The ability to use small molecule alkaloids as defensive chemicals, often acquired via trophic interactions, has evolved in many organisms. Animals with diet-derived defences must balance food choices to maintain their defence reservoirs along with other physiological needs. Poison frogs accumulate skin alkaloids from their arthropod diet, but whether they show selectivity for specific prey remains unexplored. We investigated the role of leaf litter prey availability and dietary selectivity in shaping poison frog chemical defences along a geographic gradient. Specifically, we examined skin alkaloid composition, stomach contents and leaf litter ants in aposematic diablito frogs (Oophaga sylvatica) at five sites in north-western Ecuador and in sympatric, cryptic Chimbo rocket frogs (Hyloxalus infraguttatus) at one site. Our results show that differential availability of leaf litter ants influenced alkaloid profiles across diablito populations, and low levels of alkaloids were observed in the sympatric, ‘undefended’ Chimbo rocket frog. Ants were the primary dietary component of the defended species, while the ‘undefended’ species ate other prey categories including beetles and larvae in addition to ants. A prey selection analysis suggested that defended and ‘undefended' frogs both feed on a high proportion of specific small ant genera that naturally contain alkaloids, suggesting that selectivity for toxic prey is not restricted to classically aposematic and highly toxic species. These findings suggest that poison frogs’ use of feeding resources relative to availability may be an understudied and important selection factor in the evolution of acquired defences
