Guppy Ecology, Sexual Selection and Sensory Bias

Guppies choose mixtures of simple perceptual opposites such as bright and dark colors when they mate.

Topics

Ecology
Feeding
Predators
Habitat Interactions
Social Behavior
Sex
Sexual Selection
Male Competition
Direct Benefits
Good Genes
Sensory Bias
Works Cited

Ecology

The Trinidadian guppy (Poecilia reticulata Peters, 1859) is a ray-finned fish in the cyprinodont family poeciliidae, Greek for “with different colors” (Froese and Pauly 2008). It’s also known as the “millions fish” or the “rainbow fish.” Males grow to a maximum size of about 3.5 cm, females to about 5 cm standard length. Guppies have no dorsal spines, seven to eight dorsal soft rays, no anal spines, and eight to ten anal soft rays (Rodriguez 1997, Froese and Pauly 2008). In the poeciliid family there are 293 extant species living in freshwater and brackish habitats at low altitudes in the tropics from the eastern United States to northeastern Argentina, as well as in Africa and Madagascar (Froese and Pauly 2008, Rodriguez 1997). Guppies come from Venezuela, Barbados, Trinidad and Tobago, northern Brazil and the Guyanas, (Rodriguez 1997, Froese and Pauly 2008, Andersson 1994, Houde 1997) but have been introduced widely for mosquito control.

They tend to live among vegetation in the slow-flowing water of forested freshwater springs, ponds, canals and ditches. Their distribution throughout many small streams results in populations with various evolutionary degrees of independence (Houde 1997). Some populations are estimated to have diverged as much as 600,000 years ago (Houde 1997). Gene flow distance per generation was estimated to be about 0.75 km (Endler 1977). Populations within streams are made up of loosely connected sub-populations, with gene flow sometimes cut off by barriers such as waterfalls (Houde 1997). Guppies are sometimes found in distinct populations above and below large waterfalls. It’s not known how the areas above waterfalls were colonized, but ideas include transportation by birds or windstorms, or colonization before the formation of the waterfalls (Houde 1997).

Feeding

Guppy feeding is omnivorous and opportunistic (Houde 1997). They may eat zooplankton, insect larvae and small adult insects, other invertebrates, algae, benthic detritus, their own young, the young and eggs of other fish such as Rivulus hartii (Houde 1997, Rodriguez 1997, Froese and Pauly 2008), and berries (Brooks 1996).

Predators

Guppies have different sets of natural predators in different parts of their range, including, overall, at least seven cichlids, five characins, three gobies, three pencilfishes, the killifish Rivulus hartii, the mullet Agonostomus monticola, the common snook Centropomis unidecimalis, and the prawn Macrobrachium crenulatum (Houde 1997). R. Hartii, an omnivore, is often the only predator in the upper reaches of streams, so mortality there is low relative to that in lower reaches (Reznick et al. 1997).

Guppies under higher natural predation mature at an earlier age and smaller size, allocate more resources to offspring, produce more offspring more often (Reznick et al. 1997), form more cohesive schools, remain near the banks of streams (Houde 1997), display less in courtship and attempt more “sneak copulations” (Godin 1995). Life history differences such as these between low and high predation populations have been shown to be genetic. In guppies transplanted from high to low predation sites life histories evolved toward later maturation and less investment in offspring (Reznick et al. 1997). Changes in males occurred in as little as 4 years, while females showed significant change in maturation rate by 7.5 years. Therefore male guppies appear to have greater genetic variation and rates of evolutionary change than females. O’Steen et al. (2002) showed that the ability to escape from predators can evolve very rapidly — within 26–36 generations, and that it also deteriorates rapidly in populations transplanted to low predation sites. Reznick and Ghalambor (2005) have used the results of life history experiments with guppies to predict similar changes in populations of commercially important fish, predicting evolution toward earlier maturity at a smaller size on a decade time scale.

Habitat Interactions

Microhabitat preference, courtship behavior, and swimming performance are all affected by the physical characteristics of the habitat in which guppies live. Those from the headwaters of streams prefer greater water velocity microhabitats and perform better in swimming tests than males from downstream reaches, and they tend to display to females in fast flowing water, while guppies downstream display in still pools (Kodric-Brown and Nicoletto 2005).

Social Behavior

Social organization and behavior can differ depending on predation and water quality (Andersson 1994), but shoals usually consist of 2 to 20 fish, and encounters occur about once every 14 seconds. The shoals disperse at night and reassemble in the morning, creating the potential for rapid mixing and random shoal membership. Croft et al. (2004) used mark and recapture to model guppy social interactions as a network (with guppies as nodes and “edges” representing associations between them). They found a “giant connected network,” with associations occurring in 726 out of 4851 possible cases. The average fish had 14.7 associations and was thus connected to about 15% of the network. Pair-wise female associations were significantly persistent and male-male ones were not. Guppies meet the criteria for what in network theory is called a “small world,” where information and disease are able to spread rapidly. Still, the clustering coefficient, a measure of “cliquishness” (e.g. by phenotype assortment or associations between familiar individuals) was larger than that expected in a randomly wired network. Clustering may evolve at least in part because it tends to restrict the spread of epidemics (Croft et al. 2004).

Croft et al. (2004) suggest that active preferences, at least among females, are what give rise to the persistent pairs they observed, and mention a hypothesis of preference evolution — repeated association between individuals is thought to increase familiarity, which in turn is thought to confer advantages such as reduced competition and increased predator evasion.

Sex

The family poeciliidae have internal fertilization (males transfer sperm via penis-like gonopodia) and give birth to live young (Houde 1997). Reproduction in guppies is promiscuous and occurs all year, but may be limited by environmental factors (Houde 1997). Minimum population doubling time is less than 15 months (Rodriguez 1997, Froese and Pauly 2008). They have typical X-Y sex determination and heterogametic males (Houde 1997). Color patterns are expressed in males only (Houde 1997). Those of morphs show X or Y linkage, Y more commonly than X, and are inherited as a unit (Houde 1997). Males usually inherit their fathers’ color pattern intact, but with some variation in tail coloring (Houde 1997). Males spend a large portion of every day searching for and courting with females and may mate several times in a day (Houde 1997). Females mate with two or three different males during each receptive period (Houde 1997), two or three days in each 25–30 day reproductive cycle (Houde 1997). There is a strongly male biased operational sex ratio because the number of receptive females at any given time is low (Andersson 1994). There is often strong competition for mates and high male display rates (Andersson 1994). Males may display up to 2.7 times per minute, but patterns in display rate vary (Houde 1997).

Sexual Selection

In most animals factors determining the evolution of mating preferences are not well understood (Rodd et al. 2002). Sexually selected characters in guppies have been studied much more extensively than those of most other species, and still we understand very little about how and why they evolved. Most fish species have inconspicuous or cryptic color patterns in their natural habitats (Endler 1980), but many do not. Within the poeciliid family many species show apparently sexual ornamentation, including conspicuous coloration, complex patterning, and elongated fins. But within this family guppies have the most elaborate color patterns, the greatest degree of sexual dimorphism, and the greatest amount of color pattern polymorphism (Houde 1997, Endler 1980).

The color patterns reflect a combination of selection for crypsis against predators and selection for conspicuousness to females (Endler 1980). Degree of conspicuousness can be measured as the deviation from a random sample of the background against which an animal is seen and the patch size, coloration and brightness of the pattern on the animal. Sexual selection in guppies favors higher degrees of conspicuousness by way of this deviation from the background. The balance between crypsis and sexual selection shifts when predation and/or background color pattern parameters are changed. Guppies in their natural environments experiencing higher predation show less conspicuous patterns, with reduced color patch size and fewer bright spots (Endler 1980).

Male Competition

Understanding the origin of any sexually selected trait begins with determining whether it’s predicted by competition between males for mates. A number of facts about guppy mating suggest that ornamentation is maintained by female preferences as opposed to male-male competition. Only about 10% of encounters between courting males and females are likely to result in courtship. There is little obvious sexual competition between males. Females usually decide when and with which male they will mate (Houde 1997).

Direct Benefits

Next it’s essential to determine whether the trait is directly associated with a male’s tendency to provide a female or her offspring with important resources. Male guppies usually do not provide females with any resources (Andersson 1994), and more ornamental males don’t tend to provide females or their offspring with more resources than less ornamental males. This rules out the possibility of evolution of preferences in response to direct benefits.

Good Genes

The three remaining theoretical mechanisms of sexual selection — good genes (see Trivers 1972, Zahavi 1975, Hamilton and Zuk 1982), Fisher runaway (Fisher 1930), and sensory bias (Endler and Basolo 1998) — could all be important in guppy mate preference evolution. However, Fisher runaway does not predict the complexity of guppy sexuality, because it doesn’t predict complexity at all, certainly not a large suite of sexual traits and a large amount of variation in coloration between individuals. What remains, then, in understanding the complexity of intersexual signals is to determine the applicability, or relative importance, of good genes versus sensory bias models in the evolution of guppy sexual preferences and related sexual traits.

Preference for more extensive orange (carotenoid) coloration appears to be particularly strong (Brooks 1996). Experiments have shown a correlation between the extent of carotenoid coloration and foraging ability, parasite resistance, and overall male condition (Brooks 1996). Therefore the evolution of orange coloration might be explained in terms of indirect benefits accruing to the offspring of females who prefer it. However, Grether (2000) found no covariance between female preference for orange and carotenoid limitation, and therefore a lack of support for the idea that females can predict foraging ability or mate health using orange coloration.

Sensory Bias

An alternative explanation of the preference for orange is pre-existing bias for orange because guppies consume the orange colored fruit of the cabrehash tree Sloanea laurifolia (Rodd et al. 2002). However, male attractiveness is also reduced when black area (melanin) is reduced (Brooks 1996). Brooks (1996) suggested that the black area evolved as an amplifier for orange areas, which could improve the accuracy of female discrimination.

It’s possible that a multicolored pattern rather than single color is preferred (and maintained) because of its relative stimulatory complexity, instead of magnitude. Guppy coloration is almost always a compilation of bright and dark colors. It’s subtle, in this way, rather than purely conspicuous. It’s rare, based on photos from the web (see guppy photos on Pinterest), for a male guppy to be colored with only brighter (e.g. red, orange, yellow) or only darker (e.g. blue, purple, black) colors. They tend to be orange and black, red and black, red and blue, orange and blue, or mixtures of other contrasting colors rather than being as bright as possible. This indicates females are looking for a bright — dark mixture rather than pure brightness, along with the fact that darker and brighter colors are often dispersed within each other in extremely intricate ways in their tails. The Pinterest guppy page contains photos of both pet store and wild guppies, and photos of both Poecilia reticulata and Eldler’s guppy Poecilia wingei, native to Venezuela.

The abundance of sexually selected patterns in animals and human cultural phenomena expressing guppy-like mixtures of brightness or brighter colors with darker colors suggest a widespread, sensory system-based bias favoring these mixtures. Guppies are probably like other animals, ourselves included, in finding red, orange, yellow and white exciting. These are warning colors in nature and in human culture. It’s indisputable that brightness is physically more exciting for the animal nervous system than darkness, and not unrealistic to imagine colors of one wavelength being slightly more physically excitatory to a brain than those of another wavelength. It’s hard to imagine how we could know the difference between one color and any other without gauging the wavelength. That we find brightness and redder colors more exciting is built into nonsensical, idiomatic language: “delighted,” “a red flag,” “seeing red,” “go bananas,” “rose colored glasses.” Like guppies we associate brightness with sex, as in the colors of Valentine’s Day and the expressions “take a shine to,” “love light,” “red light district” and “knight in shining armor.”Meanwhile we describe the opposite of excitement as dark and blue. A reasonable assumption is that a mixture of dark and bright, black and white or blue and red conveys an intermediate amount of excitement, an appealing sort of moderation you don’t get from one or the other by itself.

It’s common for sex organs to evolve bright coloration, but the brightness is also very commonly offset by a darker-colored component. Decorative makeup in humans is normally a bright — dark mixture of some versions of red, blue, brown and black. Our brown, blue or green irises are in contrast with the redness of our lips and the whites of our eyes. Although it seems like the guppy world is far removed from our own, we have in common an appreciation for bright — dark mixtures. This is evident in the way we dress, not like clowns and priests most of the time; plus, clowns and priests take measures to counteract the extremity of their respectively too-bright and too-dark appearances.

The lasting appeal of the classic love poem about red roses and blue violets is perhaps a manifestation of an association between mixtures of these colors and attraction. We’ve been intrigued by this and passed it on for hundreds of years. These lines from Edmund Spenser’s poem The Faerie Queene from 1590 supposedly initiated the trend (Spencer et al. 1910):

It was upon a Sommers shynie day,
When Titan faire his beames did display,
In a fresh fountaine, farre from all mens vew,
She bath’d her brest, the boyling heat t’allay;
She bath’d with roses red, and violets blew,
And all the sweetest flowres, that in the forrest grew.

In modern versions of the poem all that remains is rhyme, redness, blueness, flowers, honey and love. Spencer’s poem being responsible for the origin of the current, popular rhyme doesn’t explain the persistence of the contents from one of its lines over such a long time, its transformation into various other forms, or, really, why anyone other than the Spencer would be particularly taken by the idea. We often think of phrases and ideas as though they have some sort of self-propelling inertia that carries them through minds over time, almost against our will, without psychology or preferences or biases having much to do with it. We think a funny phrase, idiom, mythology or simple aesthetically pleasing mixture like red and blue in a flag or black and white in the Yin Yang symbol has a single random origin and it carries on by chance, fate, herd behavior, divinity or just lack of change. Another view is the same preferences leading to their invention maintain them because the preferences themselves are universal and they don’t change as easily as words, stories, symbols and bodies.

Back in the guppy world, we could assume male ornamentation originated on one occasion, in a single male, by chance, and by chance in a single, unrelated event a female preference for the ornamentation originated, in a single female, on one occasion, then both the preference and the trait spread through the populations until every guppy expressed them as they do today, for complex, interactive, ecological reasons which are nearly impossible to decipher. Otherwise, perhaps, certain simple, universal preferences, such as that for a mixture of bright and dark colors, maintain the pressure on males to evolve their ornaments at all times, and don’t change as easily as male guppy color genes. The ornamentation spreads through the population in response to relatively invariable, physical, sensory bias-based preferences, when populations can afford it, given variance in predation, because guppies like it that way by default.

Perhaps the most salient example of aesthetic bright — dark coloration in animals is the Thayer effect — the fact that most animals are darker on the upper surface and brighter on the lower surface. Research thus far has focused on this arrangement somehow being cryptic for animals in daylight, but it’s much more simply understood as a product of simple, universal sensory biases. Smaller-scale, undeniably sexual traits often show the same upward darkness and downward brightness. Whether it holds for guppies or not is difficult to tell because of the amount of variation they exhibit, but the Thayer effect shows that a bright — dark mixture is extremely common in animal body coloration. The black portions on a male guppy are functional in winning him mates (Brooks 1996), even though these patches are reflecting very little or no light into female guppy eyes. What female guppies seem to be looking for is not maximum sensory stimulation. It’s an intricate mixture of more and less stimulation.

Kodric-Brown and Nicoletto (2005) showed that where predation is high carotenoid ornamentation is positively correlated with swimming performance. Therefore females choosing the most colorful males might be choosing good genes for swimming and escape ability in their offspring. At higher altitudes, where predation is usually low, females may benefit from choosing males which display in fast moving water, thus demonstrating their vitality, and good genes (Kodric-Brown and Nicoletto 2005).

Though we may be able to explain the evolution of a preference for orange coloration, high display rates, and perhaps a number of other traits, guppy sexual ornamentation has many components, including several colors other than orange, each of which varies among males in a population (Blows et al. 2003) so much that no two individuals are alike (Endler 1980). The coloration itself is complex (Houde 1997), incorporating “combinations of black, white, red-orange, yellow, green, iridescent, and other spots, speckles and lines” forming mosaics of spots or patches which themselves vary in overall color, size, position and reflectivity (Endler 1980), and is controlled by at least 40 loci in the genome.

There are many ways for a male to be attractive to a female (Blows et al. 2003). Females may assess males and make decisions about mating based on, more or less, male coloration, including ultraviolet (Smith et al. 2002), color pattern, spot number (Ender 1980), tail length or size. Males have larger tails than females. In aquarium strains tails with an area of 220 square millimeters are favored by females about four to one over 16 square millimeter tails (Gould and Gould 1989, see also Bischoff et al. 1985). Familiarity might play a role. Females are more attracted to males from their own population than males from alien populations (Endler and Houde 1995). Rarity, novelty (Farr 1977), overall size, total length, body height, gonopodium length (Houde 1997), boldness in approaching predators (Godin and Dugatkin 1996), and probably various dynamic aspects of the courtship display contribute to guppy appeal. The dance consists of quivering and forming the body into the shape of an S, which may stimulate a female visually as well as by way of her lateral line.

This list poses a problem for the good genes model because good genes predicts the evolution of only one type of male ornament in an ideal, or error free, signaling system (Brooks 1996). The guppy is one of many species that possess multiple ornaments. One proposed explanation for multiple ornaments is “inaccurate perception” on the part of choosing females (Brooks 1996). Whatever the case may be it’s unlikely that every attractive feature of a male guppy can be explained in terms of a correlation with ecological fitness. I’ve found no reasonable explanations of a preference for blue, yellow, high spot numbers, large tails, or dancing. It may be we need more creative ways of understanding apparently ecologically detrimental features, but I find it more likely (and parsimonious) that guppies express an aesthetic sense, which might not be entirely different from our own. Good genes models have difficulty explaining polymorphism in secondary sexual characteristics, which is extreme in guppies. If ornaments are correlated with survival ability, and preferences for them evolve accordingly, then we should not expect the evolution of a large set of preferred traits and preferences, each varying widely within a population.

Guppies and humans apparently have somewhat similar ideas of beauty. These fish have become such popular pets because they are decorated in ways that appeal to humans, and because their ornamentation is extremely diverse. The ornamentation and its diversity are a product of the fishes’ own preferences, and disinterest in the status quo among guppies themselves is likely the cause of the extravagance that makes them popular. Farr (1977) found that females strongly prefer rare and novel mates, and says that this is a probable reason for the polymorphism in male traits, and that it may have evolved to support genetic diversity in the face of environmental contingency and to prevent inbreeding in founder populations. This preference for novelty is reminiscent of the way humpback whale (Megaptera novaeangliae) preferences drive rapid changes in the structure of the songs they sing.

An appreciation for novelty, taken together with the preference for moderate familiarity of mates, indicates that what female guppies, like female humpbacks, really want is a blend of the two, or moderate originality. While there are working theoretical reasons why a preference for moderate originality could evolve, it should be mentioned that this could also arise from certain essential structural characteristics of sensory systems and brains (Endler 1993):

“Given that a sensory system can have biophysical properties which are independent of evolutionary history, it is possible that these properties may actually bias the direction of evolution of signals. It is possible that pre-existing biases may exist in the brain, for similar reasons.”

In pre-existing bias models it’s usually assumed that the bias evolved in a context other than mate choice, and in direct relation to some aspect of ecological fitness. For instance, according to Endler and Houde (1994) geographic variation in traits is predicted by sensory bias models. I don’t think sensory bias necessarily predicts geographic variation. It depends on the level within sensory and cognitive systems from which the preference arises. If a bias is related to the structure of the brain at a deep enough level then it should be found consistently among species, with little geographical variation, and could be considered as a general aesthetic sense something like that envisioned by Darwin (1871) in the original theory of sexual selection. A bias of this kind would be expected to be general, for instance favoring complexity in patterns over simplicity. One type of complexity common to all animal sensory systems is that which accompanies the liquid crystalline condition of organisms and brains. Also, liquid crystals tend to be extremely colorful like guppies (Leslie 2016):

“Under polarized light, liquid crystals resemble shattered rainbows, swirling seas of dye, dazzling jewels come to life. These threads, steps, terraces, planes, droplets are as if alive, for they move, under their own power. They meet and coalesce. They copulate, forming new shapes and colours. They seem to be little life forms.”

Female guppies vary between populations in preference for the intensity, presence or absence, and the overall number of sexual traits (Endler and Houde 1995). Variation in preferences shows they are genetic and subject to evolutionary change, and seems to suggest that they are not simply part of a general aesthetic sense in guppies, or in the guppy genus, family or higher-order taxonomic group. However, it’s still possible that some preferences are the ancestral condition, and are suppressed when they sufficiently lower fitness. It’s possible that some preferences evolved along with preferred traits (perhaps orange coloration and display rate), yet others, like a preference for complex patterning and elongated fins, evolved with the physical structure of the brain itself — as by-products of the way a brain must be built in order for it to work. I say this because complex patterns and elongation of body parts are common in sexual ornamentation in many taxa, including numerous fishes, birds, insects and mammals. Houde (1997) points out that sometimes older male guppies develop extreme elongation of pigmented portions of their caudal fins, and mentions preexisting sensory bias as a possible explanation. Preexisting bias for pigmented, elongated caudal fin rays has been demonstrated in related fish (see Basolo 1990, 1995).

Pet store female guppies show a preference for the extremely elaborated tails (beyond what occurs naturally) of pet store males (Houde 1997). Although these fish have been bred for attractiveness to humans, we cannot rule out the possibility that they could evolve such elaborate tails naturally given a long span of time without predators and/or limiting resources. Lack of competition and predation in New Guinea are likely the reasons for the unmitigated ornamentation of the birds of paradise (paradisaedae), such as the tail of the ribbon-tailed astrapia (Astrapia mayeri). This bird’s body is about 25 cm long and its tail is about 90 cm (Endangered Wildlife 2001). Elongated body parts probably have an increased effect on the visual system of mates, causing more neural stimulation. Another effect of elongation is that the stimulus (e.g. a larger tail) is increasingly dynamic (e.g. more waves may travel along the tail at a given time as the fish dances).

Guppy dances and shapes, like their colors, and along with animal dances in general, are not maximally stimulating. Only part of the fish is elongated and dynamic. Dancing is not the same as simply moving rapidly in front of a mate (presumably the most stimulatory presentation), but it’s hardly static. Likewise, a dance cannot be described as either exclusively regular or random. A guppy dance is a satisfying mixture of dynamism, stasis, randomness and regularity, fluidity, roundness, length and bright and dark colors, with a certain liquid crystalline essence, in which some of the qualities are more exciting and others are less so.

It seems possible, and probable, that increasingly complex mixtures of more and less exciting stimuli are chosen by mates because they tend to cause more complicated neural stimulation, that it happens because the brain itself is complex in a similar way, and that many, if not most animals with an acute visual sense would tend to evolve similarly complex patterning and ornaments if mate choice were not limited by the need to survive.

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Life in terms of the nonliving. Universal animal preferences for mixtures of simple, more and less exciting psychological opposites.

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