Better than mere attraction - adhesive properties of skin secretion in the Common Rain Frog, Breviceps adspersus

Frogs of the genus Breviceps, distributed in Eastern and Southern Africa, have short arms, a rounded body shape, and show significant sexual dimorphism in body size. Consequently, the much smaller male is unable to effectively clasp the female during amplexus. Instead, mating pairs are bonded together by an adhesive skin secretion. While both sexes produce the sticky secretion in defence, it is not clear whether the male or female frog, or both, produce the glue during amplexus. Furthermore, there is no basic information on the physical properties of the adhesive secretion in Breviceps, such as its adhesive strength, differences in adhesive strength between the sexes, and adhesion duration. In this study, we used an electrical stimulator to induce the release of the adhesive secretion and demonstrated that it is produced by both male and female Breviceps adspersus individuals. Additionally, the adhesive strength of the frog secretion was measured for different adhesion durations. The results showed that the adhesive strength was maximal at 1-3 h (median: 8.12 N/cm(2) for females at 1 h duration and 7.20 N/cm(2) for males at 3 h), decreased significantly after one day (0.85 N/cm(2) for females at 24 h and 0.41 N/cm(2) for males) and almost disappeared after three days. A comparison of adhesive strength between the sexes showed few statistically significant differences, suggesting that both male and female secrete the same glue substance. The question of the precise origin of the adhesive secretion during amplexus is discussed.