Abstract :
[en] Amphibians are particularly sensitive to environmental water availability due to their particularities in physiology, mainly related to permeable tegument and ectothermy, to their water-dependent reproduction, and their reduced mobility compared to other vertebrates. However, amongst anurans, some fossorial species have adapted to xeric environments, either extremely dry or drastically seasonal and unpredictable in precipitation distribution. The present study aims at advancing the understanding of the complexity of life-history adaptations and behavioural strategies that allow fossorial anurans to survive in such demanding environments, through each particular stage in the life-cycle of the study organism (larvae, juveniles, adults, and finally reproduction and egg-laying). To test the research predictions, a seasonally dry tropical forest located in the Pacific Ecuador, Arenillas Ecological Reserve, was chosen as the study site. The climate at this location is characterized by severe seasonality in terms of precipitation regime, but with a minimal variation in temperature throughout the year. The project is focused on the Pacific horned frog Ceratophrys stolzmanni, the only fossorial anuran present at the study site.
The effects of water limitations were evident in the species starting with the first and most sensitive life-stage, the aquatic tadpoles. The larvae responded to both tested types of cues related to the danger of pond desiccation: an increase in tadpole density as well as a decrease in water level, by rapidly modifying their development and growth in response to the environmental conditions. When not experiencing water limitations, tadpoles were able to capitalize on favourable aquatic conditions by intensifying their growth rates, delaying their transition to terrestrial habitat and metamorphosing at large size. The size at metamorphosis was a particularly important parameter for the fitness of individuals, since the detrimental conditions experienced during larval development were carried-over to the juvenile stage, and froglets metamorphosing at a small size experienced higher mortality and achieved lesser performance in food acquiring and predator avoidance traits.
The activity of the horned frogs was strongly related to the precipitation pattern, individuals coming out of the ground mainly during or shortly after rains. This means that at the study site there are a limited number of opportunities for them to acquire food even during the short rainy season. However, the species showed intense growth rates both before and after metamorphosis, and both males and females were able to sexually mature in their first year of life. Some of the investigated behavioural strategies contribute to reduce their overall energetic expenditures: low individual mobility, the choice of burrows depth in accordance with substrate humidity and an abbreviated mating season. Additionally, to diversify the modalities of resource exploitation, the Pacific horned frogs employed some mechanisms that are rather unusual for most anurans. Amongst them, they exhibited a divergence in temporal niche between terrestrial life-stages, which probably reduces intraspecific interactions and allows increased access to food for the smaller juveniles. The other mechanism refers to the capacity to consume a wide variety of prey items, including vertebrates.
The intense growth rates and rapid maturation probably come at a cost of increased mortality, reflected in the low longevity of individuals from the studied population. In this aspect, the permanence of the species in such an unpredictable environment suggests that mass tadpole mortality caused by pond desiccation is rather uncommon. This is due at least partially to the synchronized reproduction that takes place at the first heavy storm of the rainy season, as soon as breeding pond become available, giving the tadpoles the longest interval possible to develop. Additionally, the female investment in a relatively large number of eggs and the overall short larval duration should contribute to successful yearly recruitment.
Our study enhances the understanding of this poorly-known species and of the potential risks to its long-term survival. Due to the strong influence of rainfall regime on all life-stages, the predicted changes in precipitation patterns are likely to lead to further restriction of the activity of the frogs and to reductions of recruitment or juvenile fitness. Additionally, the short reproductive lifespan of the species makes the persistence of the population vulnerable to persistent droughts of 2–3 consecutive years. The population parameters that we provide in this thesis serve as base-line information for the present welfare of the population, and can be a starting point for the detection of potential declines. This knowledge should be included in future management plans for the effective conservation of the species.
