Love bites: male frogs (Plectrohyla, Hylidae) use teeth scratching to deliver sodefrin precursor-like factors to females during amplexus

Abstract

Background Efficient transfer of chemical signals is important for successful mating in many animal species. Multiple evolutionary lineages of animals evolved direct sex pheromone transmission during traumatic mating—the wounding of the partner with specialized devices—which helps to avoid signal loss to the environment. Although such direct transmission modes of so-called allohormone pheromones are well-documented in invertebrates, they are considered rare in vertebrates. Males of several species of the frog genus Plectrohyla (Hylidae, Anura) have elongated teeth and develop swollen lips during the breeding season. Here we investigated the possibility that these structures are used to scratch the females’ skin and apply allohormone pheromones during traumatic mating in several Plectrohyla species. Results Our behavioural observations revealed that males press their upper jaw onto the females’ dorsum during amplexus, leaving small skin scratches with their teeth. Histological examinations of the males’ lips identified specialized mucus glands, resembling known amphibian pheromone glands. Whole-transcriptome sequencing of these breeding glands showed high expression of sodefrin precursor-like factor (SPF) proteins, which are known to have a pheromone function in multiple amphibian species. Conclusions Our study suggests SPF delivery via traumatic mating in several anuran species: the males have specialized breeding glands in the lips for production and secretion and use their elongated teeth as wounding devices for application. We hypothesize that these SPF proteins end up in the females’ circulatory system, where understanding their exact function will require further molecular, physiological and behavioural testing.

Recurrent Co-Option and Recombination of Cytokine and Three Finger Proteins in Multiple Reproductive Tissues Throughout Salamander Evolution

“…This passive broadcast of SPF pheromones contrasts with private transfer of SPF proteins when a male D. ocoee abrades the dorsum to a single female with his premaxillary teeth where pheromones are presumably diffused into the female bloodstream rather than stimulate the olfactory system ( Houck et al, 2007 ). Recently, an extremely similar system was discovered in Plectrohyla frogs where males scratch the female dorsum with their teeth delivering SPF expressed by glands in the upper lip ( Schulte et al, 2021 ). Despite the similarity in delivery system, the SPF pheromones of plethodontid salamanders and Plectrohyla frogs are independently derived from the SPFα and SPFβ families, respectively, that arose by gene duplication in a common ancestor of amphibians.…”

Gene expression of secretory proteins in the nuptial pads of three Lithobates species (Anura: Ranidae)

“…The 2D-TFPs found in the breeding glands of L. vaillanti fall into one clade with all SPF sequences extracted from other anuran breeding glands, which clusters with the beta-SPF clade -one of two salamander SPF clades with known SPF pheromone function (marked with alpha and beta). For further information see Bossuyt et al (2019) and Schulte et al (2021).…”

Love Hug—Functional Validation of Nuptial Pad-Secreted Pheromone in Anurans

“…Early studies suggested that in anurans, vocal communication was the primary means of interaction [2,3,15]. However, increasing evidence indicates that chemical communication also plays a significant role in the reproduction and courtship behaviors of frogs and toads [5,[16][17][18][19].…”