Do Agonistic Interactions Underlie the Segregation and Relative Abundances Between TwoLoxoscelesSpecies (Araneae: Sicariidae)?

Abstract

The medically important spiders Loxosceles intermedia Mello-Leitão and Loxosceles laeta (Nicolet) are segregated in Curitiba, southern Brazil, where L. intermedia is more abundant and widespread than L. laeta. Because they share similar microhabitat preferences and wander in search of web sites, agonistic encounters are likely to occur. The purposes of this study were to describe agonistic interactions and interpret their consequences for the relative abundances and spatial segregation of L. intermedia and L. laeta. Experimental contests were performed between residents and intruders. Asymmetries between contestants included sex, age, species, weight, and residence status. Nine behavioral categories were defined. Through discriminant analyses, it was possible to differentiate spider sex, species, and residence based on their agonistic behaviors. Intruders, juveniles, and L. intermedia individuals were better characterized by exploratory behaviors, whereas L. laeta females were differentiated by aggressiveness. By performing a multiple logistic regression, with winning or defeat as a dependent variable of sex, age, species, size, weight, and residence, it was possible to say that residents and L. intermedia individuals had the highest winning odds in contests, whereas juveniles had lower winning odds than adults. Advantages of the prior residence may help to explain the predominance of L. laeta in old colonization sites, whereas the higher winning odds of L. intermedia and less aggressive behavior toward conspecifics may lead to a successful establishment of dense populations in new sites. A better understanding of agonistic interactions as a mechanism of spacing, segregation, and species replacement among spiders may be helpful for control purposes.

Segregação espacial horizontal em pequena escala entre duas espécies da família Theridiidae (Araneae)

“…Body size is an important trait in species interactions and in the structuration of animal assemblages (Werner & Gilliam, 1984). Although in some isolated cases one spider species may be dominant over other Fischer et al, 2014), in general, large-bodied individuals are more successful in intra (Riechert, 1978;Wise, 1983;Samu et al, 1996) and interspecific web takeover (Enders, 1974;Eichenberger et al, 2009;Fischer et al, 2014). Our results are in agreement with the tendency observed in literature, suggesting that H. longicauda is not competitively superior in interference competition for webs.…”

“…), we did not observe cannibalism or intraguild predation between H. longicauda and C. intervales. In addition to nonlethal fighting during interactions, we also witnessed ritualized displays that apparently mediate communication between intruder and web owner, as observed in other experiments involving agonistic interactions (Riechert, 1978;Fischer et al, 2014). For instance, by investigating intruderresident contests on Metepeira labyrinthea, Wise (1983) recorded that winners usually displayed a greater behavior variety, such as web shaking.…”

“…Wise, 1983), apparently, the web owning condition is an advantage in intra and interspecific contests involving H. longicauda and C. intervales (and in other systems, e.g. Schuck-Paim, 2000;Fischer et al, 2014), since resident spiders remained in the web solely in approximately ¾ of all interactions, in all categories. For instance, most common outcomes were "Intruder left the web" followed by "resident won the fight".…”

“…For instance, by comparing abdomen volumes, Enders (1974) observed that web invader spiders are usually undernourished. Moreover, younger adults leaving the mothers web may not be as heavier as older spiders, and it's known that juveniles are disadvantaged in taking over alien webs (Fischer et al, 2014). As ownership is an advantage, the difference between C. intervales invaders and H. longicauda residents might not be enough to confer successful takeovers to the intruders.…”