Why do some huntsman spiders live in groups while the majority are perfectly happy living a solitary life? Dr Linda Rayor from Cornell University in New York spoke at Macquarie University recently about her work with Australian social huntsman spiders (Delena cancerides) and her excitement about finding more social huntsman species in Australia last year. Dr Rayor is passionate about social spiders and her findings could shed light on the evolution of parental care in a broad range of animals.
Sociality in arachnids is very rare – less than 1% are social beyond a short time of maternal care just after hatching. This may be due to the challenges a spider species has to overcome to become social. Aggressiveness in spiders means the majority can’t tolerate any other spiders being around them and this can lead to cannibalism (Riechert & Lockley 1984). The existence of three social huntsman species in Australia is a ‘big deal’ according to Dr Rayor as they have overcome the inbuilt aggressiveness that seems to come naturally to so many spiders.
Sociality in spiders has evolved independently at least 18 times (Yip & Rayor 2013b), so there must be something in it. Social huntsman spiders are found in south-west and south-eastern Australia and live in groups of 20 to 200 individuals, with a dominant female and her offspring of all different ages. In contrast to social insects, such as bees and termites, living together doesn’t increase the reproductive output of social spiders so there must be a different driving force behind their sociality (Whitehouse & Lubin 2005).
Research from Dr Rayor’s group suggests a lack of available habitat was the driving force for the evolution of sociality in these spiders. Their preferred habitat of tight spaces under peeling acacia bark is normally 80-100% occupied, so there’s not much room to spread out. The spiders are forced together due to a lack of available housing.
Social huntsman spiders aren’t attached to their family members; they only live together because there’s no other option. This is demonstrated when a colony has to move because their home is destroyed (usually the bark falling off the tree). The family doesn’t move as a group – the spiders go in search of new homes by themselves with no regard for their siblings and the small ones generally get eaten by predators.
These social spiders live (mostly) peacefully in family groups, but if there are big spiders trying to immigrate into their colony they will become aggressive and deny entry to the invaders (Beavis et al. 2007). This is consistent with the limited habitat concept as the spiders are protecting their home (a valuable resource). Young spiders don’t leave the family home until they are big enough to compete with others for the sparse housing options.
To test this idea the researchers looked at the relationship between the availability of suitable habitat and the occupants of the bark spaces. They found that as suitable habitat becomes rarer the number of spiders in each colony increases and there are more large spiders in the colonies (Yip 2012). In addition, the frequency of takeovers of bark spaces also increases when available habitat decreases.
Instead of using a web to trap prey, huntsman spiders roam around at night and hunt their prey (hence the name ‘huntsman’). This food is brought back to the colony for consumption. Food is shared about 5% of the time – mainly between mothers and children and sometime older siblings even share with their younger brothers and sisters (Yip & Rayor 2013a). Even this small amount of sharing is very different to solitary spiders who share food less than 1% of the time.
Prey sharing means all the spiders in the group have some food often, so there is less variability in the amount of food they consume. This is very different to spiders living alone that can have erratic food availability. D. cancerides has a lower metabolic rate than solitary spiders which means they can survive on lower amounts of food (Zimmerman 2007). The researchers aren’t sure whether sociality arose because of the lower metabolic rate in these spiders or whether sociality allows them to share prey and they have developed a lower metabolic rate as a result – it’s a bit of a ‘which came first: the chicken or the egg?’ discussion in the research group at the moment.
It would be interesting to study the metabolic rate of some closely related social and solitary huntsman species to see if all social huntsman species have a low metabolic rate or just D. cancerides. This could possibly shed some light on the evolution of sociality in this group.
To learn more:
Beavis AS, Rowell DM and Evans T (2007). Cannibalism and kin recognition in Delena cancerides (Araneae: Sparassidae), a social huntsman spider. Journal of Zoology, 271:2, 233-237.
Riechert SE and Lockley T (1984). Spiders as Biological Control Agents. Annual Review of Entomology, 29, 299-320.
Whitehouse MEA & Lubin Y (2005). The functions of societies and the evolution of group living: spider societies as a test case. Biological Reviews. 80, 347-361.
Yip E (2012). ‘Costs and benefits of group living in an unusual social spider, Delena cancerides’. PhD thesis, Cornell University, New York.
Yip EC & Rayor LS (2013a). The influence of siblings on body condition in a social spider: is prey sharing cooperation or competition? Animal Behaviour. 85, 1161-1168.
Yip EC & Rayor LS (2013b). Maternal care and subsocial behaviour in spiders. Biological Reviews, doi: 10.1111/brv: 12060
Zimmerman A (2007). ‘Assessing the Costs of Group Living: Comparing Metabolic Physiology and Growth in Social and Solitary Spiders’. PhD thesis, Cornell University, New York.