Classification and Biogeography
The Trinidadian leaf frog (Phyllomedusa trinitatis) is a neotropical tree frog belonging in the Family Phyllomedusidae. The family currently includes over 60 species in eight genera, notably Agalychnis (including the poster species, the red-eyed tree frog) and Phyllomedusa (16 species). Phyllomedusa are commonly known as leaf frogs (because of their bright green colour) or monkey frogs (because of the way they perch and grasp twigs with their long fingers and toes). Members of the family are found throughout the tropical forests of Central and South America. P. trinitatis are distributed across the forests along the northern coast of Venezuela and throughout the island of Trinidad. This distribution suggests that the species has changed very little since Trinidad split from the South American mainland. Tobago, on the other hand, was created through volcanic activity, and is not home to any P. trinitatis individuals.
P. trinitatis are sized between 50 and 90 cm, with females larger (roughly 1.5x) than males. The species shows a mostly nocturnal activity pattern, which allows escape from unfavourable hot and dry conditions present during the dry season in tropical regions. They have been seen to feed on insects present in tropical forests, with a particular penchant for grasshoppers. However, the general ecology of the species is relatively understudied, with not much more known. We made a start by using radio-trackers and thread bobbin tracking to follow where the frogs go after leaving the breeding site. Females moved more than males, with some climbing high into the trees, and others staying at lower levels. The majority of studies on the species focus on breeding ecology. All Phyllomedusa species lay egg clutches on leaves, usually above standing water, and cover the eggs by folding the leaves around them. In contrast, the egg clutches of Agalychnis are laid on leaves, but not covered.
Courtship and Mate Selection
During the wet season in Trinidad, typically starting with rains in early June and ending in November, individuals begin to gather around suitable bodies of still water at sunset. Higher densities of frogs are seen on nights showing higher levels of humidity, mostly due to rains from the preceding day. Sunset occurs at roughly 6.30pm in Trinidad, with little variation throughout the year due to the island’s location close to the Equator. Male individuals are the first to descend from the trees, taking up positions within the vegetation surrounding a pond.
From these perches, males begin emitting advertisement calls to entice females and demonstrate their superiority over other males present. Calls of a lower frequency have been associated with a greater body condition. The calls of P. trinitatis individuals are relatively quiet in comparison to other anuran species present in the forests of Trinidad, and can be drowned out by the raucous sounds of other species. An interesting and unexplored question is whether the calls of other species interfere with Phyllomedusa mating behaviour. Some frogs are explosive breeders, with large numbers gathering at a breeding site over a day or two, mating, then dispersing. Phyllomedusa does not show this pattern. Variable numbers of individuals appear at breeding sites over a period of weeks, even months. Some males appear to adopt a strategy of attending the breeding site frequently, others only occasionally. There may be a trade-off here: since the attendance of fertile females is unpredictable, frequent attendance increases the chance of mating opportunities. On the other hand, frequent attendance may be costly, through reduced opportunities to feed and increased chances of predation. Studying site attendance patterns requires the ability to recognise individuals. We found that Phyllomedusa have individually variable ventral patterns of ‘islands’ and spots, which allow the construction of a photo-catalogue of all the individuals in an area. Fortunately, these are quite docile frogs which are not fazed by being picked up to photograph their belly patterns!
Fertile females arrive after hearing males calling and select a mate from the males on display. The females are weighed down with a clutch of eggs ready to lay. They are commonly outnumbered, which can produce a high level of competition between males, resulting in conflict behaviours including scramble competition where several males crawl over each other and wrestle to achieve amplexus with a female.
Once amplexus is achieved a period of inactivity takes place, presumably for the mating pair to gain familiarity and for the female to judge the suitability of the male. After courtship is completed a mating pair will move to find an appropriate leaf on which to lay the clutch and construct a nest. The leaf selected is either overhanging still water or in close proximity to it, and broad enough to support the weight of an egg clutch (although in locations lacking broad leaves, they are able to construct nests from a number of narrower leaves, even grass stems). The pair remains in amplexus while constructing the leaf nest, using their back legs to hold the edges of the leaf together in a narrow cone shape while the female lays the egg mass which is covered in adhesive jelly to secure the nest structure. Nest construction can last anywhere between 30 and 90 minutes, after which the male departs, leaving the female to hold the leaf in place while the adhesive compounds set to cement the nest structure.
The egg mass within also contains eggless capsules which provide hydration for the developing eggs (since the sealed nest does not allow the entry of rainwater). Hatching tends to take place around 8 days after nest construction, with tadpoles falling out of the bottom end of nest into the water below where they develop until metamorphosis. The development time between tadpole and froglet is roughly 3 weeks in P. trinitatis, during which juveniles grow to over five times their original size. Upon exiting the pond froglets ascend into the surrounding vegetation where the adolescent tail is fully reabsorbed, metamorphosis is completed, and life as an adult frog begins.
Downie, J.R. et al. (2013). Nest structure, incubation and hatching in the Trinidadian leaf-frog Phyllomedusa trinitatis. Phyllomedusa 11, 13-32.
Gourevitch, E.H.Z. and Downie, J.R. (2018). Evaluation of tree frog tracking methods using the Trinidad monkey frog Phyllomedusa trinitatis. Phyllomedusa17, 233-246.
Smith, J.M. et al. (2019). Stable individual variation in ventral spotting patterns in Phyllomedusa trinitatis and other Phyllomedusa species: a minimally invasive method for recognising individuals. Phyllomedusa 18, 13-26.
IUCN (International Union for Conservation of Nature), Conservation International & NatureServe. 2008. Phyllomedusa trinitatis. The IUCN Red List of Threatened Species. Version 2020-2 (https://www.iucnredlist.org/species/55867/11382870; accessed on 8/7/2020)
Authors Cameron Boyle and Roger Downie, University of Glasgow