The mechanism that facilitates the evolution of maternal care is ambiguous in egg-laying terrestrial vertebrates: does the ability of mothers to recognize their own eggs lead them under some circumstances to begin providing care or can maternal care evolve from simply being in close proximity to the eggs (e.g. through territorial behaviour)? This question is difficult to answer because in most species, parental care is either absent altogether or present; in only a few species we have the opportunity to observe intraspecific variation in the expression of parental care. We studied a population of long-tailed skinks (Eutropis longicaudata) in which females have recently evolved maternal care from a noncaring state. Females on Orchid Island, Taiwan, remain with their eggs during incubation and when doing so, actively deter egg predation by egg-eating snakes (Oligodon formosanus); in all other populations, females lack post-ovipositional maternal care. Nest-guarding females on Orchid Island (i) showed antipredator behaviours only in the original nest site in which they laid eggs, even after we removed all of the eggs or substituted them with those of a conspecific; (ii) protect any eggs present inside the original nest site (even when the eggs belong to a conspecific); and (iii) develop this behaviour while gravid (i.e. prior to laying eggs). This supports the hypothesis that long-tailed skinks cannot recognize their own eggs, suggesting that maternal care is a directed form of territoriality only expressed towards egg-eating snakes and only during reproduction. Nest guarding is among the most primitive forms of parental care, and the recent evolution of this behaviour in a single population provides insight into one of the mechanisms by which parental care can originate in terrestrial vertebrates.