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Nectary feeding and guarding behavior by a tropical jumping spider

2017; Wiley; Volume: 15; Issue: 8 Linguagem: Inglês

10.1002/fee.1538

1540-9309

Christina J. Painting, Caleb C Nicholson, Matthew Bulbert, Yusoff Norma‐Rashid, Daiqin Li,

Insect and Arachnid Ecology and Behavior

If you were asked to imagine a spider feeding, you would probably conjure up gruesome images of fangs, venom, and large, sticky webs. Spiders are generally considered to be carnivores (Foelix 2011), but there is growing evidence that some species supplement their diet with plant-derived products, and there are even vegetarian spiders. The jumping spider Bagheera kiplingi feeds almost exclusively on Beltian bodies, the nutrient-enriched, detachable leaf tips of African acacia trees (Meehan et al. 2009). More commonly, however, spiders feed on nectar from flowers or extrafloral nectaries (Nyffeler et al. 2016>). Although nectivory in spiders can increase their longevity and reproductive output, it comes with an inherent risk. Extrafloral nectaries are primarily exploited by ants, which defend the plant against intruders and reduce herbivory in return for food rewards (Heil 2015). Other invertebrates forage opportunistically on the nectaries when the aggressive ants are not present. Here, we report observations that suggest the jumping spider Orsima ichneumon guards extrafloral nectaries through active confrontation with ants and by depositing silk barriers to inhibit their competitors. In November 2015, we took a field trip to the Gombak Field Station owned by the University of Malaya and located in Selangor, a state in the hills above Kuala Lumpur. We intended to collect specimens of the spectacularly colored and charismatic jumping spider O ichneumon (Figure 1). Our study aimed to address the controversial suggestion that the body shape and movement of O ichneumon make it an ant mimic (Reiskind 1976), which is at odds with its kaleidoscope of colors – a trait not usually associated with ants. Unfortunately, our trip quite literally met a roadblock after a large landslide wiped out the road leading to the field station. Although it was tempting to put away the collecting jars, don the binoculars, and head off bird watching in the Genting Highlands, we decided to take a closer look at the roadside vegetation, in case this proved to be useful habitat for our jumping spider. It was not long before we found several O ichneumon individuals on the leaves of a Clerodendrum villosum tree, a common forest edge species in Southeast Asia. The tree was also abundantly covered with tree-dwelling ants, including Tetraponera nigra, Crematogaster spp, and multiple species of Polyrhachis, all of which were foraging on extrafloral nectaries. While observing a female O ichneumon, we noticed that she buried her mouthparts into the extrafloral nectaries on the upper surface of the leaves, presumably feeding on the nectar produced by the plant. Although the nectivory itself was not particularly unexpected, her subsequent behavior took us by surprise. After feeding on the nectary, the spider turned around so that her abdomen pointed toward the food source, and then proceeded to wave her abdomen from side to side, apparently depositing silk around it (Figure 2a; WebVideo 1). Closer inspection revealed distinctive patches of silk around each of the nectaries that the spider had been feeding at (Figure 2b). We subsequently observed two other individuals (one male, one female) also feeding and then silk-spinning in this manner on the same tree. We also documented the same behavior in an adult male on a C villosum tree at Upper Peirce Reservoir in Singapore. Our preliminary observations suggest O ichneumon guard the nectaries from ants that are competing for access. The spiders each patrolled a single leaf surface, repeatedly feeding from the same nectaries, followed by abdomen waving. They chased away ants of similar or smaller size than themselves (WebVideo 2). When larger ants approached, the spider modified its position according to the ant's movements by shifting to the edge or underside of the leaf and waiting for the ant to finish feeding on the nectaries that the spider had previously guarded and deposited silk over, before returning to its original position (WebVideo 3). On one occasion we detected an ant stumbling as it approaching a nectary that had already been visited by the spider; the ant abandoned its foraging path and left the leaf surface that the spider was guarding. We did not witness any insects being snared by the silk patches. These observations suggest the silk acts as a deterrent or an impediment rather than as a web to trap the ants as prey. Impeding the ants may result in reduced foraging competition for the spiders, although we would need to test this hypothesis further. We have also seen Siler semiglaucus, a closely related jumping spider, feeding on nectaries of the same tree species. They did not display any silk-depositing activity, suggesting O ichneumon's behavior is potentially unique. Although the notion of a spider guarding nectaries and barricading them against competitors is enticing, there are other possible reasons for O ichneumon to deposit silk around the nectaries. Jumping spiders use silk draglines like miniature bungee cords that allow for safe maneuvering through their habitat (Blackledge 2012). Chemical cues distributed on draglines can also provide spatial and temporal information about conspecifics of the same sex, potential mates, or food. The silk may therefore provide the spider with the location of the nectaries as well as indicate the relative time since each nectary has been drained. Nectar is a replenishing resource that, once depleted, requires time to be refilled by the plant. Nectar-feeding birds, for instance, maximize their foraging activities by not revisiting flowers before they are replenished (Burke and Fulham 2003). For O ichneumon, the silk could provide similar information, thereby enhancing foraging efficiency and maximizing nutritional return from each nectary visit. However, the silk-depositing behavior seems more complicated than necessary if it only provides information about a nectary's location. Furthermore, the behavior is distinctly different from anything we are aware of being reported before. Of course, the silk could have a dual purpose – both protecting the nectaries from competitors and providing information about foraging intensity. O ichneumon's seemingly substantial investment in protecting the nectaries could simply be explained by the fact that it is an obligate nectivore. However, we have observed O ichneumon attacking flies and other arthropods in the field, and they flourish on live prey in the lab. Spiders usually feed on plants when they are food-limited in some way, such as during critical development periods for juveniles or during egg production in females (Sanders 2013; Nyffeler et al. 2016). Given the presence of hostile ants on the plants, O ichneumon may suffer from a shortage of insect prey. This could explain why nectivory is required but it does not explain why the spiders should spend time guarding the nectaries when they could move to a plant with fewer ants and more prey. The possible costs of reduced prey availability may be offset by the protective benefits of reduced predation that they receive through their close association with the ants, especially if they are ant mimics. Furthermore, the fitness benefits of staying put and guarding energy-rich nectaries might outweigh the energetic costs of producing silk deposits and warding off competitors. Moving between plants in search of food may increase the risk of predation, and will require energy to search for other nectary-rich plants that may be sparsely distributed in the landscape. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.