The Strange Genes of the Polymorph Bird

On the smaller grasslands and throughout the forests of the northern continent, the rhyncheirids comprise two major groups; in the trees are a group of climbers called boras (short for arboreals), and on the ground, the ground-living running species known as terries (short for terrestrials). Boras are found only on Serinarcta, while one species of terry is also found on Serinaustra, that being the squork.

From an ancestral stock very similar to that basal species, the northern terries differ little in build from their southern kin - save for an even more elongated tail - but most demonstrably in their diets, becoming true vegetarians. Though some still prefer marshy habitat, a great many have fully weaned themselves from their historic lifestyle of fishing at the water's edge and moved into the open grasslands and the upland forest exclusively. Their diets of most are opposite those of squorks - almost all vegetable with only a small percentage animal - supplementing a diet of seeds, fruit, and green plants with the occasional insects or bird's egg. Their stomachs are large and able to ferment this diet, the culmination of tens of millions of years of slight expansions of their carnivorous' ancestors highly reduced digestive system (the squork, intermediate in diet, has a generalized stomach able to break down some soft plant matter such as seeds and green leaves but not to ferment dry grasses or coarse foliage.)

Able to chew their vegetable diets, the northern terries compete efficiently for food resources with circuagodonts. They combine many attributes of Serina's earlier megafaunal birds; the dexterous tentacled snouts of the elefinches with the chewing abilities of vivas, as life converges again on the same effective survival strategies. The serilope similarity becomes especially pronounced as some of the grazers exhibit a reduction in their facial appendages into a single prehensile lip. Like the elefinches they are still tied to the hard-shelled egg and have none of the adaptations of the vivas to nurture their eggs internally, but by carrying their eggs away with them to safety if threatened - something elefinches did not do - they are able to protect their unborn offspring almost as well.

The terries which have evolved on the northern continent have more varied reproductive strategies than their monogamous ancestors. Many species are polygamous, with one male maintaining a harem of several females and collectively protecting their chicks. The male continues to defend a territory on which several females nest, but no longer does he assist in the actual incubation, so many now raise only a single egg that they can easily pick up and carry if the need arises. As the males evolve away from co-parenting their young with the females, they are no longer restricted by predators to remain plain and inconspicuous whilst incubating. Females are then free to choose their partners on more aesthetic grounds, and sexual selection quickly produces more ornamented males to appeal to her personal aesthetics, and larger ones which can better defend her and her chicks. The males of some species are up to five times the females' size and so different in appearance that they no longer resemble members of the same genera, let alone species.

The Polymorph Bird

The most extreme changes can be seen in a species known as the polymorph bird, which has evolved to exhibit an especially extreme degree of intraspecific variation without comparison elsewhere in the animal kingdom. The polymorph is an omnivore with herbivorous tendencies that occurs in the brushy grasslands and shallow marshes of central Serinarcta, where they feed on seeds, herbs, and grasses as well as insects and occasionally small animals. Their general build is much like the squork, though the tail is more elongated and the tentacles on the face limited to just one upper and one lower appendage. They feed by coiling their upper and lower tentacles, each having become a prehensile lip, around plant stalks and then pulling them into the hidden beak where the food is mashed up before being swallowed.

The polymorph is remarkable for the great variation they exhibit in appearance and behavior. The female's appearance is usually pretty consistent; she stands five feet tall and is colored plainly, with short, tan fur-like plumage with a paler undercarriage. The male, however, occurs in four different morphs, each with a differing, corresponding behavioral tendency. The most common morph is known as the ardor, and differs from many other terries in that he is still nearly identical to the female. He is small and rather plain, just slightly heavier than her and distinguished only by a black head crest, some longer white plumage on his neck, some pale yellow markings on the snout and yellow eye rings. He is also has old-fashioned ideals in finding love; he is a monogame and will try to win over just one female, with whom he will attempt to form an enduring bond and to assist in raising her chicks. The second most common morphs is called the keeper, and he differs drastically from the ardor in that he is a highly dimorphic form, twice the female's size and ornamented with long, dark plumes that lay down over his body like hair, a reddish underside, a thick white ruff around the neck, and a bright yellow face. These males are very territorial and polygamous. They attempt to attract harems of as many females to breed and nest in their claimed territories as possible, while driving off other competing keeper males. The third most abundant morph, known as the tramp, is physically similar to the ardor but even closer to the female's appearance and lacks the head crest and white neck feathers - behaviorally, however, he is a deadbeat. Holding no territory of his own, he merely sneaks around the territories of keeper males, mimicking one himself to avoid the male's aggression, and then mates with the females sneakily behind his back. He is extremely promiscuous and doesn't form a pair bond, nor does he assist any of his partners in raising their young. The least common morph, the bachelor is arguably the most unusual, for they don't reproduce with females at all. They are monogamous like the ardor, but predominately form homosexual pair bonds; heterosexual pairings make up just ten to fifteen percent of partnerships. They are intermediate in physical appearance between the ardor and the keeper, with dark plumage and a yellow facial mask, bigger than the ardor or the tramp but smaller than the keeper. Usually unable to hatch their own eggs, they habitually use their size advantage to intimidate and steal them from other polymorphs, usually ardor pairs, then raising them as their own. They establish and keep to a territory but are not very aggressive about defending it and normally share their range with several pairs of ardors, which usually tolerate them as they are too large to easily drive away.

The existence of the four morphs is controlled by just two closely related but distinct genes, the 'A' gene and the 'B' gene, the combinations of which produce each of the different appearances and behavioral templates seen in the polymorph. They visibly affect only the males of the species, but are carried as well by the females and represented to a less obvious degree in their behavior; all of the females also fit into one of the four morphs, depending on the gene combination they carry, but their differences are almost invisible. The most common genotypes - the keeper, ardor and tramp - are each the form produced depending on how many dominant or recessive alleles they have of the most important 'A' gene, which is tied to the production of testosterone and represented A/a. Individuals with two dominant copies, A/A, mature into keepers, the most aggression and masculine variation, with the weakest parental instincts, which make up roughly thirty percent of the species. Males with one dominant allele and one recessive allele, A/a, become ardors, the most common genotype, comprising around forty five percent of all the species. Tramps only occur when a male polymorph inherits two recessive alleles of the gene, a/a, and are thus even rarer, making up approximately fifteen percent of the population. They have low testosterone levels, preventing the development of any male-specific plumage, but are still capable of reproduction. Their reduced aggression and lessened sex drives are beneficial traits in their method of reproduction, allowing them to sneak around and masquerade as females without attracting the attention of the keeper male, and giving them the self-control to wait to fulfil their urges until he is not around to catch them in the act. If they exhibited higher sex drives and aggression, fighting the keeper and harrying the females in broad daylight, he would be driven from the harem. Unlike the ardor, the tramp does not exhibit a stronger pair-bonding or parental instinct in the absence of higher testosterone levels, these facets of his behavioral repertoire apparently being stunted versus the other morphs and never developing fully, but the genotype does make him extremely social and almost completely non-aggressive. There is no male on male aggression and tramps mate with other tramps as often as with real females; they also are mounted by keeper males, fully mimicking female receptive posture in order to completely fool the dominant male as to their true sex.

The rarest genotype, the bachelor, which occurs in around one out of twenty males, occurs when two recessive alleles of the additional modifier 'B' gene, are present in an individual sporting the double dominant allele of the 'A' gene, A/A, which would otherwise mature into a keeper morph. This second modifying gene, in this genotype, stunts the amount of testosterone produced and thus reduces the levels of aggression and the development of the plumage in these individuals as they mature while simultaneously shifting the mate preferences of the male toward the same sex. The result is an animal intermediate in size and appearance between a keeper and an ardor.

above: the four male morphs of the polymorph bird. The female, not pictured, resembles the tramp morph.

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Since this is a seemingly deleterious genetic combination - a mutation that so frequently prevents this morph from successfully spreading its genes - it may seem unusual that it hasn't simply died out. The reasons for its persistence are several-fold, but mostly can be summed up by saying that the non-breeding morph is actually the occasional, inadvertent result of sexual selection for other desirable traits produced by different allele combinations with the second gene.

The second gene in question, the 'B' gene, is believed to be the gene from which the 'A' gene mutated sometime since the separation of the polymorph bird from its relatives five million years ago. Like the 'B' gene, it is also composed of three genotypes, and these can and do occur independently of the first gene in terries. The 'A' gene is present only in the polymorph, but the the 'B' gene is found in all terry species, including the squork. Both the 'A' and 'B' genes affect the male and female differently. The affects upon the males are different for both genes, but the genes affect the female in similar ways, controlling the degree to which the female of a given species is drawn toward the males of its species. Females with the double dominant alleles of either gene are relatively loose and polygamous, readily mating with many partners. The heterozygous individuals are more contemplative about their partners and the homozygous recessive especially choosy. Females who have matching dominant 'A' and 'B' genotypes, such as A/A and B/B, are the most indiscriminate with which males they mate with, while females with double recessive genotypes are the most particular. The mate choices of males favor the A/A and B/B females, which are the easiest to mate with. But the male terries are also affected by the modifier gene. We've already discussed the way the 'A' gene affects males, producing the different morphs, but the 'B' gene also affects the development of these morphs in addition.

The 'B' gene adds an additional modification to the 'A' gene in the polymorph birds, producing even higher levels of variation. Males of either keepers, ardors, or tramps which carry two dominant alleles have higher testosterone levels than males with other 'B' genotypes, relative to their levels as a result of the 'A' gene's affects. An ardor with a double dominant set of alleles is larger and slightly more colorful than one with a heterozygous genotype, for example, but it will be be less monogamous and less appealing to a female wanting a reliable partner. A keeper with two dominant 'B' alleles is likely to be even larger and aggressive than it already was, making them very fertile but less apt to make good fathers or kind mates, meaning that females may avoid joining their harems (or that they may be too aggressive to potential mates and harm them). A tramp with the B/B genotype will have a harder time waiting for the right moment to launch a mating attempt and be more likely to be caught and driven off by the keeper males who guard the harems he infiltrates, reducing his reproductive chances. Males sporting the double-dominant 'B' genotype are thus oftenless attractive to mates (or in the case of the tramp, less able to access partners), even though they are technically more fertile, and so the double dominant genotype remains a minority in the population.

Meanwhile, males of the three most common morphs which carry the heterozygous genotype of this gene, B/b, are less fertile but more social and more paternal. They are more reliable as mates, are less aggressive to other males (reducing the chances of them killing the female's offspring if she thinks they belong to another partner), and are the most preferred by all of the females, and are thus the most common 'B' genotype. Occasionally though, matings of these preferred heterozygous males with the different female genotypes will produce double recessive male, b/b. In the polymorph, the combined effects of the A/A and b/b produce a bachelor male. The existence of the bachelor male is due to a selection by females toward heterozygous (B/b) males, which are more social and less aggressive than B/B males, being more tolerant of other males and therefore inclined toward infanticide and which make more devoted partners. Because when a heterozygous or homozygous reecessive female mates with one of these preferred heterozygous males a small percentage of their offspring are going to show other genotypes, the bachelor morph, which is so socially tolerant of other males that it normally prefers them to female company and rarely breeds, is produced inadvertently.

In all other terry species besides the polymorph, the three 'B' genotypes are much less distinctly represented than they are in the polymorph and differ minimally enough to fall under the guise of normal individual variation; homozygous dominant individuals are a little larger and more likely to be more aggressive than heterozygous individuals and homozygous recessive individuals generally smaller, more social, and more likely to form same-sex pair bonds, but they don't look like different species, and the double recessive individual still readily pairs with the opposite sex. The bachelor morph apparently only occurs in the polymorph bird, when the 'B' gene coexists with the mutant 'A' gene specific to this species. When coupled with the 'A' gene the double recessive 'B' gene's effects are very particularly tied to the genotype of the 'A' gene represented in a given individual. The recessive alleles of the 'A' gene appear to guard against the affects of the b/b allele combination, so that male birds with heterozygous or homozygous recessive 'B' alleles are affected less than males with homozygous dominant males. The keeper male, with two dominant alleles and no resistent recessive ones, is drastically changed into the smaller, non-aggressive bachelor morph when it also inherits two recessive 'B' alleles. The double recessive genotype of the polymorph, the tramp - which has two of the recessive 'A' alleles and none of the dominant ones at all - doesn't experience any further loss of testosterone or any more homosexual behavior when also carrying the double recessive 'B' alleles than it does with the other genotypes of the 'B' gene. The ardor, which carries one dominant allele of the 'A' gene, is affected to a partial degree; a small percentage of ardors, which are naturally already less aggressive and more social than the keepers, inherit both copies of the recessive 'B' allele and these individuals exhibit duller plumage almost identical to the tramp but stay very different behaviorally. Due to the affects of the recessive allele, they mature to be even more social, extremely loyal to their partners, and to show a heightened frequency of same-sex pair bonding (which is very rare or absent in ardors with B/b or B/B genotypes), though to a lesser extent than the keeper does with the b/b genotype.

Though it may seem their existence is an evolutionary accident, the fact that bachelor males swipe eggs from reproducing females and rear them may have a net benefit to the success of the species. Females who lose their eggs will lay others to replace them, doubling or tripling their reproductive output, and pairs of non-breeders may have greater success raising their adopted chicks than pairs involving a male of the second monogamous morph, the ardor, and a female due to their increased size over both.

Female terries, including polymorph birds, differ in their mate preferences depending on their own inherited copies of the 'A' and 'B' genes; double-dominants with the A/A and B/B combo are the least choosy and the least inclined to monogamy, and thus the most likely to join harems and mate with the least attractive mates, the big and aggressive keeper males with A/A and/or B/B genotypes, while the heterozygous and double-recessive females, B/b A/a, and b/b and a/a, are both more inclined to pair with less aggressive ardors, with similar A/a and B/b genotypes. In time, it might seem this would eventually lead to several speciation events, with females regularly mating only with males that match their own genotypes of both genes, eventually causing the morphs to evolve into distinct species, but the issue is much more complicated than it first seems. The preferences of heterozygous individuals - ardors - to mate with other heterozygous individuals, for example, ensures that for every two offspring produced who will share their genotypes, there will be a double recessive individual and a double dominant individual of of the 'A' and 'B' genotypes. There thus could never be a pure species of ardor without the continued production of occasional keepers, tramps, and occasionally bachelors.

The speciation of the homozygous genotypes - the keeper and the tramp - are not necessarily impossible, as these forms breed true together, but this is unlikely to ever happen with the keeper because the tramp male throws another wrench in the system. As tramps associate with keeper males and sneakily mate with all of the females in these groups, and always exhibit a totally opposite double recessive a/a genotype and varying 'B' genotypes, they ensure that all of the different genotypes, both 'A' and 'B', continue to mix and mingle, keeping the species diverse and highly varied. And without keepers, the tramp male has no reproductive strategy, tethering it permanently to this other morph for its survival. It could not persist as a species by itself.

The combined effects of the two genes in different allele combinations produce an enormous variety of diversity in the polymorph bird. Some chicks will grow up to be particular and some indiscriminate, some large and some small, some gentle and monogamous and others aggressive and polygamous. The greatest variety of offspring come from ardor pairs, but any male morph can produce any type of offspring depending on the type of females he mates with. Through the actions of a single genetic mutation and the consequences of mate choice by both sexes, the polymorph bird has adapted into one of the most widely variable species ever to evolve. With so many different options available, polymorphs are able to make the most efficient use of their environments, and different morphs predominate over others depending on the local habitat. All female polymorphs naturally lay two eggs at a time, but may not be able to raise both, depending on their circumstance. Keepers and their harems, and in turn the tramps that associate with them, have the most reproductive success in brushy territory, where the females can build a more intricate nest than most other terries in a secure sight from the sights of predators, in the company of the other females, while her mate patrols around them. If they are threatened, their mate will be busy either fighting off the predator or telling them to flee, and he doesn't help them to shelter their eggs. In the event a predator is coming at the nest, each female can pick up only one egg and flee, leaving the other to be eaten. In open country, meanwhile, individual pairs of polymorphs, consisting of an ardor and his mate, have greater success. Here they don't build much of a nest and move their egg more often throughout their territory, as both partners can take the eggs and flee if threatened, meaning they more often raise both eggs to hatching. Because a harem of many individuals is harder to steal an egg from than an isolated pair, bachelors normally avoid the large keeper males and associate with ardors. Young females are nomadic and initially move between different areas as they mature, depending on their preferences in mates, before settling down with the type of mate that most appeals to them.

Though gene flow occurs regularly between the different morphs and genotypes, issues can arise if a female and male of very different behavioral tendencies are made to mate due to a local lack of other options. A monogamous ardor will wear himself out trying to keep a promiscuous female who would rather be part of a harem to stick around, lacking the size and strength to control her as a keeper male would, and after laying her egg and seeing him take care of it she will often abandon him to find a new partner, leaving him as a single dad. Conversely, a very loyal pair-bonding female will be left in the same boat if her only choice of mate is a tramp, who will leave after the honeymoon in search of other females. A pair-bonding female will sometimes join a harem and bond to a large and aggressive keeper male, trying to drive off any others that approach her chosen mate and force her conservative relationship views on her liberal partner. In the absence of another like itself, even a bachelor will occasionally bond with a female and produce offspring, though this must occur only infrequently and usually due to a local lack of other bachelors. Perhaps the rarest pairing of all is between a bachelor and an ardor, usually a smaller one with a double recessive pair of alleles on its 'B' gene - a coupling normally still uncommon as the non-breeder is normally inclined to court other males with impressive male plumage, which the ardor mostly lacks. Bachelors will also attempt to court full-fledged keeper males unsuccessfully, as the keepers drive them off as they do other keepers. Tramps don't pair-bond, and so pairings between them and other morphs are restricted to brief sexual encounters, but even so, these must occur at least occasionally between the tramp and the bachelor given the vagrant disregard for the sex of their mates on the part of the tramp.

Polymorphs of all variations raise their young for between six months and one year. Like squorks, the chicks can walk and feed themselves within a few days of hatching. Chicks raised by single pairs stay with their parents longer than those raised by harems of females, which leave the harem younger and band together in juvenile groups for the next few years before reaching sexual maturity; the juveniles raised by lone pairs will also eventually join these mixed groups. Though all the male chicks begin life identically, their development begins to deviate by eighteen months of age as ardors grow larger than tramps, bachelors grow larger than ardors, and keepers in turn keep growing after the bachelors cease. The smallest morphs are sexually mature earliest, with tramps being capable of breeding by eighteen months, ardors by two years, bachelors forming pair bonds around 2.5 years and keepers not being fully developed until the age of three or four. Females begin to breed around the same time as the ardors. The mixed-sex bands break apart into distinct groups as the changes in the males become apparent, with females forming their own bands, often with the female-mimicking tramp males, and the keeper males, becoming intolerant of one another, going their own way to lead solitary lives until they can collect a harem of their own. Ardors and bachelors, which are more social than the keepers, will often remain in bands together after the females, tramps, and keepers have split off until they start to pair off and go on their own ways.