They often arrive in bad shape - hit by cars, zapped by high-voltage wires as they climb trees, or orphaned because superstitious locals have killed their mothers.
But life gets sweet once the gates open at Costa Rica's sloth sanctuary, one of few in the world specialising in the study of these famously sedentary and solitary mammals.
The youngest even get stuffed animals to hug in incubators.
Their digs are indeed nice: 130 hectares of lush tropical forest with a crystal-clear river flowing through it in Penshurt, 215 kilometers from the capital San Jose near Costa Rica's east coast.
The Costa Rica Sloth Sanctuary - a four-metre cement replica of one of the critters greets visitors at the entrance - was founded in 1992 by a Costa Rican named Luis Arroyo and his US wife, Judy Avey.
I cannot imagine life without them. To know them is to love them.
The idea is to protect, nurse and study the animals, but also to teach people about them.
Locals call them "osos perezosos", or lazy bears, and some even associate them with witchcraft. They are an enigma of sorts. Why don't they move, run or jump, like other self-respecting mammals do?
"It hurts me that people do not appreciate them. They are not lazy, but rather simply slow. We can learn from their calm, to maintain serenity, as they do," said Avey.
The refuge - originally supposed to be for birds in an area that is home to some 350 species - receives two kinds of sloth, two-toed and three-toed, both of which exist in Costa Rica.
Teresa Gonzalez, an employee at the sanctuary, says she has been feeding the animals for five years and knows their every quirk.
"One does not like carrots, but rather green peas. That one will let me bathe with him," said Gonzalez as she held a baby sloth named Mojo, sucking away at a bottle of goat's milk.
Look around and some sloths are perched in trees, others rest in baskets and young ones in incubators clutch stuffed animals as if they were their mothers.
The ones brought in as babies stay for good, because they do not know how to live in their native habitat. But injured adults are returned to the wild when they have recovered.
Avey points to her first resident - a specimen named Buttercup, snoozing in a hanging rattan chair. She was brought to the refuge after her mother was hit by a car and died.
"Neither the zoos nor anyone else wanted her because they did not know anything about sloths. But we fell in love with her. She climbed into my arms and stayed there. She is my spoiled one," said Avey.
Since its founding the center has taken care of more than 500 of the animals. It costs about $US400 ($A383) per head each year.
The sanctuary raises revenue with a small zoo, a hotel and guided tours of this most relaxed of biological reserves.
"We saw some on YouTube and decided to come and see them up close. We love them," said Briggs Lebeacq, a young American tourist who came to Costa Rica with his girlfriend.
What is the life of a sloth like?
Vets say they eat only leaves, do not drink water and in Costa Rica tend to live on the Caribbean coast to the east because of the humidity and abundant presence of the guarumo, or trumpet tree, the animal's favorite.
Sanctuary veterinarian Marcelo Espinosa said their metabolism is so slow it takes them a month to digest food. They eat twice a day and only come down out of the trees once a week to defecate.
They sleep 18 hours a day and eat little, as they do not burn a lot of energy.
As for sex, little is known about the two-toed variety.
But three-toed females, in heat, scream out for males to find them. What ensues could certainly test non-sloth romance: the male can take three days just to get there.
Espinosa said not a lot of research is done on sloths because he said no one cares.
But Avey, who has lived in Costa Rica for 40 years, certainly does.
"I cannot imagine life without them," she said. "To know them is to love them."