Persistence of bat defence reactions in high Arctic moths (Lepidoptera)

Evolutionary escalation: the bat-moth arms race

Echolocation in bats and high-frequency hearing in their insect prey make bats and insects an ideal system for studying the sensory ecology and neuroethology of predator-prey interactions. Here, we review the evolutionary history of bats and eared insects, focusing on the insect order Lepidoptera, and consider the evidence for antipredator adaptations and predator counter-adaptations. Ears evolved in a remarkable number of body locations across insects, with the original selection pressure for ears differing between groups. Although cause and effect are difficult to determine, correlations between hearing and life history strategies in moths provide evidence for how these two variables influence each other. We consider life history variables such as size, sex, circadian and seasonal activity patterns, geographic range and the composition of sympatric bat communities. We also review hypotheses on the neural basis for anti-predator behaviours (such as evasive flight and sound production) in moths. It is assumed that these prey adaptations would select for counter-adaptations in predatory bats. We suggest two levels of support for classifying bat traits as counter-adaptations: traits that allow bats to eat more eared prey than expected based on their availability in the environment provide a low level of support for counter-adaptations, whereas traits that have no other plausible explanation for their origination and maintenance than capturing defended prey constitute a high level of support. Specific predator counter-adaptations include calling at frequencies outside the sensitivity range of most eared prey, changing the pattern and frequency of echolocation calls during prey pursuit, and quiet, or 'stealth', echolocation.

Determining circadian response of adult male Acrobasis nuxvorella (Lepidoptera: Pyralidae) to synthetic sex attractant pheromone through time-segregated trapping with a new clockwork timing trap

Mate finding is a key lifecycle event for the pecan nut casebearer, Acrobasis nuxvorella Neunzig, as it is for virtually all Lepidoptera, many of which rely on long-range, species-specific sex pheromones, regulated largely by circadian clocks. Adult male moths were trapped at discrete time intervals during the first two seasonal flights for 6 yr to determine times of peak activity associated with male response to pheromones. From 1997 to 2002, the Harris-Coble automated clockwork timing trap was used for hourly time-segregated sampling. Analysis of variance with linear contrasts determined that circadian response of A. nuxvorella males to pecan nut casebearer pheromone began at approximately 2100 hours, the first hour of total darkness, lasting for 6-7 h. It peaked from midnight to 0400 hours and ended at the onset of morning twilight, approximately 0500 hours. The hours of peak activity are hours of minimal bat predation. The study shows that pecan nut casebearer males become responsive to pheromone several hours before females start calling and remain responsive for at least 1 h after they stop. The extended response period conforms to studies of other polygamous Lepidoptera in which a selective advantage is conferred on early responding males in scramble competition for available females.

Her odours make him deaf: crossmodal modulation of olfaction and hearing in a male moth

All animals have to cope with sensory conflicts arising from simultaneous input of incongruent data to different sensory modalities. Nocturnal activity in moths includes mate-finding behaviour by odour detection and bat predator avoidance by acoustic detection. We studied male moths that were simultaneously exposed to female sex pheromones indicating the presence of a potential mate, and artificial bat cries simulating a predation risk. We show that stimulation of one sensory modality can modulate the response to information from another, suggesting that behavioural thresholds are dynamic and depend on the behavioural context. The tendency to respond to bat sounds decreased as the quality and/or the amount of sex pheromone increased. The behavioural threshold for artificial bat cries increased by up to 40 dB when male moths where simultaneously exposed to female sex pheromones. As a consequence, a male moth that has detected the pheromone plume from a female will not try to evade an approaching bat until the bat gets close, hence incurring increased predation risk. Our results suggest that male moths' reaction to sensory conflicts is a trade-off depending on the relative intensity of the input to CNS from the two sensory modalities.

Experiments on egg discrimination in two North American corvids: further evidence for retention of egg ejection

In the absence of brood parasitism in North America, black-billed magpies, Pica hudsonia (Sabine, 1822), and yellow-billed magpies, Pica nuttalli (Audubon, 1837), may have retained egg-discrimination behaviour that evolved in Eurasian magpies, Pica pica (L., 1758), in response to parasitism by Old World cuckoos. We further examined this hypothesis by testing the egg-discrimination abilities of black-billed magpies and the American crow, Corvus brachyrhynchos Brehm, 1822, which has no history of brood parasitism. In addition, we tested an alternative hypothesis that black-billed magpies evolved or retained egg discrimination to counter conspecific parasitism by testing their ability to eject foreign conspecific eggs and by using a signal detection model to estimate the level of conspecific parasitism required for ejection to be favoured. Black-billed magpies ejected all non-mimetic eggs and 62% of mimetic eggs. Significantly more mimetic eggs were ejected during the incubation stage than during the laying stage. Magpies ejected significantly more non-mimetic eggs than mimetic eggs overall, but there was no difference in ejection frequency during incubation. American crows ejected 21% of non-mimetic eggs and 8% of mimetic eggs. There was no significant difference in ejection frequency of the two egg types. Black-billed magpies ejected 11% of conspecific eggs and a relatively high level of conspecific parasitism (22%–49%) would be required to select for conspecific ejection, which provides little support for conspecific parasitism as a current selection pressure for maintaining egg discrimination. Thus, black-billed magpies appear to have retained egg rejection in the absence of parasitism through speciation from Eurasian magpies.

Extinction of the acoustic startle response in moths endemic to a bat-free habitat

Most moths use ears solely to detect the echolocation calls of hunting, insectivorous bats and evoke evasive flight manoeuvres. This singularity of purpose predicts that this sensoribehavioural network will regress if the selective force that originally maintained it is removed. We tested this with noctuid moths from the islands of Tahiti and Moorea, sites where bats have never existed and where an earlier study demonstrated that the ears of endemic species resemble those of adventives although partially reduced in sensitivity. To determine if these moths still express the anti-bat defensive behaviour of acoustic startle response (ASR) we compared the nocturnal flight times of six endemic to six adventive species in the presence and absence of artificial bat echolocation sounds. Whereas all of the adventive species reduced their flight times when exposed to ultrasound, only one of the six endemic species did so. These differences were significant when tested using a phylogenetically based pairwise comparison and when comparing effect sizes. We conclude that the absence of bats in this habitat has caused the neural circuitry that normally controls the ASR behaviour in bat-exposed moths to become decoupled from the functionally vestigial ears of endemic Tahitian moths.

Defence against multiple enemies

Although very common under natural conditions, the consequences of multiple enemies (parasites, predators, herbivores, or even 'chemical' enemies like insecticides) on investment in defence has scarcely been investigated. In this paper, we present a simple model of the joint evolution of two defences targeted against two enemies. We illustrate how the respective level of each defence can be influenced by the presence of the two enemies. Furthermore, we investigate the influences of direct interference and synergy between defences. We show that, depending on certain conditions (costs, interference or synergy between defences), an increase in selection pressure by one enemy can have dramatic effects on defence against another enemy. It is generally admitted that increasing the encounter rate with a second natural enemy can decrease investment in defence against a first enemy, but our results indicate that it may sometimes favour resistance against the first enemy. Moreover, we illustrate that the global defence against one enemy can be lower when only this enemy is present: this has important implications for experimental measures of resistance, and for organisms that invade an area with less enemies or whose community of enemies is reduced. We discuss possible implications of the existence of multiple enemies for conservation biology, biological control and chemical control.

Quantifying an anti-bat flight response by eared moths

Using near-infrared videotaping we measured the nocturnal flight times of six species of eared moths (Amphipyra pyramidoides Guenée, Caenurgina erechtea (Cramer), Feltia jaculifera (Guenée), Phlogophora periculosa Guenée, Lymantria dispar (Linné), and Ennomos magnaria Guenée) in cages in which they flew, under randomized conditions, for 3 h in the absence and 3 h in the presence of simulated bat-attack sounds. When exposed to the ultrasound, four of the six species exhibited significant reductions in total flight time ranging from 38 to 98%. We suggest that this quantified measurement of flight time will be useful for fundamental studies on the evolution and ecology of moth hearing as well as applied studies on acoustic methods of controlling moth pests.

Auditory encoding during the last moment of a moth's life

The simple auditory system of noctuoid moths has long been a model for anti-predator studies in neuroethology, although these ears have rarely been experimentally stimulated by the sounds they would encounter from naturally attacking bats. We exposed the ears of five noctuoid moth species to the pre-recorded echolocation calls of an attacking bat (Eptesicus fuscus) to observe the acoustic encoding of the receptors at this critical time in their defensive behaviour. The B cell is a non-tympanal receptor common to all moths that has been suggested to respond to sound, but we found no evidence of this and suggest that its acoustic responsiveness is an artifact arising from its proprioceptive function. The A1 cell, the most sensitive tympanal receptor in noctuid and arctiid moths and the only auditory receptor in notodontid moths, encodes the attack calls with a bursting firing pattern to a point approximately 150 ms from when the bat would have captured the moth. At this point, the firing of the A1 cell reduces to a non-bursting pattern with longer inter-spike periods, suggesting that the moth may no longer express the erratic flight used to escape very close bats. This may be simply due to the absence of selection pressure on moths for auditory tracking of bat echolocation calls beyond this point. Alternatively, the reduced firing may be due to the acoustic characteristics of attack calls in the terminal phase and an acoustic maneuver used by the bat to facilitate its capture of the moth. Although the role of less sensitive A2 cell remains uncertain in the evasive flight responses of moths it may act as a trigger in eliciting sound production, a close-range anti-bat behaviour in the tiger moth, Cycnia tenera.

Day-flying butterflies remain day-flying in a Polynesian, bat-free habitat

To test the theory that insectivorous bats have selected for diurnality in earless butterflies I compared the nocturnal flight patterns of three species of nymphalid butterflies on the bat-free Pacific island of Moorea with those of three nymphalids in the bat-inhabited habitat of Queensland, Australia. Nocturnal flight, measured as the ratio of deep night (1 h following sunset to 1 h preceding sunrise) to twilight night (1 h before sunset to 30 min after sunrise) activity did not differ significantly between the two locations, nor did the percentage of individuals active and I conclude that living in a bat-released habitat has not produced nocturnal flight in these insects. This result is surprising considering the potential advantages of escaping diurnally active predators and suggests that physiological adaptations (e.g. thermoregulation and/or vision) currently constrain these insects to diurnal flight. Since taxonomic records suggest that gene flow does not exist with bat-exposed conspecifics, I suggest that insufficient time has elapsed since these species migrated to Moorea to have resulted in major phenotypic changes such as diel flight preferences.