Association of Male Inguinal Patch Size with Mate Assortment in the Kihansi Spray Toad, Nectophrynoides asperginis
Nectophrynoides asperginis (Bufonidae) is a small toad endemic to the spray zone of the Kihansi River waterfall in Tanzania, Africa. This species was declared extinct-in-the-wild by the International Union for Conservation of Nature (IUCN) following construction of a hydroelectric dam that reduced the flow of the waterfall. The species is now limited to ex situ populations at the Wildlife Conservation Society's (WCS) Bronx Zoo, New York, the Toledo Zoo, Ohio, and two newer facilities in Tanzania. Much of the natural history of N. asperginis remains unknown, though it is known they are a sexually dimorphic species. Typically, males are smaller than females and possess distinctive dorsolateral inguinal patches (IP). Males appear to display these patches to conspecifics, but their function remains unknown. We created a five-grade schema to categorize the IPs by visual prominence accounting for both patch size and color, with Grade 1 being the least and Grade 5 being the most prominent. We hypothesized that a positive association exists between male IP grade and the snout–urostyle length (SUL) of females they amplexed. Therefore, we measured SUL and body mass in 91 pairs of amplexing N. asperginis and assessed the relationship of IP grade to female SUL in the amplexed pairs. Statistical analysis revealed a significant positive correlation (Spearman's rho = 0.250, P = 0.017) between male IP grade and the SUL of females being amplexed. This study suggests that IP grade is associated with mate assortment in N. asperginis, and it represents one of the first glimpses into the complex social interactions of this extinct-in-the-wild amphibian.Abstract
Examples of the five IP grade categories described by Shuter et al. during this project, with lengths of inguinal patches delineated by blue bars. (A) Grade 1 = 2–6 mm, absence of dark pigmentation; (B) Grade 2 = 2–6 mm, small spots of dark pigmentation visible; (C) Grade 3 = 3–7 mm, with less than 30% of the patch made up of dark pigmentation; (D) Grade 4 = 5–7 mm, with more than 30% of the patch made up of dark pigmentation; and (E) Grade 5 = 6–9 mm, solid dark pigmentation.
Mean SUL of females amplexed by males belonging to the different IP categories.
Mean amplexed female SUL as a function of male IP grade, with male SUL segregated into tertiles (1 = smallest body length, 3 = largest body length). The IP grades 1+2 were defined as “less visually prominent patch” and 4+5 were defined as “more visually prominent patch.”
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