Conspecific visual cues: the relative importance of interference and exploitation competition among tadpoles of Rana kukunoris

Intraspecific interference retards growth and development of cane toad tadpoles, but those effects disappear by the time of metamorphosis

Competition among larval anurans can occur via interference as well as via a reduction in per-capita food supply. Previous research on intraspecific interference competition in cane toad (Rhinella marina) tadpoles found conflicting results, with one study detecting strong effects on tadpoles and another detecting no effects on metamorphs. A capacity to recover from competitive suppression by the time of metamorphosis might explain those contrasting impacts. In a laboratory experiment, we found that nine days of exposure to intraspecific interference competition strongly reduced tadpole growth and development, especially when the competing tadpoles were young (early-stage) individuals. Those competitive effects disappeared by the time of metamorphosis, with no significant effect of competition on metamorph body condition, size, larval period or survival. Temporal changes in the impact of competition were not related to tadpole density or to variation in water quality. The ability of larval cane toads to recover from intraspecific interference competition may enhance the invasive success of this species, because size at metamorphosis is a significant predictor of future fitness. Our study also demonstrates a cautionary tale: conclusions about the existence and strength of competitive interactions among anuran larvae may depend on which developmental stages are measured.

Factor in Fear: Interference Competition in Polymorphic Spadefoot Toad Tadpoles and Its Potential Role in Disruptive Selection

Disruptive selection arises when extreme phenotypes have a fitness advantage compared to more-intermediate phenotypes. Theory and evidence suggest that intraspecific resource competition is a key driver of disruptive selection. However, while competition can be indirect (exploitative) or direct (interference), the role of interference competition in disruptive selection has not been tested, and most models of disruptive selection assume exploitative competition. We experimentally investigated whether the type of competition affects the outcome of competitive interactions using a system where disruptive selection is common: Mexican spadefoot toads (Spea multiplicata). Spea tadpoles develop into alternative resource-use phenotypes: carnivores, which consume fairy shrimp and other tadpoles, and omnivores, which feed on algae and detritus. Tadpoles intermediate in phenotype have low fitness when competition is intense, as they are outcompeted by the specialized tadpoles. Our experiments revealed that the presence of carnivores significantly decreased foraging behavior in intermediate tadpoles, and that intermediate tadpoles had significantly lower growth rates in interference competition treatments with carnivores but not with omnivores. Interference competition may therefore be important in driving disruptive selection. As carnivore tadpoles are also cannibalistic, the ‘fear’ effect may have a greater impact on intermediate tadpoles than exploitative competition alone, similarly to non-consumptive effects in predator–prey or intraguild relationships.

The effect of heterospecific and conspecific competition on inter-individual differences in tungara frog tadpole (Engystomops pustulosus) behavior

Repeated social interactions with conspecifics and/or heterospecifics during early development may drive the differentiation of behavior among individuals. Competition is a major form of social interaction and its impacts can depend on whether interactions occur between conspecifics or heterospecifics and the directionality of a response could be specific to the ecological context that they are measured in. To test this, we reared tungara frog tadpoles (Engystomops pustulosus) either in isolation, with a conspecific tadpole or with an aggressive heterospecific tadpole, the whistling frog tadpole (Leptodactylus fuscus). In each treatment, we measured the body size and distance focal E. pustulosus tadpoles swam in familiar, novel and predator risk contexts six times during development. We used univariate and multivariate hierarchical mixed effect models to investigate the effect of treatment on mean behavior, variance among and within individuals, behavioral repeatability and covariance among individuals in their behavior between contexts. There was a strong effect of competition on behavior, with different population and individual level responses across social treatments. Within a familiar context, the variance in the distance swam within individuals decreased under conspecific competition but heterospecific competition caused more variance in the average distance swam among individuals. Behavioral responses were also context specific as conspecific competition caused an increase in the distance swam within individuals in novel and predator risk contexts. The results highlight that the impact of competition on among and within individual variance in behavior is dependent on both competitor species identity and context.

Geographic variation in reproductive investment of seven populations of male Brown frog (Rana kukunoris), endemic to the eastern Tibetan Plateau

Life-history theory suggests that male anurans living in harsh and unpredictable environmental conditions (e.g., low temperature and/or high seasonality) should decrease reproductive investment because of short periods of activity and poor productivity. Although geographic variations in female life-history traits have been investigated intensively across a wide range of taxa, similar studies in males have been scarce. Here, we examined geographic variation in male testis mass of the brown frog (Rana kukunoris) across seven different altitudes on the eastern Tibetan plateau, and tested the hypothesis that relative size of testis mass should increase with increasing temperature and/or decreasing seasonality. In this study, we found male body condition, age and testis mass did not increase with temperature and temperature seasonality, which do not conform to our previous hypothesis, revealing that differences in temperature and temperature seasonality across populations could not drive the evolution of testis size. However, we found a positive correlation between testis mass and body condition or age when controlling for population and the effect of temperature and temperature seasonality, indicative of a condition-dependent expression of testis size. In addition, a correlation between testis mass and operational sex ratio was not significant, suggesting that male–male competition did not lead to increasing levels of sperm competition.