ALBERT

Notes: Summary Game-theoretic analyses were completed on the territorial contest behavior of two populations of a desert spider that exhibit markedly different levels of within-species competition. Numerical payoff matrices were constructed from field data collected on the behavior and demography of each population. Payoffs were expressed in terms of expected future egg production. Three behavior patterns that a spider might exhibit following assessment of its weight relative to that of its opponent and the value of the site were considered: withdraw, display, or escalate. The model predicts for the more harsh grassland habitat an evolutionarily stable strategy (ESS) that makes ownership decisive in settling contests between opponents with small weight differences, whereas it otherwise assigns victory to the heavier opponent. Whereas the empirical data collected for this grassland population closely approximates the predicted ESS, that for a population occupying a more favorable riparian habitat deviates significantly. The ESS prediction for this latter population is that an intruding spider will withdraw from a contest if it is similar in weight to the web-owner. Withdrawal is common in this population, but so are display and threat and these actions were not predicted. We hypothesize that gene flow from surrounding habitats is preventing the riparian population from completely adapting to its local environment.

Notes: Summary Optimal foraging theory is based on the assumption that at least some aspects of foraging behavior are genetically determined (Pyke et al. 1977; Kamil and Sargent 1980; Pyke 1984). Nonetheless, very few studies have examined the role of genetics in foraging behavior. Here, we report on geographical differences in the foraging behavior of a spider (Agelenopsis aperta) and investigate whether these differences are genetically determined. Field studies were conducted on two different populations of A. aperta: one residing in a desert riparian habitat, and the other in a desert grassland habitat. Data from the spiders' natural encounters with prey demonstrated that grassland spiders exhibited a higher frequency of attack than riparian spiders towards 13 of 15 prey types, including crickets and ants. Grassland spiders also had shorter latencies to attack 12 of 15 prey types, including crickets and ants, than riparian spiders. Subsequently, we reared grassland and riparian spiders under controlled conditions in the laboratory and observed their interactions with prey to determine whether the populational differences we found in the field could be genetic. Again, grassland spiders showed a shorter latency to attack prey (crickets, ants) than riparian spiders. These latencies were not significantly affected by the hunger state or age of the spiders. Finally, we reared a second generation (F2) of grassland and riparian spiders in the laboratory and observed their interactions with prey to determine whether the populational differences in the previous generation were due to genetic effects or maternal effects. As before, grassland spiders exhibited a shorter latency to attack prey (crickets) than riparian spiders. We conclude that the foraging differences we observed between these two populations of A. aperta are genetically determined. These differences probably have resulted from either natural selection acting directly on attack frequency and the latency to attack prey, or natural selection acting on traits which are genetically correlated with these aspects of foraging behavior.

Notes: Abstract The idea that traits linked to individual fitness may differ between males and females was tested in the desert funnel-web spider,Agelenopsis aperta. The study entailed comparison of juvenile male and female behavior with respect to three traits previously shown to be linked to female body mass and ultimately to individual female fitness: habitat discrimination, territorial behavior, and agonistic behavior. As juveniles, male and female spiders behave similarly: they utilize the same habitat cues in locating web sites, maintain similar territory sizes, and exhibit the same behavior patterns in territorial disputes. Like females, males that obtain the highest-quality web sites achieve a greater body mass and are more likely to survive to maturity.

Notes: Summary ‘Decisions’ made as to what prey types to include in the diet were analysed for two populations of the spider,Agelenopsis aperta existing under markedly different prey availability and predation levels. Potential prey types were ranked as to their relative profitabilities with respect to energy gain per handling effort and predation risk. Members of the population experiencing limited prey availability but low risk of predation to visually hunting predators exhibited a significantly higher capture attempt rate towards all prey encountered than the population for which prey were abundant but for which predation was a significant problem. Neither spider population preferentially attacked prey that exhibited higher profitability rankings. An experiment was completed that indicates thatA. aperta can discriminate between more and less profitable prey. Suggestions are made as to why the population experiencing abundant food did not exhibit a narrower diet when compared to the population existing under limited food.

Notes: Summary A population of a desert spider species (Agelenopsis aperta) that is located in an unusually favorable thermal and feeding environment (AZ riparian), fails to exhibit territorial, foraging and antipredatory behaviors that are appropriate to its environment. Study of the behavior of this spider species is extended to another population that occupies a similar environment but where there is not potential gene flow from individuals occupying more arid habitats (TX riparian). The two populations are shown to experience similar prey availabilities and predation pressures when compared to a third more arid population (NM desert grassland). Analyses of the relevant behavior patterns demonstrate that the TX riparian population is well-adjusted to the ameliorated conditions. Thus, phylogenetic constraints do not appear to underlie the observed maladaptive behavior of the AZ riparian population and credibility is given to potential gene flow influences.

Notes: Summary This study investigates the possibility that gene flow underlies the apparently maladaptive behavior of a riparian woodland population of the desert spider Agelenopsis aperta with respect to territorial, foraging, and antipredatory behaviors. I found that other local populations of A. aperta in the vicinity of the riparian woodland habitat are prey-limited and exhibit an “aridlands” phenotype (high aggressiveness in competitive interactions over energy-based territories and a lack of discrimination among potential prey types). The riparian woodland population deviates from surrounding populations in the area in that prey are abundant and this population shows a mixture of “aridlands” and “riparian” (low aggressiveness towards conspecifics and discrimination of prey profitability) phenotypes. Electrophoretic analyses of population subdivision in the area indicate that significant levels of gene flow have occurred, at least, sometime in the past. Drift fence analyses of spider movement futher indicate that there is marked unidirectional movement of spiders each year from the more arid habitats into the riparian woodland. Experimental manipulation of gene flow and predation pressure demonstrates that gene flow restricts adaptation in this habitat: one generation of predation pressure in the absence of gene flow is sufficient to cause a marked shift in spider behavior towards the expected “riparian” phenotype.

Notes: Abstract Field studies of the desert spider Agelenopsis aperta revealed a primarily monogamous mating system. However polygyny, polyandry and polygynandry were superimposed upon the primary system, with 9% of the marked males and 11% of the marked females in a field population mating more than once. In the laboratory males commonly mated multiply with fertile offspring resulting, while females were less likely than males to mate multiply. Monogamy under field conditions was enforced by two factors: (1) high travel costs to males, and (2) a significant decline in female receptivity after the first mating. Heavy males were more likely to be accepted by females both in the field, and in female choice experiments conducted in the laboratory. Finally, male weight determined the outcome of male-male agonistic interactions over females. One possible explanation for female choice in this system which lacks male parental investment is that females may be using male size as an indicator of future success of their offspring.

Notes: Abstract Field studies of the desert spider Agelenopsis aperta revealed a primarily monogamous mating system. However polygyny, polyandry and polygynandry were superimposed upon the primary system, with 9% of the marked males and 11% of the marked females in a field population mating more than once. In the laboratory males commonly mated multiply with fertile offspring resulting, while females were less likely than males to mate multiply. Monogamy under field conditions was enforced by two factors: (1) high travel costs to males, and (2) a significant decline in female receptivity after the first mating. Heavy males were more likely to be accepted by females both in the field, and in female choice experiments conducted in the laboratory. Finally, male weight determined the outcome of male-male agonistic interactions over females. One possible explanation for female choice in this system which lacks male parental investment is that females may be using male size as an indicator of future success of their offspring.

Notes: Summary 1. The agonistic behavior of the funnel-web building spider, Agelenopsis aperta (Gertsch), was studied using induced encounters between adult females at natural web sites. All behavior exhibited by either individual during the course of an encounter was recorded. The results were analyzed through the use of transition matrices and the following multivariate treatments: factor analysis, ordination, and multiple regression. These latter methods were used to provide insight into possible sources of variability and their underlying causes. 2. Sequence outcome is primarily determined by the relative weight of the two contestants engaged in a territorial dispute. If the size difference is large, the larger of the two individuals wins in a significant number of cases. Home bias is evidenced in cases where body weights are close. 3. A stereotypy measure is devised that reflects the percent similarity of each sequence of events to an expected sequence (average). The frequency distribution, presence-absence or duration of all behavior patterns observed during the course of the disputes are utilized in measuring stereotypy. The territorial disputes of A. aperta exhibit low stereotypy, averaging 43% on a scale from 0 to 100%. 4. Low stereotypy is, in part, related to the utilization of 33 different action patterns by spiders in these disputes. Factor analysis is used to express these action patterns in terms of five functional groups including locating behavior, signaling behavior, threat behavior, contact behavior, and a multiple function category. The order of these categories represents increasing cost based on relative estimates of the energy expenditure necessary to complete a specific behavior pattern and the potential for injury through the use of it. 5. A Bray and Curtis ordination shows the important sources of between sequence variability to be the total energetic cost of the dispute and the complexity of behavior exhibited in it. The factors, in turn, depend on the resident-visitor status of the losing spider and on the relative size of the two contestants. The energetic cost of a dispute is markedly higher in those disputes in which the resident loses her territory. The number of action patterns and total frequency of acts observed are also greater in encounters in which the resident is the losing spider. Behavioral complexity is higher as well in cases where the weights of the two contestants are close. 6. The variation in the pathways through which the sequences progress is shown to reflect the operation of assessment strategies by A. aperta. Initial assessment of the relative weights of the opponent is made through movements on the web at a distance (locating behavior). Subsequent activities depend on the results of this assessment, the predominant strategy being ‘retaliator’ (Maynard Smith and Price, 1973) in which an individual responds to escalation with further escalation. Spiders with a large weight advantage over the opponent tend to escalate directly to threat and contact behavior (‘hawk’ strategy). The corresponding strategy for a much smaller visiting spider is immediate retreat (‘mouse’). A much smaller resident spider, however, will exhibit the ‘retaliator’ strategy to the ‘hawk’ rather than the more conservative ‘mouse’ strategy. The particular stategy exhibited, then, also depends on the energetic investment a particular individual has in the contended resource. 7. Within functional group variability is shown to significantly affect the outcome of territorial disputes. Winning spiders exhibit an average of 20% less stereotypy than losing spiders. Unpredictable behavior possibly confuses the opponent, causing it to make inaccurate assessments of the weights of the opponent relative to it. Retreat follows. This behavior is linked to the ‘protean displays’ exhibited in defense against predators (Humphries and Driver, 1967).