Christopher S. Evans

Learning specificity in acquired predator recognition

Predator recognition is often dependent upon experience. This behavioural plasticity can potentially be exploited to enhance the antipredator behaviour of captive-bred animals, but it is first necessary to understand the specificity of learning. We enhanced the responses of tammar wallabies, Macropus eugenii, to a model fox, Vulpes vulpes, by presenting this novel predator in conjunction with a human simulating a capture procedure. A control group had identical total exposure to fox and human, but with no such predictive relationship between these two events. Animals that experienced paired presentations of fox and human behaved more cautiously towards the fox after training than controls. To assess whether this learnt response was specific to the fox, we presented the animals with an array of visual stimuli both before and after training. The tammars generalized their acquired response from the predator with which they were trained to a predator with which they were not trained (cat, Felis catus), but not to a nonpredator (goat, Capra hircus). Tammars also exhibited a transient increase in response to a model wallaby after training. We suggest that this effect is more likely to reflect social behaviour than generalization of the learnt response from predator to conspecific. Two additional controls revealed that changes in behaviour after training were not attributable to the presentation device and were not caused by a general decrease in response threshold associated with training. Our results suggest that tammar wallabies perceive predators as a natural category.

Intrasexual selection predicts the evolution of signal complexity in lizards

Sexual selection has often been invoked in explaining extravagant morphological and behavioural adaptations that function to increase mating success. Much is known about the effects of intersexual selection, which operates through female mate choice, in shaping animal signals. The role of intrasexual selection has been less clear. We report on the first evidence for the coevolution of signal complexity and sexual size dimorphism (SSD), which is characteristically produced by high levels of male–male competition. We used two complementary comparative methods in order to reveal that the use of complex signals is associated with SSD in extant species and that historical increases in complexity have occurred in regions of a phylogenetic tree characterized by high levels of pre–existing size dimorphism. We suggest that signal complexity has evolved in order to improve opponent assessment under conditions of high male–male competition. Our findings suggest that intrasexual selection may play an important and previously underestimated role in the evolution of communicative systems.

Digital video playback and visual communication in lizards

Experimental analyses of dynamic visual signals have to overcome the technical obstacle of reproducing complex motor patterns such as those found in courtship and threat displays. Video playback offers a potential solution to this problem, but it has recently been criticized because of sensory differences between humans and nonhuman animals, which suggest that video stimuli might be perceived as deficient relative to live conspecifics. Quantitative comparisons are therefore necessary to determine whether video sequences reliably evoke natural responses. Male Jacky dragons, Amphibolurus muricatus, compete for territories using complex displays delivered in a rapid stereotyped sequence. We evaluated video playback as a technique for studying this visual signal. Digital video sequences depicting a life-sized displaying male were indistinguishable from live male conspecifics in the rate and structure of aggressive displays evoked. Other measures of social behaviour suggested that video stimuli were more effective in this context. Lizards produced significantly more appeasement displays and had higher rates of substrate licking and locomotor activity in response to video playback than to confined male opponents, which failed to produce aggressive displays. Lizards tracked temporal changes in the display rate of video stimuli and were also sensitive to individual differences in morphology and behaviour between video exemplars. These results show that video stimuli are appropriate for the experimental analysis of Jacky dragon aggressive displays. We compare the potential shortcomings of video playback with those of other techniques and conclude that no approach offers a panacea, but that several have complementary characteristics.

Olfactory predator recognition: wallabies may have to learn to be wary

Many species modify their behaviour in response to the scents of their predators, but species or populations living without predators may lose such abilities. This loss has been suggested to be irreversible, and to constitute a significant hurdle in restoring historical ecosystems. Olfactory predator recognition was studied in two macropodid marsupials - the tammar wallaby ( Macropus eugenii ) and the red-necked pademelon ( Thylogale thetis ). Both species are in the ‘critical weight range’ of Australian native mammals that have been negatively affected by the introduction of novel predators since European settlement. Predator-naive animals were tested by exposing subjects simultaneously to two feeders with either a predator or a herbivore faecal or urine sample beneath the food tray. The presence of predator olfactory cues beneath the feeder did not affect foraging behaviour or feeder use when compared to control stimuli (herbivore faeces or urine). Previous studies have found that predator-experienced herbivorous marsupials modify their behaviour in the presence of predator scents. In contrast, our studies of predator-naive individuals found no evidence of such selectivity, suggesting that marsupial herbivores may have to learn to modify their behaviour in response to olfactory cues from predators. This implies that the loss of olfactory predator recognition may not be irreversible. Animals translocated from predator-free areas could potentially be trained to recognise the smells of their predators.

An experimental study of behavioural group size effects in tammar wallabies, Macropus eugenii

As animals aggregate with others, the time they allot to social and nonsocial activities changes. Antipredator models of vigilance and foraging group size effects both predict a nonlinear relationship between group size and the time allocated to behaviour. Group size effects were experimentally studied in captive adult female tammar wallabies, a small macropodid marsupial, by increasing group size from 1 to 10. Tammars foraged more, looked less, groomed more, engaged in more aggressive interactions and moved about less as group size increased. Nonlinear regression models explained more variation in the time allocated to foraging, looking, locomotion and affiliative behaviour than linear models. Variation in self-grooming and aggression was better explained by linear models. Wallabies lay down significantly more, and walked significantly less, as group size increased: these relationships were significantly nonlinear. Thus, changes in perceived predation risk, which are characterized by nonlinear relationships, explain tammar wallaby group size effects for most activities. These results support the assertion that predation has played an important role in macropodid social evolution. Moreover, the findings suggest that conservation biologists should pay particular attention to group size when translocating or reintroducing endangered macropodids. Copyright 1999 The Association for the Study of Animal Behaviour.

Measuring the structure of dynamic visual signals

Correspondence: R. Peters, Department of Psychology, Macquarie University, Sydney, NSW 2109, Australia (email: richard@ galliform.psy.mq.edu.au). C. W. G. Clifford is at the School of Psychology, University of Sydney, Sydney, NSW 2006, Australia. The evolutionary significance of perceptual processes is well documented (Endler 1991; Guilford & Dawkins 1991; Pagel 1993; Dawkins & Guilford 1996; Endler & Basolo 1998). Signals must be designed to stimulate the sense organs of intended receivers (e.g. social companions, opponents, or potential mates) and this effect must typically be achieved without attracting the attention of ‘eavesdroppers’ such as parasites and predators. Signals must also be memorable (Bernard & Remington 1991; Rowe & Guilford 1996; Speed 2000), so that nuances of structure are learned quickly amongst a welter of competing stimuli impinging upon the receiver. Analysis of structure is an essential prerequisite for any exploration of signal design. The long history of successful work on acoustic communication can be traced to the development of the sound spectrograph in the 1950s (for a review see Hopp et al. 1998). Similarly, rapid advances have been made in recent years since spectral analysis (Endler 1990) has been applied in studies of static visual signals, such as ornaments and colour patterns. In contrast, much less is known about signals that are defined by movement, such as courtship and aggressive displays. Such dynamic visual signals are ubiquitous: they are used in contexts as diverse as opponent assessment (Ord et al. 2001), female mate choice (where they may act synergistically with morphology; Rosenthal et al. 1996), pursuit deterrence (e.g. Hasson 1991; Caro 1995), alarm signalling (Hennessy et al. 1981) and even camouflage (e.g. Fleishman 1985). Motion is also an important component of many multimodal signals (Partan & Marler 1999), often in combination with sound (Evans & Marler 1994). The design of these motor patterns is a classic problem in evolutionary biology (Darwin 1871) and

Social learning of antipredator behaviour in a marsupial

Abstract Socially acquired predator recognition has been demonstrated in a range of taxa, but there is no experimental evidence for this phenomenon in marsupials. We have previously shown that tammar wallabies, Macropus eugenii, can be individually trained to avoid a model fox, Vulpes vulpes. Here, we examined whether such acquired responses can be socially transmitted to predator-naive companions. Tammars were given training trials in which they observed the response of a demonstrator wallaby that was either fearful of the fox (experimental group), or indifferent to it (control group). Tammars in the experimental group subsequently responded to the fox model with significantly higher vigilance levels than controls. To examine the specificity of this acquired antipredator behaviour, we measured responses to an array of other visual stimuli after training and compared these with the results of identical pretraining tests. There was a small difference between the two groups in responses to a model cat, Felis catus, but not to a model nonpredator (goat, Capra hircus). There were also no differences between experimental and control groups during blank trials, in which no stimulus was presented, showing that the elevated vigilance response to the fox did not simply reflect a general increase in arousal. The effect of training was hence to inculcate a relatively specific response to the fox, with only limited generalization to another predator stimulus. These findings provide the first evidence for social learning in a marsupial and suggest that this group has cognitive characteristics convergent with those of eutherian mammals. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

Detection of Bilateral Symmetry in Complex Biological Images

The recognition of bilateral symmetry in simple dot patterns is reliably influenced by orientation. Performance is best when the axis of symmetry is vertical. We conducted two experiments to determine whether stimulus orientation also affects detection of the low levels of naturally occurring asymmetry in complex biological images. University students judged whether colour images displayed on a computer monitor possessed perfect bilateral symmetry. Stimuli were generated from high-resolution plan-view images of crabs and insects. In experiment 1, the asymmetric stimuli were the original animals, displayed on a standard black background. Symmetrical versions of each natural image were generated by sectioning the shape at the midline, copying and reflecting one side, and then fusing the two halves together. To facilitate comparison of results with those obtained in earlier studies, we also presented dot patterns based upon both the slightly asymmetric and perfectly symmetrical natural images. Experiment 2 was designed to assess whether symmetry detection was dependent upon the markings and patterns on the body and appendages of the animals. The natural images were converted to silhouettes and tested against matched dot patterns. In both studies, images were presented in a random sequence with the axis of symmetry vertical, horizontal, oblique left, and oblique right. Performance with the biological images was consistently better than with the dot patterns. Abolishing fine detail did not appreciably reduce this effect. A pronounced vertical advantage was apparent with all stimuli, demonstrating that this phenomenon is robust despite considerable variation in image complexity. The implications of orientation effects for perception of natural structures are discussed.

Measuring social structure : a comparison of eight dominance indices

Measurement of social status is an important component of many behavioural studies. A variety of techniques have been developed and adopted, but while there have been some analyses of index properties using simulated data, the rationale for selecting a method remains poorly documented. As a first step in exploring the implications of index choice, we compared the characteristics of eight popular indices by applying each to the same data set from interactions between male fowl Gallus gallus, the system in which social hierarchies were first described. Data from eight social groups, observed over four successive breeding seasons, were analysed to determine whether different indices produced consistent dominance scores. These scores were then used in tests of the relation between social status and crowing to explore whether index choice affected the results obtained. We also examined the pattern of dominance index use over the last decade to infer whether this has likely been influenced by tradition, or by taxa of study animal. Overall agreement among methods was good when groups of birds had perfectly linear hierarchies, but results diverged when social structure was more complex, with either intransitive triads or reversals. While all regression analyses revealed a positive relationship between dominance and vocal behaviour, there were substantial differences in the amount of variance accounted for, even though the original data were identical in every case. Index selection can hence perturb estimates of the importance of dominance, relative to other factors. We also found that several methods have been adopted only by particular research teams, while the use of others has been taxonomically constrained, patterns implying that indices have not always been chosen solely upon their merits. Taken together, our results read as a cautionary tale. We suggest that selection of a dominance index requires careful consideration both of algorithm properties and of the factors affecting social status in the system of interest.

Social responses without early experience: Australian brush-turkey chicks use specific visual cues to aggregate with conspecifics.

SUMMARY Almost all birds depend upon early experience with adults and siblings to learn recognition cues. Megapodes, such as the Australian brush-turkey (Alectura lathami), have evolved a very different life history. Eggs are incubated in mounds of decaying organic material. Chicks hatch asynchronously and receive no parental care, so imprinting cannot occur. Nevertheless, chicks subsequently form groups with similar-aged conspecifics. We explored the perceptual basis of this aggregation response, focussing on likely visual cues, such as pecking movements and body colour. Experiments were conducted under naturalistic conditions in a large aviary, using realistic robot models and colour filters. The robots successfully evoked a range of social responses resembling those of a live companion. Aggregation depended upon both behaviour and morphology. Simultaneous choice tests revealed that brush-turkey chicks preferred a pecking robot over either a static model or a scanning robot, suggesting that responsiveness depends upon particular movement patterns. In addition, chicks were sensitive to changes in appearance but only those that affected radiance at short wavelengths. The mechanism underlying social aggregation after hatching hence involves relatively specific cues. This perceptual bias seems to be largely experience independent and may exploit attributes to which potential predators are insensitive.