Research On Skull Geographical Variation Of Meriones Meridianus (Rodentia:Cricetidae) By Geometric Morphometric Methods

Midday gerbil(Meriones meridianus Pallas,1773)is a wide spread species in arid and semi-arid deserts of Eurasia,which belongs to Rodentia,Cricetidae,Gerbillinae,Meriones.It is distributed in Mongolia,Afghanistan,Iran,Caucasian region,and the following provinces in China: Xinjiang,Gansu,Qinghai,Shaanxi,Shanxi,Hebei,Inner Mongolia,and Ningxia.As to its vertical distribution,the elevation ranges from-150 meters of Turpan basin to 3200 meters of Karakoram Mountains' north slope.Both wide distributive range and complex and changeable environments show its strong adaptability.So it is an ideal model to study adaptation.By using geometric morphometric methods and regarding the skull form as standard,we investigated the pattern of midday gerbil geographical distribution in China and its relationship with environmental factors,which combines Procrustes ANOVA,Mantel test,multivariate regression analysis together.The main results are as follows:There exists significant differences among midday gerbil groups' skull form in China(p <0.001),and its the geographical distribution in China shows a north to south pattern,which was separated by Tianshan Mountains.For the skull size,individuals of the south calde are lager than the north,which is consistent with Bergmann's rule.This is because the average temperature of high altitude environment they live in is lower,and the small specific area contributes to body heat conservation.As to the skull shape,there exists significant differences in the nasal,auditory bulla,rostrum,procumbent incisor,upper tooth row,zygomatic arch,temporal fossa among the individuals of the two clades.Changes of above structures shows that:1.The volume of nasal nasal cavity is lager,which means more efficient gas exchange between the organism and external environment,and less stimulation of arid air to the respiratory system.2.Auditory bulla is smaller,which indicates its weaker sensitivity,mainly because hypoxia in the high altitude environment.But the length of external ear in this clade is longer and this also is a compensation for decrease of auditory bulla volume.3.The rostrum is wider,making the organism able to disperse more torsional stress from the prey and preventing the mandible's dislocation when volume of prey is too large;longer zygomatic arch means larger temporal fossa and stronger temporalis,and finally greater occlusal force;procumbent incisor indicates that it's not mainly used to bite but to fix the prey;slightly longer upper tooth row reflects more sufficient grinds for food.The diet of midday gerbil mainly includes insects,reptiles,and plants' stem,leaves and seeds.According to the form researches of high altitude insects and plants,we can infer that individuals of north clade groups prey on bigger insects and smaller seeds,and that the plant cytoderm is thicker.Due to the diffrences of climate,the diet of midday gerbil in China has differentiated.So the conclusion of my research is as follows:Regarding the skull form as standard,the geographical distribution of midday gerbil in China shows a north to south pattern,which was separated by Tianshan Mountains.During the long period of evolution and adaptation,the changes of climate,particular the average temperature and vapour pressure(p < 0.0001),led to the midday gerbil's differentiation in body size,sensory functions,diets,which finally led to skull form changes.