rocky habitat;therefore,the keeled box turtle has lower closing forces due to the protection of stone crevices.In addition,the average closing time of the anterior and the posterior plastron of the Indochinese box turtle was 226 s,which was significantly longer than that of the keeled box turtle(181 s).The difference between the closing forces and closing times of the two species was mainly related to their different closing structures.The results of traditional anatomy and CT(computer tomography)image analysis showed that the degree of closure of the two species varied,which was mainly related to the morphological differences of the closed structures.If the V-shaped tip on the inner side of the bridge provides enough space for the closure,it can be completely closed with the dorsal carapace,such as C.galbinifrons.However,owing to the short and thick tip of the bridge in C.mouhotii,it cannot be closed completely.To the best of our knowledge,this is the first study to explain the morphological reason for the different degrees of closure between complete and semi-closed shell turtles.Relationship between morphology and friction force of the two species:the morphological characteristics and friction force of the keeled box turtle(n=24)and Indochinese box turtle(n=24)were measured.The results showed that there was a correlation between the friction of the two species and the top of the carapace:the flatter the carapace,the greater the friction.There was also a correlation among the friction of the two species,the length and width of the carapace,and the width of the second,third,and fourth vertebral shields(CM:r~2=0.602,0.879,0.711,0.719,and 0.752,respectively,P<0.0001;CG:r~2=0.785,0.663,0.365,0.731,and 0.454,respectively,P<0.0001).There were significant differences in the frictions of the carapace of the two species.The friction force(0.54±0.05 N)of the carapace of the keeled box turtle was significantly greater than that of the Indochinese box turtle(0.45±0.06 N)because the length of the carapace and the width of the second,third,and fourth vertebral shields were significantly greater than those of the Indochinese box turtle,and the roundness was significantly reduced as compared to the latter,which indicated that the carapace of the keeled box turtle was relatively flat.It lives in stone crevices,and a flat carapace increases the contact surface with the top of the stone crevices,thus increasing the friction and reducing the risk of being pulled out of the stone crevices by predators.Correlation between resistance to mechanical pressure and morphology of the two species:the morphological characteristics and mechanical pressure resistance of the keeled box turtle(n=24)and Indochinese box turtle(n=24)were measured.The results showed that the resistance to mechanical pressure of the two species was correlated with the length of the carapace and the RHW value(the ratio of carapace height to carapace width)(CM:r~2=0.8208 and 0.3782,respectively,P<0.0001;CG:r~2=0.7327 and 0.6084,P<0.0001).The thickness of the carapace of the keeled box turtle was smaller than that of the Indochinese box turtle.There were significant differences in resistance to mechanical pressure of the two species.The resistance to mechanical pressure(2192.92±164.44 N)of the Indochinese box turtle was significantly higher than that of the keeled box turtle(1326.50±118.07 N),which was due to the domed and thick carapace of the former.Compared with the keeled box turtle,which hides in the cracks of solid rocks,the Indochinese box turtle lives in branches and leaves and is at a greater risk of exposure.Greater resistance to mechanical pressure can prevent the shell from being bitten by predators,thus protecting the head,internal organs,and limbs inside the shell.In conclusion,the morphological differences between the keeled box turtle and the Indochinese box turtle in the same region determined their different function(closing force,friction force,and resistance to mechanical pressure)to better adapt to their different microhabitats.This study revealed the morphological differences and its correlation with function in the two species,which are helpful for understanding the ecomorphological adaptation of the box turtle.Innovation1.This is the first study to explain the morphological reason for the different degrees of closure between complete and semi-closed shell turtles with the help of CT imaging analysis technology,.In the past,we only know the difference between the complete closed shell and the semi closed shell by observing the external performance,but we don’t understand its principle.2.The closing force of the box turtle was measured by bite force measuring instrument.The experimental method is innovative.3.A quantitative study on 48 morphological indexes of two species in the same region was carried out.In the past,only qualitative studies and conjectures have been made on the morphological causes of the differences in microhabitat selection in the same region. |
