Form Versus Function
Image Credit: Graham Wise, CC BY 2.0, Image Cropped
Sexual differences in weaponry and defensive behavior in a neotropical harvestman (2018) Segovia et al., Current Zoology, https://doi.org/10.1093/cz/zoy073
QUICK NOTE: Harvestmen (aka Daddy Long Legs in North America) are NOT spiders! Despite the false myth that they can’t bite you due to short fangs, harvestmen aren’t even venomous. They can’t hurt you! There, now that I got that off my chest…
Sexual dimorphism is a common phenomenon in nature whereby male and female members of a given species differ from one another physically. Think of the large bull moose or elk with its antlers, peacocks and their colorful tails, or the larger horns of male stag beetles. Because of these differences, natural selection is able to act on both their behavioral and functional differences. That is to say, differences in performance and morphology mean that males and females of the same species may experience differential selection pressures. As a result, males and females could be expected to react differently to the same challenge, such as a predator.
Harvestmen (known in North America as Daddy-Long-Legs) are a group of arachnids that, although bearing a resemblance to and being commonly mistaken for spiders, are not actually spiders. They belong to a group called Opiliones. Some males of this group have thicker legs with pronounced spines, used in male-to-male competition and anti-predator defenses. In addition to using these spines against predators, these arachnids also engage in thanatosis (“playing dead”, see Did You Know?) and use chemical defenses. Due to these morphological differences, the authors hypothesized that males and females would differ in their response to predators.
What They Did
The authors first measured male and female harvestmen (Mischonyx cuspidatus) to quantify the extent to which they differ from one another. They measured dorsal scute (spine) width and the diameter of the hind legs, as these two parameters are known to differ between the sexes.
After quantifying morphological differences the authors then tested for differences in passive defenses. The researchers held the harvestmen by one leg, picked them up and put them down a small distance away from their original position. Thanatosis was defined as the harvestmen curling up most of its legs close to the body and remaining still.
Lastly, they tested for differences between the active defenses of the males and females. Namely, they were interested in whether the males engaged more in “nipping” with their spiny legs and if the females tended to use chemical defenses more often than the males. Harvestmen were held for a period of about 10 seconds to induce defensive behaviors, and one researcher was used to control for potential differences between the trials.
Did You Know: Thanatosis
Thanatosis is the act of playing dead, a behavior used by prey animals to try and dissuade predators from consuming them. You may have heard the expression “playing possum”, and the opossum itself (at least in North America) is famous for this kind of behavior. Interestingly, this behavior relies on the predator being tricked in to thinking that the prey item is actually dead. While it may seem obvious to us, this apparent death of a prey item is enough to dissuade predators because they catch live prey. Thanatosis is not the sole domain of prey animals, as sleeper cichlids in Lake Tanganyika are know to play dead to attract scavengers. Once the scavengers are too close to escape the cichlids rights itself and consumes them.
What They Found
Not surprisngly, the physical parameters of male harvestmen were indeed larger than the females, confirming that they are a sexually dimorphic species. With regard to the passive defenses, the female harvestment engaged in thanatosis more than the males did, though the time spent in thanatosis did not differ among the sexes. For the active defenses experiment, the male harvestment “nipped” more than the females did, though there was no difference among the sexes in the usage of chemical defenses.
For the test of active defenses against predators, the authors only tested each harvestman one time. An important aspect of animal behavior is that it is a repeatable phenomenon, meaning that a given animal will repeat the same behavior over time. Given the stark differences between the sexes, it is likely that the harvestmen here are indeed displaying a repeatable behavior, but because the authors didn’t test the focal animal more than once it is impossible to say for sure.
Sexual dimorphism is an interesting aspect of organismal biology, as it can result in male and female members of a given species attempting to solve the same problem in entirely different ways. With this experiment the authors have shown that sexual dimorphic arachnids, due to their different anatomy, respond to predators in very different ways. This is compelling, as differences that first evolved due to sexual selection (i.e. spines used in male-male competition) appear to have resulted in differences in a completely different aspect of their behavior (i.e. responding to predators). More studies like this one are needed to understand the ubiquity of these results, but these experiments hint at a much larger and more complex picture of sexual dimporhism in the natural world.
Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.