Of some 40 valid Phrynocephalus species currently recognized,six (or seven) are viviparous and endemic to China.All viviparous Phrynocephalus species except R forsythii occur at elevations higher than 2500 m ASL where no oviparous Phrynocephalus species can be found.Why are high altitude Phrynocephalus lizards (Agamidae) viviparous? To answer this question,we monitored four natural nests used by reproductive female P.vlangalii to estimate air and nest temperatures experienced by lizards in May,July and September,determined the lower thermal limit at which embryonic development could take place,and examined the effects of gestation temperature on female reproductive traits and offspring phenotypes.Females ready to ovulate were collected in May and maintained in one of the six thermal regimes,of which five allowed to control body temperatures at 24,28,29,32 and 35 ℃,respectively,and the remaining one (TR treatment) allowed to thermoregulate for 12 h daily.We also collected females (NAT treatment) ready to give birth between mid-July and early August.An independent sample of pregnant females,nonpregnant females and adult males collected in mid-July 2007 was used to examine whether females shift their thermal preference when pregnant.Air temperature varied from -4.3-63.7 ℃ in May,2.2-53.9 ℃ in July,and 1.1—42.0 ℃ in September; nest temperature varied from 9.1-12.1 ℃ in May,13.8-16.5 ℃ in July,and 10.7-13.1 ℃ in September.On a daily basis air temperature varied among the 24 time points but not among the four nests,and nest temperature varied among the four nests but not among the 24 time points.Pregnant females,nonpregnant females and adult males did not differ from each other in thermal preference,and the overall mean of body temperatures selected by them was 34.5.C.100% of the females at 24 ℃ and 92.9% of the females at 28 ℃ failed to give birth.Between 64.3% and 100% of the females in the other five treatments gave birth.Morphologically,neonates produced by the NAT and TR females differed from those produced by the females at 29 ℃ and 32 ℃,whereas neonates produced by the females at 35 ℃ did not differ from those born in the other treatments.Neonates born in the five treatments differed in sprint speed,with the fastest neonates produced in the NAT treatment and slowest neonates produced in the 29 ℃ treatment.Neonates produced by the females at 29 ℃ grew more rapidly than did those born in the other four treatments accounting for mass at birth.Our data show that embryos cannot develop in nature without maternal thermoregulation.This explains why high altitude Phrynocephalus lizards viviparous.Our data also show that gestation temperature affects not only the rates of embryonic development but also a number of reproductive and offspring traits.