Now, I'm gonna take a closer look at sexual selection and sexual selection, we're gonna say is selection for traits that affect the ability to obtain mates. Alright. When we think about fitness before we've said, you know, it's the ability to survive and reproduce, but an organism may be able to survive it may be able to reproduce, but it cannot find another organism willing to mate with it. That is the heart of sexual selection. Alright.
Now this all plays out the way it does because we're gonna say here during reproduction, there is often a high investment sex and a low investment sex. And when we're talking about investment, we're talking about how much the organism invests in that single mating event. Now females are often, though not always, they're often the high investment sex, and that's because females make large gametes. Right? That's sort of the definition of being female.
They make eggs where males make sperm, and they often give more parental care. Right? So for large gametes, think of birds. Right? It's physiologically expensive to make bird eggs.
And think about parental care. Think about an elephant. Right? A female elephant is going to be pregnant for almost 2 years and then will nurse the calf after birth, and that's from that single mating event. Well, in contrast, males are often though not always they're often the low investment sex, and that's because they have small gametes, and they tend to give less parental care.
Right? Think of the male elephant. It could theoretically mate every single day that that female is pregnant, and it could have new offspring with a new female every single day if it could find enough mates. Alright. So that difference is going to lead to sexual selection, and you can tell that sexual selection is going on because you see sexual dimorphism.
And sexual dimorphism is differences in secondary sexual characteristics. And secondary sexual characteristics are differences between the sexes that aren't directly related to the ability to reproduce. So differences like size between the sexes or differences in color. Right? When you see those differences, it's a hallmark that sexual selection has occurred.
Alright. So we're gonna have 2 ways that this can play out, and this is gonna be intersexual selection and intrasexual selection. Let's look at both of them. Now intersexual selection, that inter means that it's between the two sexes. And we can also sort of think of this as mate choice.
So we have this high investment sex, again often the females. Well if they're investing a lot in this mating event, they're gonna be choosy about it. Right? They are going to be choosy, and if the females are choosy, well, that tends to select for and lead to these showy males that are trying to impress the female and win the mate. Right now, the classic example of this is peacocks, right?
These peacocks here, this giant elaborate tail, that thing is not good for survival. Right? If you're running away from a tiger in the jungle, that thing is slowing you down. But for winning mates, they pop them up, the females evaluate, they find the males that tend to have the largest, most elaborate trains, and that's who they tend to mate with. Now this happens with other organisms too.
Here we see a bowerbird. Bowerbirds, they build these sort of elaborate structures, they decorate them, and females come around and they evaluate them, and they tend to mate with the males that have the most impressive structures. Alright. So that's intersexual selection, that mate choice. We also have intrasexual selection.
This is sexual selection within one sex, and we can think of this as competition. Competition for some competition for some resource that grants access to mates. So in this case, it's typically or it's going to be the low investment sex, typically the males that compete. Right? And they're competing for something like territory, and within that territory are females.
So if you control the territory, then those males can typically control at least most of the matings in that territory. Now the classic example for this are these elephant seals here. These seals fighting for the territory that's the beach, and these seals have some of the biggest sexual dimorphism in any animals. The males are huge compared to the females, because if they win a good space of beach, they control something like 90% of the matings that happen in their territory. And there might be well, often there's like a dozen, but there could be up to like a 100 females in that territory, which they get to have most of those matings.
Again, we see it in other animals too, right? Classic like the rams, bashing heads. They're typically, you know, winning some territory, something like that that grants them access to mates. Now keeping inter and intra separate in my head, it's hard for me the way I remember it. Well, that in intersexual selection, I think that the females are kinda walking around going, do I wanna mate with you?
Right? It's that mate choice. And for the intrasexual selection, I see that raw, and I think that that low investment sex is going raw. I'm gonna win the mate. Raw.
We're competing. Right? Hopefully that helps you. Alright. Before we go on, I just wanna say warning.
Right? Mating systems are complex. Okay? So often when you see intersexual selection, there's also some intrasexual selection going on. Right?
These peacocks are competing for the best display space. When you see intrasexual selection, there's often some mate choice going on. Right? These elephant seals, they control something like 90% of the matings in their territory. That's not all of them.
The females are still choosing who to mate with at least some of the time. I also just want to say, really don't overgeneralize, especially in humans. People love to do this, but just please don't. Right? Humans, well, there is some sexual dimorphism in humans, but it's less than in other organisms.
So we really don't know what's going on, and trying to dissect human mating systems to know what is culturally imposed versus biologically imposed, that is really difficult to do, and I'll just say well beyond the scope of general bio. Alright. With that, we will look at intra- and intersexual selection more coming up. I'll see you there.