The defensive role of ultrasonic moth clicks against bat predation : a mathematical modeling approach

Abstract

Some moths emit ultrasonic clicks in response to bat echolocation calls. These clicks are believed to serve as a defence mechanism against bat predation by means of jamming, aposematism or startle. By assessing the characteristics and variation of the ultrasonic moth clicks it was possible to define the most likely function of the ultrasonic clicks by moths from sites in South Africa. Additionally, this study used Schaefer based mathematical models informed by field data on moth diversity and abundance, moth click parameters and bat population numbers and diversity, from these same sites in South Africa, to gain insight into the conditions that are required for each of these functions to work. The Jamming hypothesis proposes that moth clicks function by interfering with the bat's perceptual system, which is believed to be most effective in the terminal phase of the bat's attack were the clicks have been shown to be the most similar to the bats echolocation calls. This would place functional constraints on the clicks, decreasing the variation in the clicks. This was shown not to be the case, as a high level of variation was found between species' clicking parameters (e.g. peak frequency, inter pulse interval and intensity). A high level of individual moth variation was also found, with numerous patterns being produced by the same moths. Clicking variation was created both by manipulation of individual tymbals (e.g. changes in modulation cycle rate and durations) and between tymbals (e.g. timing of tymbal alternation). Furthermore the moth clicks are too different to bat echolocation calls to allow them to be mistaken by the bat as echoes from its own calls. The aposematic hypothesis suggests that the clicks function as a warning signal to inform the predator of the prey's unpalatability. As in visual based aposematism, the cost of. predator learning on the prey population would exert selective pressure towards higher densities of clicking moths and convergence in warning signals. This would allow one species to benefit from the predator learning induced by another species. This strategy is thus density dependent and would require a minimum proportion of clicking moths to work. Both the data from the ultrasonic moth clicks and mathematical modelling does not support this hypothesis as there is no signs of convergence and the mathematical modelling suggests that the low proportion of clicking moths (2.3%) in the natural moth population does not allow bats to form the association between moth clicks and noxiousness of the moth. In the case of jamming, the low proportion of clicking moths also does not reflect the competitive advantage supposedly gained from jamming. The startle hypothesis proposes that moth clicks confuse the bat by altering the normal sequence of expected events during the bat's attack. However, bats will habituate to these sounds if they are encountered regularly. Both the low proportions of clicking moths and the extreme variability of the click patterns hinder habituation and are consistent with the startle hypothesis, which suggest that the most likely function of ultrasonic moth clicks in these South African sites is startle. On the other hand, the aposematic and startle models do suggest that the existence of both noxiousness and clicking create the initial conditions for the evolution of an aposematic function for moth clicks. If the clicking moth is noxious the startle effect of the clicking could allow the population to grow to high enough numbers to make the aposematic function of the clicks viable.