Effect of different pupil to eye size ratios on tonic immobility in chickens

Abstract

Seven simulated eyes with varying pupil size to eye ratios were tested for their effects on the duration of tonic immobility. A pair of eyes with a ratio of approximately 11/20 mm was the only configuration which produced reliable increases in response duration. The results were discussed as they relate to the possible role of eye contact in predator-prey encounters.Tonic immobility (TI) appears to function as the terminal response in a series of distance-dependent predator defenses (Ratner, 1967; Sargeant & Eberhardt, 1975). The response, which is produced by some form of physical restraint, occurs in a wide variety of species and is characterized by a catatonic-like state of physical immobility and muscle hypertonicity. Attempts to produce TI in the laboratory eventuate in immobility episodes that vary in duration from a few seconds to several hours.Among the most potent agents for increasing the duration of TI are stimuli which are thought to produce fear (Gallup, 1974). Prominent in this list are stimulus characteristics of potential predators. In a series of studies, it has been shown that (a) the opportunity to view a stuffed hawk or a human experimenter increases the duration of TI in chickens (Gallup, Cummings, & Nash, 1972;Gallup, Nash, Donegan, & McClure, 1971) and lizards (Edson & Gallup, 1972; Gallup, 1973), and that (b) the effects of simulated predation on TI are due almost exclUSively to the visibility of the predator's eyes. Of such singular importance are eyes in sustaining TI that merely suspending artificial glass eyes over immobilized chickens (Gallup, Nash, & Ellison, 1971) or crabs (O'Brien & Dunlap, 1975) results in a marked increase in response duration.The preceding findings, when considered together, strongly suggest that eye contact plays a powerful and fundamental role in the maintenance of tonic immobility. A logical next step would be an attempt to determine which feature(s) of such eyes is critical for sustaining the reaction. 0 'Brien and Dunlap (1975) found that artificial eyes potentiated TI in crabs, but eyes of different sizes all yielded comparable effects. One likely candidate for remaining investigation is the size of the pupil relative to the rest of the eye.Requests for reprints should be sent to G. G. Gallup, Jr., Department of Psychology, State University of New York at Albany, Albany, New York 12222. METHOD SubjectsThe subjects were 86 straight-run, Production Red chickens (Gallus gallus) obtained from a local hatchery at 1-2 days of age. All birds were housed in commercial brooders and maintained on artificial light for 14 h/day . Purina Chick Chow (Growena) and water were continuously available during rearing. ApparatusThe apparatus consisted of an upright stand, fitted with a pair of 1.27-cm wooden dowels which could be suspended over an immobilized bird. Small magnets were attached to the ends of the dowels and provided a means by which stimulus materials could be attached and replaced.The stimulus materials consisted of seven pairs of circular paper eyespo...

Effects of simulated predation on tonic immobility in Anolis carolinensis: The role of eye contact

“…Although the presem:e of eyes is not a sufficient condition for inducing TI, eyes may increase fear and therefore potentiate the duration of TI, once elicited. While Gagliardi, Gallup, and Boren (1976) found the optimal pupil to eye ratio was approximately 1: 2 with chickens, in anoles a dark well-defined pupil seems to prolong TI best: The largest pupils produced the greatest effect, although an entirely black spot had no more apparent effect than did a yellow spot or small pupil to eye ratio models. In addition, while pupil size was of great importance, the overall size of the eye was not.…”

Risk discrimination of eye contact and directness of approach in black iguanas (Ctenosaura similis).

“…In the laboratory Gallup, Cummings, and Nash (1972) reported that the experimenter's eyes are of primary importance in sustaining immobility in chickens (Callus gallus) and that the ratio of pupil size to eye size was critical in the response duration (Gagliardi, Gallup, & Boren, 1976). Eyes and direct gaze also sustain tonic immobility of lizards (Gallup, 1973;Henning, 1977) and death feigning by a neonatal snake (Heterodon platirhinos; Burghardt & Greene, 1988).…”

Predatory overtones of open-field testing in chickens

“…Subjects remain immobile longer when tested with detached eyes suspended overhead than when the eyes are blocked from view (Gallup, Nash, & Ellison, 1971;O'Brien & Dunlap, 1975). Interestingly, the only pupil-to-eye size ratio found effective in potentiating tonic immobility in chickens was that which most clearly resembles the average vertebrate predator eye (Gagliardi, Gallup, & Boren, 1976). In light of these effects, Experiment 2 was conducted to determine if artificial eyes could be used to alter open-field behavior.…”

“…The adaptive significance of being able to respond differentially to a predator's eyes seems obvious. During any stage of a predatory encounter, prey which can identify the direction of a predator's attention may have the greatest chance of escaping should the predator become distracted (see Gagliardi et al, 1976).…”