Opposites don’t attract in the spider dating scene. Bad boy bridge spiders hooked up with tough female arachnids, while nice guy spiders mated with the 8-legged equivalent of the shy girl-next-door in a recent experiment.
Aggressiveness runs in the family for bridge spiders (Larinioides sclopetarius). Some spiders are natural meanies, while others are born to back down. Aggressive spiders chase and attack other spiders of the same sex when they are on the same web more frequently than passive spiders.
When biologists gave males of the aggressive and non-aggressive types the option of courting females of both “personalities,” the males tended to make their move on females with the same aggressiveness level as themselves. In another experiment in the same study, tough-guy males tended to have more offspring than passive male spiders from the same number of copulations with females of intermediate aggressiveness.
The biologists suggested in their paper published in Behavioral Ecology that by mating with spiders of their own personality type, the spiders maintain diversity of attitudes in their population. A previous study found that high-density populations of only aggressive bridge spiders tend to have higher mortality rates than mixed populations.
Bridge spiders frequently live in high densities in urban areas, so behaviors that allow more of them to live in one area gives the community a survival advantage.
The balance of personalities in the bridge spider reminds me of the “Hawk-Dove” game in evolutionary game theory. In that game, individual animals of the same species have one of two behavior types, similar to those of the bridge spider. “Hawks” will fight each other over a resource until only one gets the payoff, whereas “doves” will back down from a hawk, but share a resource with another dove.
Mathematical models can determine the ideal balance of hawks versus doves in a population. The mating strategies of the bridge spider may be doing a natural form of this calculation.
IMAGE: The bridge spider (H. Krisp, Wikimedia Commons)