Function and development of male nuptial pads in frogs: from the better-grip-theory to the transfer of sex pheromones

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Male secondary sexual characters play an important role during reproduction in many animal species. In frogs, one of the most common characteristics that can only be found in males in breeding condition are so-called nuptial pads. These pads can be found on the first digits of the hand (therefore also called thumb pads) and are often covered in dark, cornified, spiny epidermal projections (EPs). Because of these EPs, it is frequently suggested that the nuptial pads develop to give the male a better grip when clutching the female during amplexus – especially when mating in water. However, many species do not develop such EPs, but have smooth, light nuptial pads. While the epidermal structures (i.e. EPs vs. smooth) vary substantially between species, the underlying dermal structures always contain glands, that resemble amphibian specialized breeding glands shown to be involved in protein pheromone or adhesive protein production. This suggests that the breeding glands may be involved in chemical communication or in adhesion of the male during amplexus. Glands and EPs of the nuptial pads may therefore have different functions during anuran reproduction. In the proposed study we aim to investigate both, the potential roles of the glands and their secreted content, as well as the association between the EPs and the breeding ecology of the frogs. For this we suggest three work packages: WP1 is a large-scale theoretical approach that compares the nuptial pads of a phylogenetically wide range of anuran species, and systematically reviews and analyses the relationship between their outer morphology and their ecological and life history traits related to mating and reproduction. With this WP we aim to learn more about the ecological and evolutionary causes regarding the development of large, dark, and spiny nuptial pads. In WP2 we focus more on the potential production of pheromones and adhesive proteins in the nuptial pads and aim to find potential patterns among species. For this, we will sample over 40 frog species (from different families, in temperate and tropical zones) and analyse and cluster the gene expression patterns of their nuptial pads. Finally, in WP3, we will use a developmental approach. Using a smaller set of species from temperate zones, we will compare how the nuptial pads change morphologically as well as how their gene expression and co-expression networks vary between breeding versus non-breeding season. With this, we intend to advance our understanding of which structures and genes are especially important for reproduction, e.g., being involved in the dark colouration, the maintenance of glandular tissue or the production of pheromones and adhesives. Altogether, we are confident that our study will drastically improve our understanding of the reproductive significance of both, the outer nuptial pad morphology as well as the breeding glands and the herein produced proteins and secretions.