Aggressive spiders ride out cyclones

Aggressive spider colonies survive tropical cyclones better than docile ones, a new study shows.

On first reading that may seem only of interest to spiders (particularly docile ones), but the researchers from Canada’s McMaster University say it is a valuable pointer to how extreme events could play a role in shaping animal behaviours.

“It is tremendously important to understand the environmental impacts of these ‘black swan’ weather events on evolution and natural selection,” says lead author Jonathan Pruitt.

“As sea levels rise, the incidence of tropical storms will only increase. Now more than ever we need to contend with what the ecological and evolutionary impacts of these storms will be for non-human animals.”

Studying the ecological effects of infrequent and often unpredictable events is challenging, as it requires a comparison of habitats both before and after the event. 

Pruitt and his team focused on female colonies of the spider known as Anelosimus studiosus, which lives along the Gulf and Atlantic coasts of the US and Mexico, directly in the path of tropical cyclones that form in the Atlantic basin from May to November.

By anticipating the trajectory of three cyclones or storms that hit the region during 2018, the authors assessed the size of colonies in the projected paths. 

Once a storm’s path was determined, they sampled populations before landfall, then returned to the sites within 48 hours.

They sampled 240 colonies, and compared them to control sites, with particular interest in determining if subtropical storm Alberto, Hurricane Florence and Hurricane Michael caused particular spider traits to prevail over others.

A. studiosus has two sets of inherited personality traits: docile and aggressive. The aggressiveness of a colony is determined by the speed and number of attackers that respond to prey, the tendency to cannibalise males and eggs, the vulnerability to infiltration by predatory foreign spiders, among other characteristics.

Aggressive colonies, for example, are better at acquiring resources when scarce but are also more prone to infighting when deprived of food for long periods of time or when colonies become overheated.

“Aggressiveness is passed down through generations in these colonies, from parent to daughter, and is a major factor in their survival and ability to reproduce,” Pruitt says.

The analysis suggested that after a tropical cyclone, colonies with more aggressive foraging responses produced more egg cases and had more spiderlings survive into early winter. 

The trend was consistent across multiple storms that varied in size, duration and intensity, suggesting the effects are robust evolutionary responses.

But the researchers are not yet sure why it happens.

“We suggest that post-tropical cyclone environments could reduce prey availability, which could enhance selection on aggressiveness in web-building spiders because foraging is a time-sensitive opportunity in these societies,” they write in a paper in the journal Nature Ecology & Evolution.

“Post-tropical cyclone environments may also increase the motility and aggressiveness of other species of spider, which might be more likely to invade A. studiosus colonies following tropical cyclones. These intruders are known to select for more aggressive colonies in A. studiosus. 

“Finally, tropical cyclones may reduce the longevity of A. studiosus mothers engaged in parental care, which imperils the young in this species. If this is the case, then it is possible that aggressive offspring may be less reliant on the prey and defence of their mothers and, therefore, suffer less mortality.”