Correlación de la afectación en distintos territorios

design, where the position of the dishes were changed after every feeding attempt, would minimise these effects, and that the results show preference for feeding dishes, rather than spatial memory.

The results of Experiment 2 allowed rejection of the hypothesis that bats associated structural features of the food dish with reward, and were therefore attracted to features of the dishes by associative learning (Siemers 2001). However, as all the bats flew simultaneously in this experiment, we cannot exclude the possibility that some individuals sometimes explored the dishes without getting a reliable indication of reward. In the first experiment there were equally many feeding attempts at the petri dishes providing sonar cues as at the no cue dishes, which means that we found no indication that the bats could detect the prey items by using sonar alone. One can therefore hypothesize that bats having to rely exclusively on sonar may learn to recognize structures rather than prey.

There were more feeding attempts at the petri dishes providing visual cues only compared to those that provided sonar cues only. This suggests that the long-eared bats were capable of finding prey visually and that they even preferred using vision when possible. Long-eared bats emerge from their roosts late in the evening (15-55 minutes after sunset depending on the latitude; Swift 1998), which means that they normally operate in very low light levels (<1 lux). Their technique of gleaning insects off leaves also means that they do not use the bright sky to backlight prey, which has been suggested to be the case for some aerial hawking bats (Pettigrew 1980; Vaughan & Vaughan 1986). However, Eptesicus fuscus (Vespertilionidae) can discriminate differences in brightness in ambient illumination as low as 0.001 lux, conditions which resemble darkness to a human dark adapted eye, although the optimal performance is around 10 lux, which is equivalent to dusk and dawn conditions (Ellins & Masterson 1974). This suggests that bats can use visual cues under nocturnal conditions. The resolving power of vespertilionid bats typically ranges between 0.7 and 1 degree of arc (Marks 1980; Bell & Fenton 1986; Pettigrew et al. 1988), which means that an object of mealworm size (ca. 2 cm) can be detected from a distance of ca. 1 m. The eyes of long-eared bats are larger than those of most other

Vespertilionidae (Cranbrook 1963), suggesting that they presumably also have better visual acuity as well as light sensitivity (Land & Nilsson 2002). As the bats in this study usually hovered above the petri dishes on a height of 10-15 cm, the mealworms were most likely visible to the bats at least on the lit surfaces. Cranbrook (1963) noted that long-eared bats, despite their big eyes, did not seem to look for the food when feeding from bowls, but rather felt about until a prey item was captured. Studies on northern bats (Eptesicus nilssonii,

Vespertilionidae) catching ghost swifts (Hepialis humulii) suggest that vision is used primarily in an initial search phase to detect prey, rather than in the terminal attack (Eklöf et al. 2002). In a similar manner, phyllostomid bats detect landing grids visually, but use echolocation when performing the actual landing operation (Joermann et al. 1988). Hence, it seems possible that the long-eared bats used visual cues for detection, but exploited additional information, such as echolocation and passive listening during the final pursuit.

ACKNOWLEDGEMENTS

We wish to acknowledge Marc Holderied and Julian Partridge for comments on the experimental design and Jens Rydell for comments on the manuscript. We also wish to thank John Gustafsson and Catrin Bergqvist for statistical advice. The study was supported by ”Stiftelsen Paul och Marie Berghaus

donationsfond”, and ”Adlerbertska forskningsstiftelsen” (JE). Research was performed under licence from English Nature.

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