The effect of morphology and physiology on butterfly territoriality

Tsuyoshi Takeuchi1) (Department of Zoology, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan)

(Accepted: 13 December 2005)

Summary Males of many butterfly compete for territories via aerial interactions. How butterflies settle a contest is rather a mystery because it is obscure what kind of costs they can inflict on their opponents. A study by Davies (1978) on the speckled wood butterfly, Pararge aege- ria provided empirical support for the idea that residency is used as an arbitrary means of contest settlement. On the contrary, recent research on the green hairstreak, Chrysozephyrus smaragdinus indicated that contest outcome could not be explained by the bourgeois strategy, at least in its original form. In the present study, I compared several morphological and phys- iological traits of territorial residents to those of intruders to investigate whether resource- holding potential (RHP) is correlated with these traits in C. smaragdinus. The differences in body size, flight-muscle ratio, and age between residents and intruders were not signifi- cant. Residents had less lipid reserves than intruders suggesting that residents consume more energy during territorial defense. This result indicates that the tested parameters are not cor- related with RHP, and does not support the idea that superiority of territorial residents in C. smaragdinus is attributable to morphological or physiological traits.

Keywords: butterflies, resource-holding potential, territoriality, wars of attrition.

Introduction compete for limited resources in nature. Violent fighting some- times occurs attending the risk of injury or death. The outcome of physi- cal fights is greatly influenced by resource-holding potential (RHP; sensu

1) E-mail address: [email protected] © Koninklijke Brill NV, Leiden, 2006 Behaviour 143, 393-403 Also available online - www.brill.nl 394 Takeuchi

Parker, 1974), which is usually correlated with large body size or weaponry, and contests are often settled on the basis of RHP asymmetry between op- ponents (reviewed by Andersson, 1994). In contrast, animals with less elab- orate weaponry fight less violently, often relying on prolonged displays to determine the outcome of contests. This is referred to as ‘wars of attrition’, where both contestants continue displays and the one that retreats earlier is the loser (Maynard Smith, 1982). In wars of attrition, high-endurance capa- bility rather than violence should be demanded, but the determinants of RHP are rather obscure. Males of a variety of species (, Odonata, Hymenoptera, Diptera) compete for mating territories through conspicuous aerial interac- tions, which are considered wars of attrition. In many butterfly species, males defend a territory and compete with conspecific males for the area (reviewed by Kemp & Wiklund, 2001). In butterfly territorial contests, two males encir- cle each other in the air (circling flight) until one of them retreats. Generally, males rarely attack the opponent physically during the contest, and risk of injury in the battle appears to be extremely low. The combatants appear to have no effective structures with which to attack their opponent, and thus, the traits correlated with RHP are not clear. Therefore, butterflies provide excel- lent opportunities to understand fighting behavior in non-armed animals and would shed a unique light on the general evolution of contest behavior. The green hairstreak, Chrysozephyrus smaragdinus () is a small butterfly (wing span 18-23 mm), inhabiting temperate deciduous forests and emerging once a year (univoltine). In Japan, adults appear in June and July. Males of C. smaragdinus compete for ownership of territories via circling flight. In their territorial contests, residents almost always defeat intruders and defend their territories for several successive days (Takeuchi & Imafuku, 2005). High winning rate of residents has been reported in many butterfly species (reviewed by Kemp & Wiklund, 2001) as is the case in other animals (Alcock, 2001). However, why residents can achieve high winning rate is not well understood in butterflies. The ‘bourgeois strategy’ hypothesis (Maynard Smith, 1982) suggests that the resident wins because competitors adopt the rule that the resident should be aggressive and the intruder should retreat. Davies (1978) gave empirical evidence supporting the bourgeois strategy in the speckled wood butterfly, Pararge aegeria. However, studies conducted later revealed that the bour- geois strategy is rarely evolutionary stable in nature (Grafen, 1987; Kemp