DOI: 10.5061/dryad.3tx95x6ds
title: Environmental and morphological constraints interact to drive the evolution of communication signals in frogs

description: Animals show a rich diversity of signals and displays. Among the many
 selective forces driving the evolution of communication between
 individuals, one widely recognized factor is the structure of the
 environment in which signals are produced, transmitted and received. In
 particular, animals communicating by sounds often emit acoustic signals
 from specific locations, such as high up in the air, from the ground or in
 the water. The properties of these different display sites will impose
 different constraints on sound production and transmission and may
 therefore drive signal evolution. Here, we used comparative phylogenetic
 analyses to assess the relationship between the display site properties
 and the structure of reproductive calls from 161 frog species from the
 frog families Ranidae, Leptodactylidae and Hylidae. Specifically, we
 compared the dominant frequency of species that vocalize from aquatic
 versus non-aquatic sites, and its relation with body size. We found that
 the dominant frequency of frogs calling from the water was lower than that
 of species calling outside of the water, a trend that was consistent
 across the three families studied. Furthermore, phylogenetic path analysis
 revealed that the call site had both direct and indirect effects on the
 dominant frequency. Indirect effects were mediated by call site
 influencing male body size, which in turn was negatively associated to
 call dominant frequency. Our results suggest that properties of display
 sites can drive signal evolution, most likely through morphological
 constraints, in particular the ones imposed on the sound production
 mechanism. Also, variation in body size between calling sites explained
 some of the differences we found in call frequency, highlighting the
 relevance of the interplay between morphological adaptation and signal
 evolution. Changes of display site may therefore have important
 evolutionary consequences, as it may influence sexual selection processes
 and ultimately may even promote speciation.
description: For each family, we collected data on the snout–vent
 length, dominant frequency and calling site. Most of the information
 was obtained from the literature or other digital sources (see
 below). Personal measurements made by the authors of the present
 article were also included. If searched in the literature, body size and
 call dominant frequency were obtained from other comparatives studies and
 books. We restricted our search to body size of males and the dominant
 frequency of advertisement vocalizations. Information on calling sites was
 obtained mainly from verbal descriptions of frog vocal behaviour present
 in the literature, the specialized website AmphibiaWeb, and from the
 personal experience of the authors. Multimedia information available from
 AmphibiaWeb and YouTube, such as pictures and videos of calling males, was
 used to confirm ambiguous verbal descriptions. For a few species (19 of
 175 species), vmultimedia information was used as the sole criterion for
 calling site assignment. Each species was assigned to one of three
 possible calling site categories: (1) aquatic, (2) nonaquatic and (3)
 mixed. Aquatic species included frogs that vocalize either standing in
 water, or floating on the water surface. The nonaquatic category included
 species that call from the ground, or from perched positions on trees
 or rocks without direct contact with water. Species calling from
 cavities dug in the ground or cavity-like structures on vegetation (e.g.
 the axils of bromeliads) were also included in the nonaquatic
 category. The few species for which both aquatic and nonaquatic calling
 was described were assigned to the mixed category.