**2. External appearance**

The rhesus monkey exhibits pronounced sex differentiation (**Figure 1**). Females measure approximately 47 cm in length (crown-rump length, thus without tail) and weigh 5–8 kg, whereas males present values of 53 cm and 8–15 kg, respectively [6, 7]. They have a relatively short, nonprehensile tail. As the rhesus monkey is a despotic species, fights often occur related to their hierarchical rank order system causing

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

severe tail wounds all resulting in the veterinarian's decision to amputate the injured tail.

The hairs on the lateral sides of the animal are gray to brown. The inside of the arms and legs, and the belly color pale beige to white. Each finger and toe, five on each hand and foot, possesses a nail (unguis). The palms of the hands and the soles of the feet are keratinized showing epidermal ridges [7]. The face, including the ears, has very few hairs and therefore a pinkish appearance. Females possess a pair of pectoral mammary glands.

The skin covering and surrounding the genitals is also devoid of hairs. In males, the scrotum is non-pendulous and contains a pair of testes that measure 4 cm in length [8]. The penis is normally retracted within the preputium (**Figure 2**), only extracted during mating.

Menarche occurs at about 3 years of age and the length of the menstrual cycle is 28 days. The gestation length is around 165 days. Rhesus monkeys are seasonal breeders. Menstrual bleeding (menses) lasts for about 4 days. During the mating season (autumn-early winter), the skin of the face and genitals of females becomes characteristically red and/or swollen (**Figures 3** and **4**) during the period of regular cycles (sex skin coloration). These periodic changes in sex skin coloration are not valid indicators of either ovulation or menstruation. The sex skin is a secondary sex characteristic and reflects estrogenic activity. It fluctuates, as a rule, as to presence, extent, and time of year in a very unpredictable manner. In older females, the sex skin is less pronounced, and the redness may persist for longer intervals. Moreover, sex color is maintained during the entire gestational period and for several weeks after parturition: females who become pregnant during the mating season do not show the significant sex skin coloration decrease seen in nonpregnant females during the months that follow the mating season. Environmental cues, perhaps acting on a seasonal biological clock rather than social factors, are thought to be mainly responsible for the seasonal fluctuations observed. Sex skin involves the skin of the buttocks, hips, and base of tail, but the coloration and swelling can even spread in red splotches down the legs and over the calves to the heel; and there might also be a forward-tapering streak of red splotches from the symphysis to the umbilicus. Females could use color as a gauge to monitor other females' reproductive status or cyclic phase, for competitive purposes, as hindquarter color can advertise the relative timing of ovulation. The possibility

**Figure 3.** *Two examples of sex skin (coloration and swelling) in the face of females.*

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

#### **Figure 5.**

*(A) Perineum of a female rhesus monkey. This female is presented with a tail stump (1) that is visible just dorsal to the anal opening (2). The large clitoris (3) is present in between the sciatic protuberances (4). (B) This female has an intact tail (1) just dorsal to the anal opening (2). The clitoris (3) is less pronounced but still visible in between the sciatic protuberances (4). The prudential labia (5) can however, more readily be recognized in the female.*

might not be plausible for facial color, given that the relationship between face color and cyclic phase is not predictable in rhesus monkeys.

In females, the vulva is pronounced, with a large visible clitoris between the sciatic protuberances that are covered with keratinized skin patches (callositas ischii/sciatic protuberances), which is typical for Old World monkeys, both males and females (**Figure 5**).
