Sidewinders (Crotalus cerastes) and Saharan horned vipers (Cerastes cerastes) have evolved to hunt desert rodents on different continents in evolutionarily independent communities. These species are remarkably convergent, except that sidewinders possess heat-sensitive pit organs that enable them to “see” in the dark. As a constraintbreaking adaptation, this may give sidewinders an advantage when hunting in the dark. How will introducing a novel predator with a constraint-breaking adaptation affect the local species? We allowed Saharan horned vipers to hunt Allenby's gerbils (Gerbillus andersoni allenbyi) in patches with or without sidewinders at full and new moon. When horned vipers hunted alone, moonlight did not affect their foraging behavior. However, in the presence of sidewinders, horned vipers increased their activity on bright nights, but dramatically decreased it on dark nights. Although gerbils foraged equally when hunted by either snake, the combined effect of the 2 predators synergistically decreased gerbil foraging, especially during full moon when both snakes were most active. Thus, sidewinders facilitated horned vipers in full moon, but interfered on darker nights when possessing pit organs were most advantageous for sidewinders. Gerbils quickly learned and adjusted their behavior to manage risks from the novel predators, but the combined effects of both local and novel predators may prove detrimental in the long run. Comparing convergent species that differ in a constraint-breaking adaptation allows us to study the effectiveness of these key adaptations and their potential roles in biological invasions.