Factores predictores de eventos cardiovasculares mayores

Johan Eklöf

Zoology Department, Göteborg University, Box 463, SE-405 30 Göteborg, Sweden, E-mail: [email protected]

Abstract

Behavioural tests on optomotor responses establish a visual acuity threshold in four species of bats of the family Vespertilionidae. Three species of Myotis spp., which are aerial-hawking bats, responded only to a stripe pattern equivalent to 5 degrees of arc, whereas Plecotus auritus, which is a gleaner, responded down to 0.5 degrees of arc. Eye diameter was positively correlated with visual acuity, and varied from 0.9 mm in Myotis mystacinus to 1.8 mm in Plecotus auritus. These results are consistent with earlier findings. The variation in eye size and visual acuity presumably reflects differences in foraging technique (aerial-hawking vs. gleaning) and, in particular, how vision is used as a complement to sonar. Key words: Chiroptera, grating, optomotor response, resolving power, spatial resolution, vision

Introduction

The microchiropteran eyes are generally adapted for nocturnal conditions in that they have large corneal surfaces and lenses relative to the size of the eye, and generally large receptor fields, which give them good light gathering power at the expense of acuity, i.e. the ability to resolve fine spatial details (Suthers 1970; Suthers & Wallis 1970). Bat eyes are generally better suited for long- than short distance operation, and due to the short effective range of sonar, vision is probably of major importance in guidance over longer distances (Griffin 1958, 1970). Loss of vision drastically reduces the homing performance in many bats (Williams et al. 1966, Hassell 1963, 1966, Davis & Barbour 1970).

At least some bats are able to use vision over short distances as well, for example during escape and obstacle avoidance (Chase 1981, 1983, Chase & Suthers 1969, Bradbury & Nottebohm 1969). There is also evidence that some species of bats use visual cues to find prey (Bell 1985, Grant 1991, Vaughan & Vaughan 1996, Eklöf et al. 2002), a task which presumably requires relatively fine detail discrimination.

Visual acuity has been estimated theoretically, based on counts of the number of retinal ganglion cells, in several species of bats (Marks 1980, Pettigrew et al. 1988, Heffner et al. 2001), and has shown a large range of variation; from 16’ of arc in the gleaning carnivorous species Macroderma gigas (Megadermatidae) to 1.4° of arc in Rhinolophus rouxi (Rhinolophidae), an insectivorous flutter- detector (Pettigrew et al. 1988). Optomotor response tests have also shown that

the visual acuity varies considerably between species of bats(Suthers 1966, Manske & Schmidt 1976, Bell & Fenton 1986).

The evidence thus suggests that visual acuity may be correlated with the food searching technique among bats. In particular, gleaners seem to have better visual acuity than those that catch insects in the air. The purpose of this study was to test this hypothesis by examining the optomotor response in some sympatric insectivorous vespertilionid bats that use different foraging techniques (gleaning and aerial-hawking), in order to establish a behavioural visual acuity threshold for these particular species. We also tested the assumption that visual acuity is positively related to the size of the eyes among insectivorous bats. Materials and methods

The experiments were performed at the old magnetite mine of Taberg, located south of Jönköping (57ºN) in southern Sweden. The bats were caught in a mist net set outside the mine entrance. They were tested for optomotor responses immediately after capture or as soon they had come to rest. The tests were made outdoors in the evening between August and November 2002, and between March and April 2003. To achieve optomotor responses, we used a device similar to that employed by Suthers (1966) and Bell & Fenton (1986). A bat was placed in a 20 cm high, 10 cm diameter Plexiglas cylinder surrounded by a 30 cm high and 60 cm diameter, revolving drum (Fig. 1). The natural light was insufficient for direct observation of the response in most cases, so the study area was lit up by a 40 W light bulb placed ca. 2 m above and 5 m away from the set- up. This provided a light intensity of 0.1-0.7 lux inside the drum (Photometer IL1400A, International Light Inc.). The drum could be rotated freely and independently of the cylinder containing the bat. Sinusoidal grating patterns, i.e. stripes with continuously changing luminance from black to white, of different fineness was attached to the inside of the drum. The drum was then rotated around the bat by hand at ca. 5 rpm randomly in both directions, and the behaviour of the bat was observed from above. Using sinusoidal patterns instead of black and white stripes reduces the risk of optical illusions, which could otherwise elicit responses from the bats and thus make the results hard to interpret (D. Nilsson & E. Warrant personal comm.). We used six gratings with different stripe width (distance from white to white): 2.84 cm, 1.42 cm, 0.57 cm, 0.43 cm, 0.28 cm and 0.14 cm. From the bats´ point of view this is equivalent to subtending angles of 5°, 2.5°, 1°, 0.75° (45’), 0.5° (30’) and 0.25° (15’) of arc. When a response was achieved the grating was switched to a finer pattern until no response could be recorded, indicating that the bats no longer could resolve the pattern. At this point a wider pattern was reintroduced to make sure that the bats still responded to moving stripes. This also served as a control for responses to stimuli other than the stripes, such as noise originating from the drum. After testing optomotor responses, we photographed the bats, using a Pentax 645 camera, on 50 ASA medium format slide film. We held the bats by hand so that the face of the bat was perpendicular to the camera. A ruler was held next to the bats, providing us with a cm-scale. The photos were scanned and magnified 17x – 33x, and the eye size of the individual bats were measured from the

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