Guppies (Poecilia reticulata) are native to Trinidad and northeastern South America. The guppy is a popular study organism among biologists because it is characterized by wide variation in behaviour, life history characters and appearance. Aquarium fish hobbyists have long been interested in the many colours male guppies can take, and breeders have produced different guppy lines in the same way that dog breeders have produced poodles, retrievers and terriers. Guppies in their natural environment look and behave much differently than guppies found in most pet stores.
Some differences between natural and store-bought guppies:
1. Male and female store-bought fish are often the same size. Natural male guppies are smaller and have a narrower abdomen than females.
2. Female store-bought fish are often colourful. Natural female guppies are drab and cryptic against light-coloured backgrounds. Natural males are very colourful (especially orange), but the colour appears in spots and rarely appears on the face, unlike store-bought males, which have colour on their entire body.
3. Cannibalism of young seems to occur most often when guppies are in confined spaces. In nature, newborn guppies are often found in geographically distinct regions of a river from that of adults. Thus, wild-type guppies, unlike store-bought guppies rarely eat their own offspring. Reproduction in store-bought guppies is complicated by large elaborate tails in the males, thus mating is less common in pet guppies than in natural guppies.
Natural history Close to 98% of all bony fish lay eggs. Guppies, however, are unusual among fish because females give birth to live young. The number of larvae produced by a female varies widely, from as few as 1 to more than 40, but the size of a brood can be predicted based on the female's size, age and her native stream. Large females tend to give birth to more larvae. Young females and old females have smaller broods than females of an intermediate age. Females found in the same stream as large, dangerous predators such as pike cichlids (Crenicichla sp.) tend to have broods of many (~20+) small individuals, whereas females found in the same stream as small, less dangerous predators such as rivulines (Rivulus sp.) tend to have broods of a few (~12-) large individuals. Females give birth many times per year, with an average inter-brood interval being about 4 weeks.
When they are born, males and females look identical. In most populations, both males and females are sexually mature by ten weeks of age. Before they are able to produce sperm, the anal fin of males differentiates from that of females. The fan-shape of the anal fin constricts and becomes rod-shaped. This is the gonopodium, which the male uses to transfer sperm to the female. As the gonopodium develops, males become increasingly colourful. Often black pigments appear before orange pigments. Young males will also attempt to court females before they are able to produce sperm. Guppies in streams with pike cichlids mature faster and are less colourful than guppies in streams without pike cichlids.
Guppies are promiscuous, meaning that males mate with many females, and females mate with many males. Although a male is able to inseminate a female by sneaking up to her and 'thrusting' his gonopodium at her gonopore, most matings involve female choice. Because males do not provide any resources to the female beyond sperm, females decide between males based on the quality of their genes. To assess genetic quality, females can examine the health of a male. Healthy males tend to have more and brighter orange on their body. Thus females prefer to mate with bright males than dull males, especially in streams where the risk of predation is low. To advertise their colour to females, males perform a 'sigmoid' display, in which they contort their bodies into an 'S'-shape, spread their dorsal and caudal fins, and quiver. Healthy males sigmoid more often than unhealthy males, thus (other characters being equal) females prefer to mate with males that sigmoid often versus males that do not sigmoid a lot. To signal to the male that she has chosen him as a mate, the female glides toward him slowly. The male will then approach the female, swing his gonopodium towards her gonopore, and transfer sperm to her. Females have the ability to store sperm from previous matings, thus as long as they have mated once, they can produce broods even in the absence of males. Because females mate with multiple males, larvae are often born into a mix of full-siblings and maternal half-siblings. Thus, being born together does not reliably indicate full-sibling relatedness.
Guppies feed primarily on algae and small invertebrates. In streams where the risk of predation is high, guppies tend to form large shoals of mostly females. In streams where the risk of predation is low, guppies tend to form small shoals. While males join and leave shoals frequently, females will often interact in the same group over the course of several days.
In the guppy, the ability to recognize and distinguish between kin and non-kin is important in two contexts: mate choice and cooperation. Firstly, offspring of closely-related parents are less healthy than offspring of unrelated parents, thus adults should be able to discriminate against relatives when choosing mates. Secondly, individuals should prefer to cooperate with relatives than non-relatives. In many fishes, kin groups have lower levels of aggressiveness, increased growth rates and improved health relative to non-kin groups. Thus, kin recognition is important in the guppy.
Suggested reading The guppy natural history section above has drawn from many sources and is intended only to introduce the reader to the guppy story. If you wish to learn more about guppies, I recommend two books in particular. First, "Sex, color and mate choice in guppies" by Anne Houde (1997, Princeton University Press) is an excellent starting point. Second, "Evolutionary ecology: the Trinidadian guppy" by Anne Magurran (2005, Oxford University Press) is a more recent review of the guppy literature. Additionally, I drew heavily from David Reznick's (University of California, Riverside) seven-part series on life history evolution in the guppy in writing the Natural History section of this page. I recommend PBS's website "Sex and the single guppy" as a fun way to learn more about guppies.
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PhD student
Department of Biology
University of Western Ontario
London, Ontario, Canada