Chemical defenses in amphibians are a common antipredatory and antimicrobial strategy related to the presence of dermal glands that synthesize and store toxic or unpalatable substances. Glands are either distributed throughout the skin or aggregated in multiglandular structures, being the parotoids the most ubiquitous macrogland in toads of Bufonidae. Even though dermal glands begin to develop during late-larval stages, many species, including Rhinella arenarum, have immature glands by the end of metamorphosis, and their post-metamorphic growth is unknown. Herein, we compared the post-metamorphic development of parotoids and dorsal glands by histological and allometric studies in a size series of R. arenarum. Histological and histochemical studies to detect proteins, acidic glycoconjugates, and catecholamines, showed that both, parotoids and dorsal glands, acquire characteristics of adults in individuals larger than 50 mm; that is, a moment in which the cryptic coloration disappears. Parotoid height increased allometrically as a function of body size, whereas the size of small dorsal glands decreased with body size. The number of glands in the dorsum was not linearly related to body size, appearing to be an individual characteristic. Only adult specimens had intraepithelial granular glands in the duct of the largest glands of the parotoids. Since toxic secretions accumulate in the central glands of parotoids, allometric growth of parotoids may translate into greater protection from predators in the largest animals. Conversely, large glands in the dorsum, which produce a proteinaceous secretion of unknown function, grow isometrically to body size. Some characteristics, like intraepithelial glands in the ducts and basophilic glands in the dorsum, are limited to adults.

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Indirect development is considered the ancestral condition in anuran amphibians and includes a free-swimming larval stage called tadpole that grows and develops in temporal or permanent water bodies Naumann et al., 2021). Tadpoles differ greatly from their adult forms, showing a stunning diversity of habitats, morphology, and feeding behaviors .The metamorphosis of aquatic larvae into juvenile forms involves morphological, physiological, biochemical, and behavioral changes

Skin glands in amphibians are either distributed throughout the skin or aggregated in multiglandular structures such as the parotoids typical of most species of Bufonidae. Although many early divergent and derived bufonids lack a discrete parotoid in the postorbital–supratympanic (PoSt) region, they have a great macroscopic diversity in the skin morphology of this region. To understand the origin and evolution of this diversity, in particular of the parotoids, we describe the histomorphology of the skin of the PoSt and dorsal regions in 17 species of bufonids, with or without external evident parotoid, and compare it with previously published descriptions. The survey results in 27 characters that were optimized on a phylogenetic hypothesis of Bufonidae. Our results reveal that the PoSt region has a noteworthy morphological diversity of types of glands, spatial organization and differences in the secretion products. Some morphological characters represent putative synapomorphies of internal clades of Bufonidae and are related to the progressive differentiation towards defined structures (macroglands, parotoids). These morphological results, along with published information on the toxicity of the skin secretions and defensive behaviours in some representative species, allow us to infer possible relationships between these features.

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