The portrait has a blunt simplicity. A chimpanzee sits with his back facing the camera. His gently lit body is nearly hairless from alopecia, and only a few strands of gray hair remain on his sandpapery scalp. A spotlight on his head reveals a reddish scar that has yet to fully heal.

Tim Flach, a London-based photographer who has been documenting exotic animals for the past two decades, said he deliberately captured the back of the chimp’s body to provoke his viewers to question the striking similarities — and differences — between their own bodies and the chimp’s.

“You know the image isn’t human, but you find yourself wondering about it,” Mr. Flach, 55, said. “This is a way of catching one’s imagination and introducing certain ideas through the context of a picture.”

The chimp’s scar “slightly punches you,” he said.

Most of Mr. Flach’s intimate portraits of animals are intended to be a bit jarring in their simplicity — concentrating on the subtle details that impart a humanistic feel. He wants his images to engage people in debates about who we are and our relationships to animals, with the belief that “how we treat animals is often dependent on how they display characteristics we think are human.”

A collection of his work was recently published in the book “Evolution,” highlighting the vast spectrum of living species, including plants, small and large primates and underwater life.

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Capturing portraits of animals around the world hasn’t always been easy or, for that matter, glamorous, Mr. Flach said, and he has learned through trial and error the different personality traits of his subjects. One of his smallest subjects, a red-eyed tree frog, happened to be one of his most challenging. Suffering from a bit of anxiety, the frog continually jumped around the set and stuck onto Mr. Flach’s camera lens. At one point, the frog closed his eyes and pretended to be dead. “He had us all working,” Mr. Flach recalled.

There was also the time Mr. Flach traveled to Bracken Cave in Texas, home of the world’s largest colony of bats. What resulted in one of his most popular photos, one bat gripping the shoulder of another, both staring piercingly at the camera, was also accompanied by an unending stream of bat feces on Mr. Flach and his team while on the set.

To avoid those moments, Mr. Flach has already done extensive research on each of his subjects long before setting up a shoot. He also prefers to work with animals that can relate to the human condition — including subjects like the dancing featherless chicken, which was part of a larger research project in Israel on genetic mutation.

“If you go to the supermarket today, we’re more used to seeing packaged animals with no feathers and no head,” Mr. Flach said. “That’s us managing livestock, which is a whole agricultural debate.”

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Before becoming a photographer, Mr. Flach studied drawing and painting at the School of Art in London. He was introduced to photography in his late teens and became deeply fascinated by the stories of the subjects he met during his commissioned work. As his career developed, his passion for painting never waned, and he now credits most of the influences of his work to well-known painters like Pablo Picasso — not just photographers.

It is the structure of a painting that fascinates Mr. Flach most about the medium, particularly having to “trip over things that later reveal themselves to you,” he said. When studying his subjects, he looks for their most intriguing features, using single light sources while on set to accentuate textures and forms.

But it isn’t about showing every detail on the body of his subjects. According to Mr. Flach, sometimes an image is most affective by what isn’t in the frame. At a quick glance, the details of a white horse’s neck, as his flowing mane caresses his back, look more like a snow-capped mountain.

“You find yourself thinking of a ski holiday,” Mr. Flach said. “You can be looking at a horse, but there you are thinking about something else.”

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