Jens Rydell

Echolocation call design and limits on prey size: a case study using the aerial-hawking bat Nyctalus leisleri

The echolocation calls used by Nyctalus leisleri during search phase in open air space are between 9 and 14 ms long, with the peak energy between 24 and 28 kHz. The pulses are shallowly frequency-modulated with or without an initial steep frequency-modulated component. The diet consists primarily of small flies (Diptera), including many chironomids (wingspan 9–12 mm) and yellow dung flies (Scatophaga; wingspan 24 mm), but also of some larger insects such as dung beetles (Coleoptera; Scarabaeoidea), caddis-flies (Trichoptera) and moths (Lepidoptera). The echo target strength of some prey items was measured. Contrary to models based on standard targets such as spheres or disks, the echo strength of real insects was found to be virtually independent of the emitted frequency within the 20–100 kHz frequency range. A model was used to calculate probable detection distances of the prey by the bat. Using narrow-band calls of 13.7 ± 2.7 ms duration, a bat would detect the two smallest size classes of insect at greatest range using calls of 20 kHz. The results may therefore explain why many species of large and medium sized aerial-hawking bats use low-frequency calls and still eat mostly relatively small insects. The data and model challenges the assumption that small prey are unavailable to bats using low-frequency calls.

Acoustic identification of insectivorous bats (order Chiroptera) of Yucatan, Mexico

The echolocation calls of insectivorous bats of the northern Yucatan Peninsula, Mexico, with the exception of the phyllostomids and molossids, are presented. The aim is to provide a catalogue of bat sounds that can be used for acoustic inventories of insectivorous bats using the Pettersson heterodyne and time-expansion bat detectors. The acoustic method can be used alone or in combination with inventories based on mist-netting, a method more suitable for the low-intensity echolocators (mainly the phyllostomids), which are difficult to monitor acoustically. The insectivorous species of the Yucatan are generally easy to identify by their echolocation calls, particularly when combined with visual observations of foraging bats at dusk.

Low-Frequency Echolocation Enables the Bat Tadarida teniotis to Feed on Tympanate Insects

The European free-tailed bat, Tadarida teniotis, forages in uncluttered airspace by using intense narrowband echolocation calls with low frequency (11–12 kHz), and feeds on relatively large flying insects, mainly (90% by volume) of the tympanate orders Lepidoptera and Neuroptera. The use of low-frequency echolocation calls without strong harmonics appears to be a specialization for long-range detection of large, tympanate insects, which are less well represented in the diet of most other aerial-hawking bats. The results provide evidence in support of the allotonic frequency hypothesis, i. e. that use of echolocation calls with frequencies above or below the best hearing of tympanate insects is an adaptation to increase the availability of these insects.

No lunar phobia in swarming insectivorous bats (family Vespertilionidae)

The purpose of this study was to evaluate the impact of moonlight on the swarming activity of bats at an abandoned mine in southern Sweden. The mine serves as a hibernaculum for six species of insectivorous bats. Swarming activity at one of the mine entrances was measured on 12 nights between 1 August and 8 September 2000, by electronically counting the number of bats passing through an opening (about 30×40 cm) leading to a mine tunnel. The number of bat passes outside the mine was also counted, using a bat detector. Most bats were males, as confirmed by mist-netting outside the mine. Low frequency vocalizations, indicating territorial interactions, were heard frequently. The number of bats entering the mine was closely correlated with the number of bats flying outside, and neither was affected by moonlight. We conclude that the insectivorous bats at high latitudes may not have been exposed to significant nocturnal predator pressure, leading to the evolution of lunar phobia, as have many tropical bats.

Vision complements echolocation in an aerial-hawking bat.

The northern bat Eptesicus nilssonii normally hunts flying insects in the air using frequency-modulated echolocation calls. It is also known to detect and catch visually conspicuous prey (white moths) hovering low among grass stalks. To overcome the problem with acoustic clutter from the grass, which interferes with target echo detection, the bats make use of visual cues in addition to those of echolocation. We therefore investigated the minimum size of prey that the bats could distinguish by using vision, by presenting the bats with different sized dead and spread moths. We found that vision increased the chance of detection only when the moths had a wingspan of at least 5xa0cm. Smaller targets were detected using echolocation alone. The mean detection range was 3.5xa0m, suggesting that the bats need a visual acuity of 49′ of arc to detect the prey. This is consistent with results of optomotor response tests and counts of retinal ganglion cells in closely related species. Our results suggest that the visual acuity of Eptesicus bats may not be adequate for prey detection under normal conditions, but that the bats can use vision when the prey is unusually large and conspicuous. The northern bats display a flexibility in prey detection techniques not previously recognised among aerial-hawking bats and they are able to use their full visual capacity in the field.

Capture success of little brown bats (Myotis lucifugus) feeding on mosquitoes

A small colony of little brown bats Myotis lucifugus was observed feeding on mosquitoes (Culicidae) inside a barn near Fairbanks, Alaska in June 2000. Observations were made visually and with a bat detector. All capture attempts were associated with feeding buzzes, indicating that the bats used echolocation to detect and track the insects, although the light was bright and the insects were easily seen by us. The capture efficiency of the bats was 92% ( n = 100). The insects did not show any defensive behaviour in response to approaching bats.

Trace fossil associations in the Swedish Mickwitzia sandstone (Lower Cambrian): Did trilobites really hunt for worms?

Abstract The spatial and temporal associations between Cruziana, trace fossils dug by foraging trilobites, and back filled burrows made by infaunal “worms” in a Lower Cambrian sandstone are described and discussed. In three large slabs and several smaller pieces, all from the same bedding plane in the lowermost part of the Mickwitzia sandstone (File Haidar Formation) at Kinnekulle, Västergötland, Sweden, Cruziana and worm burrows coincided more frequently than would be expected by chance. Overall, however, the association was weak and independent of the size of the worm trace. In 24 out of 29 cases, the worm burrow had deformed the Cruziana, implying that the arthropod was first on the spot. No clear case of the opposite situation was found. It is concluded that for an unknown reason the infaunal worms preferentially dug in places which had been visited earlier by foraging trilobites, or, possibly, that worms and trilobites preferred to dig in the same spots independently of each other. There is no evidence from this surface that the trilobites had been searching for worms or predated on them.

Evasive response to ultrasound by the crepuscular butterfly Manataria maculata

The crepuscular nymphalid butterfly Manataria maculata was studied in Monteverde cloud forest, Costa Rica, during the dry season reproductive diapause. M. maculata has ears in the form of Vogels organs located near the base of the forewings. Its behaviour in response to bursts of ultrasonic pulses (26xa0kHz, 110xa0dB SPL at 1xa0m) was condition-dependent. At dusk and dawn the sound consistently elicited evasive responses, similar to those of moths, in flying individuals. In contrast day-roosting individuals always remained motionless although they were alert to other stimuli. The daily movements between day- and night-roosts coincided in time and light intensity with the activity of insectivorous bats. This is the first reported case of ultrasonic hearing connected to evasive flights in a true butterfly (Papilionoidea). It strongly supports the idea that echolocating bats were involved in the evolution of hearing in butterflies.

Avoidance of bats by water striders (Aquarius najas, Hemiptera)

Our study showed that one species of water strider (Aquarius najas) dominated the insect fauna (>90% of the biomass) on and near the surface of a small stream in southern Sweden, but the diet of Daubentons bats (Myotis daubentonii), regularly feeding over the same stream, contained <1% of these insects. To explain why the bats did not eat water striders in proportion to the apparent abundance of these insects, we tested three hypotheses, with the following results: (1) The water striders did not respond to artificial bat calls and therefore their defence against bats is not based on ultrasonic hearing. (2) Water striders hand-fed to the bats were eaten with similar frequency as palatable controls (caddis flies), so there is no evidence that water striders are unpalatable to bats. (3) Attempted prey-captures by the bats occurred less often than expected within 1 m of the bank of the stream (the bank consisted of a stone wall), indicating that the bats flight or, alternatively, their detection or capture of insects, was difficult within that area, possibly constrained by clutter. At dusk and just before the bats emerged to feed, the water striders moved away from the open water. They remained motionless close to the bank for as long as the bats were active.