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 5 cm. Smaller targets were detected using echolocation alone. The mean detection range was 3.5 m, 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.

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 Vogel's organs located near the base of the forewings. Its behaviour in response to bursts of ultrasonic pulses (26 kHz, 110 dB SPL at 1 m) 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.

Detection of prey in a cluttered environment by the northern bat Eptesicus nilssonii

We studied the acoustic behaviour of bats (Eptesicus nilssonii) hunting for large (wing span 5 cm) non-hearing hepialid moths (Hepialus humuli). Groups of silvery-white male H. humuli perform a short (30 min) hovering display flight over grassland at dusk. They typically hover at, or below, the tops of grass panicles and are therefore situated in a highly acoustically cluttered habitat. Occasionally, they move to a new position by making short (1–5 s) flights at higher levels. E. nilssonii is not a clutter specialist, and yet we found that they attacked H. humuli within the ‘clutter overlap zone’. The bats did not change their signal design in any marked manner for this specific task. Measurements of echoes from a moth 10 cm above or below the grass tops showed that information for detecting the moths was available to the bats. Nevertheless, the bats did not attack moths in stationary hovering display flight, only when they moved above the grass panicles. The duration of the up/down flights (movements) were almost always longer than an entire capture sequence by the bats. Apparently, the bats rely on the movement of the moth in space, monitored over successive echoes, to discriminate moth echoes from overlapping clutter echoes.

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

We investigated the bat defence reactions of three species of moths (Gynaephora groenlandica, Gynaephora rossi (Lymantriidae) and Psychophora sabini (Geometridae)) in the Canadian Arctic archipelago. Since these moths inhabit the Arctic tundra and, therefore, are most probably spatially isolated from bats, their hearing and associated defensive reactions are probably useless and would therefore be expected to disappear with ongoing adaptation to Arctic conditions. When exposed to bat-like ultrasound (26 kHz and 110 dB sound pressure level root mean square at 1 m) flying male Gynaephora spp. always reacted defensively by rapidly reversing their flight course. They could hear the sound and reacted at least 15-25 m away. Psychophora sabini walking on a surface froze at distances of at least 5-7 m from the sound source. However, two out of three individuals of this species (all males) did not respond in any way to the sound while in flight. Hence, we found evidence of degeneration of bat defence reactions, i.e. adaptation to the bat-free environment, in P. sabini but not in Gynaephora spp. Some Arctic moths (Gynaephora spp.) still possess defensive reactions against bats, possibly because the selection pressure for the loss of the trait is such that it declines only very slowly (perhaps by genetic drift; and there may not have been enough time for the trait to disappear. One possible reason may be that Arctic moths have long generation times.

Constant-frequency and frequency-modulated components in the echolocation calls of three species of small bats (Emballonuridae, Thyropteridae, and Vespertilionidae)

The echolocation calls of Rhychonycteris naso (Emballonuridae), Thyroptera tricolor (Thyropteridae), and Myotis riparius (Vespertilionidae) were recorded at the Cãno Palma Field Station in Costa Rica in February 1998. All three species produced echolocation calls at low duty cycle (signal on ~10% of the time). While T. tricolor produced low-intensity echolocation calls that were barely detectable when the bats were <0.5 m from the microphone, the other two species produced high-intensity calls, readily detectable at distances >5 m. Myotis riparius produced calls that swept from about 120 kHz to just over 50 kHz in about 2 ms. We found no evidence of harmonics in these calls. Rhynchonycteris naso and T. tricolor produced multiharmonic echolocation calls. In R. naso the calls included narrowband and broadband components and varied in bandwidth, sweeping from just under 100 kHz to around 75 kHz in over 5 ms. Most calls were dominated by the higher harmonic (ca. 100 kHz), but some also included a lower one (ca. 50 kHz). The calls of T. tricolor were 5-10 ms long and dominated by a single frequency (ca. 45 kHz), sometimes with a ca. 25 kHz component. The echolocation calls of all three species included frequency-modulated and constant-frequency components. While these terms describe the components of the echolocation calls, they do not necessarily describe the bats' echolocation behaviour.

Bat defence in lekking ghost swifts (Hepialus humuli), a moth without ultrasonic hearing

The Hepialidae represents an early branch of the Lepidoptera, whose members lack the ultrasonic hearing and other obvious predator defence systems present in other extant moths. I observed lekking male ghost swifts, Hepialus humuli, being exploited by northern bats, Eptesicus nilssonii, over a hayfield in southern Sweden. Because the moth's display flight was restricted to a brief (30 min) period at dusk, they avoided most predators temporally but were exposed to early emerging aerial-hawking bats. Against these, they apparently employed 'acoustic crypsis', achieved by flying close (< 0.5 m) to the vegetation, thereby hiding from the bats among clutter (echoes returning from the background). Nevertheless, the predation risk for the displaying moth males was very high (20% per night), mainly because they sometimes left the safety of the vegetation. The lack of 'advanced' predator defence mechanisms in H. humuli requires alternative defence strategies, which, however, restrict the behavioural repertoire and still carry a high predation risk.

Mercury vapour lamps interfere with the bat defence of tympanate moths (Operophtera spp.; Geometridae)

Bats often forage near streetlamps, where they catch moths in particular. At least two hypotheses may explain the apparent increase in the availability of moths to bats feeding around streetlamps: (1) the moths become concentrated near the light and therefore more profitable to exploit; and (2) the light interferes with the moths' evasive flight behaviour. We tested the second of these hypotheses by exposing flying male winter moths, Operophtera spp., to bursts of ultrasound (26 kHz, 110 dB sound pressure level) from an electronic source. The light from a 125 W mercury vapour lamp had a quantitative effect on the moths' evasive flight response at close range (within ca 4 m), inhibiting it totally in nearly half (43%, N=125) of the cases. By contrast, moths flying in the surrounding woodland and without interference from the lamp always responded to the sound. Streetlamps of the mercury vapour type (white lamps) thus interfere with the defensive behaviour of moths and presumably increase their vulnerability to echolocating bats. This may have implications for the conservation of both moths and bats. Copyright 1998 The Association for the Study of Animal Behaviour.

Hearing and bat defence in geometrid winter moths

Audiograms and behavioural responses to ultrasound reveal that male geometrid winter moths (Agriopis and Erannis spp.; Ennominae, and Alsophila aescularia; Oenochrominae), which have large wings and a slow flight, have good, broadly tuned ultrasonic hearing with best frequencies at 25-40 kHz, coinciding with the frequencies used by most sympatric aerial-hawking bats. Ultrasonic pulses (27 kHz 110 dB at 1 m) delivered at distances of 1-12 m evoked consistent reactions of free flying, male A. marginaria in the lab as well as in the field; those at < 5 m resulted in the moth spiralling or diving towards the ground, those at 5-12 m resulted in one or several changes in the flight path, but did not end on the ground. The differential reaction probably reflects whether the moth is likely to have been detected by the bat or not. The micropterous (and flightless), and hence cryptic, females have strongly reduced tympanic organs and are virtually deaf. Sexual dimorphism in hearing and behavioural reactions to ultrasound reflect differential natural selection on males and females by bats. Natural selection on the hearing of the males thus seems to occur although they fly in late autumn and early spring, when bat activity is much reduced.

Primary focal segmental glomerular sclerosis in adults: prognostic value of histologic variants

Primary focal segmental glomerular sclerosis (FSGS) is a clinicopathologic syndrome in which variable amounts of proteinuria are associated with the renal biopsy finding of segmental glomerular scarring in some, but not all, of the glomeruli. Additional histologic features have been described in FSGS, including the position of the scar relative to the vascular and tubular pole of the glomerulus, foam cells, hyalinosis, mesangial deposits of immunoglobulin M, diffuse mesangial hypercellularity, glomerular visceral epithelial cell hyperplasia and hypertrophy, and the extent of associated interstitial fibrosis and tubular atrophy. We performed a retrospective study on 81 patients with biopsy-proven, primary FSGS to determine whether any of the histologic features of FSGS correlated with renal function at the time of biopsy and the incidence of end-stage renal disease at follow-up. Sixty patients were nephrotic and 21 had nonnephrotic proteinuria. Only the degree of interstitial fibrosis correlated with the initial serum creatinine (r = 0.536) and none of the histologic features predicted the presence of nephrotic-range proteinuria at the time of biopsy. Segmental scars involved 21% +/- 14% of the glomeruli per biopsy specimen, but their position within the glomerulus was uniform in only 13% of the cases. Diffuse mesangial hypercellularity was present in 17% of the biopsy specimens, and glomerular epithelial cell lesions were present in 57% of the biopsy specimens. Multivariate analysis showed that only the degree of interstitial fibrosis predicted end-stage renal disease in all 81 patients and in the 60 patients with nephrotic-range proteinuria. The current data do not support different therapeutic approaches in primary FSGS based on histologic subtypes.