‘You know, sloth is a sin,’ he says softly. ‘I prefer to think of it as an adorable animal.’

Ella James, Sloth (2015)

In 2013, London Zoo decided it was time for their two-toed sloth, Marilyn, to get a boyfriend. The zoo had never successfully bred a baby sloth before, but they sensed that Marilyn might be up for it. So they flew over from Germany a boy sloth named Leander and, more in hope than expectation, waited to see what would happen.

In their natural habitat, sloths are highly solitary creatures. Three-toed sloths keep themselves to themselves, hanging around in their own tree with no apparent need or desire to communicate with other sloths. Their two-toed cousins sometimes live two to a tree, but that is still the exception, with single-sloth occupancy generally the preferred tree-dwelling manner. In either case, when a boy sloth meets a girl sloth, it can take several months before he invites her back to his place.

The same slow progress has been seen in sloth romances in other zoos, with boy–girl pairs studiously ignoring each other for months. This was certainly the pattern followed by Marilyn and Leander. After the sloths had been together for over six months, the keepers were astonished to discover that Marilyn was pregnant. As one was quoted as saying, ‘We did not even know they had acknowledged each other’s presence.’

Marilyn gave birth in May 2014 and, factoring in the expected eleven-month gestation period of the two-toed sloth, it was calculated that conception must have happened six months after Leander’s arrival, which is thought to be pretty quick going for a pair of sloths.

In the wild, accounts of sloth lovemaking are very rare. In 1926, the great naturalist William Beebe, who was one of the first to spend a good deal of time observing sloths in their natural habitat, wrote: ‘I have watched two courtships, one of an immature male, and the other of an animal of full size and colour. Both were alike in their absolute directness and simplicity.’ He went on to describe how the males climbed up to the female and tried to grab her: ‘In the first instance, where the female had a month-old baby clinging to her fur, she lunged leisurely with full force at the disturber of her peace. The other female simply mounted higher, and when she could ascend no more, she climbed down and across her suitor, leaving him stranded on the lofty branch looking vaguely about.’ The male sloth then tried to reach out for an iguana lizard, which he thought might be his beloved, before getting back to his pursuit of the female sloth. ‘This unemotional pursuit continued for an hour, when he gave up for good and went to sleep.’

Accounts of successful sloth mating in the wild are also very sporadic and provide insufficient evidence to build a reliable picture. Brazilian researchers have reported seeing sloths mating in two positions, with the male mounting the female from behind, or face-to-face. Unlike most of their other activities, lovemaking seems generally to be brief, lasting only a few minutes, but the male may then take a short rest and try again. On the rare occasions that researchers have observed sloths mating, they have tended to sense the unusualness of the occasion and written it up in great detail for the benefit of others. What follows should therefore perhaps carry a parental advisory warning.

In 2008, a team of Brazilian researchers published a paper entitled ‘First Observation on Mating and Reproductive Seasonality in Maned Sloths Bradypus torquatus (Pilosa: Bradypodidae)’. Coming straight to the point, they spare no blushes by saying: ‘A pair of maned sloths was observed copulating in September 2005 in the Atlantic Forest region of south-eastern Brazil.’ After a good deal of background information on gestation periods and possible seasonal mating behaviour of sloths, they return to the X-rated business:

The two were high in the canopy and so tightly embraced that initially we thought there was only a single large sloth in the tree. For this reason we could neither discern copulation movements nor see if their relative position was ventral-ventral or ventral-dorsal... The two adults stayed embraced for approximately 7 min and then separated from each other, remaining side by side on the same tree trunk.

The female sloth, incidentally, was nursing a baby while all this was going on. It was also pouring with rain, which could be seen as adding drama to the intense cuddling. The researchers report that when it stopped raining several hours later, they were able to climb the tree and capture the male to confirm that it was indeed sexually active and ‘had a prominent penis’.

Tabulating the known dates of sloth copulations and births, the authors say that ‘the little information that does exist indicates some reproductive seasonality’, but suggest that not all sloths, even those from the same area, fall into the same pattern. To support this, they mention earlier research involving a female kept together with a male in semi-captivity who was observed copulating with another male that entered her enclosure, ‘while the male who lived with her did not attempt to copulate nor did he show any sort of reaction’.

As far as I have been able to discover, the academic literature contains no reference to homosexuality in sloths, though my searches have revealed one appalling riddle:

Q: What do you call a gay sloth?

A: A slo-mo-sexual.

Meanwhile, back with the heterosexual sloths, two recent research papers bring some surprising revelations.

In December 2012, the online science journal PLOS One published a paper, ‘Unexpected Strong Polygyny in the Brown-Throated Three-Toed Sloth’, which revealed that male sloths are far more promiscuous than one would expect from such a sedentary creature. The research involved taking DNA samples from all the sloths in a given area over the course of two years in order to determine the parentage of the babies. The surprising result was that ‘only 25% of all resident adult males sired offspring and one individual sired half of all sampled juveniles’. In fact, of the nineteen males and twenty juveniles whose DNA was successfully sampled, one male was the father of ten children, and another four males accounted for all the others, being the fathers of four, three, two and one offspring respectively. Fourteen of the males were found not to be responsible for any of the children.

‘Jack, you have debauched my sloth.’

Patrick O’Brian, HMS Surprise (1973)

Nearly five years later, however, female sloths hit back against this picture of sexually rampant males. In 2017, the Journal of Mammalogy published a paper, ‘Individual Reproductive Strategies Shape the Mating System of Tree Sloths’ (by Mario F. Garcés-Restrepo and others), showing that sexual promiscuity among sloths is not the sole preserve of the menfolk. The research took place on a cacao plantation (sloths love chocolate, apparently), a tropical forest and a cattle pasture in Costa Rica, and involved capturing almost 400 sloths and fitting them with radio-collars that allowed very accurate tracking of their locations. In particular, they could determine when two sloths were spending time together, and the results suggested that the females were no sexual slouches. Just as some males had displayed a high degree of polygyny (multiple female mating partners), the women displayed polyandry (many males). ‘Female sloths often mated with multiple males over their lifetimes,’ they report; ‘indeed 70% of Bradypus variegatus and 50% of Choloepus hoffmanni switched mates at least once during the study... Our study, which spanned 4 breeding cycles in both species, suggests that mating with more than 1 male over time is common in sloths, and greater than previously believed.’ (To be fair to the female sloths, we should point out that after mating, the males usually wander off, and after childbirth will play no part in looking after the offspring.)

Courtship is another area which is only just beginning to be understood. Some say that females climb to the tops of trees, then emit a whistling sound to attract males. The sound only lasts for a second but may be repeated several times. This whistling cry is said by many to account for the onomatopoeic name of ‘ai’ being given to the three-toed sloth, but it is unclear whether it is specifically a mating call.

William Beebe reported that the pitch of the whistling note was ‘not far from the upper limit of human whistling’ and was always precisely the D sharp above middle C. ‘A most interesting thing is the way their hearing is exactly attuned to this note,’ he wrote, giving details of an experiment in which he placed two mother sloths in a cage, took away their offspring, then went sixty feet away and whistled at them. ‘Slowly but surely both heads turned in my direction and a male, high up on his tree, also turned at the same instant,’ but only when he whistled a D sharp. Other notes produced no reaction whatsoever. An anxious mother might show interest in a note a semitone away, but even a D natural had no effect on most sloths, ‘while D sharp aroused all the interest which their poor, dull minds could bring to bear’. Beebe does not, however, mention this D-sharp whistle specifically as part of courtship, so whether girl sloths whistle at boy sloths remains a matter for speculation.

Another, much more recent, theory of sloth courtship involves mutual poo-sniffing and has been advocated as a good reason for three-toed sloths climbing down their trees once a week always to poo in the same place. Quite how sniffing poo at the foot of a tree fits in with whistling from its top is another question that needs answering. We shall leave the sloth’s curious toilet habits and its possible link to mating behaviour until Chapter 9.

After the mysteries of courtship and mating come gestation and childbirth, and the first of these is a bit of a mystery too. The trouble is that sloths, even in captivity, have never been sufficiently closely monitored to know precisely when pregnancy has occurred and, when it does occur, detectable signs may not be seen for some time. Estimates of the length of a sloth’s gestation period therefore vary quite widely. One authority asserts that all sloths have a gestation period of ‘about six months’; another estimates ten. Four to six months for species of three-toed sloth, but ten to twelve months for two-toed, say some, while yet others insist it is five to six months for Linnaeus’s two-toed sloth but eleven to twelve for Hoffmann’s two-toed variety. All they seem agreed on is that female sloths give birth to one baby at a time. The most convincing recent research into sloth gestation periods came in a study in Germany in 2016 which used ultrasound to detect early signs of pregnancy with greater reliability than earlier attempts to do so. Their conclusion was that the gestation period of both species of two-toed sloths is between 330 and 350 days.

Finally, some news to report that is both good and bad: the first ever baby sloth delivered by Caesarean section was born at the Sloth Institute in Costa Rica in October 2014. The baby’s heavily pregnant mother had been brought there after falling from a tree and suffering head injuries. An emergency C-section was performed after the mother failed to respond to treatment and a baby boy sloth was delivered successfully. Sadly, the baby was found to have a heart murmur and died a week later. The mother died the following day from brain injuries suffered in her fall.