Inter-relating characters in the evolution of jumping spider (Araneae, Salticidae) behaviour, with special reference to Trite planiceps Simon.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Jumping spiders (Salticidae) differ from other spiders by having exceptionally acute vision and relying primarily on visual cues to mediate their behaviour. The unusual vision dependent behaviours of salticids have become popular subjects of research based on 'the functional approach', a research paradigm that tends to treat each character as independent and emphasises natural selection as a mechanism explaining evolution. Using Trite planiceps, a New Zealand salticid, as a case study, this thesis examines how additional appreciation of the ways in which behaviour evolves may be gained when treating characters as inter-related and considering alternatives to natural selection. A series of specific examples investigating different types of inter-relationships between apparently unrelated behavioural characters of T. planiceps are presented and discussed. These examples consider (1) how change or stasis in one character may influence evolution of other characters (2) how a behaviour may incur costs in apparently unrelated contexts and (3) how trade-offs between different sources of fitness may influence the decision rules underlying flexible behaviours. Much of T. planiceps' behaviour appears to be moulded around its unusual microhabitat of rolled-up flax leaves. This species' nest structure, intraspecific communication, and ability to attack and catch intruders in the absence of visual cues are all shown to be unusual in ways that suggest adaptation for this habitat. Apparently, habitat selection has had 'run-on' effects on each of these activities. On the other hand, current stasis in microhabitat preference likely constrains the evolution of each character to its current state. Like other salticids, T. planiceps females guard their broods, protecting them from predators. This character is conservative family-wide, and likely constrains the evolution of other salticid characters to pathways that do not compromise this vital activity. Trite planiceps faces trade-offs between the anti-predator benefits of appendotomizing (discarding) Legs I and costs of diminished ability during the seemingly unrelated contexts of intrasexual contests, courtship and hunting. An increased tendency to appendotomize legs might increase chances of surviving encounters with enemies, but selection for such an increased tendency is likely held in check by these opposing costs. Trite planiceps is shown to have behavioural flexibility in contexts not previously reported for any salticid. Trite planiceps females make hunger-dependent decisions about whether to eat or guard their eggs and whether to ignore or associate with draglines of male conspecifics. They also make habitat-dependent decisions about whether to oviposit or delay oviposition. By matching behaviour with context, T. planiceps is able to expose behaviours to selection only when they are appropriate. Various currencies of success appear to be traded off against one another when choosing the most appropriate behaviour. For example, eggs are valuable both as progeny and as food, and which utility wins out will depend on the relative merit of the two. In summary, I review the examples outlined in the various chapters throughout this thesis and place each in the context of what is known about salticid behaviour generally. I then question whether the functional approach has been applied appropriately in studies of salticid behaviour and whether additional insights might be gained by adopting a more holistic paradigm.