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Sexual Dimorphism Reversal and Polyandry – Part II

July 20, 2016

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.

Crockford's Club riot (Pinterest)

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 became Hercules)

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 female and male (J.Kenney F 3-10-10, M 1-10-07)

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 Peltomaki at FinnNaturi)

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, Malibu Lagoon, CA (Jim Kenney 11-23-06)

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.

Female Northern Jacana is % heavier than the male (Cherie Pittillo in Yucatan Times)

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]

  1. July 20, 2016 10:37 am

    Very interesting, thank you. Looking forward to Part III.

    Enid Hayflick Ridgewood NJ




  1. Sexual Dimorphism Reversal and Polyandry – Part III | Santa Monica Bay Audubon Blog
  2. Sexual Dimorphism Reversal and Polyandry – Part I | Santa Monica Bay Audubon Blog

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