An integrative approach to detect subtle trophic niche differentiation in the sympatric trawling bat species Myotis dasycneme and Myotis daubentonii.

Bats are well known for species richness and ecological diversity, and thus, they provide a good opportunity to study relationships and interaction between species. To assess interactions, we consider distinct traits that are probably to be triggered by niche shape and evolutionary processes. We present data on the trophic niche differentiation between two sympatric European trawling bat species, Myotis dasycneme and Myotis daubentonii, incorporating a wide spectrum of methodological approaches. We measure morphological traits involved in foraging and prey handling performance including bite force, weightlifting capacity and wing morphology. We then measure resulting prey consumption using both morphological and molecular diet analyses. These species closely resemble each other in morphological traits, however, subtle but significant differences were apparent in bite force and lift capacity, which are related to differences in basic body and head size. Both morphological and molecular diet analyses show strong niche overlap. We detected subtle differences in less frequent prey items, as well as differences in the exploitation of terrestrial and aquatic-based prey groups. Myotis dasycneme feeds more on aquatic prey, like Chironomidae and their pupal stages, or on the aquatic moth Acentria ephemerella. Myotis daubentonii feeds more on terrestrial prey, like Brachycera, or Coleoptera. This suggests that these bats use different microhabitats within the habitat where they co-occur.

Echolocation by the barbastelle bat, Barbastella barbastellus.

When searching for insects along edges, Barbastella barbastellus alternated between two signal types. Type-2 signals had durations around 6 ms and were composed of an initial shallowly downward frequency modulated component, starting at about 45 kHz and followed by a shorter more steeply modulated component that ended at about 32 kHz. Type-1 signals were rather stereotyped with durations around 2.5 ms and a very short rise time. They covered an approximately 8 kHz-wide frequency band positioned just below the 12-15 kHz-wide frequency band of type-2 signals, with no or small frequency overlap. In the recordings, type-1 signals almost had always a higher amplitude than type-2 signals, at least partly caused by head movements. Assuming that signal structure reflects function, we hypothesize that type-2 signals have the same adaptive value as the signals with a broadband and narrowband component of other vespertilionids, but with a reverse arrangement of the signal elements. Like the broadband component of the type-2 signals, type-1 signals are well suited to localize background targets. Thus, the localization component may be distributed among two signals separated in time, which has the advantage that both signals can be varied independently in the direction of emission and in amplitude.

The acoustic advantage of hunting at low heights above water: behavioural experiments on the European 'trawling' bats Myotis capaccinii, M. dasycneme and M. daubentonii.

We have demonstrated in behavioural experiments that success in capturing prey from surfaces in 'trawling Myotis' (Leuconoë-type) depends on the acoustic properties of the surface on which the prey is presented. Two types of surface structure were ensonified with artificial bat signals to probe their acoustic characteristics. We have shown that perception of prey by echolocation is easier if the prey is presented on a smooth surface (such as calm water) than if it is presented on a structured surface (such as vegetation or the ground). This is because the smooth surface reflects a much lower level of clutter echoes than the structured one if ensonified at an angle typical for bats foraging low over water. The ensonification experiments revealed that the sound pressure level of the echo was even higher for mealworms on a smooth surface than for mealworms suspended in air. This might be because waves travelling via the surface also contribute to the echo (e.g. reflection from the surface to the mealworm, back to the surface and then to the receiver). From the behavioural experiments, we conclude that 'trawling Myotis' take isolated objects on smooth (water) surfaces for prey. Those objects reflect isolated, stationary acoustic glints back to the echolocating bats. Conversely, 'trawling Myotis' will not recognise prey if prey echoes are embedded in numerous clutter echoes. We have demonstrated marked similarities between the three European 'trawling Myotis' species M. dasycneme, M. daubentonii and M. capaccinii in echolocation behaviour, search image, foraging strategy and prey perception. We propose that a combination of prey abundance and acoustic advantages could have led to repeated and convergent evolution of 'trawling' bats in different parts of the world.