To be or not to be…..in a group?

SOCIAL BEHAVIOUR IN AUSTRALIAN HUNTSMAN SPIDERS

Aggressive, hostile and cannibalistic are just some words often used to describe spiders. Commonly their murderess behaviour is directed not only towards other animals on the menu, but also to those they are most closely related to. For this reason most people expect spiders to be solitary animals, and in this case most people would be correct.

However, those rare cases are often the most interesting and spiders are no exception.

In a recent seminar Dr Linda Rayor, an expert in the field of spiders, discussed that  while the majority of spiders are solitary, some spider species have adapted towards a more cooperative group lifestyle (Auletta & Rayor 2011). This cooperative “social” lifestyle as defined by Dr Rayor, includes any individuals forming long term associations (Yip and Rayor, 2011). Dr Rayor went on to reveal why for such a naturally aggressive animal, do some spiders choose to live in groups.

The evolution of sociality for all animals is actually a bit of a conundrum (Yip & Rayor 2011). Living in close quarters with others carries not only the high cost of competition for food and mates, but also increased parasite transmission (Yip & Rayor, 2011).Thus, spiders must be obtaining some enticing benefits for such social behaviour to occur.

In other words, like with all animals, some spiders may adapt to a lifestyle of cooperation in the face of competition because it increases the chances of survival and reproductive success (Yip & Rayor 2013). It was once believed all social spiders were geared towards group living as they benefited by sharing the high costs of producing silk and capturing larger prey (Auletta & Rayor 2011). While this theory appears to be the case for the vast majority of web building social spiders, new findings by Yip and Rayor (2013) reveal there may be a few rare exceptions.

The Australian huntsman spiders, Delena cancerides, are unique among social spiders as they do not produce prey capturing webs (Yip et al, 2012). Instead they live in cramped colonies under the bark of trees and venture out individually to hunt for unsuspecting prey (Yip et al, 2012). Considering this species does not suffer the cost of web production, further research was needed to investigate the benefits driving sociality in the huntsman spider.

Incredible findings by Rayor and Yip (2013) suggest the answer to this question involves food. Firstly, a bit of explanation about the ecology of huntsman spiders is necessary. The social groups this species live in are under bark hideaways and are cramped, rare and in high demand for protection from a range of predators who ironically hunt huntsmans (Yip & Rayor 2013). In addition, groups consist not only of related individuals, but commonly involve a mother and several generations of her offspring (Yip & Rayor 2013). In terms of feeding, field observations by Yip and Rayor (2013) indicate most of the time individuals are hunting and consuming their prey outside the nest. Nonetheless, on occasion the older spiders have been seen returning to the nest with large prey items in tow. It was postulated that on these occasions younger, smaller spiders may be able to get in on the action and benefit by feeding on scraps of such ‘shared food’ (Yip & Rayor 2013).

In order to test if this prey sharing is a considerable benefit, possibly driving group living in these species, a combination of direct and indirect measurements were taken. Rayor and Yip (2013) collected and measured the body condition of spiders from over 90 colonies and compared them to the individual demographics of each colony. The findings supported the researchers’ postulations and uncovered that younger spiders living in groups with older spiders were being fed more as indicated by their heavier body weight. Larger spiders on the other hand, suffered no real impact on body weight by sharing with younger spiders (Yip & Rayor 2013).

Therefore, younger spiders appear to be benefiting by getting a really good meal every so often, while older spiders seem to gain nothing for their seemingly altruistic behaviour.

Yip and colleagues (2012) suggest kin selection/ inclusive fitness theory may be able to shed light on how, for the older spiders, such as selfless system could evolve. These theories are defined by suggesting that ‘altruistic’ behaviour may occur when individuals are helping relatives as they share a common gene pool (West-Eberhard, 1975). From an evolutionary perspective the goal for all species is too pass on their own genes and because relatives share genes, the more individuals in one family who survive to reproduce ultimately benefits everyone in the group (West-Eberhard, 1975). Thus, older spiders may only tolerate food sharing when those who benefit are close relatives.

Ultimately the significance of this research reveals that while many forms of sociality can exist, what drives the evolution of social life is often unique to each and every animal on earth, including spiders.

Sharing, caring and social…. are hopefully just a few new words you can use to describe these furry little creatures.

References.

Auletta, A. and L. S. Rayor (2011). “Preferential prey sharing among kin not found in the social huntsman spider, Delena cancerides (Araneae: Sparassidae).” Journal of Arachnology 39(2): 258-262.

West-Eberhard, Mary Jane (1975). “The evolution of social behavior by kin selection.” Quarterly Review of Biology 50.1 : 1-33.

Yip, Eric C., David M. Rowell, and Linda S. Rayor. (2012) “Behavioural and molecular evidence for selective immigration and group regulation in the social huntsman spider, Delena cancerides.” Biological Journal of the Linnean Society 106.4 : 749-762.

Yip, Eric C., and Linda S. Rayor. (2011) “Do social spiders cooperate in predator defense and foraging without a web?.” Behavioral Ecology and Sociobiology 65.10 : 1935-1947.

Yip, Eric C., David M. Rowell, and Linda S. Rayor (2012). “Behavioural and molecular evidence for selective immigration and group regulation in the social huntsman spider, Delena cancerides.” Biological Journal of the Linnean Society 106.4: 749-762.

Yip, Eric C., and Linda S. Rayor (2013). “The influence of siblings on body condition in a social spider: is prey sharing cooperation or competition?.” Animal Behaviour 85.6 : 1161-1168.

Video link.

http://www.cornell.edu/video/preying-together-older-siblings-aid-younger-social-spiders

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