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First published online January 25, 2005
Journal of Experimental Biology 208, 539-548 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01416

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Foraging in a complex naturalistic environment: capacity of spatial working memory in flower bats

York Winter^1,2,* and Kai Petra Stich²

¹ Department of Biology, University of Munich, 82152 München-Martinsried, Germany
² Max-Planck Institute for Ornithology, 82319 Seewiesen, Germany

View larger version (34K): [in a new window]   Fig. 1. Experimental feeder. (A) side view, (B) frontal view, (C) block diagram of feeder components (arrows indicate signal direction) and (D) diagram of electronic circuit. Details in (A) are: a, feeder head; b, tube for food solution; c, swivel arm; d, motor; e, slotted interruptor; f, plastic cover for cables and tube; g, valve; h, connector; i, aluminium carrier. For experimental animals only the front part of the feeder to the end of the plastic cover is perceivable (left part of drawing). Details in (B) are: a, swivel arm; b, infra-red photo trigger; c, LED; d, food port; e, PVC head; f, odour port. Development partly in cooperation with Animal Cognition Systems. For further details see supplementary material.

View larger version (16K): [in a new window]   Fig. 2. Experimental cage. Details: a, white light LED; b, electronic balance for roost; c, surveillance video camera; d, cage door motor; e, hanging roost; f, feeder; g, door; h, rope for guillotine door; i, computer-controlled guillotine door (lower quarter made of elastic rubber to prevent injury). Side walls are opaque PVC and front, back and top walls are transparent.

View larger version (25K): [in a new window]   Fig. 3. Data lines between 64-feeder array, six experimental cages and computer. Arrows show direction of data transfer. IO-cards are Decision Computer model 48-192 I/O ISA based on IC 8255. Camera switch is PIH 200-8, CS Lilin (Sintron) and motion alarm is PIH 6004, CS Lilin (Sintron), modified for computer operation. Pumps are custom stepper motor pumps, which move a pallet along a screw spindle that, in turn, moves the piston of a gas tight injector (Hamilton, 1025TTL, 25 ml). In forward drive it delivers nectar and in reverse drive the pump can empty the tube system for automatic cleaning operation. For further details see supplementary material.

View larger version (27K): [in a new window]   Fig. 4. Interleaved timing of 60 automatically administered trials during one experimental night with three animals sequentially exchanged between the 64-feeder array (trial) and their individual cages (inter-trial). Shading indicates trial at the feeder array (black), enclosure phase (horizontal stripes), animal in cage before and after a series of experimental trials (dark grey). Feeders at array are active (black circles) or inactive (open circles) but indistinguishable in external appearance (not cued) for the test animal.

View larger version (13K): [in a new window]   Fig. 5. Spatial distribution of first visits to feeders (i.e. excluding revisits during trials) at the 64-feeder array during trials 1–3 (A) and trials 11–20 (B) by all 17 individuals in all n trials. Circle diameters are proportional to the percentage of first visits directed at that feeder (same scale for A and B). Amounts of single food rewards were equal at all feeders (see Materials and methods). The shortest path between cage exits and feeder array was to the bottom left corner of the array (about 3–4 m from the different cages).

View larger version (18K): [in a new window]   Fig. 6. Spatial (A) and temporal (B) spacing of successive visits to the 64-feeder array. Data based on N=3465 (after reward) and N=1028 (after no reward). Hovering duration was not included in the calculations of the between-feeder visit intervals in (B).

View larger version (17K): [in a new window]   Fig. 7. Revisits to previously emptied feeders in comparison to revisit frequency expected under random choice of feeders. (A) Between-trials: revisits during successive trials of the experiment. The drop within the first three trials indicates the transition from a brief win-stay to a win-shift strategy. For calculations, see text. Revisits expected under random choice (line symbol) are a function of the number of feeders emptied during a trial; this number increases (from trials 1–3 to trials 11–20) as more feeders are emptied within a trial. Error bars are 95% confidence intervals of the difference (paired t-test) between measured revisits and revisits expected assuming random choice of feeders. Differences were significant for trials 1–3 (t=5.3, d.f.=14, P<0.001) and trials 11–20 (t=–2.8, d.f.=15, P=0.014) but not for trials 4–10 (t=0.5, d.f.=15, P>0.05). Data shown are means of the individual values for N=16 individuals (one individual with less than 20 visits during each of trials 1–3 was excluded from this specific analysis). (B) Within-trials: ratio of revisits to revisits expected under random choice within a trial. Data are for successive blocks of five visits within trials (based on trials 11–20, N=16 individuals, for calculation, see text). Data shown are the means of individual values (±S.E.M.). Frequency of revisits differed significantly from random expectation up to the 20–25th visit during a trial (Wilcoxon test, Z>2.1, N=16, P<0.05).

View larger version (18K): [in a new window]   Fig. 8. Proportion of revisits relative to the potential number of revisits for a given inter-visit interval during a specific trial (for calculation, see text). (A) Data from this study for Glossophaga soricina (N=17 individuals). The high values during trials 1–3 (open symbols) are an effect of the initial win-stay strategy of revisiting just-emptied feeders (see Fig. 7A). (B) Equivalent data for marsh tits (Parus palustris; Shettleworth and Krebs, 1982) and rufous hummingbirds (Selasphorus rufus; Sutherland, 1986) shown here for comparison (see Discussion).