There is a famous statement in pop culture that says ‘sex sells’. And it is true. Sex services, like prostitution and pornography, have long been recognised as some of the most lucrative businesses available. The fact that they achieve such a successful income, despite carrying a social view of being highly dishonourable and disrespectful, just highlights our imminent desire to consume sex. Thus, the question that arises from it is ‘why’? Why are humans so interested in other humans’ bodies?
Even though this is a legitimate question, it is naively asked. Humans are not exceptional in this regard. Every single living organism we know of is obsessed with reproduction. Anyone who has a pet in heat at home, knows how these animals can be agitated when the subject is the opposite sex. Just like anyone with a teenage human at home. So, the right question would be: Why are living organisms so interested in reproduction? The answer is simpler than most of us can anticipate.
Organisms do not have to have an innate instinct for reproduction. But, if they do, they will more likely reproduce. So, their genes will be passed to the next generation. Those that don’t will probably not reproduce. So, their genes will not be so abundant in the next generation. Multiply this logic over the 3.5 billion years that there has been life on this earth, and you have a bunch of species very dedicated to reproduction. Those that weren’t, you just won’t see anymore. This means that sexual behaviour has the same basis for every animal out there (the ones with separate sexes, at least). And this includes you, me, your dog, and even that tiny spider on your ceiling.
Spider’s sexual behaviour
And this is where the subject of this article starts. I decided to dedicate my life to the study of spiders’ sexual behaviour. More specifically, Hasarius adansoni sexual behaviour. And the results of such research might seem more familiar than you think.
First, we found out that spider males usually are much more willing to mate. They seek copulation almost every time we present a female to them. The females, on the other hand, quite often refuse to mate. Even in front of a voracious male. Sounds familiar? Human males are often thought to be much more prone to engage in sexual behaviour than human females. This is actually a very common pattern in animal behaviour. Men produce small gametes (sperm cells), which are very abundant. There is hardly any way a man could ‘waste’ his gametes, because sure enough he has a lot more ready to use. A woman, on the other hand, has very few, and therefore very costly, gametes. That is why natural selection has shaped women to be more selective about the sexual partners they will give a chance to. They just can’t waste many gametes with a low-quality male who won’t be able to give her what she needs and won’t help take care of the kids.
But why do spider males do the same thing? Because they follow the same rule. Actually, every animal follows this rule: males have a high quantity of low-cost gametes, while females have a low quantity of a high-cost gamete. This means that natural selection has shaped spiders (and most of other animals) to have different sexual behaviours across the two sexes, just like humans do.
So, if females are choosing which male to copulate with, what are they choosing? This is one of the most debated questions in sexual selection theory. I tried to answer this question and found an interesting pattern. Let’s take one male characteristic often linked to female preference: body size. When I measured the average male body size of those males accepted by females and those not accepted, the size is pretty much the same. So, maybe spider females didn’t read the same animal behaviour book I did. Or maybe I had to investigate this a little further. I decided, then, to measure the average male size of those males chosen by each female individually. And thus, a pattern finally emerged. Basically, some females like large males, while others like small males. Just like you and your friends, they have different ‘types’. When a female rejects a male, it doesn’t mean the male is of low quality. He’s just not that female’s type. As a male myself, that was a refreshing result. But it doesn’t end there.
There is one more clear pattern we found in those interesting spiders. If you take a male with average size (not the most preferred by any female), some females will be more likely to mate with him, while others will be less likely. This is basically to say that some females are more promiscuous, and others are more … coy, for lack of a better word.
This is all well and good, but if you are a male, you might be thinking: ‘Ok. I might be less choosy than most of my female friends, but I don’t just wanna have sex with any female that pops out in front of me. I’m not quite like those bugs’. And this is a legitimate point. In fact, males have been shown to have, at least, some choosiness in many animal species, ranging from insects to primates. However, finding out what animals are looking for when searching for a mate is not always trivial. In H. adansoni for example, males show off to females by ‘dancing’ in front of them, and by doing an interesting behaviour called tremulation.
Tremulation consists of rapid movements of the abdomen, that vibrates the substrate. Females can feel such tremulation, and they respond with their own tremulations. Their tremulation is not quite the same, however. Males perform high amplitude vibrations of long duration (0.5 seconds), while females perform low amplitude vibrations of short duration (0.25 seconds). So, even though they are doing basically the same thing, they are doing it differently. Just like when someone is flirting on the dance floor. Both the guy and the girl are dancing, but the movements aren’t exactly the same. Each sex has their own way of communicating their intentions. What information is contained in such tremulations, however, is yet to be found. Future research I will conduct will ask exactly that. What the function of such vibrations is, and if each sex is choosing their partners at least partly based on it. So, spider males might not be so promiscuous as we now think, though this is yet to be answered.
Another important question is why animals choose the partners they do. There are a number of hypotheses proposed to answer such questions. One important hypothesis is called the handicap hypothesis. This says that animals choose partners based on characters that indirectly describe their ability to escape predators. This would ensure that the animal’s offspring would escape predators just as easily. However, this hypothesis doesn’t seem to work for H. adansoni, since different males didn’t differ in their ability to escape a simulated predatory attack. In future work, I will also try to understand what is guiding the choosiness process in these spiders.
After a little over six years studying these tiny animals, I already found a lot of interesting features. And each answer opens at least two other questions. This is far from frustrating, however. For me, this is the true beauty of science. You never know enough to say you know it all. So, there is always some challenge ahead, waiting to be accomplished. But at the very least, we can say that we are not exceptional animals. Actually, we are, in many ways, quite ordinary. Hopefully, we will all remember this the next time we see a tiny spider climbing up the wall.
ReferencesCastilho, L. B., Andrade, M. C. & Macedo, R. H. (2020). Males mate indiscriminately in a the tropical jumping spider Hasarius adansoni (Audouin, 1826), Ethology, 127(1), pp. 83-90.
Castilho, L. B., Andrade, M. C. & Macedo, R. H. F. (2020). Testing the differential cost assumption of the handicap hypothesis with a tropical jumping spider, Behaviour, 157(5), p. 433–449.
Castilho, L. B., Macedo, R. H. & Andrade, M. C. (2020). Individual preference functions exist without overall preference in a tropical jumping spider. Animal Behaviour, 160, pp. 43-51.
Castilho, L. B., Maydianne, A. C. & Macedo, R. H. (2018). Mating and egg-laying behavior of Hasaraius adansoni (Araneae: Salticidae) and the influence of sexual selection. Journal of Arachnology, 46, pp. 398-403.