Adult-Juvenile interactions and temporal niche partitioning between life-stages in a tropical amphibian

Abstract

Divergence in ecological niche offers organisms the opportunity of exploiting different food and habitat resources, scaling down competition and predation both among species, and within different age or size-classes of the same species. In harsh environments, where abiotic factors determine a clustering of resources during short timespans, competition and predation between organisms is likely to be enhanced. This is the case in tropical dry forests, where amphibians have limited opportunities to feed, their activity being restricted to the short rainy season. One way to maximize resource exploitation while avoiding predation risk is by adopting different diel activity patterns. We tested this hypothesis by comparing activity patterns in adults and recently metamorphosed juveniles of Pacific horned frogs (Ceratophrys stolzmanni) during field surveys and in an experimental study. Field surveys showed that the adults are strictly nocturnal, whereas freshly metamorphosed juveniles can be found active above ground at all hours, with a peak activity during daytime. The average body condition index of juveniles found active during the night was higher than that of juveniles found active during the day, suggesting that the weaker individuals may be constrained to being active during the day. On the other hand, in a laboratory experiment, juveniles that were visually exposed to adults moved less than those in the absence of adults. Both field and experimental observations indicate a temporal niche divergence between life stages. The results of the experiment offer support to the hypothesis that the juveniles in this species display an inverse activity pattern compared to adults, which can reduce competitive interactions and predation pressure from the larger conspecifics.

Fig 1. Forest trail at the study site (Arenillas Ecological reserve, Ecuador).

Fig 2

Influence of adult presence on juvenile activity: (a) examples of tracks (i.e. individual movements) of the juveniles in the experimental design. The left and right enclosures correspond to the single (i.e. no adult) and the exposed (i.e. adult visible) treatments, respectively. Each enclosure consisted of two arenas separated by transparent glass. The coloured paths indicate the distance travelled by individuals during 1 h recording (red and yellow: juveniles; green: adult); (b) distance travelled (mean ± S.E.) in the experimental enclosure during 1 h by juveniles, without and with the presence of an adult.

Fig 3. Effect of the time of day on number of encountered horned frogs, according to life-stage: (a) adults (dark brown); (b) juveniles (light brown).

Surveys between 7:00–19:00 considered diurnal, and those between 19:00–7:00 nocturnal. Bars: mean; whiskers: S.E.

Fig 4. Differences in body condition (mean ± S.E.) between juvenile Pacific horned frogs active during the day (n = 176) compared to those active during the night (n = 108).