Size matching between fruits and bill gapes differentially affects gulper and masher frugivorous birds

Abstract

Birds feeding on fruits (frugivores) affect seed dispersal, seedling establishment and,ultimately, plant assemblage composition. Thus, the mechanisms underlying fruitconsumption by birds are central to disentangling the assembly dynamics of mutualistic interactions and their effects on ecosystem functioning. In this regard, the sizematching hypothesis states that the bill gape width is an anatomical filter that allowsa bird to ingest a fruit. However, handling behaviour could allow birds to overcomeanatomical constraints that limit the consumption of certain fruit species. Here wetested how size matching affects consumption rates and probabilities of frugivores thatswallow the fruit unprocessed (gulpers) and those that mandibulate the fruits beforeingestion (mashers). Gulpers showed a weak augment in both the rate and probabilityof fruit consumption as bill-gape width increased relative to fruit diameter. On theother hand, mashers did not respond to size matching either in consumption rate orprobability. Altogether our results highlight that other factors besides size are affectingthe consumption rate and probability of frugivorous birds. Thus, gulpers and mashersfruit consumption could be subject to different mechanisms regardless size matching. In this regard, fruit traits and animal abilities interact leading to consumption bymashing or gulping the fruit highlighting the dynamic nature of handling behaviour.Altogether, our results warn about the unevaluated overutilization of size-matchingas the main mechanism to test the multiple facets of the ecology and evolution offrugivory and seed dispersal. Thus, including handling behaviour and other factorsinto the mechanisms underlying fruit–frugivore interactions could greatly improve ourunderstanding of mutualistic interactions.