Interference competition and group size effect in sika deer (Cervus nippon) at salt licks

Competition has long been considered as a confounding factor of group size effect but the understanding of interference competition is rudimentary for the difficulty in disentangling interference competition from scramble competition adequately. Here, we analysed remote-camera video records of wild sika deer (Cervus nippon) at salt licks in southern China from March 1, 2006 to November 30, 2008 to investigate how interference competition and predation risk interacted on vigilance behaviour. Scramble competition is negligible at salt licks; therefore, we could focus our interest in interference competition. We used linear mixed model to compare vigilance, licking and aggression behaviours among females with and without fawn as well as males with different group sizes to identify the primary role of vigilance behaviour in sika deer. In total, 168 individuals were recorded and observation time was 2,733.04 min. We found that deer spent much time on vigilance and scanned frequently in spring and winter, and females with fawn spent more time on vigilance than females without fawn, suggesting vigilance for predation risk. Aggression ratio increased first and then decreased, while scan frequency continued to decline and then slightly increased when group size increased from two to seven, implying vigilance for interference competition. Our results suggested vigilance in sika deer was influenced by both predation risk and interference competition, but was mainly driven by predation risk even at sites with intense interference competition. Our results of interference competition shed some light on finding the underlying mechanism of group size effect in wild populations.