Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Male túngara frogs (Physalaemus pustulosus) give evoked vocal responses to a larger range of heterospecific and ancestral mating calls than females will approach during playback trials (Bernal et al. 2007).
Category: