• Abstract and Figures
• Public Full-text

Environmental Biology of Fishes 27: 147-152, 1990. 0 1990 Kluwer Academic Publishers. Printed in the Netherlands. Distance-to-cover and the escapedecisions of an African cichlid fish, Melunochromis chipokxze Lawrence M. Dill Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, B.C. WA IS6 , Canada Received 18.7.1988 Accepted 27.2.1989 Key words: Predation risk, Flight initiation distance, Escape velocity, Refuge cover Synopsis The risk to a prey individual in an encounter with a predator increases as the distance to protective cover increases. Prey should therefore initiate their flight to cover at longer distances from an approaching predator (i.e. sooner) and/or flee at greater velocities, as the distance to cover increases. These predictions were tested with an African cichlid fish, Melanochromis chipokae presented with a looming stimulus simulating an attacking predator. The fish varied their flight initiation distance as predicted, but there was no significant effect of distance-to-cover on escape velocity. Nevertheless, the cichlids appeared to choose a combination of flight initiation distance and escape velocity which ensured they reached cover with a constant temporal ‘margin of safety’. Introduction then distance-to-cover (DC) would be directly pro- portional to time-to-cover (TC), the critical param- Animals which flee to cover to escape their predators eter from the prey’s point of view. However, most must choose both a distance from the predator at prey can probably vary their escape velocity, and which to initiate their flight and an escape velocity. In therefore might control time-to-cover in that way, general, these are not expected to be the maximal since TC = DC/EV. As long as the prey reaches distance and velocity achievable, but rather those cover before the predator can get there, the prey which maximize the prey individual’s fitness, taking will be secure. The purpose of the study reported into account the costs of escape (primarily lost oppor- here was to test the hypothesis that small cichlid tunity to engage in other activities; Ydenberg & Dill fish escaping to the shelter of rocks will vary flight 1986). Since risk of capture should increase as the initiation distance and/or escape velocity with dis- prey’s distance-to-cover increases (see, for example, tance to cover. The results suggest that the fish do the simulation results of Dill 1973), Right initiation in fact choose values of these two behavioural par- distance might be expected to be greater for prey ameters which assure them of reaching cover with a further from cover. This appears to be true in both fiied margin of safety. the gray squirrel, Sciurus carolinensis (Dill & Hout- man 1989), and the brook charr, Salvelinus fontinuh (Grant & Noakes 1987). Escape velocity was not Methods measured in either of these studies, however, but was assumed to be invariant. The subject fish were three individuals of Melano- If escape velocity (EV) were indeed constant chromis chipokae (Johnson 1975) purchased from a 148 local aquarium store. These fish are members of ternal disturbance. A video camera was positioned the ‘mbuna’ group of rock-dwelling cichlids from between the feeder and the rocks so as to film (in Lake Malawi (Fryer & Iles 1972). They were cho- another 45” mirror beneath the tank) the feeder, sen because of their characteristic escape beha- the fish, and the ripples caused by the ball striking viour: attacked by a predator, they flee to a refuge the water surface. The ball itself was usually not in provided by rocks. The three fish averaged 4.9,5.5 camera view, and the rocks were not always visible and 5.7cm total length at the time of the experi- at the longest refuge distances. The camera also ments. Data were collected over a 10 month peri- filmed two lamps added part way through the ex- od, but the fish grew little during this time. Al- periments. One of these lit when the ball began its though the sample size is small, it proved impos- descent, and the other lit just as the ball touched sible to obtain additional specimens of this species. the surface of the water. The first lamp made it The test aquarium (Fig. 1) consisted of a long possible to recognize (and edit from the data set) rectangular tank (122 x 14.5 x 20.5 cm high) with any startle responses to the noise of the motor. The a glass bottom; water depth was 16.5 cm. Two second lamp was added due to occasional difficul- rocks positioned on reference marks on the bottom ties seeing the ripples. This lamp also allowed the provided a refuge for the fish at one end. A feeder spread of the ripples to be timed so that the instant could be hung in the tank at calibrated distances that the ball hit the water in earlier tests could be from the front edge of these rocks, and immediate- determined accurately, even when the ripples ly in front of a small 45” mirror spanning the width could not be seen immediately in any particular of the tank. A black Styrofoam ball (10.2cm dia- trial. Neither lamp was visible to the fish. meter) was positioned 117cm directly above the Between tests the fish were fed in their home centre of the mirror, attached with monofilament aquaria, using the same tablets in an orange holder, line to a pulley driven by a 6VDC motor. The in order to maintain the conditioned association. motor unwound the line, lowering the ball at a Feeding was carried out in the afternoon, 18-23 h controlled speed (averaging 14.2cm s-l). The prior to testing, so that the hunger level of the test length of the line was such that the ball reversed fish (and the lost opportunity cost of escape) would direction just as it touched the water surface. be consistently high. The feeder consisted of an orange plastic hanger A fish which had been regularly ‘hover feeding’ with a small depression drilled in it to hold a tablet in its home tank (i.e., feeding calmly and remaining of dry food (Tetratabs TM). The test fish had been within 2cm of the tablet) was chosen for testing. conditioned to approach an orange feeder in their The distance-to-cover was chosen randomly, sub- home tanks, and to graze on the tablet, much as ject to the constraint that each fish was tested only they probably graze algal slime from rocks in the once at each of 6 distances: 15,25,35,45,65, and field (Fryer & Iles 1972). The feeder was placed so 95 cm from the rocks. The test fish was gently trans- that a feeding fish had to be positioned sideways in ferred to the pre-arranged test aquarium and al- the tank, its left eye always toward the mirror. lowed free access to it for one hour, after which the Once the ball began to fall the fish experienced a loaded feeder was put in place. Once the fish had black solid looming toward it; such a stimulus is begun hover feeding the motor was switched on effective at eliciting escape in other small fishes and the response of the fish to the falling ball film- (Dill 1974a). The mirror, feeder and overhead mo- ed. A test was aborted if the fish did not hover feed tor moved as a single unit, so that the ball always within 20min. Only trials in which the fish fled fell in the same part of the mirror and at the same rapidly and for at least 10 cm toward the rocks were height as the food tablet, producing a consistent accepted. stimulus. An opaque piece of plexiglass directly The ball’s position above the water surface at the above the feeder prevented the fish from seeing the time the test fish began to flee could be determined falling ball directly. The entire apparatus was sur- from the speed of the ball and the subsequent time rounded with black plastic curtain to prevent ex- taken for it to strike the water (noted by the ripples 149 \ 5 Fig, 1. The apparatus used to measure flight initiation distance as a function of distance-to-cover. (1) orange plastic feeder, containing (2) a food pellet; (3) glass tank bottom; (4) 45” mirror in which the threat stimulus appeared to the fish; (5) 45” viewing mirror; (6) rocks serving as shelter for the fish; (7) sliding door (open during test); and (8) black Styrofoam ball connected to an electric motor with monofilament line. or the lamp). The fish’s flight initiation distance distance-to-cover was also consistent across fish was calculated as the sum of this measure plus the (dummy variable regression; Kleinbaum & Kupper distances from the mirror centre to the water sur- 1978). Although there is a suggestion of non-linea- face (5 cm) and to the food tablet (10cm). The rity in these data, the simple linear regression pro- fish’s velocity over the first 10 cm of its flight path vides an adequate fit and there is no a priori reason (or to any pause) was also measured from the vid- to fit a more complicated model. eotapes; this was a true vector velocity, measured The situation with regard to escape velocity was along the long axis of the tank. Data were collected somewhat more complex, owing to missing data intermittently from 10.7.1985 to 15.5.1986. The wa- points. In 5 of the 18 trials the fish stopped before ter temperature was 21” C, and the light intensity at reaching cover, and in 5 others the rocks were not the water surface was 750 lux.

Load more

  6

Copy Link