Amphibian larvae can make their adaptive response to environmental resources. Changes in water regime and food supply may accelerate their metamorphosis and facilitate them to evolve alternative morphs and therefore alter their foraging strategy, promoting resources partitioning, reducing intra and interspecific competition and improving their fitness.
R. D. Semlitsch's study on Ambystoma talpoideum showed a higher percentage of individuals metamorphosed from drying, low-density, and high-food ponds than from constant water level, high-density, and low-food ponds. A higher percentage of individuals became paedomorphic in constant water-level and low-density ponds than in drying and high-density ponds. Only initial density affected survival of larvae.*
To understand the proximate mechanisms that mediate the developmental response to pond drying, Robert J. Denver analyzed changes in endocrine activity in tadpoles of the western spadefoot toad (Scaphiopus hammondii) exposed to experimental water volume reduction in the laboratory. Tadpoles exposed to a declining water level accelerated metamorphosis compared with tadpoles raised in a constant high-water environment. The acceleration of development was associated with the precocious elevation of whole-body contents of the hormones that control metamorphosis, the thyroid hormones thyroxine (T4) and triidothyronine (T3), and the interrenal steroid corticosterone (CORT).
Tadpoles transferred to one liter of water exhibited significant metamorphic changes within forty-eight hours after transfer. In addition, dramatic elevations in whole-body T4, T3, and CORT contents were evident at this time point. Thus, the metamorphic response to pond drying is likely driven by the activation of the thyroid and interregnal axes.*
Environmentally induced phenotypic plasticity allows developing organisms to respond adaptively to changes in their habitat. Desert amphibians have evolved traits which allow successful development in unpredictable environments. Tadpoles of these species can accelerate metamorphosis as their pond dries, thus escaping mortality in the larval habitat.
David W. Pfennig et al. coupled field observations with experiments to ask if the degree of character displacement reflects the intensity of competition between two closely related spadefoot toads (Spea bombifrons and S. multiplicata). They found that S. multiplicata became increasingly more omnivore-like as the relative abundance of S. bombifrons increased. Moreover, in controlled laboratory populations, S. multiplicata became increasingly more omnivore-like and S. bombifrons became increasingly more carnivore-like as Pfennig increased the relative abundance of the other species. However, divergence also reflected differences in canalized traits.
When reared under common conditions, S. multiplicata tadpoles became increasingly less likely to produce carnivores as their natal pond decreased in elevation because S. bombifrons became increasingly more common with decreasing elevation. Local genetic adaptation to the presence of S. bombifrons was remarkably fine grained, with differences in carnivore production detected between populations a few kilometers apart.*
These indicate that the maintenance of trophic polymorphisms is favored by a different resource use in alternative morphs.
Mathieu Denoël et al. and others found out whether heterochronic morphs occupy particular microhabitats and focus on specific prey items. Resource partitioning was found between morphs.
Some habitat specializations were also found. Polymorphism allows animals to exploit the different resources in order to minimize intraspecific competition, but the extent of resource partitioning depends on habitat characteristics.
Discrete resource polymorphisms occur in various vertebrate species and probably occur more frequently than is generally appreciated. They are manifested in a number of ways, including morphological, behavioral, and life-history characters. Research on a number of unrelated taxa suggests that resource polymorphisms may be underestimated as an important role in speciation. In an ecological context, they are important in resource partitioning and reducing intraspecific competition.*
Drawing on data from both natural populations and a controlled experiment, David W. Pfennig presented such a test in tadpoles of two species of spadefoot toads (Spea bombifrons and S. multiplicata). These two species show exaggerated divergence in trophic morphology where they are found together (mixed-species ponds) but not where each is found alone (pure-species ponds), suggesting that they have undergone ecological character displacement.
Many amphibians metamorphose in temporary ponds, and may accelerate larval development to avoid mortality when a pond desiccates. A younger age at metamorphosis often results in reduced body size, but may also facilitate a trade-off with physiological traits that are linked to fitness in the adult stage.
Character displacement has long been considered a major cause of adaptive diversification. When species compete for resources or mates, character displacement minimizes competition by promoting divergence in phenotypes associated with resource use or mate attraction.
The experiments have proved that, according to environmental factors, amphibian larvae may develop different morphs and change foraging strategy to favor resource partitioning, reduce intraspecific competition, and increase fitness. Resource competition has been seen as an important cause eliciting intra and interspecific phenotypic variation. Natural selection may favor those individuals capable of having different phenotypes. Resource polymorphism may play an important role in speciation. These studies are representative examples of intraspecific comparative system.
* For further details, please refer to Deep Structure Studies 7: Experimental Reviews and References I.