Lorises carry a brachial gland on the inside of the upper arm. Its secretion alone is not strongly toxic; combined with saliva it becomes active. The animal grooms the secretion into the mouth, where a toothcomb of forward-pointing lower incisors and canines both delivers the mixture and creates the wound channel. Mothers also anoint infants with it before leaving them parked on a branch.
The arms-raised posture that precedes a bite is often described as a defensive display. The primary target appears to be other slow lorises: severe wounds from conspecific bites are common in wild populations, and intraspecific competition is a plausible driver. Predator deterrence and parasite defence have both been proposed as additional functions.
The raised-arms posture is also the reason for the animal's popularity in online video, and the trade that follows. Traders commonly clip or pull the toothcomb before sale, which is fatal often enough that it is a recognised welfare and conservation problem.