Have you ever had the “what would win in a fight” conversation with various different insect and arachnid combinations? Cockroach vs Mantis, Spider vs Scorpion, Ant vs Spider? Well, if you can think of it, nature has probably done it. True to form, I present you: Ant vs Spider, a battle being played out in Australian forests as we speak.
The ant is Myrmecia pyriformis a formidable opponent, measuring in at up to 2.5 cm long (making it one of, but not the biggest ants), the bull or bulldog ant is certified to be the most dangerous ant in the World. It has killed three people in the last 78 years. Wait a minute. Three people? In 78 years. No panic then.
While the bulldog ant might not be a particularly formidable opponent for us, it certainly is for its mortal enemy, its arch nemesis, the Avondale spider (Delena cancerides). The ants and spiders share certain forests in Southeast Australia, where they argue about just about everything. They both like to build their nests under the loose bark of dead trees. They both like to eat other insects and they both like to forage at night. Apparently the two simply cannot live with each other.
But the ants have a dirty trick up their sleeves. Sneaking in under cover of darkness, a group of bulldog ants attacks the Avondale nest. The mother fights back, of course, and in some cases she is able to overcome the ants and save her nest. More often, though, the spiders are driven out. But rather than use the nest or leave it for others to use, the ants fill it with twigs and leaves, whatever debris they can find, making it unusable. This appears close to what we might consider a genuine act of spite in the animal kingdom.
Why are the ants so harsh? Well it seems that the reason for their extreme measures is that the spiders are so successful that they jeopardise the survival of the ants. Driving the spiders out and destroying their homes is the last desperate attempt by colonies of ants that are being pushed to the brink, out-competed by their neighbours, the spiders. What are the spiders doing to be so successful? Well, they have a slower metabolism, making them less vulnerable to starvation and better able to out-compete the ants. But their social lives might also be an important aspect of their success. Avondale spiders (the spiders from the film Arachnophobia) are one of just a small group of spiders, and the only species of their genus, that live in groups. Young spiders are slow to mature, and so they remain with their mother for up to a year, during which time she will lay several more batches of eggs, meaning that her nest is always filled with between 100 and 300 spiders ranging from newborn up to a year old. They also accept the odd immigrant, and young spiders leaving the nest will often try to find a neighbouring nest to move into.
Although scary looking (Avondale spiders can grow up to 20cm in diameter, although that’s mostly leg), they are in fact totally harmless. They cooperate and live together in a way that few spiders mimic. Although spiders will often keep the food they forage to themselves, they have also been seen to bring it back to the nest to share with their siblings.
A recent study found that during a 2-month period, 6% of spider nests were raided by bulldog ants. Bulldog ants live in colonies of up to 1400, and unusually for ants, they can survive happily without a queen. Whereas most social insects (e.g. honeybees, leafcutter ants) have a sterile worker caste, bulldog ants have workers capable of reproduction. Despite this flexibility, and their much larger colony size, it seems the ants are being harmed by interference competition from the spiders, rather than direct combat. Interference competition can lead either to peaceful coexistence between species, or competitive exclusion of the losing species. It isn’t clear yet which way this one’ll go.
Want to Know More?
- Yip (2014) Ants versus spiders: interference competition between two social predators Insectes Sociaux
- Yip and Rayor (2011) Do social spiders cooperate in predator defense and foraging without a web? Behavioral Ecology and Sociobiology