Cuckoos versus reed warblers: Adaptations and counteradaptations

“Sign stimuli” elicit specific patterns of behavior when an organism's motivation is appropriate. In the toad, visually released prey-catching involves orienting toward the prey, approaching, fixating, and snapping. For these action patterns to be selected and released, the prey must be recognized and localized in space. Toads discriminate prey from nonprey by certain spatiotemporal stimulus features. The stimulus-response relations are mediated by innate releasing mechanisms (RMs) with recognition properties partly modifiable by experience. Striato-pretecto-tectal connectivity determines the RM's recognition and localization properties, whereas medialpallio-thalamo-tectal circuitry makes the system sensitive to changes in internal state and to prior history of exposure to stimuli. RMs encode the diverse stimulus conditions referring to the same prey object through different combinations of “specialized” tectal neurons, involving cells selectively tuned to prey features. The prey-selective neurons express the outcome of information processing in functional units consisting of interconnected cells. Excitatory and inhibitory interactions among feature-sensitive tectal and pretectal neurons specify the perceptual operations involved in distinguishing the prey from its background, selecting its features, and discriminating it from predators. Other connections indicate stimulus location. The results of these analyses are transmitted by specialized neurons projecting from the tectum to bulbar/spinal motor systems, providing a sensorimotor interface. Specific combinations of such projective neurons – mediating feature- and space-related messages – form “command releasing systems” that activate corresponding motor pattern generators for appropriate prey-catching action patterns.

Neuroethology of releasing mechanisms: Prey-catching in toads

Imagine a zebra in the African savannah. At each moment in time this zebra has to weigh up alternative courses of action before deciding which will be most beneficial to it. For instance, it may want to graze because it is short of food, or it may want to head towards a water hole because it is short of water, or it may want to remain motionless in order to avoid detection by the predator it can see lurking nearby. This is an example of the problem of action selection: how to choose, at each moment in time, the most appropriate out of a repertoire of possible actions. This thesis investigates action selection in a novelway and makes threemain contribu¬ tions. Firstly, a description is given of a simulated environment which is an extensive and detailed simulation of the problem of action selection for animals. Secondly, this simulated environment is used to investigate the adequacy of several theories of ac¬ tion selection such as the drive model, Lorenz's hydraulic model and Maes' spreading activation network. Thirdly, a new approach to action selection is developed which determines themost appropriate action in a principled way, and which does not suffer from the inherent shortcomings found in other methods.

Computational mechanisms for action selection

Sensory traps affect mate choice when male courtship signals mimic stimuli to which females respond in other contexts and elicit female behavior that increases male fertilization rates. Because of the supernormal stimulus effect, mimetic signals may become quantitatively exaggerated relative to model stimuli. Viability selection or a decrease in responsiveness to signals that are exaggerated beyond their peak supernormal effect may limit signal elaboration. Females always benefit by responding to models and they may often benefit by responding to mimetic courtship signals. If the response as a preference is costly, it may be maintained by frequent and strong selection for the response to the model. I review five examples of courtship that illustrate the kinds of studies that can provide evidence of sensory traps. The strategic designs of mimetic courtship signals arise not from selection of responses to them but from selection for responses to models. This results from deceit by mimicry and the evolution ...

G.P. Baerends

Papers

The herring gull and its egg .2. the responsiveness to egg-features