Arachnologische Mitteilungen 58

Arachnologische Mitteilungen / Arachnology Letters 58: 97-102 Karlsruhe, September 2019 Is cooperation in prey capture flexible in the Indian social spider Stegodyphus sarasinorum ? Ovatt Mohanan Drisya-Mohan, Neisseril Anirudhan Kashmeera & Ambalaparambil Vasu Sudhikumar doi: 10.30963/aramit5813 Abstract. Among social spiders, cooperation is a key characteristic behaviour. Cooperation in prey capture increases the probability of successful prey capture and to some extent reduces the individual costs associated with foraging. We assessed spider cooperation in prey capture under natural conditions in relation to the number of spiders in the colony and the type and size of the prey captured by the social spider Stegodyphus sarasinorum Karsch, 1892 (Araneae: Eresidae). First, we determined natural prey in the spider webs and found that beetles (Coleoptera) were the most frequent prey followed by grasshoppers (Orthoptera). These two prey types were then used to study the cooperative hunting behaviour of this spider. We investigated prey capture frequency, recruitment and immobilization time when spiders are more active in the mornings and less active around midday. The study revealed that the immobilization time and recruitment time were shorter when hunting beetles, the smaller sized prey, while larger numbers of spiders were recruited in response to grasshoppers, the larger prey. The study concluded that cooperative behaviour in S. sarasinorum depends on the size of prey present. Keywords: cooperative behaviour, immobilization, predatory efficiency, recruitment time Zusammenfassung. Gibt es flexible Kooperation beim Beutefang der indischen sozial lebenden Spinnenart Stegodyphus sara- sinorum ? Unter sozialen Spinnen gehört Kooperation zum charakteristischen Verhaltensinventar. Kooperation erhöht die Chancen auf erfolgreichen Beutefang und reduziert den individuellen Aufwand, der damit verbunden ist. Wir untersuchten die Kooperation beim Beutefang von Stegodyphus sarasinorum Karsch, 1892 (Araneae: Eresidae) unter natürlichen Bedingungen. Zuerst bestimmten wir die Beute in den Netzen und fanden am zahlreichsten Käfer (Coleoptera) vor Heuschrecken (Orthoptera). Diese beiden Beutetypen dienten dann für genauere Studien des Beutefangverhaltens der Art. Die Häufigkeit und die Dauer des Verhaltens (Beuteerwerb und -fixierung) ist am Morgen höher als in der Mittagszeit. Die Dauer war bei den Käfern, der kleineren Beute, kürzer als bei den Heuschrecken, der grö- ßeren Beute. Das Kooperationsverhalten von S. sarasinorum hängt von der Größe der verfügbaren Beute ab. Among invertebrates, social life has evolved in two taxa: spiders and insects. In spiders, cooperation is considered a characteristic of a social species (Brach 1975, Jackson 1979, Krafft 1970, Riechert et al. 1986). Among the permanently social spiders, there are approximately twenty species of co- operative spiders distributed across seven families and most of them show remarkable convergent evolution of a suite of traits associated with their social way of life (Lubin & Bilde 2007, Bilde & Lubin 2011) . The genera Anelosimus and Ste- godyphus contain both social and subsocial species with mul- tiple independent origins of permanent sociality (Agnarsson 2006, Johannesen et al. 2007). In permanent associations, the individuals share the same web and co-operate in different activities: web construction, prey capture, brood care and web maintenance (Lubin & Bilde 2007). Organisms foraging in groups experience increased fora- ging efficiency in comparison to solitary foragers by capturing large or greater numbers of prey, reducing the likelihood of prey escape, hunting risk and lower variability in prey cap- ture (Rypstra 1989). Therefore it decreases the individual consumption rate, which buffers the group against starvation (Caraco et al. 1995) and enables an increase in dietary niche (Guevara & Aviles 2007). Also, resource distribution is a key ecological factor influencing group dynamics (Packer & Rut- tan 1988). Hence group living increases the competition for resources with group size (Krause & Ruxton 2002, Majer et al. 2018). Because of this, most species of social spiders live in tropical regions of the world and lowland rain forest where insect size and density is highest, but several sub-social spe- cies reach into the Eastern United States and other temperate areas (Powers & Aviles 2007, Guevara & Aviles 2007). New world Anelosimus occur in the most productive continental biome i.e., tropical rain forests while the Old World Stego- dyphus inhabit drier savannah habitats. Low precipitation seasonality supports abundance in social spiders (Majer et al. 2015). Stegodyphus species are restricted to areas with rela- tively high vegetation productivity and insect biomass (Majer et al. 2013). Social spider nests can contain hundreds or thousands of individuals, who build communal webs to capture insect prey. The communal two or three-dimensional webs that social spi- ders build function ecologically as single units that intercept prey through their surface (Aviles 1997). Thus the surface area of this webbing exposed to the environment should de- termine the frequency with which prey items enter the webs (Majer et al. 2018). It is observed that the mean available web surface per individual decreases from solitary to social species ( Jackson 1978, Majer et al. 2018). So it can be assumed that in order to increase their rate of energy removal per individual and per web surface unit social spiders must have developed several strategies. For these purposes social spiders could (a) increase the capture ratio of available prey, (b) enlarge their prey size range and capture very large prey that is not availa- ble to solitary spiders or increase their prey size range in rela- tion to dietary niche, or (c) reduce capture web production in relation to colony size (Majer et al. 2018). Cooperation is expected to be of mutual benefit (Downes 1995), either by direct or indirect (kin-selected) benefits like altruism, mutualism, strong reciprocity and group selection (Lehmann & Keller 2006,West et al. 2007). According to the risk-sensitive foraging theory, group hunting occurs in two situations where average prey availability exceeds the mini- mum necessary for survival (Uetz & Hieber 1997), or where a single prey item is too large to be consumed by a single This contribution was presented at the 31st European Congress of Arachnology, Vác, Hungary, 2018 July 8–13 Ovatt Mohanan Drisya-Mohan, Neisseril Anirudhan Kashmeera, Ambalaparambil Vasu Sudhikumar, Centre for Animal Taxonomy and Ecology, Department of Zoology, Christ College, Irinjalakuda, Kerala, India; E-mail: drisyamohan2@gmail.com, kashmeera.n.a@gmail.com , avsudhi@rediffmail.com submitted 4.8.2018, accepted 19.7.2019, online 13.9.2019