Graded alarm call behavior in wild fox squirrels (Sciurus niger)

Distress or alarm calls are vocalizations made when animals are in stressful situations or faced with a predator. Squirrels (Sciuridae) are known for being very vocal; however, most studies on alarm vocalizations are limited to ground squirrels. We investigated the acoustic behavior of the arboreal fox squirrel (Sciurus niger) under different conditions. Specifically, we tested the hypothesis that fox squirrels modify acoustic alarm behavior in response to different perceived threat levels and that this response is affected by sex and individual experience. Squirrels were trapped, and acoustic data were collected during periods in which the squirrels were alone, approached by humans, manipulated in traps, and handled by humans. Calls were categorized based on acoustic features, and we quantified the call rate (calls/s) across conditions. Threat level significantly affected vocal rate, with squirrels producing more calls overall when alone but shifting the proportion of emitted call types as threat level increased. Sex, capture history, and individual had no effect on call rate. These results suggest that fox squirrels use a graded alarm call response system to respond to threatening situations.

Assessing anti-predator decisions of foraging eastern chipmunks under varying perceived risks: the effects of physical and social environments on vigilance

Animals foraging under risk have to trade-off resource acquisition and predator avoidance. Environmental factors can modulate the level of risk and should thus influence the expression of anti-predator behaviours such as vigilance. In this study, we investigated the effects of physical and social environments on eastern chipmunks’ (Tamias striatus) vigilance, by varying the perceived risk through playback experiments of alarm calls and neutral environmental sounds, and by integrating habitat and weather characteristics, as well as neighbour density. Chipmunks showed higher levels of vigilance when foraging in more open habitats, under high wind conditions, when they heard alarm calls and when surrounded by a high neighbour density. The effects of wind intensity and neighbour density on vigilance were also stronger during the broadcast of alarm calls rather than neutral sounds. Our results emphasize how both the physical and social environments can modify risk perception and therefore risk-taking decisions of foraging individuals.

Spying on small wildlife sounds using affordable collar-mounted miniature microphones: an innovative method to record individual daylong vocalisations in chipmunks

Technological advances can greatly benefit the scientific community by making new areas of research accessible. The study of animal vocal communication, in particular, can gain new insights and knowledge from technological improvements in recording equipment. Our comprehension of the acoustic signals emitted by animals would be greatly improved if we could continuously track the daily natural emissions of individuals in the wild, especially in the context of integrating individual variation into evolutionary ecology research questions. We show here how this can be accomplished using an operational tiny audio recorder that can easily be fitted as an on-board acoustic data-logger on small free-ranging animals. The high-quality 24 h acoustic recording logged on the spy microphone device allowed us to very efficiently collect daylong chipmunk vocalisations, giving us much more detailed data than the classical use of a directional microphone over an entire field season. The recordings also allowed us to monitor individual activity patterns and record incredibly long resting heart rates, and to identify self-scratching events and even whining from pre-emerging pups in their maternal burrow.

Evolving communicative complexity: insights from rodents and beyond

Social living goes hand in hand with communication, but the details of this relationship are rarely simple. Complex communication may be described by attributes as diverse as a species' entire repertoire, signallers' individualistic signatures, or complex acoustic phenomena within single calls. Similarly, attributes of social complexity are diverse and may include group size, social role diversity, or networks of interactions and relationships. How these different attributes of social and communicative complexity co-evolve is an active question in behavioural ecology. Sciurid rodents (ground squirrels, prairie dogs and marmots) provide an excellent model system for studying these questions. Sciurid studies have found that demographic role complexity predicts alarm call repertoire size, while social group size predicts alarm call individuality. Along with other taxa, sciurids reveal an important insight: different attributes of sociality are linked to different attributes of communication. By breaking social and communicative complexity down to different attributes, focused studies can better untangle the underlying evolutionary relationships and move us closer to a comprehensive theory of how sociality and communication evolve.

Alarm signals of the Sichuan sika deer Cervus nippon sichuanicus

Sichuan sika deer (Cervus nippon sichuanicus) is an endangered and endemic subspecies of sika deer to Sichuan Province, China. According to our observations in the wild, the Sichuan sika deer makes alarm signals in the presence of actual or potential predators. In order to test the variation of the rhythmic alarm calls in some sex/age classes and different risk contexts, we recorded alarm calls of Sichuan sika deer from 2 October to 30 November 2008 and from 4 April to 5 September 2009 in the Tiebu Nature Reserve, Zoige County, Sichuan Province, China, and made acoustic analysis of these alarm calls. The results showed that the fundamental frequencies of alarm signals of Sichuan sika deer tended to decrease with age, and were significantly higher for females than for males. Duration tended to increase with age, and was significantly longer for males than for females. The fundamental frequencies and duration of alarm calls in adults were significantly higher and shorter respectively in high-risk than in moderate-risk contexts.

Alarm signals of the great gerbil: acoustic variation by predator context, sex, age, individual, and family group

The great gerbil, Rhombomys opinus, is a highly social rodent that usually lives in family groups consisting of related females, their offspring, and an adult male. The gerbils emit alarm vocalizations in the presence of diverse predators with different hunting tactics. Alarm calls were recorded in response to three predators, a monitor lizard, hunting dog, and human, to determine whether the most common call type, the rhythmic call, is functionally referential with regard to type of predator. Results show variation in the alarm calls of both adults and subadults with the type of predator. Discriminant function analysis classified an average of 70% of calls to predator type. Call variation, however, was not limited to the predator context, because signal structure also differed by sex, age, individual callers, and family groups. These variations illustrate the flexibility of the rhythmic alarm call of the great gerbil and how it might have multiple functions and communicate in multiple contexts. Three alarm calls, variation in the rhythmic call, and vibrational signals generated from foot-drumming provide the gerbils with a varied and multi-channel acoustic repertoire.

Repetitive calls of juvenile Richardson's ground squirrels (Spermophilus richardsonii) communicate response urgency

Richardson's ground squirrels, Spermophilus richardsonii, produce both repetitive and non-repetitive antipredator calls. While many hypotheses have been advanced to explain non-repetitive calls, the function of repetitive calling has received relatively little attention. We presented juvenile Richardson's ground squirrels with a predator model at distances ranging from 1 to 8 m and recorded the subsequent repetitive calls on digital audiotape. The rate of calling was inversely correlated with the distance between the model and the caller, with distance explaining almost 24% of the variation in call rate. To determine whether call recipients use that information, we manipulated the intersyllable latency of a single repetitive call exemplar to form 3 test stimuli varying only in call rate. Across 16 Richardson's ground squirrel colonies to which these calls were broadcast, the proportion of squirrels assuming the highly vigilant, alert posture increased with the rate of the repetitive call presented. Hence, juvenile Richardson's ground squirrels appear to communicate the proximity and presumably the degree of threat posed by potential predators.

Juvenile Richardson's ground squirrels, Spermophilus richardsonii, discriminate among individual alarm callers

Richardson's ground squirrels, Spermophilus richardsonii, issue vocal alarm responses to avian and terrestrial predators. Recipients of those calls presumably benefit from enhanced detection and subsequent avoidance of predators. If receivers of alarm calls differentiate among callers, they could use that information to tailor their behavioural response to the perceived level of threat. A neighbour's call may indicate more imminent danger than that of a non-neighbour; with individual recognition, recipients could adjust their response according to the reliability of the caller. I elicited and recorded alarm calls of juvenile Richardson's ground squirrels using an avian predator model. Series of five calls were then played back to focal juveniles in the field. Within each series, playbacks involved an initial 'habituation series' of four calls of either a neighbour or a non-neighbour. In 'control' series, the fifth call was from the same individual but differed from the four preceding it. In 'experimental' series, the fifth call was from a different individual than those played earlier. Juveniles showed greater vigilance in response to playbacks of neighbours' calls relative to non-neighbours' calls, but habituated rapidly to call playbacks, showing a significant decline in vigilance by the second call. In control series, a different call from the same individual did not significantly increase vigilance. In experimental series, a call from a different individual restored vigilance to a level similar to that recorded prior to habituation. These results suggest that juvenile Richardson's ground squirrels discriminate among callers and use that ability to respond differentially to alarm calls issued by neighbours and non-neighbours. Copyright 1998 The Association for the Study of Animal Behaviour.