Slow lorises use venom as a weapon in intraspecific competition.

Animals have evolved an array of spectacular weapons, including antlers, forceps, proboscises, stingers, tusks and horns [1]. Weapons can be present in males and females of species needing to defend critical limiting resources, including food (rhinoceros beetles, Trypoxylus) and territories (fang blennies, Meiacanthus) [1-3]. Chemicals, including sprays, ointments and injected venoms, are another defence system used by animals. As with morphological weapons, venom can serve multiple purposes, including to facilitate feeding, in predation, and in defence when attacked [4]. Although rare, several taxa use venom for agonistic intraspecific competition (e.g. ghost shrimp, Caprella spp.; sea anemones, Actinia equina; cone snails, Conidae; male platypus, Ornithorhynchus anatinus) [4-6]. Another group of venomous mammals are the nocturnal slow lorises (Nycticebus) [7]. Slow loris bites often result in dramatic diagnostic wounds characterised by necrotic gashes to the head and extremities. Although these bites are the major cause of death of lorises in captivity, the function of this aggressive behaviour has never been studied in the wild [7]. Here, through an 8-year study of wounding patterns, territorial behaviour, and agonistic encounters of a wild population of Javan slow lorises (Nycticebus javanicus), we provide strong evidence that venom is used differentially by both sexes to defend territories and mates. VIDEO ABSTRACT.

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