DESIGN OF AN INTENTION SIGNAL IN THE BLUEHEAD WRASSE (THALASSOMA-BIFASCIATUM)

The 'design' of any animal signal is affected by at least three select ion pressures: (i) the type of information being conveyed (the message ); (ii) the transmission properties of the medium; and (iii) the senso ry and psychological properties of the receiver. We describe a field s tudy of the bluehead wrasse (Thalassoma bifasciatum) and examine the ' circling' display of the male in the light of these three pressures. A nalysis of videotape records of the display shows that it carries accu rate information about when a spawning dash will occur. The speed with which the pectoral fins are moved increases steadily over the 10 s be fore spawning so that the exact time of spawning can be predicted. Dar k spots (not present when the male is chasing other males) appear on t he ends of the pectoral fins and may 'amplify' perception of fin movem ents. The colour change shown by the male when chasing other males and when courting females was measured in situ by using Munsell colour ch arts, and was subsequently converted to dominant wavelength and excita tion purity equivalents. The circling display appears to be designed t o convey information about intention to spawn to a female that is rela tively close whilst not conveying this information to other males chat are further away.