Evolution of treefrogs' calls in tropical islands might be under directional selection

Animals that present migration restrictions are especially interesting to study isolation effects in the context of island biogeography theory. Among these effects, behavioral traits are one of the least studied subjects. The treefrog Boana albomarginata presents gigantism in some Atlantic islands and, therefore, examining its advertisement calls may be an opportunity to test the Island Rule with a behavioral trait. Hence, we compared calls, body and tympanum sizes of populations of this species in Brazilian islands and mainland sites. We tested if call frequencies of island individuals differed from mainland ones, also considering the influence of body size and its relationship with tympanum diameter. One of the island population had larger body size and lower call frequencies than any other populations. We suggest that to be a result of directional selection. We also observed lack of correlation between body size and advertisement call frequencies in some populations, implying that males and females cannot use calls as a proxy of the calling male's size, which may indicate a rare case of relaxed selection. Therefore, this is the first evidence of relaxed selection in animal communication, as a consequence of recent insularity. Frequency of advertisement calls of island treefrogs does not present the expected negative relation with body size. Also, this frequency is much lower than expected by its size. Probably it represents an effect of directional and relaxed selection, respectively, that could explain the gigantism in island populations.