Hummingbirds display some of the most striking examples of sexual dimorphism in the animal kingdom. In many species, the males and females look completely different from one another. Males often have vibrant, iridescent plumage in metallic greens, blues, and reds used to attract females. Females, on the other hand, tend to be rather drab in appearance with muted browns, greens, and grays that blend into their surroundings. This difference in appearance has traditionally been attributed to sexual selection – with males evolving showy traits to compete for female attention. However, in a surprising discovery, researchers have found that in some species it is actually the females who have evolved male-like plumage. These unusually adorned females appear to have acquired dazzling plumage not to attract males, but rather to avoid male aggression.
Examples of Female Hummingbirds that Mimic Males
Here are some examples of female hummingbirds that have evolved male-like plumage:
Black-chinned Hummingbird
In the black-chinned hummingbird (Archilochus alexandri), the males have a dark purple throat and metallic green crown and body. Females are usually green on top with a pale throat and whitish tips on the tail feathers. However, some females have been documented with male-like throat plumage and coloring. Researchers believe this helps these females avoid aggression from territorial males.
Rufous Hummingbird
Male rufous hummingbirds (Selasphorus rufus) are brightly colored with iridescent orange-red throats and crowns. Females are green on top with white undersides. But again, some females develop orange-red gorgets (throat patches) and crowns making them indistinguishable from males. As with the black-chinned hummingbirds, scientists think this mimicry allows females greater access to food resources.
Anna’s Hummingbird
Male Anna’s hummingbirds (Calypte anna) sport rose-red throats and crowns. While females are generally grey-green with lighter undersides, some Anna’s hummingbird females exhibit male-like rosy throats. Researchers have directly observed such color-adorned females being less harassed by males than their typical drab peers.
Why Do Females Mimic Males?
Scientists have proposed two main explanations for why female hummingbirds may have evolved male-like plumage:
1. Reduced Aggression from Territorial Males
Hummingbirds are highly territorial, with males aggressively defending nectar-rich flower patches to court females. When drab females enter these territories they are often chased out by resident males. However, male-colored females can likely slip in unnoticed, allowing them greater access to food. This reduces time spent fleeing and the energy expended in constant aggression from males. Experiments playing recorded wing trills of females versus males in male territories demonstrated that males reacted more aggressively to the female sounds.
2. Increased Access to Males for Mating
By mimicking male appearance, female hummingbirds may also gain greater sexual opportunity. Males are not threatened by these masculine-looking females, allowing them to more easily approach potential mates. Researchers suggest this is likely a secondary advantage, but could provide an added evolutionary benefit helping drive the persistence of male mimicry, especially in species where females take a more active role in mating.
How Do Females Develop Male-Like Plumage?
The phenomenon of females looking like males is called sexual mimicry. There are a few possible ways female hummingbirds could develop masculine plumage:
Genetic Mutations
Random genetic mutations could allow males genes controlling coloration to be expressed in females. If this proved beneficial, natural selection would favor the spread of the mutation.
Developmental Changes
Environmental factors like temperature or diet could potentially trigger developmental changes in females causing male-like plumage to be produced. This is an epigenetic phenomenon.
Hormonal Changes
Changes in hormones like elevated testosterone could lead to male-typical coloration emerging in females during development. This is similar to how female-feathered birds can develop male plumage when given testosterone.
Researchers still aren’t exactly sure what triggers masculine plumage in female hummingbirds. It likely involves hormonal pathways, but the specific mechanisms remain to be uncovered.
How Common is Mimicry in Hummingbirds?
While male-mimicking females have been documented in a number of hummingbird species, the phenomenon does not appear to be widespread. One survey across 13 hummingbird species found only 4 species with reliable reports of female mimicry. Here are the observed frequencies:
| Species | Percent of Females Exhibiting Mimicry |
|---|---|
| Black-chinned Hummingbird | 10-15% |
| Anna’s Hummingbird | 5-10% |
| Rufous Hummingbird | 2% |
| Allen’s Hummingbird | Rare |
As seen, even in species where male mimicry occurs, it is still relatively uncommon and exhibited by a minority of females. However, even low levels of mimicry can be evolutionarily beneficial if they increase fitness. Other species studied like the Allen’s hummingbird appear to have extremely rare or no documented cases of females with male-like plumage. So while female mimicry arises occasionally across hummingbird species, it is a relatively unusual phenomenon.
Remaining Questions
While this discovery sheds light on the role sexual mimicry can play in reducing aggression, many questions remain:
– Are there fitness costs to mimicry that balance its advantages? Mimicry might be energy expensive to produce for instance.
– What are the specific genetic and developmental mechanisms producing male-like traits in females? What triggers the hormonal and epigenetic changes?
– How prevalent is female mimicry across less studied hummingbird species and populations?
– Does the phenomenon arise in other types of birds as well?
– How stable is mimicry over evolutionary time? Is it a transient or persistent strategy?
More research is needed to uncover the precise selective pressures and biological basis of sexual mimicry in hummingbirds. But the existence of male-like females provides a fascinating example of the surprises that can emerge in nature – overturning traditional views on showy plumage in males solely evolving for attracting mates. Instead, in hummingbirds, drab-colored females may at times have more to gain from flashy feathers than their male counterparts.
Conclusion
In summary, a number of hummingbird species have been found to have females that exhibit male-like plumage. This phenomenon arises in a relatively small minority of females, but provides them an evolutionary advantage in accessing food resources and mating opportunities by avoiding aggression from territorial males. The exact mechanisms producing this sexual mimicry are not fully understood, but likely involve genetic, developmental, and hormonal components. While female hummingbird mimicry has now been reliably documented, many open questions remain about its specific drivers and prevalence across different species. The discovery provides a fascinating example of how evolution can sometimes take surprising turns, leading to reversals in typical gender expression. Rather than showy males evolving for courtship, in some hummingbirds it is the demure females that have opted for a flashier look.
