Various mammalian taxa have evolved different morphological adaptations to aid in preventing death by predators, including spines, quills, dermal plates, and noxious chemical sprays. The development of these traits has previously been linked to intermediate body size, an openness of habitat, and smaller relative brain sizes, as well as a low metabolic rate and insectivorous diet. One family, Tenrecidae, contains several species that have evolved spines despite a small body size. We investigated the ecological factors that favored the evolution of spines within this group, focusing on conspicuousness to predators through body size and openness of habitat. We compiled hair and spine measurements along with natural history data and ran comparative phylogenetic analyses to study the morphological and ecological factors that favored the evolution of these antipredator defenses. We show that as tenrecs evolved a larger body size and moved into a more open habitat, they were more likely to evolve spines, and as more robust spines evolved, relative brain size decreased. We discuss the suite of changes that occur during the transition from non-spiny forest-dwelling tenrecs into larger spiny tenrecines with smaller brains and aposematic coloration.