David W. Kikuchi

Predator Cognition Permits Imperfect Coral Snake Mimicry

Batesian mimicry is often imprecise. An underexplored explanation for imperfect mimicry is that predators might not be able to use all dimensions of prey phenotype to distinguish mimics from models and thus permit imperfect mimicry to persist. We conducted a field experiment to test whether or not predators can distinguish deadly coral snakes (Micrurus fulvius) from nonvenomous scarlet kingsnakes (Lampropeltis elapsoides). Although the two species closely resemble one another, the order of colored rings that encircle their bodies differs. Despite this imprecise mimicry, we found that L. elapsoides that match coral snakes in other respects are not under selection to match the ring order of their model. We suggest that L. elapsoides have evolved only those signals necessary to deceive predators. Generally, imperfect mimicry might suffice if it exploits limitations in predator cognitive abilities.

Imperfect mimicry and the limits of natural selection.

Mimicry—when one organism (the mimic) evolves a phenotypic resemblance to another (the model) due to selective benefits—is widely used to illustrate natural selections power to generate adaptations. However, many putative mimics resemble their models imprecisely, and such imperfect mimicry represents a specific challenge to mimicry theory and a general one to evolutionary theory. Here, we discuss 11 nonmutually exclusive hypotheses for imperfect mimicry. We group these hypotheses according to whether imperfect mimicry reflects: an artifact of human perception, which is not shared by any naturally occurring predators and therefore is not truly an instance of imperfect mimicry; genetic, developmental, or time-lag constraints, which (temporarily) prevent a response to selection for perfect mimicry; relaxed selection, where imperfect mimicry is as adaptive as perfect mimicry; or tradeoffs, where imperfect mimicry is (locally) more adaptive than perfect mimicry. We find that the relaxed selection hypothesis has garnered the most support. However, because only a few study systems have thus far been comprehensively evaluated, the relative contributions of the various hypotheses toward explaining the evolution of imperfect mimicry remain unclear. Ultimately, clarifying why imperfect mimicry exists should provide critical insights into the limits of natural selection in producing complex adaptations.

High-model abundance may permit the gradual evolution of Batesian mimicry: an experimental test

In Batesian mimicry, a harmless species (the ‘mimic’) resembles a dangerous species (the ‘model’) and is thus protected from predators. It is often assumed that the mimetic phenotype evolves from a cryptic phenotype, but it is unclear how a population can transition through intermediate phenotypes; such intermediates may receive neither the benefits of crypsis nor mimicry. Here, we ask if selection against intermediates weakens with increasing model abundance. We also ask if mimicry has evolved from cryptic phenotypes in a mimetic clade. We first present an ancestral character-state reconstruction showing that mimicry of a coral snake (Micrurus fulvius) by the scarlet kingsnake (Lampropeltis elapsoides) evolved from a cryptic phenotype. We then evaluate predation rates on intermediate phenotypes relative to cryptic and mimetic phenotypes under conditions of both high- and low-model abundances. Our results indicate that where coral snakes are rare, intermediate phenotypes are attacked more often than cryptic and mimetic phenotypes, indicating the presence of an adaptive valley. However, where coral snakes are abundant, intermediate phenotypes are not attacked more frequently, resulting in an adaptive landscape without a valley. Thus, high-model abundance may facilitate the evolution of Batesian mimicry.

Costs of Learning and the Evolution of Mimetic Signals

Predators must use the appearance of their prey to decide whether it is likely to be defended. Most theory assumes that predators have completed learning about prey appearance, yet we do not understand how predators learn which aspects of appearance to use for classifying prey. If sampling prey can be risky, predators might forgo opportunities to learn about the relationship between prey appearance and defense. Using Bayesian inference and dynamic programming, we modeled how the immediate risks and future rewards of learning about prey appearance influence how predators learn. In addition, we explored how variation in predator learning affects the evolution of mimicry, which occurs when two prey evolve to share a common signal to predators. We found that when learning about prey with distinct appearances was expensive, optimal predators tended to lump them into the same category or exhibit an unwillingness to sample at all (neophobia). This resulted in a reduction in selection for defensive mimicry. However, the same predator behavior favored the evolution of aggressive mimicry, because in that case, mimics benefited from being sampled. When prey were very rare and costs of sampling them were high, predators exhibited neophobia, refusing to attack. This behavior could forestall the evolution of mimicry and instead select for polymorphism.

Terrestrial and understorey insectivorous birds of a Peruvian cloud forest: species richness, abundance, density, territory size and biomass.

Terrestrial insectivorous birds in a cloud-forest on the north-western slope of the Peruvian Andes were described in terms of species richness, abundance, density, territory size and biomass. Abundance, density and territory size were also characterized for several understorey insectivores. The three terrestrial insectivore species, all in the genus Grallaria , had 35.25 territories on a 26.2-ha plot, defended territories of (mean ± SD) 1.65 ± 1.34 ha, dwelled at an average density of 4.4 ± 2.7 pairs per 10 ha per species and constituted a biomass of 2470 g per 10 ha. Eight understorey insectivore species had 122.75 territories on the plot, held territories 0.86 ± 0.62 ha in size, and lived at an average density of 5.0 ± 2.7 pairs per 10 ha per species. Six of the 11 species studied each occupied over 50% of the plot. Data on terrestrial insectivores from this study were compared with data from other Neotropical plots to examine how properties of guild structure relate to one another. Increasing densities, smaller territory sizes and higher biomasses appeared to be linked with decreasing species richness and increasing elevation, suggesting consistent patterns of covariance.

Batesian mimicry promotes pre‐ and postmating isolation in a snake mimicry complex

We evaluated whether Batesian mimicry promotes early‐stage reproductive isolation. Many Batesian mimics occur not only in sympatry with their model (as expected), but also in allopatry. As a consequence of local adaptation within both sympatry (where mimetic traits are favored) and allopatry (where nonmimetic traits are favored), divergent, predator‐mediated natural selection should disfavor immigrants between these selective environments as well as any between‐environment hybrids. This selection might form the basis for both pre‐ and postmating isolation, respectively. We tested for such selection in a snake mimicry complex by placing clay replicas of sympatric, allopatric, or hybrid phenotypes in both sympatry and allopatry and measuring predation attempts. As predicted, replicas with immigrant phenotypes were disfavored in both selective environments. Replicas with hybrid phenotypes were also disfavored, but only in a region of sympatry where previous studies have detected strong selection favoring precise mimicry. By fostering immigrant inviability and ecologically dependent selection against hybrids (at least in some habitats), Batesian mimicry might therefore promote reproductive isolation. Thus, although Batesian mimicry has long been viewed as a mechanism for convergent evolution, it might play an underappreciated role in fueling divergent evolution and possibly even the evolution of reproductive isolation and speciation.

Mimicry's palette: widespread use of conserved pigments in the aposematic signals of snakes.

Mimicry, where one species resembles another species because of the selective benefits of sharing a common signal, is especially common in snakes. Snakes might be particularly prone to evolving mimicry if all species share some of the same proximate mechanisms that can be used to produce aposematic/mimetic signals. We evaluated this possibility by examining color pigments in 11 species of snakes from four different families, three species of which participate in a coral snake mimicry complex involving convergence in coloration. We found that all 11 species used combinations of two pteridine pigments and melanin in their coloration, regardless of whether or not they were mimics. Furthermore, the presence or absence of red pteridines was strongly correlated with the relative excitation of medium‐ and long‐wavelength photoreceptors in birds, thereby linking shared pigmentation to perception of those pigments by likely agents of selection. Thus, precise color mimicry might be relatively easy to evolve among snakes owing to symplesiomorphies in pigmentation.

Pollinators and pollen dispersal of Piper dilatatum (Piperaceae) on Barro Colorado Island, Panamá

Biology of Extinction ; bees ; clonal growth ; Encyclopedia of Life ; Forces of Change ; BCI ; Barro Colorado Island ; Gatun Lake ; Panama Canal ; STRI ; filename_problems

Population densities of curassows, guans, and chachalacas (Cracidae): Effects of body size, habitat, season, and hunting

ABSTRACT Understanding the factors that determine population densities is critical for conserving viable populations of threatened species. Half of the 50 species in the family Cracidae have experienced population declines. We conducted a literature review to explore the relations of population densities of cracids with body size, habitat, season, and hunting. We compiled 103 density data points for 27 species in 37 localities from Mexico to Argentina. There was no correlation between body mass and density. The larger cracines tended to have lower densities than penelopines, but densities in both subfamilies spanned a similar range of values. Intraspecific and interspecific densities varied among sites over 2 orders of magnitude (1–100 birds km−2). Some cracids exhibited plasticity in habitat use, with variable densities among habitats. There is evidence that some species performed local movements related to seasonality in rainfall or resource availability, leading to aggregations around water sources during the dry season or around seasonally abundant food sources. Hunting had a negative effect on population densities, but in some cases low to moderate hunting did not cause a decrease in density in comparison to nonhunted sites. Despite having similar ecologies, densities of cracid species are very variable, and each population seems to respond idiosyncratically to local factors, which requires care if data are extrapolated across populations or species. Future studies that aim to characterize cracid populations for conservation purposes should take into account possible intraspecific density variations related to seasonality, local movements, and habitat heterogeneity.

Evolutionary biology: Life imperfectly imitates life

Some species evolve to resemble another species so as to protect themselves from predation, but this mimicry is often imprecise. An analysis of hoverflies suggests why imperfect imitation persists in the face of natural selection. See Letter p.461 Batesian mimics are potential prey species that are harmless to predators but gain protection through a resemblance to unpalatable prey species. Surprisingly, many Batesian species seem to be fairly mediocre mimics, despite presumably strong evolutionary pressure to improve the resemblance. This paper presents a morphological and phylogenetic analysis of harmless hoverfly species that mimic — with various degrees of success — stinging hymenopteran species. The authors rule out several hypotheses, such as that the imperfect mimicry is an artefact of human perception and that the imperfect mimics are actually hedging their bets by resembling several hymenopteran species at the same time. Instead, the authors find a link between imperfect mimicry and small body size, which suggests that the imperfect mimics are simply not subject to particularly intense selection.