The effects of morphology and substrate diameter on climbing and locomotor performance in male spiders

Allometry in desert ant locomotion (Cataglyphis albicans and Cataglyphis bicolor) - does body size matter?

Desert ants show a large range of adaptations to their habitats. They can reach extremely high running speeds, for example, to shorten heat stress during foraging trips. It has recently been examined how fast walking speeds are achieved in different desert ant species. It is intriguing in this context that some species exhibit distinct intraspecific size differences. We therefore performed a complete locomotion analysis over the entire size spectrum of the species Cataglyphis bicolor, and we compared this intraspecific dataset with that of the allometrically similar species Cataglyphis albicans. Emphasis was on the allometry of locomotion: we considered the body size of each animal and analysed the data in terms of relative walking speed. Body size was observed to affect walking parameters, gait patterns and phase relationships in terms of absolute walking speed. Unexpectedly, on a relative scale, all ants tended to show the same overall locomotion strategy at low walking speeds, and significant differences occurred only between C. albicans and C. bicolor at high walking speeds. Our analysis revealed that C. bicolor ants use the same overall strategy across all body sizes, with small ants reaching the highest walking speeds (up to 80 body lengths s-1) by increasing their stride length and incorporating aerial phases. By comparison, C. albicans reached high walking speeds mainly by a high synchrony of leg movement, lower swing phase duration and higher stride frequency ranging up to 40 Hz.

Locomotion and kinematics of arachnids

A basic feature of animals is the capability to move and disperse. Arachnids are one of the oldest lineages of terrestrial animals and characterized by an octopodal locomotor apparatus with hydraulic limb extension. Their locomotion repertoire includes running, climbing, jumping, but also swimming, diving, abseiling, rolling, gliding and -passively- even flying. Studying the unique locomotor functions and movement ecology of arachnids is important for an integrative understanding of the ecology and evolution of this diverse and ubiquitous animal group. Beyond biology, arachnid locomotion is inspiring robotic engineers. The aim of this special issue is to display the state of the interdisciplinary research on arachnid locomotion, linking physiology and biomechanics with ecology, ethology and evolutionary biology. It comprises five reviews and ten original research reports covering diverse topics, ranging from the neurophysiology of arachnid movement, the allometry and sexual dimorphism of running kinematics, the effect of autotomy or heavy body parts on locomotor efficiency, and the evolution of silk-spinning choreography, to the biophysics of ballooning and ballistic webs. This closes a significant gap in the literature on animal biomechanics.

Peculiar relationships among morphology, burrowing performance and sand type in two fossorial microteiid lizards

Associations among ecology, morphology and locomotor performance have been intensively investigated in several vertebrate lineages. Knowledge on how phenotypes evolve in natural environments likely benefits from identification of circumstances that might expand current ecomorphological equations. In this study, we used two species of Calyptommatus lizards from Brazilian Caatingas to evaluate if specific soil properties favor burrowing performance. As a derived prediction, we expected that functional associations would be easily detectable at the sand condition that favors low-resistance burrowing. We collected two endemic lizards and soil samples in their respective localities, obtained morphological data and recorded performance of both species in different sand types. As a result, the two species burrowed faster at the fine and homogeneous sand, the only condition where we detected functional associations between morphology and locomotion. In this sand type, lizards from both Calyptommatus species that have higher trunks and more concave heads were the ones that burrowed faster, and these phenotypic traits did not morphologically discriminate the two Calyptommatus populations studied. We discuss that integrative approaches comprising manipulation of environmental conditions clearly contribute to elucidate processes underlying phenotypic evolution in fossorial lineages.

Attachment performance of stick insects (Phasmatodea) on convex substrates

Phasmatodea (stick and leaf insects) are herbivorous insects well camouflaged on plant substrates as a result of cryptic masquerade. Also, their close association with plants has allowed them to adapt to different substrate geometries and surface topographies of the plants they imitate. Stick insects are gaining increasing attention in attachment- and locomotion-focused research. However, most studies experimentally investigating stick insect attachment have been performed either on single attachment pads or on flat surfaces. In contrast, curved surfaces, especially twigs or stems of plants, are dominant substrates for phytophagous insects, but not much is known about the influence of curvature on their attachment. In this study, by combining analysis of tarsal usage with mechanical traction and pull-off force measurements, we investigated the attachment performance on curved substrates with different diameters in two species of stick insects with different tarsal lengths. We provide the first quantitative data for forces generated by stick insects on convex curved substrates and show that the curvature significantly influences attachment ability in both species. Within the studied range of substrate curvatures, traction force decreases and pull-off force increases with increasing curvature. Shorter tarsi demonstrate reduced forces; however, tarsus length only has an influence for diameters thinner than the tarsal length. The attachment force generally depends on the number of tarsi/tarsomeres in contact, tarsus/leg orientation and body posture on the surface. Pull-off force is also influenced by the tibiotarsal angle, with higher pull-off force for lower angles, while traction force is mainly influenced by load, i.e. adduction force.

Male black widows parasitize mate-searching effort of rivals to find females faster

Mate-searching success is a critical precursor to mating, but there is a dearth of research on traits and tactics that confer a competitive advantage in finding potential mates. Theory and available empirical evidence suggest that males locate mates using mate-attraction signals produced by receptive females (personal information) and avoid inadvertently produced cues from rival males (social information) that indicate a female has probably already mated. Here, we show that western black widow males use both kinds of information to find females efficiently, parasitizing the searching effort of rivals in a way that guarantees competition over mating after reaching a female's web. This tactic may be adaptive because female receptivity is transient, and we show that (i) mate searching is risky (88% mortality) and (ii) a strongly male-biased operational sex ratio (from 1.2 : 1 to more than 10 : 1) makes competition inevitable. Males with access to rivals' silk trails moved at higher speeds than those with only personal information, and located females even when personal information was unreliable or absent. We show that following rivals can increase the potential for sexual selection on females as well as males and argue it may be more widespread in nature than is currently realized.

Metabolic and behavioral responses of predators to prey nutrient content

Predators feed on a diversity of prey that can vary widely in nutrient content. While prey nutrient content is known to have important consequences for life history traits, less is known about how it affects physiology and behavior. The purpose of this study was to test how diet affected the physiology and behavior of the wolf spider Hogna carolinensis. We hypothesized that higher protein intake would result in a lower metabolic rate due to less energy intake. Further, we also expected the high protein group to exhibit increased activity levels and aggression in an attempt to increase energy intake. Spiders were maintained on three different treatment diets in order to simulate prey with differing macronutrient composition: high protein, intermediate, and high lipid. Spider respiration was measured to quantify the baseline metabolic rate (SMR), digestive metabolic rate (SDA), and active metabolic rate (AMR). We found no significant effect of diet on metabolic rates. However, the SDA coefficient (i.e. digestive cost relative to prey content) was higher in the high protein group, meaning that this group metabolized a greater portion of their prey during digestion and had a lower net energy intake from prey. In our behavioral assays, spiders in the high protein group were significantly more active and attacked prey more quickly in their first trial. Our results demonstrate that diet had relatively little effect on predator metabolism but more of an effect on behavior. These findings suggest that diet regulation should be analyzed by studying multiple responses together, including metabolism and behavior, to gain a more comprehensive understanding of the effects of diet on organism performance and fitness.

Small size does not confer male agility advantages in a sexually-size dimorphic spider

Selection pressures leading to extreme, female-biased sexual size dimorphism (SSD) in spiders continue to be debated. It has been proposed that males of sexually size dimorphic spiders could be small because gravity constrains adult agility (locomotor abilities). Accordingly, small males should achieve higher vertical climbing speeds and should be more prone to bridge. The curvilinear model of the gravity hypothesis predicts a negative relationship between vertical climbing speed and male body size only over a threshold of 7.6 mm, 42.5 mg. Because males of most species with extreme SSD fall well below this threshold, the relationship between male size and agility at this scale remains vague. Here, we tested three hypotheses on how male size, mass and age (after maturation) relate to vertical climbing and bridging ability in Nephilingis cruentata, a highly sexually dimorphic orb-weaver with males well below the size threshold. We placed males of different sizes and adult ages in a vertical platform and recorded their climbing speeds. Contrary to the original study testing male bridging ability as binary variable, we measured the duration of the crossing of the bridging thread, as well as its sagging distance. Male body size and mass positively related to the vertical climbing speed and to the distance of the sagging thread during bridging, but had no influence on the bridging duration. The detected positive correlation between male size/mass and vertical climbing speed goes against our first prediction, that small males would have vertical climbing advantage in Nephilingis cruentata, but agrees with the curvilinear model. Against our second prediction, small males were not faster during bridging. Finally, in agreement with our third prediction, threads sagged more in heavier males. These results suggest that small male size confers no agility advantages in Nephilingis cruentata.

Major ampullate silk gland transcriptomes and fibre proteomes of the golden orb-weavers, Nephila plumipes and Nephila pilipes (Araneae: Nephilidae)

Natural spider silk is one of the world’s toughest proteinaceous materials, yet a truly biomimetic spider silk is elusive even after several decades of intense focus. In this study, Next-Generation Sequencing was utilised to produce transcriptomes of the major ampullate gland of two Australian golden orb-weavers, Nephila plumipes and Nephila pilipes, in order to identify highly expressed predicted proteins that may co-factor in the construction of the final polymer. Furthermore, proteomics was performed by liquid chromatography tandem-mass spectroscopy to analyse the natural solid silk fibre of each species to confirm highly expressed predicted proteins within the silk gland are present in the final silk product. We assembled the silk gland transcriptomes of N. plumipes and N. pilipes into 69,812 and 70,123 contigs, respectively. Gene expression analysis revealed that silk gene sequences were among the most highly expressed and we were able to procure silk sequences from both species in excess of 1,300 amino acids. However, some of the genes with the highest expression values were not able to be identified from our proteomic analysis. Proteome analysis of “reeled” silk fibres of N. plumipes and N. pilipes revealed 29 and 18 proteins, respectively, most of which were identified as silk fibre proteins. This study is the first silk gland specific transcriptome and proteome analysis for these species and will assist in the future development of a biomimetic spider silk.

Interactive effects of leg autotomy and incline on locomotor performance and kinematics of the cellar spider, Pholcus manueli

Leg autotomy can be a very effective strategy for escaping a predation attempt in many animals. In spiders, autotomy can be very common (5–40% of individuals can be missing legs) and has been shown to reduce locomotor speeds, which, in turn, can reduce the ability to find food, mates, and suitable habitat. Previous work on spiders has focused mostly on the influence of limb loss on horizontal movements. However, limb loss can have differential effects on locomotion on the nonhorizontal substrates often utilized by many species of spiders. We examined the effects of leg autotomy on maximal speed and kinematics while moving on horizontal, 45° inclines, and vertical (90°) inclines in the cellar spider Pholcus manueli, a widespread species that is a denizen of both natural and anthropogenic, three‐dimensional microhabitats, which frequently exhibits autotomy in nature. Maximal speeds and kinematic variables were measured in all spiders, which were run on all three experimental inclines twice. First, all spiders were run at all inclines prior to autotomization. Second, half of the spiders had one of the front legs removed, while the other half was left intact before all individuals were run a second time on all inclines. Speeds decreased with increasing incline and following autotomy at all inclines. Autotomized spiders exhibited a larger decrease in speed when moving horizontally compared to on inclines. Stride length decreased at 90° but not after autotomy. Stride cycle time and duty factor increased after autotomy, but not when moving uphill. Results show that both incline and leg autotomy reduce speed with differential effects on kinematics with increasing incline reducing stride length, but not stride cycle time or duty factor, and vice versa for leg autotomy. The lack of a significant influence on a kinematic variable could be evidence for partial compensation to mitigate speed reduction.

Energetics, scaling and sexual size dimorphism of spiders

The extreme sexual size dimorphism in spiders has motivated studies for many years. In many species the male can be very small relative to the female. There are several hypotheses trying to explain this fact, most of them emphasizing the role of energy in determining spider size. The aim of this paper is to review the role of energy in sexual size dimorphism of spiders, even for those spiders that do not necessarily live in high foliage, using physical and allometric principles. Here we propose that the cost of transport or equivalently energy expenditure and the speed are traits under selection pressure in male spiders, favoring those of smaller size to reduce travel costs. The morphology of the spiders responds to these selective forces depending upon the lifestyle of the spiders. Climbing and bridging spiders must overcome the force of gravity. If bridging allows faster dispersal, small males would have a selective advantage by enjoying more mating opportunities. In wandering spiders with low population density and as a consequence few male-male interactions, high speed and low energy expenditure or cost of transport should be favored by natural selection. Pendulum mechanics show the advantages of long legs in spiders and their relationship with high speed, even in climbing and bridging spiders. Thus small size, compensated by long legs should be the expected morphology for a fast and mobile male spider.

Meek males and fighting females: sexually-dimorphic antipredator behavior and locomotor performance is explained by morphology in bark scorpions (Centruroides vittatus)

Sexual dimorphism can result from sexual or ecological selective pressures, but the importance of alternative reproductive roles and trait compensation in generating phenotypic differences between the sexes is poorly understood. We evaluated morphological and behavioral sexual dimorphism in striped bark scorpions (Centruroides vittatus). We propose that reproductive roles have driven sexually dimorphic body mass in this species which produces sex differences in locomotor performance. Poor locomotor performance in the females (due to the burden of being gravid) favors compensatory aggression as part of an alternative defensive strategy, while male morphology is coadapted to support a sprinting-based defensive strategy. We tested the effects of sex and morphology on stinging and sprinting performance and characterized overall differences between the sexes in aggressiveness towards simulated threats. Greater body mass was associated with higher sting rates and slower sprinting within sexes, which explained the greater aggression of females (the heavier sex) and, along with longer legs in males, the improved sprint performance in males. These findings suggest females are aggressive to compensate for locomotor costs of reproduction while males possess longer legs to enhance sprinting for predator evasion and mate finding. Sexual dimorphism in the metasoma ("tail") was unrelated to stinging and sprinting performance and may best be explained by sexual selection.

Repeatability of escape response performance in the queen scallop (Aequipecten opercularis)

In order for natural selection to operate, physiological and behavioural traits must exhibit both inter-individual variability and intra-individual consistency (i.e. repeatability) in performance. In this study, we describe individual variation and temporal repeatability in the escape responses of the queen scallop Aequipecten opercularis and determine whether individuals exhibited consistently high or low rankings in different aspects of the escape response. Five measures of individual performance were recorded on 4 occasions (days 0, 2, 7 and 28) providing proxies for sensory acuity (response latency), immediate and sustained swimming performance (burst and average clap-rates) and swimming endurance (total number of claps and total time spent clapping). All components of the escape response exhibited significant inter-individual variability (all P<0.0001). Escape response latency, burst clap-rate, total number of claps and total duration spent clapping maintained significant repeatability over 28 days (all P<0.016). Average clap-rate was repeatable in the short term (2 days, P<0.0001) but repeatability declined by 28 days (P=0.097). Concordance analysis indicated that individuals maintained the same performance rankings over time for each component of the escape response (all P<0.001). In addition, some individuals ranked as consistently high or low performers across response latency, burst and average clap-rate and total number of claps. An individual's ability to evade predators through the provision of an escape response of an appropriate magnitude, subject to physiological, behavioural and organismal constraints, will have clear fitness-related consequences.

Terrestrial locomotion in arachnids

In this review, we assess the current state of knowledge on terrestrial locomotion in Arachnida. Arachnids represent a single diverse (>100,000 species) clade containing well-defined subgroups (at both the order and subordinal levels) that vary morphologically around a basic body plan, yet exhibit highly disparate limb usage, running performance, and tarsal attachment mechanisms. Spiders (Araneae), scorpions (Scorpiones), and harvestmen (Opiliones) have received the most attention in the literature, while some orders have never been subject to rigorous mechanical characterization. Most well-characterized taxa move with gaits analogous to the alternating tripod gaits that characterize fast-moving Insecta - alternating tetrapods or alternating tripods (when one pair of legs is lifted from the ground for some other function). However, between taxa, there is considerable variation in the regularity of phasing between legs. Both large and small spiders appear to show a large amount of variation in the distribution of foot-ground contact, even between consecutive step-cycles of a single run. Mechanisms for attachment to vertical surfaces also vary, and may depend on tufts of adhesive hairs, fluid adhesives, silks, or a combination of these. We conclude that Arachnida, particularly with improvements in microelectronic force sensing technology, can serve as a powerful study system for understanding the kinematics, dynamics, and ecological correlates of sprawled-posture locomotion.

Functional relations between locomotor performance traits in spiders and implications for evolutionary hypotheses

BACKGROUND

Locomotor performance in ecologically relevant activities is often linked to individual fitness. Recent controversy over evolution of extreme sexual size dimorphism (SSD) in spiders centres on the relationship between size and locomotor capacity in males. Advantages for large males running over horizontal surfaces and small males climbing vertically have been proposed. Models have implicitly treated running and climbing as functionally distinct activities and failed to consider the possibility that they reflect common underlying capacities.

FINDINGS

We examine the relationship between maximum climbing and running performance in males of three spider species. Maximum running and climbing speeds were positively related in two orb-web spiders with high SSD (Argiope keyserlingi and Nephila plumipes), indicating that for these species assays of running and climbing largely reveal the same underlying capacities. Running and climbing speeds were not related in a jumping spider with low SSD (Jacksonoides queenslandica). We found no evidence of a performance trade-off between these activities.

CONCLUSIONS

In the web-spiders A. keyserlingi and N. plumipes good runners were also good climbers. This indicates that climbing and running largely represent a single locomotor performance characteristic in these spiders, but this was not the case for the jumping spider J. queenslandica. There was no evidence of a trade-off between maximum running and climbing speeds in these spiders. We highlight the need to establish the relationship between apparently disparate locomotor activities when testing alternative hypotheses that yield predictions about different locomotor activities. Analysis of slopes suggests greater potential for an evolutionary response on performance in the horizontal compared to vertical context in these spiders.