Dwarf mongooses responded to the presence of fork-tailed drongos by significantly reducing their anti-predator vigilance. Individuals halved the period they spent on sentinel duty, allowing them to re-allocate this time to other behaviours. Because dwarf mongoose sentinels are known to suffer an elevated risk of predation (Rasa 1987
), individuals probably gained direct survival benefits also. Mongooses also reduced the frequency with which they interrupted foraging to scan for danger. This is likely to significantly improve foraging efficiency in a species that preys on highly mobile invertebrates. The causality of the relationship between vigilance and the presence of a co-foraging bird species was confirmed by the playback experiments (), which simulated the presence of a drongo using vocal cues.
Dwarf mongooses presumably reduced their vigilance when accompanied by drongos because they capitalized on the birds' predator warnings (although the mongooses' use of heterospecific alarms (Rasa 1983
) is yet to be confirmed experimentally). The mongooses are unlikely to have benefited from a dilution effect (Inman & Krebs 1987
) because only one or two drongos normally accompanied a group of 10–26 mongooses (mean 18.5). The mongooses appeared to treat the drongos as a reliable sentinel species, responding more strongly to drongo-initiated alarms than to alarms initiated by any other species. At least one mongoose fled to cover for 92 per cent of drongo-initiated alarms compared with 42 per cent of alarms initiated by hornbills (Tockus leucomelas
or T. erythrorhynchus
) and 50 per cent of alarms initiated by tree squirrels (Paraxerus cepapi
The drongos initiated alarms almost exclusively in response to raptors that gained close proximity to the group (either by concealed waiting or stooping), and they aggressively mobbed these birds. Theoretically, these raptors were a threat to both mongooses and drongos, although we never observed a drongo attacked. Ridley et al. (2007)
demonstrated experimentally that fork-tailed drongos alarm at terrestrial predators when accompanying ground-foraging pied babblers (Turdoides bicolour
) but not when alone, and we observed drongos assisting the mongooses to mob puff adders (Bitis arietans
) and an African civet (Civettictis civetta
), neither of which poses a threat to drongos.
Dwarf mongooses are known to coordinate sentinel behaviour among group members (Rasa 1986
), but our findings suggest that they can also facultatively adjust their behaviour to take into account contributions to vigilance by another, unrelated, species. Our experiments do not exclude the possibility that the mongooses were responding to the presence of a vocalizing co-foraging bird species, rather than to a drongo specifically. However, they were clearly able to discriminate between bird species on the basis of vocal cues, and they did not lower their vigilance in the simulated presence of a non-co-foraging species (c
This is, to our knowledge, the first study to demonstrate experimentally that a mammal responds to the presence of an avian co-forager by reducing its vigilance. Observations of mixed-species foraging parties in Kenya found a negative correlation between number of sentinels posted by dwarf mongooses and number of birds present (Rasa 1983
). Similarly, red colobus (Colobus badius
) look down less frequently when foraging with Diana monkeys (Cercopithecus diana
; Bshary & Noe 1997
). However, these relationships were not tested experimentally and short-term environmental factors could explain the findings. For example, a lack of predator activity, a locally abundant food source or the lessening of predation risk owing to weather, cover or habitat type could lead to both a relaxation of vigilance and a tendency for heterospecific individuals to gather. Among birds, fork-tailed drongos are known to be associated with reduced sentinel behaviour in small (but not large) pied babbler groups (Ridley & Raihani 2007
), and both downy woodpeckers and white-breasted nuthatches are less vigilant when foraging in mixed-species flocks. Downy woodpeckers reduced their vigilance when played the contact calls of heterospecific flock members (Sullivan 1984
), and both species increased vigilance after the removal of two flocking species (Dolby & Grubb 1998
In conclusion, this study shows that the benefits of using heterospecific predator warnings can extend beyond a presumed reduction in mortality rates by allowing individuals to divert time and resources from vigilance to alternative uses. These findings strengthen our understanding of the importance of this form of interspecies communication, not only its capacity to shape the behaviour of individual species or foraging assemblages, but also its potential role in coevolution.