Which androgens specifically induce nuptial pad formation in frogs

While testosterone is the primary androgen produced by the testes, the most potent hormone directly responsible for inducing the intense cellular changes in nuptial pads is dihydrotestosterone (DHT).

Here is the specific pathway:

1. The testes produce testosterone, which circulates throughout the body.
2. In the target skin cells of the thumb pads, an enzyme called 5α-reductase converts testosterone into DHT.
3. DHT binds to androgen receptors in the cells with much higher affinity and stability than testosterone. This powerful binding is what drives the rapid cell division, thickening of the skin, and formation of keratinized spines that characterize the fully developed nuptial pad.

Experimental injections of DHT have been shown to induce pad formation in male frogs even outside of the breeding season.