Abstract The extent to which decision-making processes are constrained in animals with small brains is poorly un- derstood. Arthropods have brains much smaller and sim- pler than those of birds and mammals. This raises ques- tions concerning limitations on how intricate the decision- making processes might be in arthropods. At Los Baños in the Philippines, Scytodes pallidus is a spitting spider that specialises in preying on jumping spiders, and Portia labiata is a jumping spider that preys on S. pallidus. Scy- todid spit comes from the mouth, and egg-carrying fe- males are less dangerous than eggless scytodids because the female uses her chelicerae to hold her eggs. Held eggs block her mouth, and she has to release them before she can spit. The Los Baños P. labiata sometimes adjusts its tactics depending on whether the scytodid encountered is carrying eggs or not. When pursuing eggless scytodids, the Los Baños P. labiata usually takes detour routes that enable it to close in from behind (away from the scyto- did’s line of fire). However, when pursuing egg-carrying scytodids, the Los Baños P. labiata sometimes takes faster direct routes to reach these safer prey. The Los Baños P. labiata apparently makes risk-related adjustments spe- cific to whether scytodids are carrying eggs, but P. labiata from Sagada in the Philippines (allopatric to Scytodes) fails to make comparable risk-related adjustments. Keywords Salticidae · Portia labiata · Scytodidae · Scytodes pallidus · Assessment Introduction Application of models from behavioural ecology to real animals will usually depend on an understanding of ani- mal cognition, with perceptual, information-processing, and decision-making abilities being especially relevant to understanding assessment strategies (Yoerg 1991; Belisle and Cresswell 1997; Kamil 1998; Dukas 1999; Shettle- worth 2001). Research on intricate and flexible assess- ment strategies in arthropods is important for clarifying how decision-making and other cognitive abilities might be limited by the size and complexity of the animal’s brain. During conflict with conspecific rivals, animals tend to rely on assessment strategies where the goal can be envis- aged as making decisions based on estimates of potential risks and payoffs (Maynard Smith and Price 1973; Parker 1974; Enquist et al. 1990). However, direct detection of risk may often be impossible, and animals usually rely in- stead on detection of cues that are correlated with par- ticular risks and payoffs (Huntingford and Turner 1987; Bouskila and Blumstein 1992). Assessment may also be important in predator–prey systems involving different species (Huntingford 1976; Curio 1978; Rowe and Owings 1978; Lloyd 1986; Lima and Dill 1990). Predators are obviously dangerous to prey, and examples are known where prey discriminate be- tween different species of predators (Leger and Owings 1978; Owings and Leger 1980; Seyfarth et al. 1980; Slo- bodchikoff et al. 1991; Cheney and Seyfarth 1988; Gyger et al. 1987; Greene and Meagher 1998), or even between particular individuals belonging to single species of predator. This includes discerning the likelihood that a predator will attack (Hammerstrom 1957; Buitron 1983; Pettifor 1990), intrinsic variation in how dangerous an in- dividual predator might be should it attack (Rowe and Owings 1978; Curio et al. 1983; Owings and Loughry 1985; Hennessy 1986; Helfman 1989; Loughry 1989; Foster and Ploch 1990; Walters 1990; Rowe and Owings 1996; Swaisgood et al. 1999a) and contextual factors such as where the encounter takes place and the prey’s repro- Robert R. Jackson · Simon D. Pollard · Daiqin Li · Natasha Fijn Interpopulation variation in the risk-related decisions of Portia labiata, an araneophagic jumping spider (Araneae, Salticidae), during predatory sequences with spitting spiders Anim Cogn (2002) 5 : 215–223 DOI 10.1007/s10071-002-0150-y Received: 23 November 2001 / Revised: 15 July 2002 / Accepted: 11 August 2002 / Published online: 4 September 2002 ORIGINAL ARTICLE R.R. Jackson · N. Fijn Department of Zoology, University of Canterbury, Private Bag 4800, Christchurch, New Zealand S.D. Pollard (✉) Canterbury Museum, Rolleston Avenue, Christchurch 8001, New Zealand e-mail: spollard@cantmus.govt.nz, Tel.: 64-3-3669429 ext. 815, Fax: 64-3-3665622 D. Li Department of Biological Sciences, National University of Singapore, Singapore 119260 © Springer-Verlag 2002