RESEARCH: MATING SYSTEM AND REPRODUCTIVE BEHAVIOR OF NYCTIBATRACHUS WRINKLED FROGS
Wrinkled frogs (genus Nyctibatrachus) derive their name from the unusually wrinkled skin on their backs. There are over a dozen described species, all endemic to the evergreen forests of the Western Ghats (southwestern India), and many of them are endanged due to continuing loss of habitat. Until recently, nothing was known about their biology but since 1997 many fascinating facts about their mating system and behavior have been uncovered, mainly from our observations on N. petraeus, a species that Krushnamegh Kunte discovered at Castle Rock, northern Karnataka, in 1997 and described as a new species with Indraneil Das in 2005.
Nyctibatrachus petraeus is a prolonged breeder and reproduces for several months during the monsoon. This is unlike bull frogs and common toads, which are "explosive breeders" that form very large reproductive congregations and finish courting, mating and egg-laying in a matter of days at the beginning of the monsoon. Male N. petraeus establish resource-rich territories at the beginning of the monsoon, each male guarding, on average, two metres of a running stream in evergreen forests. Although they are mostly aquatic, they perch on vegetation overhanging flowing water and transmit their melodious sexual advertisement calls (yes, melodious!, from a frog) out in the night to attract females. Nightly, females travel through the territories of several males, and visit them up on the vegetation when they want to lay eggs. Again, unlike bull frogs and common toads that lay thousands of very small eggs, female N. petraeus lay very small egg-clutches (usually less than 30 eggs per clutch) of very large eggs, thus their investment per egg is unusually high. Males call from spots where they want the females to lay eggs, so males decide where the eggs are laid. When a gravid female approaches, the male moves a few centimetres away from its calling post and continues to call from the new spot. The female lays eggs exactly at the spot from where the male was previously calling. Over many nights, males attract several females to lay eggs in their territory, and females lay several clutches over the breeding season in several males' territories.
Perhaps the most remarkable feature of the reproductive behavior of N. petraeus is its absence of amplexus. Amplexus is near-universal in anurans (generally, frogs and toads) and only a few exceptions (e.g., some poison-dart frogs, Family Dendrobatidae) are known in which amplexus is absent. In N. petraeus, however, males move over when gravid females visit them and lay eggs, and then move back to the spot where the females laid eggs and then fertilize the eggs. There is usually no physical contact between the pair. It turned out that N. petraeus was the first known Old World anuran in which amplexus is completely absent.
Another remarkable feature of its egg-laying is where the eggs are laid. Usually, frogs and toads lay eggs either in water (as in most familiar frogs and toads), or on vegetation inside foam nests that the female and male make together as they are mating and laying eggs. In Nyctibatrachus, however, eggs are laid bare on vegetation. Males presumably guard eggs at night, but they retreat to their resting places during the day. When tadpoles have finished developing inside the eggs, they wriggle out of the egg-jelly and fall in the water below, where they develop further. They may also wriggle out of the egg-jelly prematurely if they are disturbed by predators during development. This is a smart anti-predator strategy, first discovered and studied in detail in the Central American red-eyed tree frogs (Agalychnis callidryas) by Karen Warkentin.
This is fascinating, mainly because the combination of reproductive behaviors that make up N. petraeus's mating system – (a) absence of amplexus, (b) resource-based male territoriality, (c) very small clutch size, (d) multiple mating by both males and females, (e) oviposition on vegetation outside water, without a foam nest, and (f) plastic development time that is sensitive to risk of predation – is unique among anurans. Interestingly, each of these behaviors is uncommon among anurans. Also, these behaviors show similarities with the Neotropical poison-dart frogs (Dendrobatidae), glass frogs (Centrolenidae) and red-eyed tree frogs (Agalychnis spp). This unique mix of characters and intriguing similarities with Neotropical frogs make it an attractive behavioral study system.
Kunte, K. 2004. Natural history and reproductive behaviour of Nyctibatrachus cf. humayuni (Family Ranidae: Anura). Herpetological Review, 35:137–140. PDF file (112KB).
Das, I. and K. Kunte. 2005. New species of Nyctibatrachus (Anura: Ranidae) from Castle Rock, Karnataka State, Southwest India. Journal of Herpetology, 39:465-470. PDF file (176KB).