Weaponry, color, and contest success in the jumping spider Lyssomanes viridis
Highlights
► Males having longer chelicerae than their opponents were more likely to win contests. ► Males who won, despite being smaller, had less red chelicerae than their opponents. ► Male and female cheliceral and foreleg lengths correlated tightly with body size. ► Chelicerae and forelegs showed stronger positive allometry in males than in females.
Introduction
Male–male competition for access to females is a widespread phenomenon in the animal kingdom. Many such contests seem to be resolved by visual signals, which typically fall into two categories: displays of weaponry and badges of status (Maynard Smith and Harper, 2003, Searcy and Nowicki, 2005). Weapons are specialized structures or elaborated body parts designed to inflict damage during physical fights, such as antlers, teeth, crustacean chelae, and spider chelicerae. Weapon size could, in theory, be evolutionarily co-opted to function as a visual signal of fighting ability or aggressiveness. Badges of status are theorized to be patches of color(s) whose hue, brightness, darkness, contrast, size, shape, or pattern indicate a willingness or ability to fight. Badges and weaponry may, in some species, evolve dual signaling functions in both intrasexual and intersexual contexts (Berglund et al., 1996).
Although weapon size has been found to correlate with body size in both vertebrate and invertebrate taxa (e.g. Huxley, 1932, Pomfret and Knell, 2006), little is known as to whether or not weapons tend to function as visual signals of fighting ability. In the Cervidae (deer and their allies), for example, there is no conclusive experimental evidence that antler size or branching patterns are used to assess fighting ability (reviewed by Clutton-Brock, 1982). Chela size appears to be a rough predictor of contest success in snapping shrimp (Alpheus heterochaelis) (Hughes, 1996) and slender crayfish (Cherax dispar) (Wilson et al., 2007). However, evidence suggests that these signals do not honestly convey actual fighting ability (Hughes, 2000, Wilson et al., 2007). Further work is needed to clarify the signaling function of weaponry in both vertebrate and invertebrate taxa.
Experimental manipulations of avian color badges have provided strong support for the theory that color patches can signal fighting ability or aggressiveness (e.g. Fugle et al., 1984, Rohwer, 1985, Pryke et al., 2002, Pryke and Andersson, 2003). However, little is known about the role of color in the agonistic interactions of other taxa, particularly invertebrates. The few studies that have explored the relationship between invertebrate color patterns in aggressive interactions suggest that invertebrates are capable of associating fighting ability with the size of a color patch (Grether, 1996) or the patterning of light and dark elements (Tibbetts and Dale, 2004, Tibbetts and Curtis, 2007). However, the roles of other spectral properties, like dominant wavelength and spectral purity, are unknown.
Jumping spiders (Salticidae) make excellent invertebrate subjects for studies of both weaponry and badges of status because in many species, males possess exaggerated chelicerae, which function as weapons in escalated contests, as well as colorful patches of hair or cuticle on the face and/or abdomen, which may function as badges of status. Females do not have exaggerated chelicerae and are typically more cryptically colored. Salticids are diurnal and have excellent vision; measurements of Portia fimbriata's photoreceptor spacing and optics suggest a resolvable angle of 0.04° (Williams and McIntyre, 1980). This is much finer than the 0.24° inter-ommatidial angle of the compound eye of the dragonfly Anax junius, which possesses the finest known spatial acuity of any insect (Land and Nilsson, 2004). The saliticid Menemerus confusus was found to possess four different photoreceptor classes whose peak sensitivities span the spectrum from UV (360 nm) to yellow-orange (580 nm) (Yamashita and Tateda, 1976). Behavioral observations of Lyssomanes suggest a strong role for vision in predation and intraspecific interactions, although Lyssomanes’ visual acuity may be somewhat lower than that of Portia (see Blest and Sigmund, 1984). Male Lyssomanes and other salticid species wave their legs at each other in stereotypical patterns when competing over females, so it seems likely they are visually evaluating one another.
Despite their colorfulness, the relationship between natural variation in male salticid color patterns and success in agonistic encounters has not been examined. In Habronattus pyrrithrix, Taylor et al. (2011) found the inflection point and spectral purity of red facial coloration, as well as the darkness of green forelegs, to be condition-dependent. Whether conspecific observers actually use the information embedded in coloration, however, remains unknown. In several salticid species, including Phidippus johnsoni, Euophrys parvula, Zygoballus rufipes, Trite planiceps, Plexippus paykulli, Evarcha culicivora, and Phidippus clarus, larger-bodied males have been found to be more likely to win male–male competitive interactions and to gain access to females (Jackson, 1980, Wells, 1988, Faber and Bayliss, 1993, Taylor and Jackson, 1999, Taylor et al., 2001, Cross and Jackson, 2007, Elias et al., 2008), but the relationship between weapon size and contest success remains unexplored.
The goal of the present study was to evaluate the relationships between contest success, weapon size, and color in the sexually dimorphic jumping spider Lyssomanes viridis. Male L. viridis have reddish-brown chelicerae that are exaggerated in length, as well as boldly striped legs and a larger patch of red hairs on the forehead than females. Females are cryptically colored translucent green, except for a red ring of hairs against a background of white hairs on top of the head, and have comparatively short, green chelicerae (Fig. 1). In escalated contests, males press their chelicerae and forelegs against each other and push until one of them gives way and retreats. Thus, for the purposes of this paper, we considered both the chelicerae and forelegs to be weapons. Specifically, we staged contests between pairs of males, and asked the following: (1) Does male weapon length (i.e. cheliceral and foreleg length) correlate with contest success? (2) Does the spectral composition or overall brightness of male foreheads or chelicerae correlate with contest success? and (3) Does weapon length correlate with body size?
Section snippets
Subjects and housing
Thirteen juvenile male and eight juvenile female Lyssomanes viridis were collected by beating sweet gum (Liquidambar styraciflua) trees along the Black Creek Greenway (35°49.3′N, 78°47.1′W) in Wake County, North Carolina, USA, in February 2009. Eleven additional mature males and one immature female were collected from the same area and from the Jordan Lake State Recreation Area (35°50.0′N, 78°58.0′W) in Chatham County, North Carolina, USA, in May 2009. All spiders molted to sexual maturity
Behavioral observations
During a typical trial, each male walked about the experimental arena and explored female silk cues with his pedipalps. Males periodically spotted one another and displayed until one of them retreated or neutrally disengaged. The same male invariably won all three interactions; there were only two experimental trials in which the winner and loser were reversed over the course of the trial. There was no clear winner in 7 of 68 (10%) of experimental trials; these trials merely consisted of a
Weaponry
Cheliceral length was an excellent predictor of success in male–male agonistic encounters, and was tightly correlated with foreleg breadth and prosoma diameter. Salticid displays usually include an extension and/or waving of the legs (Richman, 1982). Our results suggest that such displays could function as honest indicators of male body size. The highly disproportionate increase in male cheliceral length with body size, compared to the same relationship in females, was particularly striking,
Acknowledgements
We would like to thank John Endler for suggesting salticids as a study system and Dustin Wilgers for teaching collecting techniques. Thanks to Chris Castorena, Rob Colautti, Joe Lucas, and Louise Roth for advice on statistics. We appreciated the comments of G. B. Edwards, John Endler, George Uetz's Animal Communication class, Manuel Leal, and two anonymous reviewers on the manuscript. This work was funded by the National Science Foundation's Graduate Research Fellowship, the James B. Duke
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