Do the elongated eye stalks of Diopsid flies facilitate rival assessment?

Behav Ecol Sociobiol (2009) 63:12431246 DOI 10.1007/s00265-009-0774-x

COMMENTARY

Do the elongated eye stalks of Diopsid flies facilitate rival assessment?

Yoni Brandt & John G. Swallow

Received: 21 January 2009 /Revised: 3 May 2009 /Accepted: 4 May 2009 /Published online: 16 May 2009 # Springer-Verlag 2009

Abstract The elongated eye span of male Diopsid flies is a sexually selected character that scales positively with body size. Previously, the duration of agonistic contests was found to increase as rival body size and eye span disparities decreased. Hence, along with its role in mate choice, eye span seems to facilitate mutual assessment of rival size. However, such results are also expected in the absence of rival assessment, when each individual persists according to its own size-dependent internal threshold. Here, we reanalyze these contests to distinguish between these two hypotheses using two measures of size: body length and eye span. Mutual assessment predicts that contest duration should increase with loser size and decrease with winner size. In contrast, our results were more consistent with self-assessment: We found a positive relationship between loser size and contest duration, whereas winner size did not affect contest duration. Thus, flies did not appear to assess the size of their rivals, indicating that the mutual assessment function of eye span elongation may be less important than previously suspected.

Keywords Self-assessment . Rival assessment .

Fighting . Size . Armament . Stalk-eyed flies .Aggressive interactions

Introduction

In agonistic interactions, large individuals are better able to inflict injuries, are less susceptible to injurious attacks, and enjoy higher social status than smaller individuals (Riechert 1998). Armaments such as horns and antlers may improve the efficacy and precision of rival assessment when incremental increases in body size are accompanied by greater incremental increases in armament size (Gould 1973; Gould 1974; Petrie 1988; Hongo 2006), a scaling pattern known as hyperallometry (Huxley 1932). Certain stereotyped postures that bring armaments into close proximity may further permit rivals to directly compare armament size. The elongated eye stalks of Diopsid flies (Diptera: Diopsidae) are among the better-known examples of such armaments (de la Motte and Burkhardt 1983). Eye span shows a close positive association with body size, as well as considerable interspecific variation (Baker and Wilkinson 2001). In sexually monomorphic species, both...