Guppy Study: Darking's Book Reference

The Guppy Study, conducted by evolutionary biologist John Endler in the 1970s, explored the wide variation in colouration among male guppies in different Trinidadian streams. Endler's research revealed a strong correlation between a guppy's habitat and its colouration, with males from one pool sporting vivid blue and orange splotches, while those downstream had only modest dots of colour near their tails. Endler's findings also indicated a link between the presence of specific predators and the colouration of guppies in different pools. Through his study, Endler aimed to understand why guppies exhibited such diverse colouration, despite the risks of standing out to predators.

Guppies' colouration and the correlation with their environment

Guppies are born without any colouration and develop their hues gradually as they mature. Their colouration is influenced by a combination of genetic and environmental factors, such as temperature, diet, and light. Guppies have four types of colour cells: xanthophores, erythrophores, iridophores, and melanophores, which are derived from neural crest cells during embryonic development. The distribution, density, size, shape, and arrangement of these colour cells create the diverse patterns and hues seen in guppies.

Genetics play a significant role in guppy colouration, with different strains and breeds exhibiting distinct colour patterns and combinations. The genes that control guppy colouration are mostly located on the sex chromosomes and primarily affect males. Females have two X chromosomes, while males have one X and one Y chromosome. The Y chromosome carries genes that suppress or enhance the expression of colour genes on the X chromosome, resulting in different colour combinations inherited from their mothers.

Hormones also influence guppy colouration by affecting the activity and responsiveness of colour cells. Thyroid hormones regulate the differentiation and migration of neural crest cells into colour cells during embryonic development. Melanin-concentrating hormone and melanocyte-stimulating hormone affect the dispersion of pigment granules within melanophores, making them appear lighter or darker in colour.

Environmental factors, such as water quality, diet, and lighting, can also enhance or diminish the expression of colour genes and affect the activity of colour cells. For example, soft water with a low pH can intensify blue colours, while hard water with a high pH can enhance red colours. Guppies obtain the bright colours in their skin from carotenoids in their diet, such as algae, spirulina, shrimp, and krill. Natural sunlight is the best source of light for guppy colouration, providing a full spectrum of wavelengths that reflect off iridophores and enhance carotenoids in the skin.

The role of thyroid hormones in male guppies' colour patterns

The thyroid hormone is a key regulator of post-embryonic vertebrate development. It has been found to play a role in the colour diversity of male guppies. Male guppies display a wide variety of colour patterns, which are a result of a complex interplay between sexual selection, predation, and other environmental factors. The heterogeneity of the environment affects the ornamental traits in male guppies through genotype-environment interaction.

Thyroid hormones (THs) are important regulators of the ontogeny of fish and serve as a link between environmental changes and phenotypic development. THs have been found to affect the timing and rate of the development of coloration in male guppies, leading to an increase in phenotypic variability. The experimental assessment of the effect of THs on the variability of ornamental traits in Poecilia wingei males revealed that alterations in the TH status caused changes in the timing and rate of the development of coloration. This, in turn, affected the transformation of various elements of the colour pattern in males, leading to the appearance of ornamental traits in the male colour patterns that were characteristic of closely related species of Poecilia.

The data obtained from the experiments indicate a potentially important role of thyroid hormones in the diversification of guppy colour patterns. This opens up new prospects for studying the role of endocrine regulatory mechanisms in the adaptive evolution of poeciliid fish.

Guppies' mating system and its benefits and disadvantages

Guppies have a mating system called polyandry, where females mate with multiple males. Multiple mating is beneficial for males as their reproductive success is directly related to how many times they mate. The cost of multiple mating for males is very low because they do not provide material benefits to the females or parental care to the offspring.

Conversely, multiple mating can be disadvantageous for females as it reduces foraging efficiency and increases the chances of predation and parasitic infection. However, females gain some potential benefits from multiple mating. For example, females that mate multiple times are found to be able to produce more offspring in shorter gestation time, and their offspring tend to have better qualities such as enhanced schooling and predator evasion abilities.

Female guppies mate again more actively and delay the development of a brood when the anticipated second mate is more attractive than the first male. Experiments show that remating females prefer a novel male to the original male or a brother of the original male with similar phenotypes.

Inbreeding ordinarily has negative fitness consequences (inbreeding depression) and as a result, species have evolved mechanisms to avoid it. In the case of guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between the sperm of rival males for achieving fertilization.

Female guppies' preference for brightly coloured males

The guppy, or *Poecilia reticulata*, is a small fresh water fish with sexually dimorphic males and females. Males are known for their bright, colourful spots and splotches, which stand out against the drab olive-grey colouration of the female.

The guppy is an example of a species where females prefer to mate with brightly coloured males. This preference is seen in numerous species, and is a key component of sexual selection theory. The hypothesis suggests that if the expression of costly preferred traits in males (e.g. conspicuous colours) is proportional to the male's overall quality, then a well-developed trait should indicate good condition and/or high viability. Therefore, a female choosing such a male would stand to gain direct or indirect fitness benefits, or both.

In the case of the guppy, the visual conspicuousness of the male's colour pattern has been found to correlate positively with boldness toward, and escape distance from, a cichlid fish predator. This means that bolder individuals are more informed about nearby predators and more likely to survive encounters with them.

In a study by Godin and Dugatkin, female guppies were given the opportunity to observe the behaviour of potential male mates toward a fish predator. The results showed that females preferred to mate with colourful males, but when given the choice, they would select bolder males irrespective of their colouration. By choosing colourful males, female guppies are thus choosing relatively bold, and perhaps more viable, individuals.

A later study by Sakai and Kawata investigated the genetic mechanism underlying colour vision variation and its relationship with female preference. They found that one of the opsin genes (LWS-1) that codes for a protein that detects visible light in the orange-red spectrum, showed allelic polymorphisms. These variants are maintained by natural selection in wild populations. The study also showed that the expression levels of multiple opsin genes varied between individuals with different LWS-1 genotypes and that rearing light environments also affected the expression levels of opsin genes. Using behavioural experiments, the researchers demonstrated that the variation in LWS-1 expression affected the sensitivity of individuals to different-coloured light.

These results indicate that the expression variation in opsins is caused by both genetic polymorphisms and light environment during growth, which in turn generates variation in light sensitivity and female preference for male colour patterns.

Guppies' risk-taking behaviours when encountering predators

Guppies exhibit a range of risk-taking behaviours when encountering predators. One such behaviour is predator inspection, where some guppies approach the predator to assess the danger. This behaviour is risky as it increases the likelihood of being noticed and attacked by the predator. To mitigate this risk, guppies avoid the predator's attack cone and approach from the side or back. They also form groups for protection, with larger groups observed in high-predation populations.

Guppies also display a conditional-approach strategy, where they will inspect the predator initially but may retaliate if co-inspectors do not participate or do not approach the predator closely enough. This strategy resembles the tit-for-tat hypothesis, where the guppies' behaviour is conditional on the actions of their co-inspectors.

Another risk-taking behaviour observed in guppies is the darkening of their irises from silver to jet black when they detect a predator. This draws the attention of the predator to the guppies' head instead of their body's centre of mass. This diversionary tactic allows guppies to pivot out of the way as the predator lunges, mimicking the movement of a matador.

Guppies also tend to school together to avoid predation. Schooling is more common in populations under high predation pressure, exerted by either predator type or density. Additionally, the colouration of guppies evolves in response to predation pressure. In high-predation environments, male guppies evolve to be duller in colour with fewer and smaller spots, making them less conspicuous to predators.

Guppies' risk-taking behaviours, such as predator inspection, are evolutionarily stable when mechanisms are in place to prevent selfish individuals from taking advantage of "altruistic" inspectors. The conditional-approach strategy may serve as such a mechanism, where guppies retaliate against defectors, maintaining the stability of the group.

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