ABSTRACT
Before a ‘m echanism ’ can be proposed to ‘explain’ a phenom enon, it is necessary to state clearly the characteristics w hich it has to account for.
First, all behaviour patterns have a lim ited duration: they start, proceed fo r a certain tim e and then stop. A ny contro l m echanism m ust therefore incorporate starting and stopping devices (which m ay be independent o f one another). Starting devices are often related to special and highly specific external stim uli sometimes called re leasing stimuli; fo r exam ple, the sound o f a bat’s high-frequency ‘radar’ note m ay cause noctuid m oths to take avoiding action [229, 230]. In this case the behaviour appears to be switched on by the stimulus. B u t some behaviour patterns begin ‘spontaneously’, w ith searching or appetitive behaviour w hereby the anim al appears to seek special stim uli w hich evoke further reactions: a hungry anim al searches fo r food; a sexually m otivated male, for a female. Stopping devices are also sometimes related to external stim uli, b u t sometimes not. A n escaping anim al ceases to flee w hen under ‘cover’, bu t it ceases to eat o r drink w ithou t any new external stimulus to m ake it do so. S tarting and stopping systems m ust therefore respond to in ternal states as w ell as to external stimuli.
