Sometimes it’s the exception that proves the rule.

Part one of this series introduced the concepts of sexual monomorphism and sexual dimorphism, with a brief discussion of why female choosiness of mates appears, why this drives sexual selection, and why it can cause extremes of sexual dimorphism. The “rule” was introduced:

Greater equality in breeding duties means greater similarity in appearance. The lesser the similarity in appearance, the lesser the involvement in breeding duties by the male, who is the more colorful bird.

British gentlemen “amicably” discussing sexual selection (Pinterest)

Details of all of the above are elements of the topic of sexual selection, and have been a touchy subject indeed since explained by Charles Darwin in The Descent of Man and Selection in Relation to Sex (1871). Many Victorian-era male scientists and educated elite welcomed Darwin’s Theory of Evolution, yet balked at the idea that females of any species, especially humans, could have any say in the matter of mate-selection, let alone actually driving the process of evolution by the mere fact of choosing her mate. The very idea struck at the core of male chauvinistic society. In this matter, as in all matters, males were firmly in charge and made all important decisions.

Humans display sexual dimorphism: female left (Beauty Dart Bathing Beauty); male right (a young Steve Reeves before he starred as Hercules)

In Western society, that prejudicial belief of the male’s importance began waning some decades ago, only to be replaced by a different prejudicial belief: However animal biology might influence animal behavior, humans are certainly not mere animals and human biology can not dictate human psychological behavior; human beings have “free will,” it is not nature that determines sexual pairing.

Of more than 10,300 species of birds, nearly all are either monomorphic or sexually dimorphic with the male more brightly colored. However, there are a very few exceptions, examples of sexual dimorphism reversal, in which the female possesses the more brightly colored plumage: the Plains-Wanderer and Eclectus Parrot of eastern Australia, two species of Painted-Snipes, Eurasian Dotterel, Belted Kingfisher, and all three species of Phalarope. Further, with one exception, sexual dimorphism reversal is linked to polyandry and sexual role reversal.

Two definitions:

Polyandry: females breed with multiple males, either the more common serial polyandry (after the first egg clutch is laid, she leaves the male to find a second mate, then perhaps a third and fourth), or less common simultaneous polyandry (she maintains relations and broods with multiple males at the same time). Some polyandrous species become temporarily monogamous when the numbers of males are reduced. About 40 bird species (0.4%) are polyandrous, primarily found in the Gruiformes (Storks and Allies) and Charadriiformes (Sandpipers and Allies).

Sexual role reversal: males perform duties usually performed by females in normal sexually dimorphic species; females may have no parental time investment when a particular paring is polyandrous.

First we’ll deal with the sole example of monogamy combined with sexual dimorphism reversal.

Belted Kingfisher, sexual dimorphism reversed
female left, male right; Malibu Lagoon, Ca.
(Jim Kenney, female March 10, 2010, male January 10, 2010)

Belted Kingfisher (Ceryle alcyon): It is still uncertain why males lack the female’s incomplete chestnut breast band, but there are two reasonable explanations. First, females migrate south for winter, but many or most males stay on-territory. When she returns in the spring, males are busily defending their territory and the female’s bright chestnut breast may keep him from driving her away. Second, when on-territory, females are more aggressive and territorial than males. If a higher testosterone level in the female drives this behavior, it may also affect the laying down of red pigmentation in the plumage. Females may be dominant over males in the early stage of nest-construction, when the male begins scraping at the surface of a sandy or earthen bank while she watches and calls to him. Together they then finish the three-to-fifteen-foot-long tunnel. 1

Temminck´s Stint, Varanger, Norway, June 2004
(Jari Peltomäki at Finnature)

A possible evolutionary precursor of serial polyandry is found in Temminck’s Stint, Little Stint, Mountain Plover, and Sanderling. In these species, the female lays a clutch that is incubated by the male, followed by a second clutch which she alone incubates. This two-clutch system can be envisioned as a step toward the sort of serial polyandry seen in the Spotted Sandpiper, but females of that species will only incubate a clutch alone if their mate has been killed.

There are a few cases where sexual size dimorphism reversal without plumage dimorphism reversal is linked to polyandry.

Spotted Sandpiper, still spotted, has normal plumage sexual monomorphism with reversed sexual size dimorphism
(Jim Kenney, Malibu Lagoon, Ca., November 23, 2006)

Spotted Sandpiper (Actitis macularia): They exhibit serial polyandry and reversed sexual size dimorphism, as females on average are 11% larger than males. Females are capable of laying up to five clutches of four eggs each, and compete among themselves for the males. An interesting note is that the female of the Common Sandpiper (Actitis hypoleucos) – the Spotted Sandpiper’s “sister-species” (their lineages split 16.6 million years ago [MYA]) – is also slightly larger than the male, yet is monogamous.

Northern Jacana, sexual plumage monomorphism, sexual size dimorphism reversal. Bottom female averages 82% heavier than the male.
(Cherie Pittillo in Yucatan Times)

Jacana (family Jacanidae): Scattered across the world’s tropics, all eight Jacana species exhibit reversed sexual size dimorphism, with the females larger than the males, but as with the Spotted Sandpiper, do not show any significant plumage dimorphism. The monogamous female Lesser Jacana is only slightly larger, but the other seven species exhibit either or both simultaneous and serial polyandry, depending on circumstances, and significant sexual size differences: females are on average 82% larger in six species, and 100% larger in the Pheasant-tailed Jacana of Southeast Asia. This is the greatest reversed sexual size dimorphism shown by any bird or mammal species in the world. 2

In Part III, we introduce those polyandrous species that display sexual dimorphism reversal.
Notated references are at the end of Part III.
[Chuck Almdale]

Sometimes it’s the exception that proves the rule.

Female Wilson’s Phalarope, Malibu Lagoon, Ca.
(Grace Murayama, June 6, 2016)

Beginning birders soon notice that in many species, the male and female look different from each other. This divergence in appearance linked to sex appears in both birds and mammals. In mammals, for example, male lions have large manes that females totally lack, gorilla males are larger than females, male deer annually grow and shed their large antlers, and, of course, human males and females differ in body size and shape.

Irish Elk Group (Tabitha Paterson, TwilightBeasts)
Irish Elk size comparison (prehistoric-wildlife.com)

This distinct difference in features between genders is commonly called sexual dimorphism, or sometimes sexual bimorphism (two forms). The single-form alternative is sexual monomorphism (one form).

Sexual monomorphism in Snowy Egrets, Malibu Lagoon, Ca.
(Jim Kenney,  November 2006)

Some examples of sexual monomorphism commonly seen in California are: geese, swans, loons, grebes, cormorants, pelicans, most herons and egrets, vultures, most sandpipers, gulls, terns, alcids, typical owls, swifts, corvids, chickadees, thrashers, some warblers, and most sparrows.

Common Murre (bridled variety), an alcid, exhibit sexual monomorphism.
Hornoya, Norway. (Joyce Waterman, May 30, 2016)

In California, readily seen examples of sexual dimorphism are: ducks, quail, grouse, phalaropes, hummingbirds, kingfisher, woodpeckers, gnatcatchers, most warblers, tanagers, blackbirds and orioles, finches, House Sparrow.

Hooded Mergansers exhibit sexual dimorphism; male front, female behind
(Jim Kenney, February 17, 2011)

Field guides generally show if a species is monomorphic or dimorphic. This is done with little male & female signs (♂♀), for dimorphism. If it says either “adult” or nothing, it’s monomorphic.

The reasons why a species is one or the other are complex. Dissertations and books have been- and still are being – written on the subject.

Four important factors to help understand sexual morphism:
Operational Sex Ratio (OSR): The ratio of the numbers of sexually receptive males to females.
Potential Reproductive Rate (PRR): The offspring production per unit of time each sex would achieve if unlimited mates were available.
Minimum Time Investment (MTI): The shortest amount of time either sex must contribute to produce a fertilized egg.
Parental Time Investment (PTI): The amount of time either sex contributes to parental duties.

For the purposes of this discussion, we’ll use the following definitions:
Breeding duties: All reproduction-related behaviors from courting, mating and egg-laying, through nest-building, incubation of eggs and feeding, protection and education of the young.
Nesting duties: Those activities listed above except mating and egg-laying.

Theoretically, the male and female can have the same MTI, and for many animals that is true. But in birds, because females must produce the large fertilized egg with a protective shell, white and yolk, while the males contributes only tiny sperm, the female’s MTI is always larger. Therefore, if either sex begins to make a larger parental time investment (PTI) than the other, as often happens in evolution, it is almost always the female. This greater (MTI+PTI) investment drives the evolution of the female’s greater choosiness in mate selection, picking – as best she can – the most fit male available. Female choosiness in turn causes males to compete in numerous ways for breeding opportunities. Such competition between males sets up the evolution of variations in ability to sing, gather food, build a nest, fight battles, and especially to display plumage.
[Article: Operational sex ratios & roles, dimorphism, monomorphism]

Allen’s Hummingbird shows extreme sexual dimorphism; female left, male right (Jim Kenney: female September 4, 2009, male January 9, 2007)

Thus it is the female selection of attractive, healthy, large or strong characteristics that shapes the male’s appearance and so produces sexual dimorphism. Because males cannot take on the female’s large initial time investment in egg-laying, males evolve in other ways: nest building, bringing food to the female, territory protection, and predator distraction. His overall parental time and energy investment in these activities may exceed the female’s combined egg production and parental time investment. Further, by performing such necessary duties, the female can concentrate on incubation, etc. Thus a sexual division of labor appears – different roles for females and males.

What does this division of labor have to do with monomorphism and dimorphism? In summary the “rule” which I alluded to at the beginning is:

Greater equality in breeding duties means greater similarity in appearance.

Eggs are always produced by females. In monomorphic species everything else is shared as equally as possible: site-selection, nest-building, incubation of eggs, territory defense, protection, feeding and education of the young. In sexually dimorphic species, duties of nesting and parenting diverge between sexes, with the manner and amount varying widely between species.

This brings up the second part of the “rule”:

The lesser the similarity in appearance, the lesser the involvement in breeding duties by the male, who is the more colorful bird.

When sexual dimorphism is extreme, the male may do nothing beyond inseminating the female. She then performs all the duties of nest-building, incubation of eggs and protection, feeding and education of the young, etc., with no assistance from the male. The avian families of Pheasants, Hummingbirds, Cotingas, Manakins, Birds-of-Paradise and Bowerbirds are filled with such species.

In Part II, we discuss the topics of double-clutching, sexual dimorphism reversal, sexual size dimorphism reversal and polyandry, with illustrative cases.
In Part III, we introduce those polyandrous species that display sexual dimorphism reversal.
[Chuck Almdale]