Sexual Dimorphism Reversal and Polyandry – Part III

Sometimes it’s the exception that proves the rule.

Part one of this series introduced the concepts of sexual monomorphism, sexual dimorphism, briefly discussed why female choosiness of mates appears, why it drives sexual selection, and 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.

Part two furthered the discussion of sexual selection and sexual dimorphism, introduced the topics of polyandry, sexual dimorphism reversal, and sexual size dimorphism reversal. Two cases of monogamous sexual dimorphism reversal were described, followed by descriptions of double-clutching and of polyandry in conjunction with monomorphism and sexual size dimorphism reversal, likely forerunners of polyandry combined with sexual dimorphism reversal.

We now introduce those polyandrous species that display sexual dimorphism reversal, and we will begin to see that the above “rule” is not quite right.

Left: Greater Painted-Snipe (male left, female right) shows reversed sexual dimorphism (Anwar Khan)
Right: Sexually monomorphic South American Painted-Snipe
(Gastón Cassus at Cornell)

Painted-Snipe (family Rostratulidae): Ancestral lines of Painted-Snipe and Jacana diverged about 47 MYA, making them each others closest relatives. An interesting case is presented by the three currently recognized species of Painted-Snipe: South American (Nycticryphes semicollaris), Greater (Rostratula benghalensis) and Australian (R. australis). The two latter species are sometimes considered subspecies of R. benghalensis; females of both species are serially polyandrous and display sexual dimorphism reversal. This is our first example of association of serial polyandry with sexual dimorphism reversal. These two species are estimated to have diverged about 16.5 MYA ago. Their lineage split from the monomorphic and monogamous N. semicollaris of South America about 34 MYA. So in this tiny family of only three species, we see the clear association of sexual dimorphism reversal with polyandry, and either monomorphism or non-reversed sexual dimorphism with typical monogamy. 3

Eclectus Parrot pair, sexual dimorphism reversed; male left, female right
(Papagoi Keskus – Erispapagoi)

Eclectus Parrot (Eclectus roratus): Eclectus Parrots live in tropical eucalyptus-dominated Australasian rainforests, ranging from Indonesian Sumba and Halmahera Islands in the west, throughout New Guinea, eastward through the Solomon Islands, and in a small patch of rainforest in northeastern Australia. Both sexes are brightly plumaged: females are a colorful scarlet-and-blue; males are green, with ultraviolet pigments invisible to the human eye and less visible to hawks than to other Eclectus Parrots. They were long thought to be separate species due to this sexual dimorphism reversal. Their breeding behavior is complicated and habitat-driven.

Australasia has no woodpeckers; cavities are few and in high demand by cavity-nesting birds and marsupials. Eucalyptus wood is hard but brittle, and useful cavities appear primarily when high winds break large limbs from the trees, which leaves a hole or a large scar which a bird or marsupial might shape into a suitable cavity. The scarcity and high value of suitable nesting cavities caused this parrot to evolve a very rare polygynandrous breeding system, where both males and females mate have multiple simultaneous mates. The female defends her nest tree, which comprises her entire territory; her bright scarlet plumage warns other females that her tree is occupied and defended, and signals males of her presence. She does all incubation and nest defense, while her mates, as many as seven, keep her supplied with food. Her perpetual proximity to the nest protects her from predation from hawks. The male spends all his time foraging for fruits, nuts, etc.; his green plumage blends well with tree foliage, providing protection from hawk predation. His large territory can contain many nest trees, each with a female and her young. He mates with many females and supplies food to all of them. Thus each male has a dispersed collection of mates, all of whom they feed, and each female has multiple mates, all of whom feed her.

Plains-Wanderer, sexual dimorphism reversed, male left, female right
(David Baker-Gabb, Melbourne Herald Sun, November 16, 2012)

Plains-Wanderer (Pedionomus torquatus): The only species in the monotypic family Pedionomidae, the Plains-Wanderer is a classic example of serial polyandry associated with reversed sexual dimorphism, plus sexual size dimorphism reversal. These nocturnal residents of the eastern Australian grasslands are sedentary, traveling only when forced to by conditions such as overgrazing or fire, and the male does all the work of egg incubation, feeding, and protecting and raising of the young. The classification of Plains-Wanderer has changed significantly and frequently over the past few decades. Currently its closest affinities are to Seedsnipe, from whom they split 42 MYA. Their previous split was from the linage leading to Painted-Snipe and Jacana; that split occurred 51 MYA. This twig of the avian class abounds with sexual dimorphism reversals of size and plumage, as it consists of Plains-Wanderer, Seedsnipe (minor sexual size dimorphism reversal), Painted-Snipe and Jacana. 4

Eurasian Dotterel, female left, male right, sexual dimorphism reversed
(Killian Mullarney, Kaunispaa, Norway, May 27, 2016)

Eurasian Dotterel (Charadrius morinellus): This species of plover possesses a most peculiar breeding system. Their breeding range extends from northern Scotland, crosses Norway to far eastern Siberia, with a few additional mountainous locales in southern Europe and Siberia. They exhibit not just reversed sexual dimorphism and polyandry, but add a reversed lek*-system, followed by the female choosing of mates. In the lek the females compete, with success typically going to the most brightly colored; dominant females then choose their mate. After mating and laying her first clutch, the female may continue northwards and mate with another male. This may be repeated. With her last mate of the season her behavior can change, and she may share or completely take over nesting duties. Despite decades of study, it is difficult to identify any consistent mating behavior across this wide-ranging, far-northern species. Can it get more confusing? 5, 6

*Definition:
Lek: An aggregation of males gathered to engage in competitive displays to establish dominance. Females observe, then select their mate – usually the dominant male – then leave to nest and raise her chicks with no further help from the male.

Red-necked Phalarope, sexual dimorphism reversed; female left, male right
(Joyce Waterman, Lake Myvatn, Iceland, May 27, 2014)

This brings us to the three species of Phalarope (family Scolopacidae, subfamily Phalaropodinae), a familiar example of serial polyandry associated with sexual dimorphism reversal. Females are much brighter plumaged than the males. When males are numerous, the female lays her eggs in the nest prepared by the male whom she has selected; she then moves on to another mate. The first male alone does the work of incubating the eggs and feeding, protecting and rearing the young. When males are few, females may become monogamous. One of Southern California’s earliest “fall” migrants, females begin to move through as early as early June.

The Red (Phalanopus fulicarius) and Red-necked (P. lobatus) Phalaropes are two “sister species” whose ancestral line diverged about 7 MYA. The more distantly related Wilson’s Phalarope (Steganopus tricolor), separated from them about 21 MYA. They diverged from the more-typical sandpipers about 28 MYA; the Terek Sandpiper (Xenus cinereus), is their closest relative.

Red Phalarope, sexual dimorphism reversed
female left (Joe Fuhrman – Vireo), male right (Garth McElroy – Vireo)
Both from National Audubon Society

Red and Red-necked Phalaropes are holarctic nesters, nesting in suitable areas surrounding the Arctic Ocean. Red Phalaropes winter along the southwestern coast of South America, and central and southern west coast of Africa. Red-necked Phalaropes winter along the central west coast of South America, the southern Arabian coast, and in Philippines and Indonesia archipelagos.

Wilson’s Phalarope, sexual dimorphism reversed
Female left (Grace Murayama, Malibu Lagoon, Ca., June 8, 2016)
Male right (Jason Crotty, Redwood City, Ca., May, 2012)

Perhaps reflecting their more distant relationship with the other two phalaropes, Wilson’s Phalaropes are entirely New World birds, nesting from southern Alaska across prairie states and provinces to eastern Ontario, and wintering across southern South America from Ecuador to southern Brazil and south to Tierra del Fuego.

In Southern California, the Wilson’s and Red-necked Phalaropes arrive with the spring (April-May) and fall (June-October) migrants. The Red Phalarope appears as a spring (April-May) migrant, but may also winter on California coasts during the September-February period. 7

Lobed toes on diving phalarope, reflected
(Bates Littlehales at National Geographic Society)

Phalaropes differ from other sandpipers not just in their serial polyandry and sexual dimorphism reversal, but in having thick, duck-like downy plumage which enables them to float without becoming waterlogged. They can do their famous twirling feeding behavior because they have lobed toes (as do coots) and can paddle quite well. Twirling creates a vortex in the water, bringing small plankton to the surface where the phalarope can suck them up by capillary action between their thin upper and lower mandibles. All-in-all, they are unusual and special birds. 8

Summation
Polyandry, wherein the female mates with multiple males, either serially (Phalarope) or simultaneously (Jacana), is very uncommon in the avian world; it is primarily found in the Gruiformes (Storks and Allies) and Charadriiformes (Sandpipers and Allies). Only 0.4% of avian species, about 40 species, are polyandrous, with variation in both the degree of polyandry and the sexual dimorphism displayed. This variety seems to fall along the following progression.

1. The species is both sexually monomorphic and monogamous, but females double-clutch, leaving the first clutch for the male to raise and raising the second clutch herself. Examples: Mountain Plover, Sanderling, Temminck’s Stint.

2. The species exhibits sexual monomorphism, sexual size dimorphism reversal, polyandry when sufficient males are present, monogamous otherwise, and the degree of sexual role reversal likewise varies. Examples: Jacana, Spotted Sandpiper.

3. The species exhibits sexual dimorphism reversal with or without sexual size dimorphism reversal, polyandry when sufficient males are present, monogamous otherwise, degree of sexual role reversal likewise varies. Examples: Painted-Snipe, Plains-Wanderer, Eurasian Dotterel, the Phalaropes.

With the exception of Belted Kingfisher, discussed above, all species where the female is more brightly colored than the male are polyandrous. Conversely, the females of all polyandrous species are either larger, or more brightly colored, or both. The vast majority of birds are normally monogamous, and the greater their sexual dimorphism, the greater the difference in nesting duties. Monomorphic species share virtually all nesting duties, medium-range sexually dimorphic species share some duties and not others, and extreme sexually dimorphic species share few or no nesting duties.

So, for over 10,300 species, with one exception, the “rule” is:

Greater equality in nesting duties means greater similarity in appearance.  Lesser similarity in appearance means lesser involvement in nesting duties by the more colorful bird.

We have eliminated the reference to the male although in nearly all sexually dimorphic species, it is the male who has the more brightly colored plumage. Polyandry is very rare, and instances of brighter-plumaged females is even more rare. Yet the clear linkage – in species such as Phalaropes – of sexual dimorphism reversal to both polyandry and sexual role reversal, demonstrates that they are the exceptions that prove this restated rule.  [Chuck Almdale]

References (Web links are in the text)
Note: All lineage splitting dates are from Tree of Life website and are based on current data, which can easily change as studies continue: To find a species or family, open  search bar (click little binoculars icon), enter desired text into search box at bottom, click “Next Hit.” Mouse navigates the screen: click-and-drag to move around, scroll-wheel to zoom in/out.
Photo Credits: In photo captions with links supplied.
Additional reading: The Many Types of Avian Mating Systems
1. Handbook of Birds of the World (HBW), Vol. 6. del Hoyo, J., Elliott, A. & Sargatal, J. eds. (2001) Lynx Edicions, Barcelona. Pgs 248-249.
2. HBW Vol. 3. (1996) Pgs. 277-280
3. HBW Vol. 3. (1996) Pgs. 292-300
4. HBW Vol. 3. (1996) Pgs. 534-538
5. HBW Vol. 3. (1996) Pg. 438
6. Killian Mullarney, personal communication
7. Birds of Southern California: Status and Distribution. Garrett, Kimball & Dunn, Jon. (1981) Los Angeles Audubon Society, Los Angeles. Pg. 50.
8. HBW Vol. 3. (1996) Pg. 446