Sophistication and simplicity: conventional communication in a rudimentary system

Sophistication and simplicity: conventional communication in a rudimentary system Paweł Re˛k 0 1 Tomasz S. Osiejuk 0 1 0 The Author 2010. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions , please 1 Department of Behavioral Ecology, Institute of Environmental Biology, Faculty of Biology, Adam Mickiewicz University , Umultowska 89, 61-614 Poznan ́ , Poland Conventional communication, like human speech or the bee dance, consists of arbitrary signals associated with meaning through convention. Acoustic conventions seem to be frequent among passerine birds having song repertoire variability, but such behaviors are unknown in nonpasserine species producing simple calls. Because the variability of vocalizations is relatively small in nonpasserine birds, no study has investigated their ability to create conventional signals. We experimentally show here that Corncrake males (Crex crex), modifying rhythm in simple 2-syllable calls, signal their neighbors about their aggressive motivation. Males responded more aggressively (approached the speaker and attacked it) to playbacks with intermittent than with monotonous rhythm, and males calling with a more intermittent rhythm were more aggressive. The presence of convention in this system requires that the production of alternative signals is not linked with differential production costs. In Corncrakes, aggressiveness is signaled by a signal with lower production cost (lower call rate) than the lack of aggressiveness, and our results indicate that the honesty of the rhythm is maintained by a receiver retaliation cost rather than by production costs. Our observations suggest that temporal organization of signals in Corncrakes is an example of syntax, equivalent to a very simple Morse code system. Key words: aggressiveness, call rhythm, Crex crex, motivation, syntax. [Behav Ecol 21:1203-1210 (2010)] - Carbitrary with respect to the signal’s design and therefore onventional signals are the signals with a meaning that is dependent for meaning on an agreement between the signaler and receiver (sense 2 of Guilford and Dawkins 1995) . They are honest by definition if the interests of the sender and receiver are identical in an evolutionary sense; however, they are highly vulnerable to deception if the interests of both sides are opposing (Searcy and Nowicki 2005) . Therefore, the honesty of conventional signals in agonistic interactions must be connected with some stabilizing cost (Zahavi 1975; Maynard Smith 1994; Zahavi Am and Zahavi As 1997; Vehrencamp 2000; but see Lachmann et al. 2001) . Nevertheless, in order to maintain signal arbitrariness, costs should not be paid directly during the production of the signal but they should be receiver dependent (Guilford and Dawkins 1995) . Hence, conventional signals have been popularly understood as cost-free signals, whereas they should be rather considered as minimal signals (Maynard Smith and Harper 1995, 2003; Sza´mado´ 2003) , that is, the signals for which production cost is used only in signal transmission but that is not used to verify signal meaning (Sza´mado´ 2003) . Models proposed 2 kinds of receiver-dependent costs: receiver retaliation (Enquist 1985) and vulnerability handicap (Zahavi 1987; Adams and Mesterton-Gibbons 1995) . Whereas the role of the first one in conventional signaling has been empirically proved (Molles and Vehrencamp 2001) , the evidence for the second one is contradictory (Zahavi 1987; Sza´mado´ 2003, 2008; Laidre and Vehrencamp 2008; Searcy et al. 2008) . The most expressive examples of conventional communication are the bee dance and human speech; however, there are many others. Typical examples are color plumage badges in birds ( Rohwer 1982 ; Møller 1987 ; Pa¨rt and Qvarnstro¨m 1997 ; Pryke et al. 2002 ) signaling the status of the sender. Although they cannot be varied on a short timescale, they are low-cost signals that have evolved to be linked with fighting ability and motivation. Nevertheless, the assertion that badges of status are conventional signals is problematic. Because deception has not been observed in nature (Searcy and Nowicki 2005) , they can be so-called indices, the correlates of physical attributes of senders. Other possible examples of visual signals that may be conventional signals include eyespots of Anolis lizards (Anolis carolinensis) (Korzan et al. 2000) and the striped barring in Swordtail fish (Xiphophorus) (Morris and Casey 1998; Moretz and Morris 2003) . Conventional signals may also include olfactory (Poole 1989) and acoustic signals. Acoustic conventional signals, Anolis eyespots, and fish bar marks can be changed on a relatively short timescale, and they differ from plumage badges because they allow for short-term communication. Research to date has shown that song-type matching and switching rate are conventional signals in the Banded Wren ( (...truncated)