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The Saharan sand viper, Cerastes vipera, and the crowned leafnose, Lytorhynchus diadema, are small coexisting psammophilic desert snake species, consume primarily lizards and do not engage in male to male combat. They differ in that C vipera is ovoviparous and L diadema is oviparous, C vipera moves by sidewinding while L. diadema uses serpentine movement and C vipera is a sit-and-wait ambusher while L. diadema is an active hunter. Results showed that C vipera was heavier than L. diadema per unit snout-vent length (SVL). We concluded that this was related to foraging, as L diadema had to move quicker than L diadema, and to reproduction, as C vipera needed more abdominal space for developing embryos. Sexual size dimorphism was found in adults in both species and it appeared that ecological advantages took precedence over phylogenetic affinity in determining size differences. The relatively small size, ovoviparity, sandy habitat with sidewinding, and lack of male to male combat for C. vipera whereas oviparity and sandy habitat for L diadema may explain partly sexual size dimorphism in these species. Females showed seasonal fluctuations in BCI in both species but males did not and C vipera females showed greater fluctuations than L diadema females. We concluded that the former resulted from higher energy costs of females than males and the latter because ovoviparity is costlier than oviparity. (C) 2014 Elsevier Ltd. All rights reserved.