Intersexual social dominance mimicry drives female hummingbird polymorphism

Abstract

Female-limited polymorphisms, where females have multiple forms but males have only one, have been described in a variety of animals, yet are difficult to explain because selection typically is expected to decrease rather than maintain diversity. In the white-necked jacobin (Florisuga mellivora), all males and approximately 20% of females express an ornamented plumage type (androchromic), while other females are non-ornamented (heterochromic). Androchrome females benefit from reduced social harassment, but it remains unclear why both morphs persist. Female morphs may represent balanced alternative behavioural strategies, but an alternative hypothesis is that androchrome females are mimicking males. Here, we test a critical prediction of these hypotheses by measuring morphological, physiological and behavioural traits that relate to resource-holding potential (RHP), or competitive ability. In all these traits, we find little difference between female types, but higher RHP in males. These results, together with previous findings in this species, indicate that androchrome females increase access to food resources through mimicry of more aggressive males. Importantly, the mimicry hypothesis provides a clear theoretical pathway for polymorphism maintenance through frequency-dependent selection. Social dominance mimicry, long suspected to operate between species, can therefore also operate within species, leading to polymorphism and perhaps similarities between sexes more generally.

Keywords: Trochilidae; female-limited polymorphism; mimicry; polymorphism; resource-holding potential; sexual dimorphism.

Figure 1.

Burst power and wing loading in heterochrome females, androchrome females and androchrome males. Points and lines represent means and bootstrapped 95% CI. Shaded areas represent the distribution of individual data points. p-values for differences in means are indicated by N.S., p > 0.05; **0.01 > p > 0.001; p < 0.0001. Juveniles were excluded from both graphs. (a) Females of both plumage types were similar in burst power, but males lifted significantly more than both female types. (b) Similar wing loading measures were found between female plumage types, and between androchrome females and males. Heterochrome females had slightly but significantly lower wing load than androchrome males. (Online version in colour.)

Figure 2.

Principal components for seven measures of body size in white-necked jacobins. The first and second PC axes are depicted here, which explain 38.5% and 25.4% of the total variation. Green points and shading represent heterochrome female morphology, while blue and grey represent androchrome females and androchrome males, respectively. Both female types overlap, while most males are distinct from either female type. (Online version in colour.)

Figure 3.

A comparison of space use (mean number of feeders visited) and feed frequency (mean feeder visits) as a measure of feeding strategy. Best-fit linear and quadratic relationships are shown in each graph. If both non-territorial and territorial strategies are used, we expect a negative quadratic component (upside-down U) to this relationship, which was only found in androchrome males. Females of both types only exhibited a positive relationship, indicating a lack of detected territoriality. Individuals of all three types therefore can be observed using a non-territorial feeding strategy, but some males show patterns consistent with territoriality. (Online version in colour.)