Anti-predator adaptations and strategies in the Lepidoptera

This thesis examines visual anti-predator strategies employed by the Lepidoptera. I examine key aspects of pattern and behaviour and how they relate to the reduction of an individual’s predation risk. Symmetrical patterns have been found to be easier to remember and pick out, suggesting that symmetry is beneficial to aposematic displays. This suggests that symmetry may be maladaptive in cryptic patterning and asymmetry beneficial. In Chapter one, I report the results of a field experiment using artificial prey and wild birds to investigate how asymmetry and symmetry affect the efficiency of cryptic patterning to reduce predation. I found that asymmetry does not affect predation rate, in agreement with previous work. Yet, there is still the problem of how to mesh this with the potentially conflicting conclusions of symmetry studies. Chapter two examines aspects of the intimidation hypotheses of Lepidopteran eyespots. These address the generally larger and more centrally placed spots found on Lepidopteran wings and state that they startle or intimidate predators, providing time for escape. While it is agreed that eyespots intimidate or startle predators, the mechanism has not been agreed. There are two competing lines of thought 1) that ‘eyespots’ intimidate because they resemble the eyes of the predators’ own predators and 2) that it is the conspicuous colouration of the pattern that induces the startle or avoidance behaviour. The first experiment utilised artificial prey with differing ‘directions of gaze’ in a field setting. If purely conspicuous patterns direction of gaze should have no influence on prey survival. The results indicate that patterns imitating staring or upward gazes provide the greatest protection, suggesting that in some cases eyespots may be being perceived as eyes and not simply as conspicuous patterns. I wanted to see if it would be possible to find a way in which to measure or quantify the reaction of an animal to ‘real’ eyes, in order to compare it to the reaction to eyespots. Recent trials investigating human reactions to eye contact suggested a computer based method may be possible. In this second experiment we examined whether the direct gaze of a predator might produce a measurable effect in human subjects. I was not able find any effect, but it is unclear as to whether this is due to problems with the experimental set up. In Chapter three I investigate a factor often over looked in the study of crypsis, that of the behavioural adaptations that can enhance its efficiency. The larvae of the early thorn moth (Selenia dentaria) masquerade as twigs, using both colouration and behaviour adaptations. I compared the angle at which the larvae rested, to the angle at which real twigs deviate from the main stem. The results found that the larvae showed variation in their angle of rest and do not appear to match the angle of real twigs on the host tree. This result suggests that perfectly matching the angles of real twigs is not necessary to twig mimicry. While carrying out this experiment it was noticed that a breeze appeared to increase larval activity and induced a ‘swaying’ behaviour. This led me to examine whether mimic species may utilise the visual ‘noise’ produced by windy conditions to camouflage movement. Firstly, a small ‘proof of concept’ pilot was carried out, followed by a larger study using 2 different twig mimic species. The study involved measuring movement and swaying behaviour in 3 conditions (still air, wind setting 1 and 2). The results suggest that cryptic and mimetic lepidopteran species may use windy conditions to camouflage their movements and that some species may employ specialised ‘swaying’ behaviours. Cryptic species are limited in opportunities to move between foraging sites without increasing detection by predators, therefore, any adaptation that might reduce detection is extremely advantageous. In Chapter four I examine how conspicuousness and colouration are affected by living in a group, particularly in relation to other group members. A field experiment using groups of artificial prey, with differing densities and group sizes was used to explore the effect of group size and density on the predation risk and detectibility of cryptic prey. My results show that, as expected, larger groups are more likely to be detected, but that the increase is much slower than a linear increase. This suggests that groups must increase considerably in size before any individual group member will suffer increased predation risk. The second experiment examines the ‘oddity effect’ and how it affects predation. This hypothesises that when confronted by grouped prey, predators can increase their kill rate by concentrating their efforts on capturing unusual or ‘odd’ prey, a strategy that reduces the ‘confusion effect’. A field experiment was conducted with groups composed of differing proportions of two artificial cryptic prey types. Groups with odd individuals did not suffer an increase in conspicuousness and were not attacked more often. However, once located and attacked the groups did suffer a greater predation rate. Odd individuals were predated at a greater rate than normal individuals and the rate did not change as more or less odd individuals were added to the group. A computer based ‘game’ was used to further investigate the oddity effect. The results from the initial run of the game appeared to show strong evidence for the oddity effect, with a further significant increase in this effect when attention is split between foraging for prey and scanning for predators. To be confident of this result the experiment was repeated with the ‘odd’ and ‘normal’ seed patterns reversed. The new data set strongly suggested that much of the effect seen in the previous experiment was due to a difference in pattern visibility between the two seed patterns. Nevertheless, the results indicated that selecting odd seeds is quicker than selecting normal seeds. The results from both the field and computer trials suggest that preference for odd prey may improve predator foraging speed and efficiency. Chapter five investigates whether cryptic and non-defended prey could reduce their predation risk by grouping with aposematic and defended prey. This was tested using artificial prey in a field setting. My results show that undefended non-aposematic prey can benefit by grouping with aposematic prey with no evidence that predation rates for aposematic prey were adversely affected by this association. If confirmed this might illuminate the origins of Batesian mimicry. I have investigated a range of anti-predator adaptations and strategies in the Lepidoptera and in particular pattern elements and use of crypsis and aposematic displays. These anti-predator strategies are important in that they modify predation rate and so directly influence the evolution of species. While I have been able to provide evidence for some current hypotheses, in many respects my results demonstrate that there is still a lot to learn about visual anti-predatory strategies.