For many human males, baldness is a genetic inevitability. One day you being noticing more of your forehead or the back of your scalp. From then on, it’s an unstoppable force that bulldozes over the promises of hair growth pills, Rogaine or even hair transplant surgery. In T.S. Eliot’s poem“The Love Song of J. Alfred Prufrock,” the onset of balding is one of the ways the narrator bemoans the passage of time:
“Time to turn back and descend the stair,
With a bald spot in the middle of my hair —
(They will say: “How his hair is growing thin!”)”
Balding men don't actually lose hair; the hairs just become progressively smaller until they are microscopic, the same way they are at birth. But what about the rest of the animal kingdom? Growing hair is one of the defining traits that makes a mammal a mammal—yes, even dolphins and whales have some hair—but some still experience hair-related woes at some point in their lives.
Certain primates literally pull each other's hair out until it causes baldness; Andean bears can get alopecia, resulting in such uncomfortably itchy baldness symptoms that zoos sometimes consider euthanizing them for humane reasons. Only a handful of animals, however, experience permanent pattern baldness similar to what aging human males go through: the stumptailed macaque, dachshunds and greyhounds.
“A lot of it can be cured,” says Tim Nuttall, a senior lecturer in small animal veterinary dermatology at the University of Edinburgh, of animal baldness. “Unfortunately, things like pattern baldness can’t really be cured because it’s a genetic abnormality that causes it.” Nutall has studied a possible treatment for the itching that comes along with Andean bear alopecia, which isn’t genetic but caused by an immune-mediated process similar to eczema in humans.
Animal hair can be lost through a number of different mechanisms, from temporary damage due to hair being scratched out to more permanent damage from trauma. These include fungal infections like ringworm, parasitic mites that cause mange or “anything that’s inflammatory or infectious that would destroy the hair follicles,” says Nuttall. Ringworm directly damages the hair shaft while in a case particular to hedgehogs, mites feed around the base of the spines, causing the follicle opening to widen so that the spines fall out.
Yet baldness is generally not anywhere near as common in wild animals as it is with humans, says Desmond Tobin, a professor of cell biology at the University of Bradford in England. After all, a healthy coat of hair is often critical to keeping warm or camouflaging in the wild.
“There’s so much evolutionary selective pressure to grow hair,” he says. “There are so many genes involved in the genome of these animals to ensure that they don’t lose their coats. From that perspective it’s actually rare to see alopecia in the total wild outdoor populations of animals compared to these domesticated, regulated animals.”
Certain types of captive or domesticated animals are also susceptible to losing their hair. One of the main causes is breeding practices that can tangle up the basic genes responsible for keeping mammals covered in a healthy coat. “The genetics of those animals are pretty messed up,” Tobin says, adding that since hair follicles are tightly connected to the thyroid gland, the sources of many baldness issues are hormonal.
Tobin and Nuttall both say that some of the dogs most susceptible to these issues are dachshunds and greyhounds. Others, such as the Mexican hairless dog and the Chinese crested dog, are bred specifically for baldness through a condition called follicular dysplasia, in which hair follicles that break down due faulty structuring. (Coatimundis, a species of Latin American mammals related to raccoons have also been found to experience a kind of natural follicular dysplasia).
Apart from breeding specifically for a lack of hair, baldness can be caused when breeders attempt to create dogs with particular colors, like grey Labradors. Nuttall says that in some individuals, breeding for grey or silver results in weakened hair shafts, which can result in progressive hair loss. And as you might expect, purebred dogs are more at risk for baldness—along with a host of other issues—than mixed breeds, which bring a mix of different genes together.
Allison Heagerty, a social housing coordinator at the Oregon Health and Sciences University’s primate research center, recently investigated why captive rhesus macaques experience temporary bouts of baldness in patches. In a recent study published in the American Journal of Primatology, she and coauthors watched different groups of macaques, ranging in size from about 25 to 60 individuals, for about half an hour a day to discover which animals were interacting with which and to determine the pecking order of these highly hierarchical animals.
The team wanted to see whether social hair-pulling—a behavior commonly employed by the primates—was the underlying cause of these temporary bald spots. They aren’t entirely sure why the monkeys pull hair, but they found the behavior didn’t always correlate with social rank or adhere to any particular spot in the hierarchy.
It turned out that hair-pulling wasn’t the only cause of baldness. Heagerty says that hair loss among the captive macaques at the center was associated as well with other factors such as lack of sunlight, lack of natural habitat features like grass and dirt. It also has something to do with seasonal changes; the rhesus macaques Heagerty works with have the best, fullest coats on average in late summer and in early fall in preparation for the colder weather.
Another cause of hair loss among Rhesus macaques mirrors that of humans. Heagerty says that pregnant females sometimes lost hair similar to the way women occasionally lose hair during pregnancy or shortly after birth as hairs sometimes go into a resting phase. “In humans also when woman are pregnant, there tends to be a change to the timing of their hair regrowth cycle and we also see a change in the pattern of hair regrowth in our pregnant monkeys here,” Heagerty says.
A more permanent form of baldness that affects humans and certain other primates like Rhesus macaques is alopecia areata, also known as spot baldness: an immune-mediated condition in which immune systems target the hair follicles, according to Heagerty's colleague Cassandra Cullen, a veterinarian at OHSU.
A related species of primates, stumptailed macaques, experiences something similar tohuman male pattern baldness. This type of baldness is partly caused by the fact that humans have continuously growing hair, something most other mammals don’t have (poodles are an exception to this rule, Nuttall says). Only with stumptailed macaques, the condition does not distinguish between males and females–both sexes experience a type of eight-ball baldness in their old age.
“For animals you don’t really have anatomical compartmentalization of the scalp,” Tobin says, adding that the similarity between the condition experienced by stumptailed macaques and humans has led to the use the primates as test animals for hair-growth drugs like minoxidil (Rogaine). Clearly, hair loss is far from a uniquely human conundrum. Fortunately, in other species, it probably doesn't lead to midlife crises or poorly executed comb-overs.