Behavioral Ecology Advance Access originally published online on August 18, 2004
Behavioral Ecology 2005 16(1):145-152; doi:10.1093/beheco/arh141
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Allometry and sexual selection of male weaponry in Wellington tree weta, Hemideina crassidens
Department of Zoology, University of Toronto at Mississauga, Mississauga, Ontario, Canada L5L 1J7
Address correspondence to C.D. Kelly. E-mail: cdkelly{at}utm.utoronto.ca.
Received 2 October 2003; revised 14 May 2004; accepted 1 June 2004.
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ABSTRACT |
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Both male and female Wellington tree weta, Hemideina crassidens, use cavities in trees as diurnal shelters. That these galleries are often limiting in nature offers males the opportunity to increase their reproductive success by monopolizing galleries and the females residing in them. Male H. crassidens, can mature at either the 8th, 9th, or 10th instar, whereas females mature at the 10th instar only, and male head (and mandible) size positively covaries with ultimate instar number. It has been suggested that males fight for control of galleries by using their enlarged mandibles as weapons, and males with larger mandibles control galleries with more females. In the present study, I present a statistical examination of sexual dimorphism, showing that traits related to head size are on average significantly larger in males, whereas traits related to body size are on average significantly larger in females. I tested three predictions addressing the hypothesis that sexual selection is driving megacephaly in male H. crassidens. First, as predicted, traits related to head size show a positive allometric relationship with body size in males but not in females. Second, adapting a novel statistical technique based on maximum likelihood and bootstrapping revealed that males, but not females, exhibit a multimodal distribution in head and body size traits. This is likely a consequence of males maturing at one of three instars, which results in positive covariance between the ultimate instar number and morphological traits. Third, as predicted, single adult males with larger heads reside in galleries housing larger groups of adult females.
Key words: allometry, harem success, sexual selection, tree weta, weaponry.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDICUSSIONREFERENCES |
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Males in many animal species control resources required for breeding by sexually receptive females or by competing directly for control of harems (Emlen and Oring, 1977
). This situation creates intense intrasexual selection among males and can lead to selection for behaviors (Andersson, 1994) or traits (e.g., elongate mandibles, antlers, and horns of particular beetles and flies; for review, see Dodson, 2000; Emlen and Nijhout, 2000) that enable males to be more competitive in contests for resources or harems.
One result of intense competition among males for mates is selection for alternative reproductive strategies and tactics in which dominant males exhibit one phenotype and subordinate males exhibit another (for review, see Brockmann, 2001). The production of two or more distinct phenotypes can result in a discontinuous or multimodal distribution of characters such as weapon size (Brockmann, 2001; Emlen and Nijhout, 2000). The evolution of two different alternative strategies, each with its own morphology and behavior, can evolve and is common (Shuster and Wade, 2003); however, the evolution of three or more alternative phenotypes is uncommon (Sinervo and Lively, 1997; Zimmerer and Kallman, 1989; but see Shuster, 1987).
Characters under sexual selection often show allometric values greater than 1.0 when scaled on body size; that is, larger individuals have disproportionately larger trait values than do smaller individuals of a species (for review, see Andersson, 1994; Emlen and Nijhout, 2000; but see Eberhard, 2002). For example, positive allometry is exhibited by traits used by females in choosing a male mate (Alatalo et al., 1988; Green, 1992; Petrie, 1988, 1992) and traits used by males in combat with rivals (Baker and Wilkinson, 2001; Eberhard, 2002; Eberhard and Gutiérrez, 1991; Forsyth and Alcock, 1990; Gould, 1974; Hanley, 2001; Palestrini et al., 2000; Simmons and Tomkins, 1996; Tatsuta et al., 2001). Although the reason why sexually selected traits tend to exhibit positive allometry is not certain, Baker and Wilkinson (2001) suggested that it is because smaller-bodied individuals have relatively little to gain from high investment in such traits. On the other hand, Petrie (1988) suggested that only traits used to signal competitive ability to conspecifics should exhibit positive allometry, and there is no reason to expect allometric slopes greater than one in structures that function only as weapons. Empirical evidence appears to contradict Petrie's (1988) suggestion, however, as Eberhard and colleagues (Eberhard and Gutiérrez, 1991; Eberhard et al., 1998) have demonstrated positive allometry in structures in beetles (horns) and earwigs (cerci) that apparently serve strictly as weapons, although a signaling function for these devices has not been ruled out (Eberhard, 2002).
The net effect of sexual selection, therefore, could be the evolution of alternative reproductive phenotypes within a sex with each phenotype expressing differently a trait that is positively allometric and unimodal in distribution.
Hemideina tree weta (Orthoptera: Tettigonioidea: Anostostomatidae) are a group of large, flightless, nocturnal insects endemic to New Zealand (Gibbs, 2001). Anostostomatids exhibit various forms of male weaponry, including elephantine tusks (e.g., Motuweta isolata, Gibbs, 2001) and enlarged mandibles (e.g., Hemideina tree weta and Anostostoma king crickets, Field and Deans, 2001; Gwynne and Jamieson, 1998; see also Hudson, 1920; Koning and Jamieson, 2001; Moller, 1985; Spencer, 1995).
In the sexually dimorphic Wellington tree weta, Hemideina crassidens, males use their enlarged heads and mandibles in fights for possession of shelters in tree cavities (Kelly CD, unpublished data). Within these cavities, known as galleries, one to several females refuge during the day (Field and Sandlant, 1983; Hudson, 1920). Mating system theory predicts that a limited number of galleries within a location will create the opportunity for males to monopolize them and, thus, females (Emlen and Oring, 1977). Furthermore, galleries of large size or high quality are expected to accommodate more females and thus create the opportunity for males to monopolize several females simultaneously (Emlen and Oring, 1977). Males that control galleries with several females have the potential for higher reproductive success because copulations occur within the cavity and at the cavity entrance (Field and Jarman, 2001). Therefore, competition for galleries containing more females should be more intense, and thus, males with larger mandibles should control them.
Little is known about the reproductive behavior of H. crassidens in the wild. Similar to its congeners, this sexually dimorphic species is hypothesized to be resource-defense polygynous (Field and Jarman, 2001). Another unconfirmed aspect of H. crassidens reproductive biology is the suggestion that males adopt alternative reproductive strategies (Field and Deans, 2001; Spencer, 1995). In the laboratory, male H. crassidens, but not females, mature at the 8th, 9th, or 10th instar (Barrett, 1991; Spencer, 1995; Stringer and Cary, 2001). A demonstration that precocially maturing males possess smaller heads, on average, than do males that delay maturation to the 10th instar (Figure 1) led Spencer (1995) to suggest that an irreversible alternative mating strategy (Brockmann, 2001) may exist with smaller males adopting a wandering or sneaking tactic to obtain matings opportunistically whereas larger males fight for control of galleries.
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In this article, I present a statistical examination of sexual dimorphism in H. crassidens. I also test three predictions to address the hypothesis that sexual selection is driving the enlargement of head size in male H. crassidens. First, if male head size is under sexual selection, then I predict that traits related to head size will scale in a positively allometric manner against body size. Second, if laboratory-reared males exhibit a trimodal distribution in head size because they mature at one of three instars and head size is positively related to instar number (Figure 1), then I predict that wild-caught males will also exhibit a trimodal distribution in head size. I test this hypothesis by using a novel statistical technique developed to determine the number of instars present in samples of fossilized arthropods (Hunt and Chapman, 2001
). Third, if males with larger heads are more likely to monopolize galleries, then I predict that single adult males with larger heads will reside in galleries housing larger groups of adult females. In contrast to previous studies of Hemideina tree weta (Gwynne and Jamieson, 1998), my observations are on a large (approximately several thousand) apparently interbreeding population of H. crassidens. |
METHODS |
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This study was conducted February 2001, March–May 2002, and March–April 2003 on Te Hoiere/Maud Island, New Zealand (41°02' S, 173°54' E), a 309-ha scientific reserve free of alien predators (e.g., rodents [Mus and Rattus spp.] and stoats [Mustela erminea]). The endemic morepork, Ninox novaeseelandiae, is the only known predator of adult tree weta on the island.
On Maud Island, H. crassidens readily use bird nest-boxes as shelter. These boxes were installed on trees in the early 1990s by the Ornithological Society of New Zealand for a rifleman, Acanthisitta chloris, breeding program. All nest-boxes have since been abandoned by the birds and, in addition to H. crassidens, are now occupied by common geckos, Hoplodactylus maculatus. Each nest-box (hereafter referred to as artificial gallery) measured approximately 150 x 100 x 95 mm with a 25-mm entrance hole. Each census involved permanent removal of all occupants and was separated by several days to allow re-occupation. I also destructively sampled approximately 180 natural galleries by splitting open branches on dead trees. This sampling method did not permit me to determine gallery size. I report data for only those galleries containing weta. Galleries without weta are not reported because it is not possible to determine whether the cavity was or was not suitable for occupation by tree weta.
In all years, I opportunistically collected adult H. crassidens by scanning vegetation at night. In 2002 and 2003, I also sampled natural and artificial galleries. For each censused cavity, I counted all individuals and noted the sex and relative age (immature versus adult) of each occupant.
By using digital calipers (Mitutoyo Digimatic), I measured each adult weta to the nearest 0.05 mm for each adult left and right hind tibia length, left and right femur length, left and right mandible length, total head length, head width at eyes, and ovipositor length (of females). Body weight was measured to the nearest 0.1 g with a 10-g Pesola scale. All weta were released back into the wild after being measured and individually marked with bee tags (H. Thorne Ltd.); marking prevented resampling individuals. I note that I did not recapture any individuals marked in previous years, suggesting that H. crassidens either live for 1 year only as an adult or they move to areas not censused by me. This life history contrasts with that of the mountain stone weta, H. maori, in which adults live for more than 1 year (Jamieson et al., 2000).
Data analysis
Before statistical testing, data were graphically inspected for normality and heteroscedasticity by using normal quantile plots and box plots, respectively (Quinn and Keough, 2002; Sokal and Rohlf, 1995). All morphometric variables were normal for females and did not require transformation; however, the number of females in a gallery could not be normalized. Because all male variables were not normal, I attempted to transform each by using the Box-Cox procedure (Sokal and Rohlf, 1995); however, only the distribution of male mean tibia length was made normal. All data are presented mean ± SE.
All measurements of the head (length, width, left mandible length, right mandible length) and body (weight, mean tibia length, mean femur length, ovipositor length) were pairwise correlated by using either Pearson correlation (female data only) or Spearman rank correlation (male data only). To correct for multiple comparisons of correlation coefficients, the sequential Bonferroni test (Rice, 1989) was used. I describe the allometric relationship between log head size and log body size in males and females by using major axis regression and assume equal variance in both variables (McArdle, 1988). Koning and Jamieson (2001) showed that in H. maori, femur length is a good indicator of body size, and head length and head width are appropriate indicators of sexual dimorphism. All head traits and body traits for H. crassidens are positively correlated in both sexes (Table 1). I report the relationship between femur length and head width and femur length and head length separately. All variables in this analysis were log transformed. Differences in major axis regression slopes were tested by using a procedure analogous to Student's t test (Clarke, 1980).
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I use the approach of Eberhard and Gutiérrez (1991)
to detect a nonlinear relationship between the trait of interest and body size. This technique not only allows the determination of whether a dimorphism exists but also establishes the exact body size at which a developing individual is most likely to switch from one morph to another. To determine if the relationship between head size and body size is linear, they recommend the model
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is the random component with assumed normal distribution, mean zero, and common variance. If ß2 does not differ significantly from zero, then the relationship between head size and body size shows no significant deviation from linearity, and consequently, determination of the body size switchpoint is not justified (Eberhard and Gutiérrez, 1991).
Eberhard et al. (2000) and Kotiaho and Tomkins (2001) suggest modifying Expression 1 to discriminate morphs based on the dimorphic character itself rather on some correlate of it, most commonly, body size. To accomplish this, they recommend simply substituting the Y with X and X with Y to get
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If the relationship between male head size and body size is discontinuous and a switchpoint exists, I tested whether additional switchpoints were present as outlined in Eberhard et al. (2000). If males exhibit a trimorphsim in head size, then it is possible that two switchpoints exist (i.e., a separate slope for each instar or morphotype).
It is possible for a trait to be multimodally distributed and exhibit a linear relationship with body size if each mode exhibits continuous variation in trait size. Therefore, males could have alternative strategies based on morphotype without exhibiting the clearly demarcated switchpoints seen in beetles and earwigs (see Eberhard and Gutiérrez, 1991). Detecting multimodality is important to this study because adult male H. crassidens are known to exhibit trimodality in the laboratory associated with variation in instar number. I am testing the prediction that trimodality exists in the wild as well. Therefore, in cases in which I found a linear relationship, I identified whether trait distributions comprised a mixture of normal distributions in both males and females in two separate steps. First, I visually inspected normal quantile plots to determine whether each trait was normally distributed. Normal quantile plots are recommended (Sokal and Rohlf, 1995) because other methods have either low power (e.g., Kolmogorov-Smironov) or perform poorly when there are tied data (e.g., Shapiro-Wilks; Zar, 1999). If the trait was not normal, I used maximum-likelihood analysis of mixture models (mixture models analysis [MMA], Hunt and Chapman, 2001), which was developed to evaluate hypotheses that multiple normal distributions comprised trait distributions (e.g., different arthropod instars). This statistical approach provides an objective comparison of various plausible hypotheses of grouping when faced with a data set suspected of comprising more than one subdistribution. Briefly, MMA achieves this by (1) choosing the model with the number of groups that best fits the data (i.e. the best-supported log-likelihood score for the hypothesized number of groups) and (2) estimating the parameters (means, variances, and mixing proportions) of each of the subdistributions for the best-fitting model (Hunt and Chapman, 2001). Next, MMA uses parametric bootstrapping to compare hypotheses that different numbers of groups comprise the data. The power of a bootstrap test was calculated when the bootstrap failed to reject the null hypothesis (Hunt and Chapman, 2001).
All statistical analyses were performed by using JMP 5.0 (SAS Institute, 2002) for Mac OSX and MMA software package for MS-DOS available at www.paleodb.org/paleosource (see also Hunt and Chapman, 2001).
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RESULTS |
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Male and female morphology
A total of 110 (59 males and 51 females), 325 (143 males and 182 females), and 317 (159 males and 158 females) adult tree weta were collected and measured in 2001, 2002, and 2003, respectively. There were no significant within-sex differences among years for any measured morphological trait (one-way ANOVA, all tests p > .05). Therefore, data for the 3 years were pooled for each sex.
There was male-biased sexual dimorphism in the four traits related to head size and female-biased dimorphism in the three traits related to body size (Table 2). Males showed more variation than did females in all measured traits as reflected in relatively high coefficients of variation (CVs) (Table 2). As in other species of Hemideina (Field and Deans, 2001; Gwynne and Jamieson, 1998), there was directional asymmetry in mandible length, with the left mandible always being longer than the right in both sexes.
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Both head size traits were positively correlated with body size in all 3 years for each sex (Tables 1 and 3). Because the slopes of the relationship between each head size trait and femur length did not differ among years for each sex, the data were pooled by trait and by sex. The relationship between head length and femur length was positively allometric in males, as the slope was significantly greater than two (Figure 2 and Table 3). In females, however, the slope did not differ from one and was therefore isometric (Figure 2 and Table 3). The relationship between head width and femur length was also positively allometric in males (slope > 1.0) but not in females (Figure 2 and Table 3). Males had significantly steeper slopes than did females for each trait (test for heterogeneity of slopes: head length, T = 9.78, df = 354.50, p < .0001; head width, T = 8.35, df = 355.34, p < .0001).
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The relationship between head size and body size did not differ significantly from linearity (log10 head length ß2 = 0.99 ± 0.97, t = 1.02, df = 354, p = .31; log10 head width ß2 = 1.17 ± 0.70, t = 1.66, df = 356, p = .10 ), nor did the relationship between body size and head size (model 2; log10 head length ß2 = 0.06 ± 0.09, t = 0.69, df = 354, p = .49; log10 head width ß2 = –0.09 ± 0.13, t = –0.68, df = 356, p = .50). Given that the slopes (ß2) did not differ from zero in any case, all relationships were linear and did not warrant further analysis of body size switchpoints. A continuous relationship is expected, despite the possibility of a trimodal distribution, if at least one individual is present in each trait-value bin and there are no clear breaks in the trait distribution.
As predicted, inspection of normal quantile plots for data pooled for 3 years showed that female head length, head width, and femur length were normally distributed (Figure 3a–c) whereas male head length, head width, and femur length were clearly not normal (Figure 3d–f). Male head length was not normal in each of the three sampling years (Figure 4), suggesting that the nonnormal distribution of the pooled data is not an artefact of biased sampling for a particular head size in any one year. That is, all traits measured in both males and females exhibited similar distribution patterns among years.
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MMA suggests that there is a significantly higher likelihood that the distribution for each male trait is bimodal rather than trimodal (Table 4). I am confident that a two-group model better explains the distributions of both head length and femur length (body size) better than does a one-group model. However, I am more cautious about choosing between two and three groups for femur length because this test was nearly significant at the 0.05
-level (p = .072) (Table 4) and because tests for head length and femur length each suffered from low statistical power (0.19 and 0.17, respectively) (Table 4). Head width trimodality was the only hypothesis rejected with high power (Table 4). Low power suggests that even if the best-supported three-group model were true, data such as these would be able to reject the two-group model only about 19% and 17% of the time (Table 4). That the mean head lengths calculated under a three-group hypothesis (Table 5) fall either within (intermediate = 21–22 mm and large = 25–26 mm) or close (small = 15–16 mm) to each putative mode identified through visual examination provides some support for a trimodal distribution. Assuming a three-group distribution in my data, the subdistributions show considerable overlap (Figure 3); a result consistent with Spencer's (1995: see also Figure 1) laboratory data (because Spencer's raw data were unavailable to me, I could not subject them to MMA). This overlap explains the low statistical power of the analysis because MMA has higher resolving power when modes are clearly demarcated (see Hunt and Chapman, 2001). That each mode overlaps another also explains why the regressions between head size traits and body size were linear and continuous (see above) as opposed to having a clear break in the distribution and, thus, a discontinuous relationship (e.g., beetle horns; Eberhard and Gutiérrez, 1991). When taken together, regression analysis and MMA clearly show that head traits are multimodally distributed but have a linear relationship with body size.
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Harem size and male head size
My surveys of cavities revealed up to seven adult females residing in the same cavity with a single adult male (Figure 5). Modal harem size was zero (range = 0–5, n = 29) in natural cavities and 1.0 (range = 0–7, n = 79) in artificial cavities (both years pooled). Resident male head length was significantly positively correlated with the number of females in an artificial cavity in 2003 (rs = .34, n = 55, p = .011) but not in 2002 (rs = .27, n = 24, p = .21). Male head length was significantly positively correlated with harem size in both natural galleries (rs = .52, n = 29, p = .004) and in artificial galleries (pooled data for 2002 and 2003, rs = .33, n = 79, p = .003). Pooling data for both cavity types over both years produced a strong positive correlation (rs = .39, n = 108, p < .0001).
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DICUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDICUSSIONREFERENCES |
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I found female-biased body size dimorphism and male-biased head size dimorphism in Wellington tree weta. Not only do males possess larger heads relative to females but they also exhibit more variation in head size. This large variation could be attributed to the apparent trimodal distribution in male head length which is likely a consequence of males maturing at different instars (Spencer, 1995
; Stringer and Cary, 2001) (see also Figure 1).
My study provides the first evidence that the multimodal distribution of head sizes observed in the laboratory (Barrett, 1991; Spencer, 1995; Stringer and Cary, 2001) for H. crassidens exists in nature. Unlike other Hemideina species, male H. crassidens can mature at one of three different instars while females mature at the 10th instar only (Barrett, 1991; Spencer, 1995; Stringer and Cary, 2001). That the bimodal, and possible trimodal, distribution in male head length and body size observed in the present study represents individuals maturing at a different instar is strongly supported by patterns of head length distribution observed in laboratory-reared male H. crassidens (Spencer, 1995). The head length of adult males in Spencer's (1995) study showed a clear trimodal distribution, with each mode representing a final instar that was positively correlated with head length (Figure 1). More specifically, 8th instar males had a smaller modal head length (11.0–11.5 mm, approximately 5.0% of all males) than did 10th instar males (21.0–23.5 mm, approximately 8.4% of all males) and 9th instar males exhibited a modal head length (15.0–15.5 mm, approximately 6.8% of all males) intermediate to the other two (Spencer, 1995) (Figure 1). That the distribution of head sizes I observed in nature differs from Spencer's (1995) is expected given the greater variation in larval feeding history in the wild compared with the laboratory. Also, Spencer's (1995) laboratory-reared males may have exhibited overall smaller head sizes than those reported here because the quality of the laboratory diet could have been poor compared with a natural diet. Alternatively, an intermediate mode may be rare on Maud Island because it is selected against or alleles influencing head polymorphism conform to Hardy-Weinberg equilibrium and renders the intermediate mode more rare (see Shuster and Wade, 1991). I suggest that males are trimorphic with 9th instar males being more rare than either 8th or 10th instar individuals.
The Wellington tree weta is the only orthopteran and one of only a few animals (e.g., the isopod, Paracerceis sculpta: Shuster, 1987; side-blotched lizard, Uta stansburiana: Sinervo and Lively, 1997; pygmy swordtail, Xiphophorus nigrensis: Zimmerer and Kallman, 1989) known to exhibit trimorphism in weaponry, body size, or ornamentation. Moreover, the Wellington tree weta is unique in that the trimorphic character displays continuous variation and is not discrete as is the body morphology of male isopods, color of lizards, or even the clearly dimorphic horns of beetles.
As predicted, male H. crassidens with larger heads, on average, resided with larger groups of females. Smaller, 8th instar male Wellington weta are hypothesized to assume a sneak mating strategy whereby they opt not to control cavities and instead acquire copulations via sneaking or forced copulation (Spencer, 1995). However, my data show that smaller males (< 20 mm head length) (Figure 3) do indeed associate with females in cavities. Perhaps this is not an irreversible strategy (Brockmann, 2001) but simply males in poorer condition (see below) "making the best of a bad job" (Dawkins, 1980), thus maturing earlier and using a conditional strategy based on the prevailing environmental conditions and their own phenotype (Brockmann, 2001). Whether the phenotype of a male tree weta is determined by either a genetic polymorphism or condition dependence remains to be investigated.
Male H. crassidens exhibited a positively allometric relationship between each head size trait and body size, whereas the relationship in females was isometric for those same traits. In both sexes, head length exhibited a steeper slope than did head width, suggesting that length may be more important than is width in male-male combat. That all relationships were also linear suggests that males of intermediate body size produce intermediate head sizes compared with species that exhibit a sigmoidal or discontinuous scaling relationship (Emlen and Nijhout, 2000). Males of species in the latter category have either minimal or complete expression of the trait over a narrow range of intermediate body sizes (Emlen and Nijhout, 2000).
In the congeneric H. maori, Jamieson and colleagues (Gwynne and Jamieson, 1998; Koning and Jamieson, 2001) have found conflicting evidence of allometry in cephalic weaponry. Gwynne and Jamieson (1998) did not find a slope significantly greater than 1.0 either between mandible length and tibia length (slope = 0.76) or between head width and tibia length (slope = 0.76). In contrast, Koning and Jamieson (2001) found positive allometry and a linear scaling relationship with femur length in male head length (slope = 1.31) and width (slope = 1.26). These differences may be the result of each study focusing on different or inappropriate characters; perhaps head width, as in H. crassidens (the present study), is not as important in combat as is length, and femur length may be a better indicator of body size than is tibia length. Given that slopes were inappropriately calculated by using model I least-squares regression in both studies, a direct comparison of scaling relationships between H. crassidens and H. maori is not possible.
The allometric values reported here for male head length are similar to values reported for weaponry in other insect species. For example, in their comparative analysis of 42 earwig species, Simmons and Tomkins (1996) found that males in 11 species showed positive allometry in forceps length (range of model I slopes = 1.47–3.50), a trait used by males in aggressive interactions with rival males (Briceño and Eberhard, 1995). Adult male staphylinid beetles, Leistotrophus versicolor, use their enlarged mandibles in combat with conspecific males over access to dung and carrion, which is used by sexually receptive females as a foraging site (Forsyth and Alcock, 1990). Forsyth and Alcock (1990) reported allometric values of 2.1 and 1.8 for the relationship between mandible length and head width and mandible length and wing length, respectively. A sexual selection hypothesis is supported in that species for the evolution of enlarged mandibles because males with larger mandibles are better fighters and mate with more females (Forsyth and Alcock, 1990). Cephalic horn length in Onthophagus beetles is considered to be important in male-male combat (Eberhard, 1982, 1979; Emlen, 1994), and allometric values of 7.5 (Palestrini et al., 2000) and 6.23 (Eberhard et al., 1998) in O. taurus and O. incensus, respectively, support the notion that sexual selection is operating on this trait. Contrary to these examples, Eberhard reported that the front tibia in the male sepsid fly, Palaeosepsis dentatiformis, function as weapons yet do not exhibit allometric values significantly greater than 1.0 (slope from OLS regression = 0.96) and are only slightly sexually dimorphic. He suggested that because the front tibia also serve a highly specialized function in courtship, their further specialization as weaponry is compromised. Similar to that of P. dentatiformis, the front femora of males in two species of Diptera (Chymomyza mycopelates and C. exophthalma) act as weapons but exhibit isometric relationships with body size (slopes from OLS regressions = 0.919 and 0.745, respectively; Eberhard, 2002). These structures do not serve in courtship or mating and are apparently not precluded from specialization as weapons (Eberhard, 2002). Eberhard (2002) suggested that perhaps among species not all sexually selected traits will have allometric values significantly greater than 1.0 if the costs of building and carrying such a structure outweigh the benefits. Controlled laboratory studies are required to better understand how head traits scale with body size for each adult instar in male H. crassidens and to estimate the fitness benefits of maturing at different instars.
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ACKNOWLEDGEMENTS |
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I thank Darryl Gwynne, Marlene Zuk, and two anonymous reviewers for comments on the manuscript; Steve Ward (Department of Conservation, Marlborough Sounds, New Zealand) for logistical support on Maud Island; and Peter Gaze and Ian Miller (Department of Conservation, Nelson, New Zealand) for research permits. A special thanks to Anita Spencer for generously permitting me to use a modified figure from her thesis and to the Insect Behaviour Group of Erindale College for valuable discussion. This research was supported by the National Science Engineering and Research Council (NSERC) and National Geographic Society grants to D.T.G.
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