Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth (Bradypus variegatus)

Abstract

Sloths are considered to have one of the lowest mass-specific metabolic rates of any mammal and, in tandem with a slow digestive rate, have been theorized to have correspondingly low rates of ingestion. Here, we show in a study conducted over five months, that three captive Bradypus variegatus (Brown-throated sloths) had a remarkably low mean food intake of 17 g kg−1day−1 (SD 4.2). Food consumption was significantly affected by ambient temperature, with increased intake at higher temperatures. We suggest that the known fluctuation of sloth core body temperature with ambient temperature affects the rate at which gut fauna process digesta, allowing for increased rates of fermentation at higher temperatures. Since Bradypus sloths maintain a constantly full stomach, faster rates of fermentation should enhance digestive throughput, increasing the capacity for higher levels of food intake, thereby allowing increased energy acquisition at higher ambient temperatures. This contrasts with other mammals, which tend to show increased levels of food intake in colder conditions, and points to the importance of temperature in regulating all aspects of energy use in sloths.

The metabolic response of theBradypussloth to temperature

“…It is notable, however, that the sloths did attempt to minimize heat loss by retracting their limbs and reducing the exposed surface area of their bodies. The nominal sloth ‘TAZ’, corresponding to their metabolic peak at 26–30 °C, coincides closely with average daytime temperatures in tropical forests ( Giné et al, 2015 ), when sloths are most active and feed the most ( Chiarello, 1998 ; Cliffe et al, 2015 ; Giné et al, 2015 ). At these temperatures, we expect heat production to exactly balance that lost due to the small difference in sloth body and environmental temperature (of some 4 °C).…”

“…We suggest that sloths depress VO 2 at higher T a values in order to prevent hyperthermia. Due to slow rates of digestion limiting the rates of energy acquisition ( Cliffe et al, 2015 ; Foley, Engelhardt & Charles-Dominique, 1995 ), all sloths are considered to exist under severe energetic constraints ( Pauli et al, 2016 ). Delicate adjustments of metabolic rate—in part as a response to T a —are one way in which sloths adjust and minimise their energy expenditures.…”

A spontaneous mitonuclear epistasis converging on Rieske Fe-S protein exacerbates complex III deficiency in mice

“…The fact that the variant was homoplasmic in an apparently healthy WT mouse colony suggested that it must be non-pathogenic. Indeed, analysis of mtDNA sequences deposited to GenBank showed that the three-toed sloth species (Bradypus) of South America, known for their very low metabolic rate 16,17 naturally carry this variant (Fig. 1a).…”

“…For example, mt-Cyb p.D254N alone was sufficient to increase fatty acid oxidation (low RER) and induce hepatic Fgf21 expression. The mtDNA sequence data in GenBank show the presence of mt-Cyb p.D254N in three-toed sloths (Bradypus), which feed on an extremely energy-poor diet 16 . It is tempting to speculate that the variant might even be beneficial under some circumstances.…”

Myology of the pelvic limb of the brown‐throated three‐toed sloth (Bradypus variegatus)

“…Lower body mass is associated with arboreal habits (Cartmill, 1985), especially in high-canopy dwellers (Rupert et al, 2014), but it is not completely understood how reduced low muscle mass is related to specialization for suspensory locomotion in sloths, and this may further seem counterintuitive to the flexor muscle strength that is required from the limbs as a whole for suspension (Fujiwara et al, 2011;Olson et al, 2018). Moreover, comparatively less is known about the physiology and behavior in species of the genus Bradypus (e.g., Cliffe et al, 2015Cliffe et al, , 2018Pauli et al, 2016) than Choloepus.…”