Colonial Web-building Spiders Research Articles

Kleptoparasites: a twofold cost of group living for the colonial spider, Metepeira incrassata (Araneae, Araneidae)

Several species of kleptoparasitic and araneophagic spiders (Araneae: Family Theridiidae, Subfamily Argyrodinae) are found in colonial webs of the orb-weaving spider Metepeira incrassata (Araneae, Araneidae) from Mexico, where they steal food and/or prey upon their spider hosts. Census data from natural M. incrassata colonies reveal that the incidence of these species increases with colony size. This pattern may reflect the presence of several other orb-weaving spiders, each with their own kleptoparasitic species, invading larger M. incrassata colonies. As the number of these associated spiders increases, so does the density and number of Argyrodinae species in M. incrassata colonies, suggesting that associated spiders might reduce their own kleptoparasite load by building their webs within M. incrassata colonies. This represents a twofold cost to M. incrassata, as a field enclosure experiment revealed that a primarily kleptoparasitic species (Argyrodes elevatus) may reduce prey available to their hosts, but a kleptoparasitic/araneophagic species (Neospintharus concisus) inflicts high mortality upon M. incrassata. However, the cost of kleptoparasitism and predation by these species may be offset in part for M. incrassata individuals in large colonies by certain defensive mechanisms inherent in groups, i.e., “attack-abatement” and “selfish herd” effects. We conclude that increased occurrence of kleptoparasitic and/or predatory Argyrodinae spiders is a consequence of colonial web building and is an important potential cost of group living for colonial web-building spiders.

Antipredator benefits of group living in colonial web-building spiders: the ‘early warning’ effect

The Mexican colonial web-building spider Metepeira incrassata is frequently attacked by predatory wasps from a number of families. Previous studies have shown that wasps often attack more than one spider during a ‘run’ on a colony, but capture success declines as colony size increases, and that spiders in the central core of the colony have lower risk (Rayor & Uetz 1990, Behavioral Ecology and Sociobiology, 27, 77–85; Uetz & Hieber 1994, Behavioral Ecology, 5, 326–333). We used data from direct observation of attacks and field experiments to test the hypothesis that colonial web-building spiders benefit from ‘early warning’ of predator approach through vibrations in the colony web. Analysis of 135 naturally occurring wasp attack ‘runs’ (attacks on 454 spiders) showed that the per-attack run and per-spider capture success of wasps decreased significantly with increased spider colony size. Spider defensive and evasive behaviours observed in a subset of these attacks varied with the attack sequence. Evasive responses by spiders were more frequent later in the attack, suggesting advance warning of predator approach. Experiments using a predator-simulating vibration source demonstrated that mean reaction distance of spiders increased with increasing colony size. Adult female spiders in core positions reacted at greater distances than those on the periphery, but immature spiders, whose capture risk is lower, showed no difference. Behaviour of spiders during simulated attacks was similar to observed encounters with wasps: evasive responses were more frequent and response latency was shorter in spiders attacked later in the sequence, and in many cases, spiders took evasive action prior to any contact with the stimulus. Additional experiments testing isolated cues (web contact, airborne vibration, web-borne vibration) suggest spiders respond to web-borne vibrations generated by predators and evasive behaviours of other spiders. Together, these results support the ‘early warning’ hypothesis of antipredator benefits for colonial web-building spiders.

Site fidelity and the costs of movement among territories: an example from colonial web-building spiders

Species differ in whether they exhibit strong site fidelity or move readily among sites. To understand the adaptive significance of site fidelity it is necessary to measure the cost of relocating to new sites, but this is very difficult with most species because it entails following individuals during the entire course of their transition. We studied the colonial web-building spider Metepeira incrassata (Araneae: Araneidae), which defends individual orb webs within a large, shared frame web. Colonies are heterogeneous habitats, with more prey but also more predators at the periphery of the colony relative to the central core. We measured relocation costs in this species by breaking them into the following components, (i) searching costs, (ii) establishment costs, and (iii) lost-opportunities costs, or the potential benefits of alternative behaviors that an animal might engage in if it were not relocating. We observed spiders that were in the process of establishing their orb webs at the start of the foraging day and conducted experiments to examine the costs of relocation by transferring spiders between locations within the colony. Transferred spiders had more fights with conspecifics than non-transferred spiders did, and the number of fights was positively correlated with the distance moved. However, searching and establishment costs seem unimportant, as individuals moving within their territories were as active as individuals moving to new sites. Transferred spiders faced greater lost-opportunities costs in the form of time spent on prey-capture orbs, as they significantly delayed orb construction. Lost-opportunities costs seem to be the most important component of relocation costs in this species and are likely to be generally important among species. This study is one of the few to document movement costs through direct observation.

Site fidelity and the costs of movement among territories: an example from colonial web-building spiders

Site fidelity and the costs of movement among territories: an example from colonial web-building spiders

A comparison of agonistic behaviour of colonial web-building spiders from desert and tropical habitats

The agonistic behaviour of two species of colonial spiders, Metepeira sp. a (atascadero), and M. incrassata, from desert and tropical habitats, respectively, in Mexico, were compared by observing induced interactions between immature and/or female spiders at natural web sites. Individuals of the desert species showed a greater tendency to escalate to potentially damaging behaviour, whereas the tropical species exhibited primarily low-intensity aggressive behaviour. In both species, greater body mass determined which spider gained possession of the web in unequally matched contests, whereas in equally matched contests, web residents won more often than did intruders. Desert spider conflicts were of significantly longer duration than tropical spider conflicts. The significance of these results for understanding the evolution of social behaviour in spiders is discussed.

Group size and predation risk in colonial web-building spiders: analysis of attack abatement mechanisms

Group size and predation risk in colonial web-building spiders: analysis of attack abatement mechanisms

Ontogenetic shifts within the selfish herd: predation risk and foraging trade-offs change with age in colonial web-building spiders.

We examine costs and benefits associated with spatial position relative to spider age (size) in colonial web-building spiders. Predator attack and capture rates vary with position in the colony, and suggest that risk is higher for the smallest and the largest spiders on the periphery, and lower in the central core of the colony. Foraging success is greater on the periphery for small and medium spiders but does not differ significantly with position for larger spiders. Decreased predation risk may be the reason why larger spiders aggressively seek and defend positions in the colony core, demonstrating a "selfish herd effect" (Rayor and Uetz, 1990). Smaller (immature) spiders, unable to compete for protected web positions in the core, must trade-off potentially higher risk of predation to take advantage of higher prey availability on the periphery. Increased foraging success on the periphery may allow juvenile spiders to achieve the larger size necessary to compete successfully for protected core positions as adults. Spatial variation in size-related fitness trade-offs between predation risk and foraging success may explain why colonies are dynamic entities - with individual spiders exhibiting ontogenetic shifts in web location as they grow larger and mature-accounting for the characteristic age (size) structure ofMetepeira incrassata colonies.

Trade-offs in foraging success and predation risk with spatial position in colonial spiders

Colonial web-building spiders respond to trade-offs between selective forces relative to spatial position within colonies and thus provide support for the selfish herd theory. The size distribution of spiders within colonies of Metepeira incrassata, a colonial orb-weaver (Araneae: Araneidae) from tropical Mexico is nonrandom; larger (mature) spiders and females guarding eggsacs are more prevalent in the center, whereas more small (immature) spiders are found on the periphery. Experimental field studies with spiders of selected size classes show that larger spiders actively and aggressively seek protected positions in the center of the colony webbing, even though prey availability and capture rates are significantly higher on the periphery. Attacks by predatory wasps, other spiders, and hummingbirds are more frequent on the periphery than in the core of the colony. Reproductive females on the periphery are at greater risk because they are captured more often than smaller spiders, and if their egg sacs consequently remain unguarded, chances of cocoon parasitism are increased. As a result, spiders in the core of the colony have greater reproductive success, producing more egg sacs with greater hatching frequency. Colonial spiders thus appear to be making a trade-off between foraging and protection from predation and show a spatial organization predicted by the selfish herd theory. The influence of such trade-offs on individual fitness and the structure of colonies is discussed.

Group forating in colonial web-building spiders

Colonial orb-weaving spiders from Mexico were studied to test predictions of risksensitive foraging theory: 1. group foraging increases prey capture/individual, and reduces prey variance; 2. spiders should be expected to exhibit risk-averse behavior (forage in groups) when the average level of prey exceeds individual needs, and exhibit risk-prone behavior (forage solitarily) when prey are searce. Laboratory and field studies show that group foraging increases capture efficiency and reduces variability in prey captured per spider. In desert/mesquite grassland habitat, where prey availability is low, M. atascadero forage solitarily in most cases. In tropical rainforest/agriculture sites, M. increassata forage in large colonies of thousands of webs. In intermediate habitats, M. spinipes forages solitarily or in groups, depending on prey availability. Over a range of sites with varying levels of prey, M. spinipes shifts from a risk-prone to a risk-averse group foraging strategy as prey increases. Group foraging behavior observed in colonial Metepeira fits the predictions of risk-sensitive foraging models. These findings explain why spiders tend to group webs together only in areas of superabundant prey. The role of risk-sensitivity in the evolution of coloniality in spiders is discussed.