In the fall of 1870, Theodore Roosevelt Sr. sat his 12-year-old son down for a conversation that would have condemned a lesser person to a lifetime of depression and despair. He had the right mind for success, his father told him, but not the body to support it: “You must make your body," he said. "It is hard drudgery. .  .  . But I know you will do it." Theodore Jr. went on to "do it." He pumped iron at Wood's gymnasium on the Upper East Side, canoed in the Adirondacks, chased down bison in the West, thus developing his famous barrel chest—supplemented, over the course of time, by a stomach to match. The latter would eventually earn him, among his African carriers, the nickname "Bwana Tumbo," unflatteringly translated as "Mr. Unusually Large Belly."

Growing up, Theodore Jr.— "Teedee" to his parents, he hated being called "Teddy"—had physical problems galore: constant headaches, stomach aches, coughs, fevers, nervous diarrhea, and asthma. Without his glasses he was helpless. No one seemed less equipped to become a Rough Rider than Teedee. Sickness prevailed, as well, in the world around him: His father would die of stomach cancer, and his mother died of typhoid fever on the same day as his wife, whose kidneys failed soon after childbirth.

"I am an Oyster Bay type of guy," a much later president, George H. W. Bush, proudly told a visitor, referring to the Roosevelt home on Long Island. The elder Bush had no idea: He was a born athlete while Roosevelt's mental and physical robustness—never an assured thing—came from relentless effort. No American president had worked so hard at becoming what he was not. Daniel Aaron once wrote about the "artifact" Roosevelt and the ways in which TR had reinvented himself, taking the public along for the ride. Now, Darrin Lunde's fascinating study, more than any other book about Theodore Roosevelt that I have read, reminds us of the crucial role that animals, big and small and mostly dead or soon to be dead, played in that constant process of reinvention.

The Naturalist is not really a book about Roosevelt the hunter, the topic of so many gushing tributes, and Lunde is not particularly interested either in Roosevelt's admirable legacy of conservation, although he touches on both subjects. Apparently, Roosevelt, myopic from birth, wasn't even a very good shot (the hunting buddy of his later years, son Kermit, seems to have been even worse). From the beginning, there was something a little theatrical about Roosevelt's exploits, even when he didn't yet have a national audience. Eager to be taken seriously as a cowboy, he felt the need for a proper cowboy suit, which involved tracking down the one old lady who still knew how to make such a thing.

The paraphernalia he needed for his expeditions—the hobnailed boots, the leather knee-patch pants, the waterproof mackinaw, the helmet—were of the utmost importance to him, and in Lunde's account, we find out more about the library he carried with him into the field (books were trimmed down to make them just the right size for Roosevelt's pocket, and were bound in durable pigskin) than about the animals he dispatched. But as he found out, no amount of role-play could keep tragedy at bay: "I will bring you home the head of a great buffalo bull," he promised his pregnant wife, leaving her for six weeks to shoot animals that he already knew were disappearing from the American landscape.

Roosevelt remained self-conscious about his wilderness skills his entire life. The notebooks he kept in Africa, published recently in Michael R. Canfield's comprehensive Theodore Roosevelt in the Field, contain multiple sketches of animals he killed, their bodies perforated by bullet holes, a bizarre account of his triumphs and failures as a marksman. When an admirer asked Henry David Thoreau what he should carry with him on a hike he had been planning, Thoreau's recommendation was unambiguous: "I would advise not to take a revolver or other weapon of defense. It will affect the innocence of your enterprise." Roosevelt's ventures into the field—admittedly, a much more dangerous environment than Thoreau ever set foot in—were not innocent or defensive ones: He went there not to be surprised and transformed but to rediscover what he already knew, to "bag" what he could lay his hands on.

Lunde describes for us what happened when Roosevelt, nearly blind without his glasses, looked his first elephant in the eye: "Having carefully studied a sectioned elephant skull in the American Museum before his trip, he visualized his shot." As the elephant turned, Roosevelt fired. He missed. No amount of museum experience can prepare you for the movements—quick, intense, lethal—of the living, breathing, terrified animal. Consider the mess Roosevelt made on Lake Naivasha in Kenya, where he had gone to shoot one hippopotamus and ended up needlessly killing four because the animals had panicked—as had he: "We shall have to let the papers know," he sighed to the journalist who had been following him around.

Roosevelt seems alien to us today, Lunde says, in one of the more introspective moments of his narrative. Indeed, it requires a certain suspension of disbelief to accept, without hesitation, his argument that, since the white rhinoceros was about to become extinct, what one needed to do was to shoot at least one more so that it could be placed in a museum for all to see. But as Lunde argues, this is precisely the logic we need to understand if we want to understand Roosevelt: He was a naturalist, a specimen collector, before he was a hunter, writer, president, and soldier. And no one is better equipped than Lunde, a practicing naturalist at the Smithsonian's National Museum of Natural History, to tell that part of the story.

Roosevelt's obsession with natural history began when he saw a dead harbor seal on display in a grocery store on Broadway and took the animal's head home to clean and inspect it. The Naturalist brims with accounts of how to skin an animal, from the birds Roosevelt hunted in Oyster Bay to the elephants he bagged in Africa. Whatever the animal, Roosevelt was mentally dismembering it from the moment they first met. His real tool was not the gun but the knife. When he was only 14, during a family tour of Europe and Egypt, he would regularly clog up the hotel room sinks with the innards of the birds he had acquired during the day. His own guts might have been in disarray, but when it came to animals, he was the emperor of entrails. Decades later, in Kenya, Roosevelt could be seen hovering next to the heaps of corpses he had made—lion, elephant, giraffe, hippopotamus, rhinoceros, antelope—wading in the spilled contents of their stomachs. Surrounded by the carcasses of his animals, Roosevelt knew that he had conquered his own wheezing, coughing, oozing body. As Lunde reminds us, preserving an elephant was not a job for the faint of heart: An elephant's skin had to be cut off in up to five sections, depending on its size. It would take Edmund Heller, Roosevelt's zoologist during his Africa trip, several days to "process" properly such a large animal: removing the guts, "roughing out" the muscles from the skeleton bones, scraping the skin and then salting it, which would leave one's hands raw and sore for a long time. All this so that the animal, now deader than a doornail, would (at a much later point) be reassembled and resurrected in taxidermic splendor, with a new, perfectly sculpted, body inside its old, mended skin, gracing the hall of a great museum of natural history.

In September 1901, the assassin's bullet that killed William McKinley put Theodore Roosevelt in the White House. Two years after his return from Africa, another assassin's bullet nearly terminated Roosevelt's life. He was in Milwaukee at the time—again campaigning for the presidency, this time as the candidate of his new Progressive party—when John Schrank, a deranged barkeeper from New York, walked up to his automobile and shot him, from five feet away, with a .38 caliber pistol. It was only when Roosevelt slipped his hand inside his heavy coat and it came back covered with blood that he realized he was hurt.

"It takes more than that to kill a bull moose," he shrugged, referring to his party's nickname, insisting that he deliver his 90-minute speech as planned. If the magnificent elephants of Africa came crashing to the ground when he shot them, Theodore Roosevelt, the expert artificer of his own body, had taken a bullet and was still standing: a superior specimen, the best in his collection. Ironically, what had helped saved his life was a reminder of his infirmity, the steel-reinforced eyeglass case he carried in his pocket.

Christoph Irmscher, provost professor of English at Indiana University, is the author, most recently, of Louis Agassiz: Creator of American Science.