The world still often demands a “What I did during summer vacation” essay, and I’m happy to oblige. I attended the Animal Behavior Society conference in Toronto, Canada, in June, where I noticed that spiders are the new super stars in animal behavior research. There were talks about spiders in sessions on predation, foraging, mate choice, aggression, evolution, social behavior, parental care, cognition and learning.
My favorite spider talk was by Anita Aisenberg. She is an expert on the nocturnal wolf spider, Allocosa senex, a common and important species in her native Uruguay. This spider lives in sand dunes by the beach that have been heavily impacted by human activity. It’s considered an indicator species because its presence provides evidence of a healthy ecosystem. This nocturnal wolf spider can be studied inexpensively which is critical, as funding in Uruguay is monumentally difficult to obtain.
Aisenberg is interested in mating systems, and this spider has an unusual one. Males and females are both choosy — a technical term meaning that they will not mate with just any individual, but rather that they are selective. It’s common across the animal kingdom for females to be choosy and for males to mate with any willing partner. This pattern reflects the amount of parental investment by each sex.
Females generally invest more time and energy in their offspring than males do. From an evolutionary perspective, it is a poor strategy to invest these extensive reproductive efforts in offspring from low quality males, which costs them the opportunity to invest in offspring from higher quality males. To maximize their reproductive success, it is vitally important that they only reproduce with the best possible males.
With a lower investment in their offspring, males can optimize their reproductive success by mating as often as they can, regardless of female quality. Mating with less desirable females does not cost them future mating opportunities with higher quality females. Though it’s uncommon for males to be the choosy ones, such sex role reversal occurs when males have high levels of investment in their offspring but females don’t.
If both males and females invest heavily in their offspring, both are choosy, and the species is said to exhibit partial sex role reversal. In this nocturnal wolf spider, males dig burrows and produce pheromones to attract females. Digging a burrow is a lot of work, so the males are investing a lot in their reproductive effort. Males choose younger females with bodies in good condition, especially virgin females. This choice by males is advantageous to them, because younger, unmated females lay more eggs than older ones do. Females, who produce many eggs at a great energetic cost, choose the males with the longest burrows, which provide better protection for their eggs, and also indicate that a male is healthy, strong and likely to be passing on good genes.
And that, plus a family trip to Portugal, Spain and France where I paid no attention to spiders, is the story of how I spent my summer vacation.