Induced or routine vigilance while foraging

Vigilance is time-consuming and, since time is usually a limited resource for free-ranging animals, it may conflict with other activities such as feeding. For several taxonomic groups, however, some authors have questioned the importance of this historical tradeoff (Arenz and Leger 1997, Lima and Bednekoff 1999, Guillemain et al. 2001). Illius and Fitzgibbon (1994) proposed that when food intake rate is limited by chewing and swallowing rates (i.e. process 3 in Spalinger and Hobbs 1992), rather than by food encounter rate (i.e. process 1 or 2 in Spalinger and Hobbs 1992), the remaining ‘‘spare time’’ (i.e. the time spent chewing after detection of the next bite) can be used by foragers for other activities, such as vigilance, without reducing the rate of food intake. However, vigilance is shaped not only by the ingestion process (cropping, chewing and swallowing), but also by stimuli of elevated predation risk. Hence, vigilance behaviour is expected to have two components: ‘‘routine vigilance’’ when the animal is simply monitoring its surroundings during spare time or ‘‘induced vigilance’’ when responding to a stimulus. The scarcity of studies stating the importance of sudden stimuli in vigilance behaviour is puzzling (Lima and Bednekoff 1999, Ruxton and Roberts 1999, Fortin et al. 2004a, Bednekoff and Lima 2005). In addition to the conceptual importance of a distinction between induced and routine vigilance, we believe that this gap in the current research on vigilance behaviour may strongly influence our understanding of the tradeoff between foraging and vigilance. Clearly, routine vigilance is closely related to the typical form of vigilance fitting both the assumptions and predictions of theoretical models (Illius and Fitzgibbon 1994, Fortin et al. 2004a, 2004b). When foraging on patches of high biomass (i.e. handlinglimited foraging, process 3 in Spalinger and Hobbs 1992), animals that can handle food while in the act of vigilance (i.e. head raised), have the opportunity to scan their surroundings while chewing the current bite before cropping the next (Illius and Fitzgibbon 1994, Cowlishaw et al. 2004, Fortin et al. 2004a). Fortin et al. (2004a) reported that bite size sometimes exceeded mouth dimensions, so that foragers could be vigilant while vegetation gradually entered their mouth. In these situations, foragers are able to match scanning with spare time, thereby allowing for the continuity of the ingestion process, so that vigilance appears to be largely cost-free (Illius and Fitzgibbon 1994, Fortin et al. 2004a). Unlike routine vigilance where animals can match with precision the particular ingestion process and scanning behaviour, induced vigilance may disrupt the ingestion process since it requires animals to react to unpredictable stimuli whatever the phase of feeding. Accordingly, Fortin et al. (2004a) reported that the proportion of foraging time spent by free ranging elk, Cervus canadensis, and bison, Bison bison, scanning their surroundings was only a small fraction of the spare time available to them. The negative impact of induced scanning on intake rate is even more important when the stimulus occurs just after an animal has swallowed. In such instances, foragers have nothing to chew while scanning. Moreover, animals engaged in induced vigilance should reduce chewing considerably, even with a mouthful of food. One major reason for animals to reduce chewing when responding to a threat stimulus is because chewing is noisy (so much so that some authors have deployed audio transmitters on animals in order to record bite/chew rates, Laca and Wallis De Vries 2000, Logan and Sanson 2002) and thus may impede a prey’s ability to evaluate the level of predation risk. Moreover, the acoustic signals generated by chewing are primarily transmitted by bone conduction and so our own mastication sounds can appear much Oikos 116: 1603 1608, 2007 doi: 10.1111/j.2007.0030-1299.15799.x, Copyright # Oikos 2007, ISSN 0030-1299