Amphibian gland secretions specialized for defence, social communication and reproduction

Glandular secretions of the Amphibia are considered according to their possible role as signal molecules relevant for survival strategies or social communication. Actually, the antipredatory arsenal of these vertebrates consists of poisons released from skin glands, serous in nature, randomly distributed on the body, or localized in “critical” skin regions, as the parotoid ones of some toads and frogs. According to the species, the activity of these glands produces different bioactive substances aging as repellent, alarm substance or “venom” with peculiar toxicity and pharmacological actions. Besides toxin secretions, glue-like products from mucous glands may serve as antipredatory devices. In addition, odours produced by other cutaneous glands, possibly mucous in nature, are involved in a variety of social interactions. These signal molecules, usually named “pheromones”, may be used to locate conspecifics and identificate their status, sex and reproductive condition. Competitive interactions often include the establishment and defense of a territory, and pheromones are critical to give advertisement of territorial boundaries. Later, production of courtship pheromones, more common in the urodeles than in the anurans, increases the male’s chances to obtain female receptivity and, thus, to realize insemination or fertilization. Cutaneous glands may also produce “glue” for clasping pairs. In some frogs and toads, males possess thumb nuptial pads or ventral glands that cause them to adhere to the female during amplexus. Finally, secretions from integumentary components play a critical role in some parental care adaptations, as in the case of incubation and/or transport of eggs and embryos on the back skin of a nurse adult.