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Sexing Xenopus tropicalis

Although determining the sex of an immature trop is difficult, mature male and female frogs are easily distinguishable by a few obvious landmarks. In general, mature males tend to be smaller than mature females, although this can be variable depending on a variety of different factors. So we always utilize additional characteristics in order to properly identify the sex of the animals. First, males and females differ in body proportions. While males are narrow and long, females have a plumper abdomen and are more pear-shaped.


Second, males can also be distinguished by the presence of nuptial pads on the underside of their forelimb. Somewhat rougher than the rest of the skin, nuptial pads give males a good grip on the female during amplexus. On trops, they are dark lines, which can extend from the elbow to the wrist on the ventral forelimb, varying in intensity across strains. The presence of a nuptial pad is a very useful sign of being a fertile male.

Female ventral forelimb - No nuptial pad
Male ventral forelimb – Nuptial pad

Thirdly, females can be distinguished by the presence of a swollen cloaca. The cloaca is the opening through which the digestive, excretory and reproductive tracts empty. In females, the cloaca is larger and protrudes caudally. Also, when a female is induced to lay eggs, her cloaca reddens.

Female - Enlarged cloaca
Female - Enlarged red cloaca after HCG treatment.

By combining an evaluation of size with these other characteristics, it is straightforward to properly determine the sex of the animals.

Contributed by Kristin Trott
Thanks to the Grainger site.