The graphs in Figure 11.52 show the variation in lifetime...

The graphs in Figure 11.52 show the variation in lifetime reproductive success of male versus female elephant seals (Le Boeuf and Reiter 1988 ). Note that the scales on the horizontal axes are different. Why is the variation in reproductive success so much more extreme in males than females? Draw a graph showing your hypothesis for the relationship between number of mates and reproductive success for male and female elephant seals. Why do you think male elephant seals are four times larger than females? Why aren't males even bigger?

Key Concepts

Life History Trade-offs and Constraints

Life history trade-offs involve the allocation of limited resources among various biological processes such as growth, reproduction, and survival. Even though larger males may have a competitive advantage, natural selection balances size against other factors like energy requirements, agility, and survivability, preventing indefinite increases in body size.

Mating Systems and Reproductive Skew

Mating systems, such as polygyny where one male mates with multiple females, generate skewed distributions of reproductive success. In such systems, a small number of males may achieve a high number of matings while many others have few or none. This concept explains why variation in male reproductive success is typically more extreme compared to females.

Sexual Dimorphism

Sexual dimorphism is the difference in size or appearance between males and females of the same species. Often, these differences arise because one sex (usually males) evolves larger body size or other ornaments to outcompete rivals for access to mates via physical contests or displays, while females may invest more in reproduction.

Sexual Selection

Sexual selection refers to the process by which traits that influence an individual’s ability to obtain mates evolve, often leading to pronounced differences between the sexes. In many species, competition for mates results in a few individuals (typically males in polygynous systems) securing most of the reproductive opportunities, which creates extreme variation in reproductive success.

Transcript

00:01 Elephant seals.

00:03 So in elephant seals, the males have sort of harems.

00:09 They establish territories on a beach or an island, and they fight it out for access to the females and the winners, the males who are winners, get access to a lot of females and the males who are losers get access to none.

00:28 And so this is an example of intracosexual selection.

00:34 And that scenario that i just described really answers the question on why there's so much variation in the between males and females in reproductive success.

00:47 So male reproductive success is kind of an all or nothing deal, right? either you're a lucky male who manages to secure a harem and for maybe a year or two or three, however long you can keep the harem going and fend off other males, you get to mate with all those females.

01:09 If you're a male who's unlucky, and here, of course, luck may be genetic, you never get to mate with a female, right? the females only mate with the winners or rarely sneak out on the side.

01:22 And so you end up with this huge variation where some males get all the matings and other males get none.

01:31 For the females, of course, you have a lot less variation, and it's sort of what we think of as normal variation.

01:40 A female mates with a male and then raises her offspring.

01:45 And obviously sometimes, you know, females are either good at rearing their offspring or have luck rearing the offspring and they end up having greater reproduction or they have less reproduction.

01:56 Perhaps they die young or something like that.

01:59 So that's the scenario here.

02:01 We're asked to make a graph and so i just quickly show you how this can be done.

02:06 You can have number of mates on the x -axis and the number of offspring on one.

02:15 This can either be like for one season or for one year or one, for a lifetime, but i'm thinking about it mainly for one season.

02:23 And if you think of a female, chances are good.

02:26 She's only going to have one mate...

Please give Ace some feedback