Sloth Mating: Not as slow as you think
Sloths are mysterious animals—people usually only see them sleeping or resting on their trees. It is rare for anyone to witness sloths in the act of mating, and thus there is little information about sloth reproduction.
What there is to know on the topic, however, can be found in the Slothopedia!
How does one tell a male sloth from a female sloth?
Male and female three-fingered sloths can be easily distinguished once they’ve reached sexual maturity, as males have a distinctive orange and black stripe on their backs called a speculum. The speculum fur is shorter and downier and covered with a slick oil that gives the fur in that area rich yellow and orange colors.
Females have a much more subtle stripe on their backs, and may not have any stripe at all. They typically have a dark line surrounded by lighter fur in the same spot as a male would have a speculum.
The exception to this is the endangered three-fingered maned sloth, which does not have a speculum. The sex of maned sloths cannot be determined without an exam. Both sexes have a shaggy black mane around their necks. The appearance of the mane is unique to each individual, and it is not currently obvious if there is any sexual dimorphism in regards to its appearance.
In two-fingered sloths, distinguishing between males and females is notoriously difficult. This has led to some embarrassing mistakes at zoos and rescue centers, where two sloths thought to be of the same sex have been put into the same enclosure, only to produce a newborn baby some months later!
It is not possible to determine the sex of a two-fingered sloth without a very detailed physical examination, as they do not have any external secondary sex characteristics like the three-fingered sloths do.
This kind of exam is only possible under veterinary supervision and generally is also under anesthetic for the safety of the examiner. Two-fingered sloths are more aggressive and slightly faster than three-fingered sloths.
Someone who is experienced in the science of sloth anatomy can clearly sex two-fingered sloths in such an examination, but many institutions lack knowledgeable personnel with the required expertise.
Many animals mate only during certain times of the year. This mating season is related to the length of the days, temperature, latitude, and food availability, among other variables. By only mating during a specific season, the females will give birth at an optimum time of the year to increase their offspring’s chances of survival.
There are mixed reports as to whether sloths have a mating season. In areas where the climate is favorable year-round, females come into estrus monthly. However, in regions with distinct dry and wet seasons, the sloths may have their fertile periods such that the birth of their young coincides with the most favorable conditions.
Very little research has been done on the topic as it relates to sloths. Researchers suspect the mating seasons of sloths differ depending on the region—Central and South America have several microclimates which make mating seasons hard to determine.
In the South Caribbean of Costa Rica, sloths mate all year round and do not follow distinct seasons. In areas with more extreme seasonal changes, researchers have reported some breeding patterns, but these have yet to be fully investigated.
How to find a mate
A female three-fingered sloth will enter estrus once a month for approximately seven days. During this time her activity levels will increase by about 200% (that’s a lot for a sloth!) and she will emit high-pitched vocalizations to attract the attention of nearby males.
These vocalizations, or “screams”, sound like bird calls or shrill whistles. She will do this for eight to ten days every single month, with the vocalizations increasing in frequency until she is screaming every 10 to 15 minutes.
The male three-fingered sloths get very excited when they hear this call and will go in search of the female making it.
Two-fingered sloths don’t vocalize; instead, they secrete pheromones from their genital glands, which they rub all over trees and branches for potential mates to follow.
Some researchers observe that females will come down to go to the toilet every day while in heat. It is thought that this is to do with pheromone secretion and her feces will advertise her status to males in the area.
Once a female sloth is in heat she will wait for the males to come to her. All male sloths in the area will move through the canopy towards the female, and if more than one male responds, they will fight over mating rights.
Often the aim of a sloth fight is to knock the opponent out of the tree. The victorious male will move forwards and take up a position in the same tree as the female.
These fights can be surprisingly vicious, and the losing male will signal defeat by “crying”—another high-pitched vocalization that sounds similar to the female estrus call.
Despite two-fingered sloths having been bred successfully in zoos, sloth copulation itself is rarely witnessed and there is little known about the act. Footage has shown sloths mating in all different orientations; hanging upside down, front to back, face to face, hanging precariously from branches… sloths do it any way they like.
How long does it take?
Sex is the only thing sloths do quickly. Copulation lasts less than a minute. After it is over, the dominant male will stay nearby for several days, mating with the female frequently and fighting off any other males that wander too close.
This diligence takes its toll, however, and being a sloth, he also needs to take regular naps. When the dominant male is sleeping it opens a window of opportunity for any males that have been waiting nearby to sneak in. The female will mate with these other males while the dominant male is sleeping.
Gestation and birth
The gestation period for two-fingered sloths is 11 months, after which the female will usually give birth to one baby. Twins are very rare.
No one knows the gestation period for three-fingered sloths, as they have not successfully reproduced in captivity. Preliminary estimations of wild three-fingered sloths indicate the gestation period is probably much shorter than the two-fingered sloths, at around six months.
Are sloths monogamous?
Like most mammals, sloths are ´promiscuous’. The male will mate with as many females as he can find, and the females will mate with all the males who are able to approach her.
Homosexuality in sloths
Sloths of the same sex can form close bonds with each other when raised together in captivity, but there have been no reports of same-sex mating acts in the wild.
Breeding in captivity
Three-fingered sloths are very delicate creatures and do not thrive in captivity. Most three-fingered sloths are very likely to die within a few months of being captured and do not live long enough or are healthy enough, to breed.
This is a double-edged sword for the species. On the one hand, it prevents the breeding of three-fingered sloths in captivity for the purpose of exploitation or the pet trade, however, it also reduces the options for species conservation if three-fingered sloths lose their wild habitats.
Two-fingered sloths breed relatively well in captivity. However, the females do not show any visible signs of pregnancy, so without an ultrasound, it’s nearly impossible to tell when a female is expecting.
-Dr. Rebecca Cliffe
Founder and Executive Director