Taking it to extremes: what drives extreme web elongation in Australian ladder web spiders (Araneidae: Telaprocera maudae)?

Golden Orbweavers Ignore Biological Rules: Phylogenomic and Comparative Analyses Unravel a Complex Evolution of Sexual Size Dimorphism

Instances of sexual size dimorphism (SSD) provide the context for rigorous tests of biological rules of size evolution, such as Cope's rule (phyletic size increase), Rensch's rule (allometric patterns of male and female size), as well as male and female body size optima. In certain spider groups, such as the golden orbweavers (Nephilidae), extreme female-biased SSD (eSSD, female:male body length $\ge$2) is the norm. Nephilid genera construct webs of exaggerated proportions, which can be aerial, arboricolous, or intermediate (hybrid). First, we established the backbone phylogeny of Nephilidae using 367 anchored hybrid enrichment markers, then combined these data with classical markers for a reference species-level phylogeny. Second, we used the phylogeny to test Cope and Rensch's rules, sex specific size optima, and the coevolution of web size, type, and features with female and male body size and their ratio, SSD. Male, but not female, size increases significantly over time, and refutes Cope's rule. Allometric analyses reject the converse, Rensch's rule. Male and female body sizes are uncorrelated. Female size evolution is random, but males evolve toward an optimum size (3.2-4.9 mm). Overall, female body size correlates positively with absolute web size. However, intermediate sized females build the largest webs (of the hybrid type), giant female Nephila and Trichonephila build smaller webs (of the aerial type), and the smallest females build the smallest webs (of the arboricolous type). We propose taxonomic changes based on the criteria of clade age, monophyly and exclusivity, classification information content, and diagnosability. Spider families, as currently defined, tend to be between 37 million years old and 98 million years old, and Nephilidae is estimated at 133 Ma (97-146), thus deserving family status. We, therefore, resurrect the family Nephilidae Simon 1894 that contains Clitaetra Simon 1889, the Cretaceous GeratonephilaPoinar and Buckley (2012), Herennia Thorell 1877, IndoetraKuntner 2006, new rank, Nephila Leach 1815, Nephilengys L. Koch 1872, Nephilingis Kuntner 2013, Palaeonephila Wunderlich 2004 from Tertiary Baltic amber, and TrichonephilaDahl 1911, new rank. We propose the new clade Orbipurae to contain Araneidae Clerck 1757, Phonognathidae Simon 1894, new rank, and Nephilidae. Nephilid female gigantism is a phylogenetically ancient phenotype (over 100 Ma), as is eSSD, though their magnitudes vary by lineage.

How foresight might support the behavioral flexibility of arthropods

The small brains of insects and other invertebrates are often thought to constrain these animals to live entirely 'in the moment'. In this view, each one of their many seemingly hard-wired behavioral routines is triggered by a precisely defined environmental stimulus configuration, but there is no mental appreciation of the possible outcomes of one's actions, and therefore little flexibility. However, many studies show problem-solving behavior in various arthropod species that falls outside the range of fixed behavior routines. We propose that a basic form of foresight, the ability to predict the outcomes of one's own actions, is at the heart of such behavioral flexibility, and that the evolutionary roots of such outcome expectation are found in the need to disentangle sensory input that is predictable from self-generated motion versus input generated by changes in the outside world. Based on this, locusts, grasshoppers, dragonflies and flies seem to use internal models of the surrounding world to tailor their actions adaptively to predict the imminent future. Honeybees and orb-weaving spiders appear to act towards a desired outcome of their respective constructions, and the genetically pre-programmed routines that govern these constructions are subordinate to achieving the desired goal. Jumping spiders seem to preplan their route to prey suggesting they recognize the spatial challenge and actions necessary to obtain prey. Bumblebees and ants utilize objects not encountered in the wild as types of tools to solve problems in a manner that suggests an awareness of the desired outcome. Here we speculate that it may be simpler, in terms of the required evolutionary changes, computation and neural architecture, for arthropods to recognize their goal and predict the outcomes of their actions towards that goal, rather than having a large number of pre-programmed behaviors necessary to account for their observed behavioral flexibility.

Fitness consequences of plasticity in an extended phenotype

Like regular phenotypes, extended phenotypes have demonstrable fitness advantages and their properties may vary plastically across environments. However, the fitness advantages of plasticity are only known for a select few extended phenotypes. It is known that the form and functions of spider orb webs can be manipulated by laboratory experiments. For instance, the physical and chemical properties of the spiral and gluey silks vary in property as protein intake varies. Orb web spiders thus represent good models for extended phenotypic plasticity studies. We performed experiments manipulating the protein intake of two vertically aligned orb web building spiders to determine whether variations in the chemical and physical properties of their spiral and gluey silk affect prey retention in their webs. We found in both spider species that individuals deprived of protein had a greater gluey silk glycoprotein core volume, and this correlated strongly with spiral thread stickiness and increased prey retention by the webs. Moreover, we found strong positive correlations between glue droplet volume and glycoprotein core volume for spiders in the protein-deprived treatment, but weaker correlations for protein-fed spiders. We interpreted these findings as the spiders investing more in glycoprotein when nutrient deprived. We attribute the associated increase in prey retention capacity as a fitness consequence of plasticity in the spiral properties.

Modular patterns in behavioural evolution: webs derived from orbs

Imperfect knowledge of ancestral behaviour often hampers tracing behavioural evolution. This limitation is reduced in orb weaving spiders, because spider orb web construction behaviour and the cues used by modern orb-weavers are well-studied and highly conserved. Several species in orb-weaving families build non-orb webs that are clearly derived from orbs, allowing transitions from ancestral to modern behaviours to be described with high confidence. Three major patterns of general evolutionary significance were found in 69 phylogenetically independent transitions in 15 groups in 8 families: ancestral traits were often maintained as units; the most frequent of the eight different types of ancestral trait change was transfer of an ancestral behaviour to a new context; and ‘new’ traits that had no clear homology with ancestral traits were also common. Changes occurred in all major stages of orb construction. This may be the most extensive summary of evolutionary transitions in behaviour yet compiled.

Optimal foraging, not biogenetic law, predicts spider orb web allometry

The biogenetic law posits that the ontogeny of an organism recapitulates the pattern of evolutionary changes. Morphological evidence has offered some support for, but also considerable evidence against, the hypothesis. However, biogenetic law in behavior remains underexplored. As physical manifestation of behavior, spider webs offer an interesting model for the study of ontogenetic behavioral changes. In orb-weaving spiders, web symmetry often gets distorted through ontogeny, and these changes have been interpreted to reflect the biogenetic law. Here, we test the biogenetic law hypothesis against the alternative, the optimal foraging hypothesis, by studying the allometry in Leucauge venusta orb webs. These webs range in inclination from vertical through tilted to horizontal; biogenetic law predicts that allometry relates to ontogenetic stage, whereas optimal foraging predicts that allometry relates to gravity. Specifically, pronounced asymmetry should only be seen in vertical webs under optimal foraging theory. We show that, through ontogeny, vertical webs in L. venusta become more asymmetrical in contrast to tilted and horizontal webs. Biogenetic law thus cannot explain L. venusta web allometry, but our results instead support optimization of foraging area in response to spider size.

Optimal web investment in sub-optimal foraging conditions

Orb web spiders sit at the centre of their approximately circular webs when waiting for prey and so face many of the same challenges as central-place foragers. Prey value decreases with distance from the hub as a function of prey escape time. The further from the hub that prey are intercepted, the longer it takes a spider to reach them and the greater chance they have of escaping. Several species of orb web spiders build vertically elongated ladder-like orb webs against tree trunks, rather than circular orb webs in the open. As ladder web spiders invest disproportionately more web area further from the hub, it is expected they will experience reduced prey gain per unit area of web investment compared to spiders that build circular webs. We developed a model to investigate how building webs in the space-limited microhabitat on tree trunks influences the optimal size, shape and net prey gain of arboricolous ladder webs. The model suggests that as horizontal space becomes more limited, optimal web shape becomes more elongated, and optimal web area decreases. This change in web geometry results in decreased net prey gain compared to webs built without space constraints. However, when space is limited, spiders can achieve higher net prey gain compared to building typical circular webs in the same limited space. Our model shows how spiders optimise web investment in sub-optimal conditions and can be used to understand foraging investment trade-offs in other central-place foragers faced with constrained foraging arenas.

Mass predicts web asymmetry in Nephila spiders

The architecture of vertical aerial orb webs may be affected by spider size and gravity or by the available web space, in addition to phylogenetic and/or developmental factors. Vertical orb web asymmetry measured by hub displacement has been shown to increase in bigger and heavier spiders; however, previous studies have mostly focused on adult and subadult spiders or on several size classes with measured size parameters but no mass. Both estimations are suboptimal because (1) adult orb web spiders may not invest heavily in optimal web construction, whereas juveniles do; (2) size class/developmental stage is difficult to estimate in the field and is thus subjective, and (3) mass scales differently to size and is therefore more important in predicting aerial foraging success due to gravity. We studied vertical web asymmetry in a giant orb web spider, Nephila pilipes, across a wide range of size classes/developmental stages and tested the hypothesis that vertical web asymmetry (measured as hub displacement) is affected by gravity. On a sample of 100 webs, we found that hubs were more displaced in heavier and larger juveniles and that spider mass explained vertical web asymmetry better than other measures of spider size (carapace and leg lengths, developmental stage). Quantifying web shape via the ladder index suggested that, unlike in other nephilid taxa, growing Nephila orbs do not become vertically elongated. We conclude that the ontogenetic pattern of progressive vertical web asymmetry in Nephila can be explained by optimal foraging due to gravity, to which the opposing selective force may be high web-building costs in the lower orb. Recent literature finds little support for alternative explanations of ontogenetic orb web allometry such as the size limitation hypothesis and the biogenetic law.

High-performance spider webs: integrating biomechanics, ecology and behaviour

Spider silks exhibit remarkable properties, surpassing most natural and synthetic materials in both strength and toughness. Orb-web spider dragline silk is the focus of intense research by material scientists attempting to mimic these naturally produced fibres. However, biomechanical research on spider silks is often removed from the context of web ecology and spider foraging behaviour. Similarly, evolutionary and ecological research on spiders rarely considers the significance of silk properties. Here, we highlight the critical need to integrate biomechanical and ecological perspectives on spider silks to generate a better understanding of (i) how silk biomechanics and web architectures interacted to influence spider web evolution along different structural pathways, and (ii) how silks function in an ecological context, which may identify novel silk applications. An integrative, mechanistic approach to understanding silk and web function, as well as the selective pressures driving their evolution, will help uncover the potential impacts of environmental change and species invasions (of both spiders and prey) on spider success. Integrating these fields will also allow us to take advantage of the remarkable properties of spider silks, expanding the range of possible silk applications from single threads to two- and three-dimensional thread networks.