Determining the sex of a mature guppy is relatively easy. You just need to know what to look for.
Three characteristics in particular differ between males and females:
Color: The most popular type of guppy among hobbyists and the type most frequently sold for home aquariums is the fancy guppy.
- Male fancy guppies are colorful. They may be blue, green, yellow, orange, red or a combination. Color is visible on their long, expansive tails, their fins, and often their bodies. Female fancy guppies lack the flashy color of their counterparts. They're usually pale gray or silver, occasionally with a light yellow cast at best.
- Swordtails, another type of guppy, have the same coloring as fancy guppies. The only difference in their appearance is that the males' tails come to a swordlike point either at the top, the bottom or both; female swordtails have stubby tails.
- The third type of guppy is known as the common or feeder guppy, and as the name implies, they're bred primarily to serve as food for larger fish. The males have a pastel or neon color mix, while the females are clear and colorless.
* Size: Male guppies have bigger tails than females, but they're smaller overall -- they're shorter in length and have slimmer bodies. You may be able to tell the males and females apart by examining their bellies after they've been well-fed. The females have more noticeable potbellies, while the males appear to carry their weight in their chests.
- Anal fins: The male's anal fin, which is just beneath its tail, is long, thin and pointed, whereas the anal fin on a female is rounded or fanned out at the bottom. Often you can see a dark spot right above the anal fin of the female. This spot, called the gravid spot, is most visible when a female is pregnant; the farther along she is in her pregnancy, the bigger and darker it is. Female guppies are pregnant more often than not, so the gravid spot is a good marker to look for.
Knowing the gender of your guppies helps you to control their breeding and, subsequently, the population of your aquarium. Adult guppies have a tendency to eat the babies as soon as they're born, so if you want to increase the newborns' chances of survival, your best bet is to catch them in a fish net and separate them from the adult fish by placing them either in a breeding box or net that floats at the top of your larger tank, or in a separate tank. You need to know the sex of the babies as soon as possible to further separate them by gender. Females are capable of becoming pregnant at 3 months of age. Male babies are more active than their female counterparts, and are more likely to be seen darting around and chasing their tank mates.
Female guppies breed almost constantly until they reach about 2 years of age. Their average life span is two to three years. Gestation lasts 28 to 30 days, and a litter consisting of as many as 126 young may be born every four weeks. Females have the ability to store sperm for months, so it's possible for them to give birth six to eight months after being in the company of a male.