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THE ROLE OF CHEMICAL CUES , IN THE PREDATORY AND ANTI-PREDATORY BEHAVIOUR OF JU~PING SPIDERS (ARANEAE, SAL TICIDAE) A Thesis submitted in fulfilment of the requirements of the degree of Doctor of Philosophy in, Zoology at the University of Canterbury by Robert John Clark 2000 CONTENTS Abstract 1· Chapter 1: Introduction 3 Chapter 2: Theoretical background 9 Chapter 3: Chemical cues elicit prey capture in P. fimbriata 56 Chapter 4: Web use during predatory encounters between P. fimbriata, an araneophagic 91 jumping spider, and its preferred prey, other jumping spiders Chapter 5: Speculative hunting by an araneophagic jumping spider 108 Chapter 6: Chemical cues from ants influence predatory behaviour in Habrocestum pulex 125 (Hentz), an ant eating jumping spider (Araneae, Salticidae) Chapter 7: Reactions of Habrocestum pulex, a myrmecophagic salticid, to potential 147 kairomones from ants Chapter 8: Dragllnes and assessment of fighting ability in cannibalistic jumping spiders 160 Chapter 9: Relationship between violent aggression in saltlcids and use of pheromones to 178 obtain information on conspeclfics Chapter10: Discussion 189 Acknowledgements 198 References 199 2 9 MAR 2000 1 ABSTRACT The role of chemical cues in prey-capture behaviour is studied in jumping spiders (Salticldae). Prior to this study, little attention has been given to how chemical cues influence the predatory behaviour of these spiders with complex eyes and visual acuity unrivalled In any other animals of comparable size. Three categories of predation are considered: salticids preying on conspecifics (cannibalism), salticids preying on non-conspecific spiders (araneophagy) and salticids preying on ants (myrmecophagy). Primary study animals are Portia spp. and Habrocestum pulex. Portia spp. and Habrocestum pulex are known to prefer spiders and ants, respectively, as prey, and each uses specialised prey-capture behaviour against its prefered prey. Here the predatory behaviour of these salticids is shown to be influenced in a variety of ways by chemical cues from prey. A general conclusion is suggested: that reliance on chemical cues is especially pronounced In predators that specialise on particularly dangerous prey. In Queensland, Portia fimbriata preys on other genera of salticids, with Jacksonoides gueenslandicus being the dominant salticid prey species taken. Besides actively stalking J. gueenslandlcus in the open, E. fimbriata also launches attacks from webs and details of how E. fimbriatg uses Its web against J. gueenslandicus are investigated. Contact and olfactory chemical cues from J. gueenslandicys are shown to have three distinct effects on the predatory behaviour . of Queensland Portia fimbriata: (1) attracting E. fimbriata to, or indUCing E. fimbriata to remain in, areas where there are cues from J. gueenslandicus; (2) changing E. fimbriata's behaviour in ways that facilitate prey capture; (3) heightening E. fimbriata's attention to optical cues from J. gueenslandicus. No evidence was found that any other prey species has comparable influences on E. fimbrlata. 2 Undirected leaping (erratic leaping with no target being evident) is one of the Queensland E. fimbriata's responses to chemical cues from ,4. gueenslandicus. That tllis behaviour functions as hunting by speculation is investigated. Experiments show that undirected leaping induces ,4. gueenslandicus to move and thereby reveal its location to E. fimbriata. Intraspecific conflict in Sri Lankan Portia labiata is particularly violent, often ending in cannibalism. Using size matched conspecifics, two types of testing show that females of this species and population of Portia discriminate between conspecifics on the basis of fighting ability. Other Portia, and other salticid genera, were tested as well, but none of these are as prone to violent aggression and cannibalism. There was no evidence for recognition of fighting ability in any salticid other than Sri Lankan E. labiata. Habrocestum pulex is shown to rely on chemical cues from ants. Chemical cues from ants induce .!::f. pulex to: (1) remain on soil which has previously housed ants; (2) enter an experimental arm of a V-shaped olfactometer more often if it contains air from a cage with ants, or if it contains 6-methYI-5-hepten-2-one (an ant alarm pheromone); (3) change behaviour in ways that facilitate ant capture; (4) enhance attention to optical cues from ants. 3 CHAPTER 1: INTRODUCTION When animals evolve specialisation for particular tasks this may be at the cost of limiting their proficiency at performing other tasks. This is the hypothesis of adaptive tradeoffs, an idea that has had a predominant place in much of the ecological, ethological and evolutionary literature (e.g., Levins, 1968, Dukas & Real, 1993), although it has not always been stated explicitly. Sometimes its status has perhaps appeared to be more like an assumption than a hypothesis. Morphology may provide the most clear cut examples of adaptive tradeoffs. To take an example from spiders, Myrmarachne plateoides D.P. Cambridge is a salticid spider with pronounced sexual dimorphism. The males of this species have exceptionally long fangs, presumably a consequence of sexual selection for display ornamentation (Pollard, 1994). The usual function of fangs, however, is for injecting venom into prey. Myrmarachne plateoides females have fangs which are much shorter than those of males, and females have functional fangs. That is, the female has complete ducts connecting the tip of the fangs to the venom gla.nds in the female's body. However, the male's fangs are ductless. It appears that, as a consequence of sexual selection, the male's fangs have become too long and thin for functiona.l ducts to be feasible. Complete ducts, even if present, would be unlikely to function because of the mechanical problems associated with forcing venom through a long slender tube. It appears that, in evolution, the male has traded off prey capture efficiency when fangs became specialised to function in display. Unlike morphology, the degree to which adaptive tradeoffs might apply to behaviour is not so clear. Both behaviour and morphology are part of an animal's phenotype, but there are 4 important differences related to the time frames over which changes are possible for each. An animal's morphology as more or less fixed for extended periods. In contrast there is greater potential for rapid switching in behaviour. For example, birds may rapidly change feeding strategies depending on prey types encountered but do not grow alternative beak types each time a different food type is encountered. Predatory versatility, which is known for a wide range of animals (Curio, 1976), appears to illustrate how behaviour differs from morphology. The versatile predator has a conditional behavioural strategy, consisting of a repertoire of different tactics specialised for particular prey. The potential for repertoires of predatory morphology appear to be vastly more limited because of the time frame required for most morphological transformations, suggesting that the idea of adaptive tradeoffs may not apply in the same way, or to the same degree, to behaviour and morphology. There is an alternative way to envisage limitations on behaviour repertoire size. Adaptive tradeoffs in the evolution of behaviour might derive from cognitive limitations. That is, a versatile predator must have a nervous system that can sort and organise the use of the various tactics in its repertoire, and we can expect limits on what nervous systems can do. Cognitive ability might be limited by the size of an animal's nervous system (Staddon, 1983; Dukas & Ellner 1993; Wehner, 1997). Perhaps versatile predators with smaller nervous systems are constrained to have repertoires of tactics which are smaller than the repertoires possible for predators with much larger nervous systems. It is widely accepted that there is a distinction between the cognitive abilities of vertebrates and invertebrates. Vertebrates tend to be large animals, whereas most invertebrates are considerably smaller. As nerve cells cannot be shrunk indefinitely, smaller animals must suffer limitations in how many neurones are available for the control of behaviour. Cephalopod 5 molluscs are invertebrates with especially large brains, and it is in the octopus and its relatives that we expect to 'find invertebrates with especially complex, flexible bellaviour. Arthropods, on the other hand, tend to have brains that are orders of magnitude smaller than the brains of better known vertebrates, including all mammals, and it might be in the arthropods that we would expect to find especially clear evidence of adaptive tradeoffs in the evolution of behaviour. Recent studies suggest that the limitations set by brain size might have been overestimated in arthropods. Perhaps the most challenging examples come from araneophagic (Le, spider-eating) spiders in the salticid genus Portia. The species in this genus have complex, 'flexible predatory strategies used for catching and feeding on a wide array of different types of prey including web-building spiders, spiders that do not build webs, spider eggs, and insects. Web-building spiders are not simply stalked or chased down. Instead, aggressive-mimicry signals are made to deceive and manipulate the behaviour of the victim. Different aggressive mimicry tactics are used against different kinds of web-building spiders, and Portia takes on a very wide range of web-building species. It is far from clear how Portia's predatory strategy