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1. Spiders are the most sexually size dimorphic terrestrial animals and the evolution of this dimorphism is controversial. Patterns of sexual size dimorphism (SSD) in spiders have been related to individual performance and size. In 2002 Moya-Laraño, Halaj & Wise proposed the ‘gravity hypothesis’ to explain patterns of sexual size dimorphism in spiders whereby species building webs high in the vegetation are predicted to show greater SSD than those that build lower down. They advocated an advantage in climbing speed in smaller males searching for females in high places. The gravity hypothesis predicts a negative relationship between male size and climbing speed. In 2007 Brandt & Andrade questioned this interpretation and proposed that the pattern of SSD in spiders is better explained by an advantage for larger males of low-dwelling species to run faster along the ground.
2. We induced male spiders to run a standard distance up vertical poles of different diameters to examine the predicted relationship between size and climbing speed. We tested two species of extremely size-dimorphic orb-web spiders, Argiope keyserlingi and Nephila plumipes, that differ in the height at which females tend to build webs, and one species of jumping spider, Jacksonoides queenslandica, with low levels of size dimorphism. We also examined morphological determinants of horizontal motility by inducing males to run along a raceway.
3. Substrate diameter was consistently found to influence climbing performance. In N. plumipes, climbing speed was slowest on the widest diameter substrate. In A. keyserlingi, size-adjusted leg length and substrate diameter interacted to determine climbing speed, while in J. queenslandica, there was an interaction between body size and substrate diameter on climbing speed. In the effect of substrate diameter, we have identified a potential bias in previous tests of the gravity hypothesis.
4. Our results do not support the prediction of the gravity hypothesis. There was no evidence of a negative relationship between body size and climbing speed in the two orb-web species with high levels of SSD. Our results are also not consistent with a recent modification of the gravity hypothesis that suggests a curvilinear relationship between climbing speed and size.
5. Body size was positively associated with maximum running speed only in the cursorial hunter J. queenslandica. For this spider, results are more consistent with Brandt & Andrade’s explanation for variation in SSD in spiders, that larger males are selected for superior running ability in low-dwelling species, rather than selection for smaller size for climbing to females in high-dwelling species.
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- Materials and methods
Sexual size dimorphism (SSD) is common, taxonomically widespread and highly variable in animals (Andersson 1994; Fairbairn 1997; Badyaev 2002; Blanckenhorn 2005; Fairbairn, Blanckenhorn & Szekely 2007). SSD is presumed to be adaptive since size is commonly linked to fitness (e.g. Andersson 1994; Roff 2002). If size is heritable and the genetic correlation between males and females is less than unity, SSD is considered to evolve as a result of different net selection pressures acting on male and female body size (Lande 1980; Lande & Arnold 1983; Reeve & Fairbairn 2001). Males are normally the larger sex in birds and mammals and that is usually attributed to sexual selection conferring fitness advantages to larger males that are better equipped for aggressive interactions or are preferred as mates by females (Darwin 1871; Andersson 1994). The female biased SSD often shown by invertebrates, as well as cold-blooded vertebrates and raptors, has been most often ascribed to fecundity selection favouring larger females (Darwin 1871; Coddington, Hormiga & Scharff 1997; Prenter, Elwood & Montgomery 1999). It has also been attributed to sexual and natural selection for agility in species ranging from small aerial insects to large terrestrial birds (e.g. McLachlan 1986; McLachlan & Allen 1987; Steel & Partridge 1988; Crompton, Thomason & McLachlan 2003; Raihani et al. 2006; Moya-Laraño, El-Sayyid & Fox 2007) and selection for increased mobility in giant terrestrial insects favouring smaller males (Kelly, Bussière & Gwynne 2008).
Spiders are remarkable for showing by far the most extreme SSD among terrestrial animals as well as for lower correlation between male and female size than in most other animal taxa (Ghiselin 1974; Andersson 1994; Fairbairn 1997; Vollrath 1998). They are particularly noted for extraordinary cases of female-biased SSD. In some web-builders and crab spiders, males are miniscule compared to females (e.g. Head 1995; Vollrath 1998; Hormiga, Scharff & Coddington 2000; Foellmer & Moya-Laraño 2007). The evolution of such extreme SSD in spiders has long been debated, and yet remains controversial (Darwin 1871; Vollrath & Parker 1992; Coddington, Hormiga & Scharff 1997; Prenter, Montgomery & Elwood 1997; Prenter, Elwood & Montgomery 1998, 1999; Hormiga, Scharff & Coddington 2000; Moya-Laraño, Halaj & Wise 2002; Blanckenhorn 2005; Foellmer & Fairbairn 2005a, b; Moya-Laraño et al. 2007a, b;Brandt & Andrade 2007a, b; Foellmer & Moya-Laraño 2007). Phylogenetic and comparative analyses suggest selection for female fecundity as an important factor driving female-biased SSD in certain orb-web spiders (Coddington, Hormiga & Scharff 1997; Prenter, Elwood & Montgomery 1999; Hormiga, Scharff & Coddington 2000). However, in the absence of opposing selection, it is usually expected that genetic correlation between males and females would lead to increased male size in response to selection on females (Lande 1980; Reeve & Fairbairn 2001). It is unclear why males of sexually dimorphic spiders have remained small, and in some cases even decreased in size thereby increasing SSD (Prenter, Montgomery & Elwood 1997; Prenter, Elwood & Montgomery 1998; Walker & Rypstra 2003; Foellmer & Fairbairn 2004; Foellmer & Fairbairn 2005a, b;Foellmer & Moya-Laraño 2007), although survival and mating advantages have been proposed as potential explanations (Vollrath & Parker 1992; Schneider et al. 2000). Since extreme female-biased SSD is associated with animals that are typified by mobile, mate-searching adult males and more sedentary females, small male size has been repeatedly hypothesized to promote movement and dispersal in males (Ghiselin 1974; Vollrath & Parker 1992; Moya-Laraño, Halaj & Wise 2002).
Moya-Laraño, Halaj & Wise (2002) identified a pattern of greater female-biased SSD in spider species occupying vertical structures such as trees, compared to those inhabiting shrubs and the ground layer. To explain this they proposed a biomechanical model that states that size (or mass) is inversely proportional to the speed that males can achieve on vertical surfaces (i.e. a negative relationship between size and vertical speed). It suggests that small males should be favoured during mate searching in those species in which males are required to travel in a three-dimensional habitat and climb to high habitats to reach females. Furthermore, a recent revision suggests a curvilinear rather than the original linear relation between climbing speed and male size (Moya-Laraño et al. 2009). While referred to as the ‘gravity hypothesis’ its basic tenets relate directly to motility in mate searching, postulating that males with smaller body size are more mobile and have superior performance in a situation of scramble competition. Some unrelated studies provide logical support. For example, weight reduction through self-inflicted amputation of a reproductive organ (pedipalp) in males of the spider Tidarren sisyphoides is known to result in increased locomotor performance in terms of maximum speed and endurance (Ramos, Irschick & Christenson 2004).
The gravity hypothesis has recently been sharply disputed by Brandt & Andrade (2007a,b), and stoutly defended by its proposers (Moya-Laraño et al. 2007a). Brandt & Andrade (2007a) proposed an alternative motility-based explanation of the patterns of SSD observed by Moya-Laraño, Halaj & Wise (2002). They hypothesized that the trend of greater female-biased SSD in tree-dwelling spiders arose not through motility advantages for small climbing males in tree-dwelling spiders but instead through motility advantages for large males of low-dwelling spiders. Brandt & Andrade (2007a) supported their argument with evidence from a study on male black-widow spiders (Latrodectus hesperus) that live close to the ground. In this species, larger males ran faster than small males on horizontal surfaces over short distances, whereas there was no difference in the speed that large and small males climbed up short sticks.
In the present study, we investigate morphological determinants of vertical motility in male spiders over a standard distance on three different diameters of wooden dowels. We can, therefore, also evaluate the effect of substrate diameter on climbing performance in males. Substrate diameter is known to influence locomotor performance in other animals (e.g. Losos & Sinervo 1989; Losos & Irschick 1996) but this possibility appears not to have been studied previously in spiders. An interaction between climbing speed and dowel diameter would indicate the possibility of biases in previous studies that have employed single substrates of different diameter. We also examine horizontal motility in a standardized raceway. We examine two Australian species of orb-web spiders that exhibit high degrees of female-biased SSD but have different tendencies in the heights at which females build webs, and one Australian jumping spider species that is a cursorial hunter with relatively low SSD. Thus, we seek to evaluate whether and how vertical and horizontal motility in male spiders is influenced by their morphology in species with different degrees of SSD.