Sexing or Gender Determination in Small Mammals

Introduction

External reproductive anatomy is obvious in some adult small mammals such as the ferret, sugar glider, hedgehog, rat, guinea pig, and hamster. Gender determination or sexing can be challenging in some species like the chinchilla, and in many neonatal rodents. In these cases, reliance on anogenital distance or the distance between the rectum and the urogenital region is considered best practice. The anogenital distance is relatively short in females and longer in males.

Rabbits

To sex a rabbit (Oryctolagus cuniculus), apply gentle pressure to the genitalia. A bullet-shaped penis will evert in bucks (Fig 1), while the vulva in does has a more slit-like appearance (Fig 2). Male rabbits also have a hairless scrotum that sits cranial to the penis. Open inguinal rings allow the testicles to be retracted into the abdomen. Bucks do not have an os penis or nipples.

Figure 1. Note the hairless scrotal sacs cranial to the bullet-shaped penis in this male rabbit (Oryctolagus cuniculus). Photograph provided by Matt Rosenbaum. Click on image to enlarge.

Figure 2. The vulva appears as a slit-shaped structure in rabbit (Oryctolagus cuniculus). Photograph provided by Matt Rosenbaum. Click on image to enlarge.

Females of some breeds normally have a dewlap, a flap of skin that hangs beneath the chin (Fig 3).

Figure 3. Note the dewlap or “double chin” present in this overweight female rabbit (Oryctolagus cuniculus). Photograph provided by Jody Nugent-Deal. Click on image to enlarge.

Rodents

Anogenital distance is longer in male rodents (Fig 4). Rodents also have an open inguinal ring and most species can retract the testes into the abdomen. An os penis or baculum is present in hamsters, gerbils, rats, guinea pigs (Cavia porcellus), chinchillas (Chinchilla lanigera) and mice.

Figure 4. The anogenital distance is relatively long in male rodents (left) and short in female rodents (right). Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Chinchillas

Sexing can be challenging in chinchillas since the clitoris and penis appear quite similar, however the anogenital distance is about twice as long in males (Fig 5). Male chinchillas lack a true scrotum and the testicles sit within the inguinal canal. Both genders have one pair of inguinal mammary glands and two pairs of lateral thoracic mammary glands. Female chinchillas are usually larger. See Lowcountry Chinchillas and Meadowbrook Chinchillas for images illustrating male and female chinchilla external genitalia.

Figure 5. Close up of male chinchilla (Chinchilla lanigera) genitalia. Photo credit: Salix via Wikimedia commons. Click image to enlarge.

Degus

Both the prepuce in male degus (Octodon degus) and the clitoris in females are located ventral to the anus, but the prepuce is considerably larger. The male also lacks a scrotum. Intra-abdominal testicles sit within two separate hemiscrotal sacs. Females have four pairs of mammary glands.

Gerbils

Male Mongolian gerbils (Meriones unguiculatus) are larger, with darkly pigmented scrotum, and more prominent marking glands on the flanks. The male also has a prominent genital papilla. This papilla is less pronounced in females, and female gerbils also have nipples.

Guinea pigs

The penis can be everted from the prepuce using gentle digital pressure on the boar’s genitalia (Fig 6). Sows have a Y-shaped vaginal closure membrane between the anus and the urethra. The vulva is at the junction of the “Y” (Fig 7).

Figure 6. External genitalia in a male guinea pig (Cavia porcellus) or boar. Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Figure 7. External genitalia in a female guinea pig (Cavia porcellus) or sow. Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Hamsters

The scrotum creates an obvious bulge giving the rear of the male hamster a rounded appearance. The female has a relatively blunt rear and 6 pairs of nipples. Male hamsters are also relatively large when compared to females.

Mice

As in other rodent species, anogenital distance is longer in male mice (Fig 8). Male mice (Mus musculus) have a prominent scrotal sac. Males are also typically twice the size of females. The presence of nipples is also an indication of female gender.

Figure 8. External genitalia of female (A) and male (B) mice (Mus musculus). Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Figure 9. The presence of a vaginal plug may be used to confirm mating several hours after breeding in some rodent species including rats, mice, guinea pigs, degus, and chinchillas. A white mass of glandular secretions produced by the male mouse fills the vagina. The arrow indicates a white vaginal or copulatory plug in a female mouse (Mus musculus). Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Rats

A distinct scrotum is located between the anus and prepuce in the male rat (Fig 10). Only female rats have nipples (Fig 11).

Figure 10. External genitalia in a male rat (Rattus norvegicus). Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Figure 11. External genitalia in a female rat (Rattus norvegicus). Photograph provided by Dorcas O’Rourke. Click on image to enlarge.

Ferrets, sugar gliders, and hedgehogs

The prepuce of the male ferret (Mustela putorious furo) or hob sits on the ventral mid-abdomen, and the os penis is easily palpable. Although many American ferrets are neutered before sale, the testicles may be located cranioventral to the anus. Males are also typically larger, while jills are relatively small and petite.

Male sugar gliders (Petaurus breviceps) have a pendulous scrotum, and a forked penis sits behind the scrotum. Males also possess a ventral chest gland as well as a scent gland on top of the head.

Male African hedgehogs (Atelerix albiventris) have a relatively long anogenital distance. The prepuce sits midway along the ventral abdomen. Hedgehogs lack a scrotum but the testicles are palpable within subcutaneous tissues. As in rodents, the anogenital distance is short in female hedgehogs and the vulva is located close to the anus. A vaginal plug is present after mating in female hedgehogs.

Conclusion

Although external reproductive anatomy is fairly obvious in some adult exotic companion mammals, gender determination or sexing can be challenging in some species like the chinchilla, and in many neonatal rodents. In these cases, examine anogenital distance to determine gender. The distance between the anus and the urogenital region is relatively short in female rodents and longer in males. Sexing can be challenging in chinchillas since the clitoris and penis morphology appear quite similar, however the anogenital distance is approximately twice as long in males.

References