Proboscis monkey showing the white-ringed 'clerical collar' and big belly typical of the big males. It has four stomachs and is the only monkey that has actually been observed to ruminate. Borneo.

Flanged orangutan male. In the wild, it typically takes males around 20 years to grow a large mask. It's clearly something that makes an impression on the females. They prefer to mate with males with a well-developed mask. Tanjung Puting National Park, Borneo.

Wild mandrill alpha male keeping an eye on smaller troop members feeding in the canopy. Weighing 30+ kg, more than three times as much as the females, he spends most of the day on the ground. But he will sleep high up in a tree with the rest. From Gabon.

Drill alpha male. With the scientific name Mandrillus leucophaeus, it is the second member of the Mandrillus genus, next to the mandrill. These large, semiterrestrial monkeys are more closely related to the drill-mangabeys (Cercocebus) than they are to the baboons. Pandrillus.

Smiling black crested macaque. Tangkoko Nature Reserve, Sulawesi.

SNOW FLAKE / A young Japanese macaque, maybe a year-and-a-half, with a single snow flake caught on the exceptionally thick, warm, and waterproof fur on its head.

Portrait of a black crested macaque alpha male in the monsoon forest of Sulawesi. Tangkoko Nature Reserve.

Snow monkey in snow blizzard. Japan.

Gelada male from the Simian Mountains National Park, Ethiopia.

Golden snub-nosed monkey from the Qinling Mountains of China.

Happy weekend with this cheerful-looking mandrill photographed in the wild. Red nose, long canines, and a well-groomed yellow beard; mandrill males go a long way to look good (and powerful). Gabon.

RARE GOLD / The beautiful, thick-furred golden langur, with a coat that truly does glitter like gold, was only officially discovered by science in 1953. Restricted to a small area in NE India and Bhutan, it is one of the world's rarest and most endangered monkeys. Assam.

ANTHROPOID / Eye contact with an adult chimpanzee male in a relaxed pose. Kibale National Park, Uganda.

THE INNER WORLD / A wild proboscis monkey bachelor posing with closed eyes showing his pale blue eyeshadow, looking almost like he is daydreaming. Even with closed eyes there is so much expression in the faces of primates. Borneo.

PINK THREAT / Gelada alpha male sending one of the most dramatic threats seen amongst mammals, the 'flip-lip' display. The gelada has evolved unique muscles to make this facial expression, allowing it to display its long daggers framed in pink so you really notice them. Ethiopia.

Orangutan young have one of the longest childhoods of all animals. The high energetic investment of larger-brained species into brain development during infancy results in a delay in the physical development of the body, and thus a delayed onset of the reproductive period.

To celebrate International Women's Day, here four strong and charismatic fellow female primates from around the world. Drill (Nigeria), gelada (Ethiopia), snow monkey (Japan), and proboscis monkey (Borneo).

Close-up of a large drill male. The intensity of the red coloration of the 'cheek badge' is controlled by testosterone. A juvenile drill treated with testosterone developed red flushing on the badge as well as in the groin. A bright red badge is a signal of high status and power.

THE CHARMER / A male black crested macaque smiling to a female in response to her backing up to him and presenting him with her colorful behind (the pink splurge of color in the bottom right of the photo). His facial expression reveals that he knows love is in the air. Sulawesi.

EYES OF A MOTHER / A female snow monkey cradles her sleeping young to keep it warm in the winter cold. She sits absolutely still so as not to wake up the offspring, but her eyes are vigilant and with the firm look of a protective mother telling you not to disturb her child.