Guppies are popular aquarium fish that are easy to maintain and breed. These colorful fish are native to South America, Barbados, and Trinidad, where they can be found in shallow rivers, lakes, lagoons, and estuaries of fresh or brackish water. A guppy’s average lifespan is approximately 12 months, with sexual maturity reached at 8 weeks and full size reached at 6 months.


The average brood size is usually between 25 and 40 young. Guppies have a gestation period of approximately 28 days, after which young are born live. Fertilization occurs when the male passes sperm packets, called spermatophores, into the oviduct of the female. The females can store these packets in their ovaries and use them to fertilize eggs at a later date. One insemination may result in up to 8 months’ worth of offspring.

Male guppies possess the gonopodium, a modified anal fin (fig. 1 and 2). It appears narrow and is somewhat longer than the females’ anal fin. The gonopodium is located just behind the urogenital pore. In front of that pore are two pelvic fins. Fertilization is accomplished when the three modified fins swing forward together and make a temporary tube through which spermatophores are transferred into the female (fig. 1).

John Endler:

John Endler, mentioned in the lab manual, has been studying guppies in mountain streams of Venezuela and Trinidad since the 1970’s. His studies have focused on natural and sexual selection. He observed that guppy predators were located in every section of the stream, with each section isolated from each other by waterfalls. The headwaters of the stream only had one predator present, while downstream more predators could be found in every section, with all 7 predators present at the base of the stream.

Why are guppies colorful at all? Male guppies must mate, and to mate it must stand out from the gravel and from the school – he must catch the eye of the female guppy. The gaudier the male, the better his sex life. In the quiet areas of the headwaters, a gaudy male will live a long life with many chances to mate, but lower downstream where there are more predators, he may not be able to father any offspring before being eaten. The less colorful a male is, the less attractive he will be to a female, but he will live longer, thus having more chances to mate, especially in the predator-infested waters downstream.

To test his theory of natural and sexual selection, Dr. Endler developed an experiment. He captured guppies from every section of natural streams, mixed the populations, and let them breed together for many generations in artificial streams without the danger of predators. He then added predators to some of the streams. He quickly had the results he expected. The safe ones did get gaudier, and the ones in streams with enemies developed less coloration.

Both natural and sexual selection were occurring. If not, the results would not have occurred so quickly. Without natural selection, all the fish would have gotten gaudier, and without sexual selection, all of the fish would have developed more camouflage to insure survival. Currently, Dr Endler thinks guppies’ spots, mating habits, and color vision are all evolving simultaneously, with change in one of these driving changes in all the others.


A male guppies have several different types of courtship behaviors, such as gonopodial swinging, body curving, thrusting, and copulation, or he may attempt a "sneak copulation" in order to inseminate. These behaviors are described in more detail in your lab manual. When choosing an experiment, think about the following -- rates, intensity or frequency of displays -- as items to measure.

For example:

Would number of males present in the tank affect behaviors?

Does size of male or female matter?

What kind of females are males trying to attract?

What may affect rate of sneak copulations?

What about social organization?

Are males territorial?

Do they form long or short term associations?

Develop a specific hypothesis for your experiment.

Please see figures below to determine anatomy and sex of both male and female guppies.

Fig. 1

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Male above and female below.

Fig. 2

                    (From: How to Know "The Freshwater Fishes" by Samuel Eddy.)

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The gonopodium and ventral fin forms the male organ of insemination

Fig. 3

        (From: How to Know "The Freshwater Fishes" by Samuel Eddy.)

(1) mouth, (2) nostril, (3) eye, (4) operculum covering gills, (5) pectoral fin, (6) pelvic fin, (7) dorsal fin
(8) caudal fin or tail

Fig. 4

picture of gonopodium
Can you see the gonopodium?

Fig. 5

mated guppy

Can you find the gravid spot, or the darkened spot on the ventral side distinguishing a mated female?

Fig. 6

male & female guppies

Which are male or female? How can you tell?