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  • Author or Editor: Daniel W. Leger x
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Abstract

Antipredator vigilance behaviour is affected by a number of variables, but studies reporting experimental manipulation of visual obstructions are generally lacking in the literature. We examined the individual antipredator behaviour of free-ranging thirteen-lined ground squirrels in 7 city parks in Lincoln, Nebraska, USA as they fed on peanut butter within small plexiglass boxes. Boxes differed in the position and amount of visual occlusion for a squirrel feeding in it: opaque, clear, opaque-sides, and opaque-top (also opaque on the end). We experimentally controlled environmental and contextual variables that have been identified as affecting vigilance behaviour. We filmed 38 ground squirrels as they fed in the boxes, and the results indicate that ground squirrels treat lateral occlusion as functionally identical to complete occlusion. Additionally, there were no significant differences in the squirrels' behaviours in the clear versus the opaque-top box. In 35 approach tests, a person walked toward individual ground squirrels as they foraged in one of the 4 box types. The distance at which the squirrels reacted did not differ among the 4 box types. The results of these two experiments suggest that thirteen-lined ground squirrels compensate for visual occlusion, primarily through increased withdrawals coupled with low-level vigilance postures and that the position of visual occlusion can be of greater importance than the absolute amount of occlusion.

In: Behaviour

Abstract

This report presents two studies of captive squirrel monkeys living in mixed-sex social groups: In the first study, we quantified social interaction patterns and spacing in two groups during their regular daily feeding period. Dispersal of food was rapid, owing to the animals' tendencies to drop items on the cage floor, and competitive interactions were infrequent. Adult males and females were spatially segregated, and between-sex interactions were relatively infrequent. Dyadic proximity data indicated that each group contained several cliques of from two to three individuals, usually of the same sex. Members of a clique not only spent more time near each other, but favored the same areas in the cage, even when they were not in proximity. Measures of proximity and agonism were positively correlated. In the second study, the same groups were presented with either one or two bottles of fruit drink, a highly desired, non-dispersible resource, or with one or two bottles of plain water. During water-bottle sessions, between-sex interactions were no more frequent or agonistic than they were in the normative study; in the highly competitive fruit-drink sessions, however, males frequently directed agonistic behavior against females and against each other, and they successfully dominated access to the fruit-drink bottles. In contrast, female agonism was directed mainly toward other females. Our observations suggest that membership in a clique may provide a competitive advantage. In one group, a clique consisting of a mother and her daughter successfully excluded other females from the fruit-drink bottles; in another group, the only females to compete vigorously against males were members of the same clique; after one of the two males in this group was removed, females for the first time made occasional joint attacks on the remaining adult male, and he appeared to become less assertive and successful in competitive tests. We suggest that the social and spatial peripherality of males is not simply the result of coercive exclusion by females, but also reflects a "choice" by males to avoid interacting with females under most circumstances. When male motivation is strong, however, they readily approach females, aggress against them, and assert themselves successfully in competitive interactions, even against a substantial female majority.

In: Behaviour

Abstract

The predator-evoked calling of California ground squirrels (Spermophilus beecheyi) was studied in the field during the reproductive season. Three different sources of data indicated that adults call more after, than before their young have reached the age of first emergence from natal burrows. During exposure to a tethered rattlesnake (Crotalus viridis oreganus) and to a freely-moving dog (Canis familiaris), and in natural encounters with a coyote (Canis latrans) or bobcat (Lynx rufus), calling was more frequent after than before young first emerged. We concluded that California ground squirrels call in order to warn their offspring about predators, like other ground squirrel species do. In order to see the increase in mammalian predator evoked calling after pup emergence, we had to separate calling on the basis of its temporal organization. Nonrepetitive calling involved spacing a few vocalizations irregularly in time. Calls patterned in this way were more common early in an encounter, became more frequent after pup emergence, and more consistently elicited immediate reactions. Such calling was probably used to warn pups. Repetitive calling comprised rhythmic emission of a series of vocalizations. Calls organized repetitively were more common later in an encounter, were not emitted more frequently after pup emergence, and less consistently evoked immediate reactions. These and other differences between the two temporal patterns of vocalizing led us to propose that repetitive calling represented a "tonic" communicatory effort (as in SCHLEIDT, 1973). Repetitive inputs to other squirrels may act "cumulatively" in a longer time scale than nonrepetitive calling, so as to cultivate or maintain vigilance in other squirrels. The repetitive caller could benefit by using the enhanced reactions of these more vigilant squirrels as a source of information about the predator. We propose that predator-prey episodes may be understandable from an "epigenetic" perspective. That is, the first alarm calls during an encounter should shift squirrels from an unwarned to a warned status; subsequent calling must then function in some other way than as a warning.

In: Behaviour

Abstract

In this paper we report the results of our first efforts to evaluate the functional significance to signaler and perceiver of variation in tail flagging (Fig. 1 and Fig. 2A-C) by California ground squirrels (Spermophilus beecheyi). We first report a series of anecdotes in which we describe the circumstances of a variety of tail movements by California ground squirrels, including the different kinds of tail flagging. Secondly and primarily we identify the information afforded by snake-elicited tail flagging. Tail flagging is a signal used by California ground squirrels primarily when they are harassing a potential snake predator (Fig. 4). It attracts other squirrels who may also begin harassing the snake. The risk to squirrels in encounters with snakes continuously varies, and the squirrels adjust their behavior accordingly. Consequently in this situation we expected to find shifts in the information afforded by different tail-flagging variants. We view the information afforded by tail movement and other signals as a consequence, not of selection for making that information available, but of the correlations resulting from situational constraints on the signaler's behavior, e.g., correlations between tail movement variation and variation in significant events. We used two complementary approaches to help determine the information afforded by tail flagging. In one, we asked whether information important to percipients is afforded by tail flagging. In the second, we searched for situational correlates of tail-flagging variants. We applied the second approach to each individual separately and to the group comprising these individuals. This allowed us to look for idiosyncrasies in signaler behavior. Although a relatively simple signal, tail flagging varies along several structural and temporal parameters. From video recordings we quantified a structural parameter- number of movement cycles in a bout of tail flagging - and two temporal parameters- rate and temporal clustering of tail flagging. Our results show that squirrels adjust their tail-flagging behavior in the following ways. 1. When a rattlesnake rattles, harassing squirrels increase the number of cycles per bout of tail flagging. 2. Lone snake-directed squirrels temporally clump their flagging bouts more than snake-directed squirrels accompanied by other snake-directed individuals. 3. The structure of flagging varied with the squirrel's behavior vis-a-vis the snake. While dealing directly with a snake, squirrels emitted relatively few 1-cycle bouts of flagging. While in the vicinity and monitoring snake-related events, but engaged primarily in other activities such as feeding or grooming, squirrels emitted relatively greater numbers of 1-cycle bouts. As one would predict from this difference, individuals were farther from the snake on the average while emitting 1-cycle bouts than while emitting 2-cycle bouts. Adults used far more 2-cycle bouts than 1, and 3 or higher. In the field, bouts of 4-cycles or greater were very rare. We found that the information afforded by a bout of tail flagging was much greater when we considered structural variation, than when we did not. For example, flaggers were more likely on the average to "pause" before than after tail flagging. However, although the same difference held for 1-cycle bouts, just the reverse was true for 3-cycle bouts, and there was little difference in the probability of pausing for 2-cycle bouts. We found similar differences for other behavioral correlates of tail flagging. A percipient could much more precisely predict a tail flagger's behavior by considering signal variation. Our results indicate a percipient can infer from a high proportion of 3-cycle bouts that the flagger is beginning an episode of snake-directed activity, whereas 1-cycle bouts indicate a temporary cessation of snake-directed behavior. We expect to find even greater situational specificity of tail flagging when we simultaneously consider multiple structural dimensions, such as axis of movement and number of cycles. When we looked at the information afforded by a bout of tail flagging separately for individual squirrels, it was clear that the specific behavioral profiles associated with different variants of tail flagging were somewhat idiosyncratic. Thus, a percipient squirrel should be able to infer more from the tail flags of a familiar squirrel than an unfamiliar squirrel. We propose that signals are used to elicit a particular kind of performance from the target(s). The effectiveness of such action depends upon a knowledge of the current status of the individual's target(s). We conclude that variation in tail flagging is constrained in at least three ways: 1) by the number and quality of targets of tail flagging (e.g., snakes, squirrels); 2) by the signaler's certainty about the status of the target(s) (by eliciting behavior in targets, tail flagging may be used in part to extract information about the target's state); and 3) by the quality and availability of feedback (the success of tail flagging is continually assessed on the basis of feedback). The understanding of the functional significance of tail flag variability therefore becomes the problem of understanding how the flagger uses signal adjustments to deal with changes in its circumstances in terms of these three general constraints.

In: Behaviour