Frequency-dependent selection by wild birds promotes polymorphism in model salamanders

Abstract

Background: Co-occurrence of distinct colour forms is a classic paradox in evolutionary ecology because both selection and drift tend to remove variation from populations. Apostatic selection, the primary hypothesis for maintenance of colour polymorphism in cryptic animals, proposes that visual predators focus on common forms of prey, resulting in higher survival of rare forms. Empirical tests of this frequency-dependent foraging hypothesis are rare, and the link between predator behaviour and maintenance of variation in prey has been difficult to confirm. Here, we show that predatory birds can act as agents of frequency-dependent selection on terrestrial salamanders. Polymorphism for presence/absence of a dorsal stripe is widespread in many salamander species and its maintenance is a long-standing mystery.

Results: We used realistic food-bearing model salamanders to test whether selection by wild birds maintains a stripe/no-stripe polymorphism. In experimental manipulations, whichever form was most common was most likely to be attacked by ground-foraging birds, resulting in a survival advantage for the rare form.

Conclusion: This experiment demonstrates that frequency-dependent foraging by wild birds can maintain colour polymorphism in cryptic prey.

Figure 1

Examples of polymorphic plethodontid salamanders. Each pair of individuals was captured in the same place at the same time. a, Southern Red-backed Salamanders, Plethodon serratus, Great Smoky Mountains National Park (Tennessee, USA). b, Southern Zigzag Salamanders, P. ventralis, Knoxville (Tennessee, USA). c, California Slender Salamanders, Batrachoseps attenuatus, Napa County (California, USA).

Figure 2

Experimental subjects. (a) Polymorphic model salamanders. (b) Blue Jay (Cyanocitta cristata) foraging in the study area.

Figure 3

Relative fitness of model salamanders. Daily survival rates of striped relative to unstriped models (right-hand axis, filled symbols) in comparison to the frequency of striped models (left-hand axis, open symbols). Dashed line illustrates equal frequency and equal fitness.