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- Materials and methods
Because winners of intramale agonistic interactions often get primary access to vital resources (including females), fighting ability is an important determinant of male Darwinian fitness in many animals. Therefore, identifying the characteristics that make up a winner is central to understanding the evolution of the male phenotype. Until recently, most previous studies have focused on individual morphological, behavioural and endocrinological characters relevant to establishing social dominance (e.g. review in Andersson 1994; Berglund, Bisazza & Pilastro 1996; Molina-Borja, Padron-Fumero & Alfonso-Martin 1998; Kemp 2003), while whole-animal performance traits have received far less attention (Perry et al. 2004). This is unfortunate, because knowing the performance abilities relevant to the outcome of an interaction should help identify the morphological and physiological attributes that determine fighting ability, and explain why they do so (Arnold 1983; Huey et al. 2003a). For instance, a correlation between sprint speed and success in agonistic interactions would aid in understanding the evolution of longer limbs (e.g. in species in which fights involve rapid chases); an association between bite force and dominance would help explain the development of larger heads (e.g. in species that bite each other during combat). Moreover, the behavioural options open to an individual may be constrained by its whole-organism performance (Perry et al. 2004; see also Huey, Hertz & Sinervo 2003b).
Thus, several studies on lizards have examined relationships among physiological performance ability and success in agonistic interactions. The results are disparate. Garland, Hankins & Huey (1990) found a positive relationship between dominance and sprint speed in Sceloporus occidentalis, but dominance and stamina were unrelated in this species. Robson & Miles (2000) found that dominant Urosaurus ornatus males had higher sprint capacities and higher stamina than subordinate males. López & Martín (2002) found a negative relationship between sprint speed and social dominance in male Lacerta monticola. Finally, Perry et al. (2004) reported a positive correlation between dominance and endurance (but not sprint speed) in male Anolis cristatellus. Presently, the number of species studied is insufficient to test whether these seemingly inconsistent outcomes actually reflect interspecific differences in the types of agonistic behaviour displayed.
A possible shortcoming of previous studies is that the measurement of whole-animal performance was limited to two aspects of locomotory capacity, namely sprint speed and endurance. Speed and endurance are routinely used as indices of overall physiological capacity in lizards, and several authors have implied that they may be directly linked to dominance (but see Garland et al. 1990). Robson & Miles (2000) suggested that sprint speed is correlated with the frequency of head bobbing and push-up displays in Urosaurus ornatus, and Perry et al. (2004) linked endurance to the prolonged vigorous interactions in Anolis cristatellus. However, we can think of several other performance variables that are (more) likely to be relevant to fighting ability in lizards. One of them is acceleration capacity, the ability to develop speed from standstill, which is not often measured in lizards (Huey & Hertz 1984; Irschick & Jayne 1998). In many lizard species, the opponents in a conflict typically approach each other slowly, and then, as the conflict escalates, start launching short, forceful attacks at one another. In such a situation, acceleration capacity may be a more direct correlate of fighting ability than sprint speed, which is generally measured by chasing lizards over relatively long (> 2 m) race tracks. Moreover, sprint speed and acceleration capacity are not necessarily correlated (B. Vanhooydonck, unpublished data).
Another performance variable that seems particularly relevant is bite force. In many lizard species, escalated conflicts will result in fierce fighting during which opponents will bite each other severely on the head and other parts of the body. Although it has repeatedly been suggested that the sexual dimorphism in head size characteristics of many lizard species results from intrasexual selection for higher bite force, a correlation between bite force and success in agonistic interactions has to our knowledge been described only once (Lailvaux et al. 2004): in interactions between the bigger male Anolis carolinensis, bite force influences dominance. However, until now, performance capacities and fighting ability were always examined from a univariate perspective, neglecting the possibility of interacting effects of performance.
The goal of the present study is to establish relationships between morphological characteristics, whole-animal performance and success in agonistic interactions in males of the lacertid lizard Gallotia galloti. In this paper, four estimates of whole-animal performance are assessed in each individual lizard, and multivariate analyses are used to evaluate the relative influence of the different aspects of performance on the outcome of aggressive interactions.
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- Materials and methods
Our results show that winners of agonistic interactions between male Gallotia galloti lizards differ from losers in their physiological performance. Until now, dominance in lizards has been associated with locomotor performance (Garland et al. 1990; Robson & Miles 2000; Perry et al. 2004; but see López & Martín 2002), but our findings suggest a more prominent role for bite force capacity. As far as we know, only one other study has demonstrated a correlation between bite force and social dominance in lizards (Lailvaux et al. 2004).
According to our results, badge size conveys information about a male Gallotia galloti's dominance. This confirms previous findings in other lacertids (Olsson 1992, 1994; Martín & Forsman 1999; López, Martín & Cuadrado 2004) and other lizard families (Cooper & Vitt 1988; Zucker 1994; review in Whiting, Nagy & Bateman 2003). However, since no correlations between badge size and bite force or head size were found, badge size seems to signal a component of fighting capacity other than bite force (e.g. motivation). In an earlier study, Molina-Borja et al. (1998) found no relationship between dominance and the number of lateral patches in Gallotia galloti. This does not contradict our findings, since there was no necessarily significant correlation between the number of badges and the total badge size in our sample. It should be noted that our results on the effect of badge size must be interpreted with caution, since our measurements concerned spots visible by the human eye only. Ultraviolet photography and spectrophotometry in Gallotia galloti have shown that the blue badges reflect UV and are bordered by areas of non-reflective skin (Font & Molina-Borja 2004). The badges may therefore communicate information through other aspects than mere size. Clearly, this is an area that deserves more detailed research in the future.
In our experiments, a substantial part of the intraspecific variation in bite force could be explained by differences in body mass, head size and hindlimb length. The fact that larger individuals and individuals with relatively large heads could bite harder is in line with earler findings (e.g. Herrel, Van Damme & De Vree 1996; Herrel et al. 1999; Verwaijen, Van Damme & Herrel 2002). The negative correlation between bite force and hindlimb length is more enigmatic; we know of no studies that have established direct or indirect (e.g. via hormones) relationships between limb length and bite capacity.
Because male Gallotia galloti are highly aggressive in the field and will attack and bite other males, it is tempting to link dominance and bite capacity directly. This would concur with the sexual dimorphism in (relative) head size (Bischoff 1971; Molina-Borja et al. 1997; Herrel et al. 1999), the disproportionately high bite forces in males (Herrel et al. 1999), and the importance of head size in dominance in this species (Molina-Borja et al. 1998). Previous studies on other lizard species have provided indications of the potential importance of bite force capacity in settling territorial disputes. Intraspecifically, male head size is correlated with dominance in male Uta palmeri (Hews 1990), Iguana iguana (Pratt et al. 1992), Cyclura nubila (Alberts et al. 2002), Lacerta monticola (López, Munoz & Martín 2002) and Anolis cristatellus (Perry et al. 2004). Interspecifically, head size dimorphism and the occurrence of male combat are associated in the Eublepharidae (Kratochvíl & Frynta 2002) and herbivorous Iguaninae (Carothers 1984). Since head dimensions are directly related to bite force (Herrel et al. 1996, 1999; Verwaijen et al. 2002), it seems likely that bite force, through its effect on dominance, is a performance trait under sexual selection. However, the design of our experiment does not allow us to rule out the possibility that bite force and thus social dominance are influenced by a third, unmeasured variable. For instance, Garland et al. (1990), Robson & Miles (2000) and Perry et al. (2004) have already argued that testosterone may affect both dominance and locomotory measures of physiological capacity. Moreover, in Sceloporus undulatus, experimentally elevated plasma testosterone levels induced a greater sprint speed ability and burst stamina (Klukowski, Jenkinson & Nelson 1998). Although we are unaware of studies that have investigated the effect of sex hormones on bite force in any species, there are several reasons to expect such a relationship. First, it is well known that sex hormones influence mandibular bone growth in mice (Fujita et al. 2004), rats (Verdonck et al. 1998; Gebhardt & Pancherz 2003) and humans (Verdonck 1997; Verdonck et al. 1999). Second, sex hormones may influence bite performance through their effects on the muscles of mastication. For instance, in adult guinea pigs (Lyons, Kelly & Rubenstein 1986) and mice (Eason et al. 2000), androgens alter the composition of masseter muscle fibres and are required for the maintenance of sexual dimorphism in that muscle. On the other hand, it is also possible that these hormones are actually influencing social dominance, through their effects on bite performance.
Our results seem to stand in contrast with earlier findings that dominance in lizards is correlated with locomotor performance (Garland et al. 1990; Robson & Miles 2000; Perry et al. 2004). At this time, it is hard to assess whether and why lizard species differ in the performance variables that determine their dominance status. Differences in social and sexual behaviour may select for different performance variables: in species in which disputes are decided in fast agonistic interactions (such as in Sceloporus occidentalis and Urosaurus ornatus), sprint speed or acceleration might be important; in species in which conflicts involve lengthy sequences of display behaviour (such as in Anolis cristatellus), endurance may be more relevant; in species that readily engage in physical combat, bite force might be decisive. Of course, interspecific variation in performance variables is likely to be subject to differences in other selective pressures (e.g. predation intensity) as well. In any case, the paucity of species studied, discrepancies in the methodology used by different authors (e.g. in the measurement of endurance) and the fact that bite force has not often been measured before, currently prevent tests of this idea. Our finding that some of the performance variables may be intercorrelated further stresses the need to measure different, potentially important variables simultaneously. In addition, inferences based on experiments in the laboratory need to be verified with observations made in (more) natural settings (as in Perry et al. 2004).