The key is you have to have a lot of mates, okay?
So how many mates do most organisms have?
Well, when you look across the animal kingdom, multiple mates for males or
females are quiet common.
Then you're maybe familiar with this idea of social monogamy.
That's when you have the social bond of one male and one female at a time.
People tend to think of ducks, so this is actually a poor example.
People tend to think of ducks because they see the male duck and
the female ducks swimming around together.
Now, separate from social monogamy, there's also sexual monogamy.
That's when partners are sexually exclusive with each other.
Now with the growth of DNA based assays for paternity.
What people have found is a lot of these species that appear to be
socially monogamous, are actually not sexually monogamous.
In fact, extra-pair copulations are quite common.
Now the extra-pair copulations are when you're essentially breeding with somebody
outside of your social pair.
This is quite common across much of the animal kingdom.
And again with more genetic data we see this much better.
Now if we assume there are multiple mates, then the increase in number of matings, or
increased number of partners, will often increase your potential reproduction.
Again, this assumes non-monogamy, right.
So this is an important facet, to this.
We are assuming non-sexual monogamy.
Now what's interesting is the difference between sexes
in potential reproductive rate, is very important.
Like basically, how many offspring could you have?
Well, let's look at the world records for humans,
though I don't usually like using humans for these sorts of examples.
The world record that's know for a human female is about 69 children.
That is obviously huge.
Now, what would you think the world record for a human male would be?
Now, potentially you can think, well, how many kids could a male sire over
the course of a year versus a female sire over the course of a year?
The female, you're looking at one to two,
maybe a little bit more if they have multiple kids at a time.
But with males, the potential reproductive rate is much higher.
And, in fact, the world record for a human male is somewhere between 867 and
1,000 [LAUGH] over their lifetime.
Again, the potential reproductive rate for
males is much higher than the potential reproductive rate for females.
Now why this difference?
Well, part of the reason is probably that females often invest more in resources and
in care, and that limits their reproductive opportunities.
All right, so for example, the human female.
If they're gonna give birth to a child, they have this long period of labor
through which they cannot be reproducing anymore.
They are stuck with that period.
And then even after that,
there is this care post-reproduction that also slows them down.
Now, an estimate from one study suggested that, on average,
females devote 100 to 1000 times more resources to egg production
than males devote to ejaculate production.
That's a pretty big difference.
Just as one extreme example many of you are probably familiar with
the bird, the kiwi.
Well it's egg is 15% of its overall weight.
So just that physical investment in creating this one offspring is huge.
Now a lot of these ideas fall from an interesting principle.
This is referred to as Bateman's principle.
So let's look at this first in terms of the females, and
then in terms of the males.
With the females there is typically low variation in female mating success.
And, by that, I mean that most females will tend to mate, and
they will tend to produce offspring, okay?
Now, interestingly, more matings by female does not necessarily
lead to more offspring, because again, they're sort of capped,
that they can copulate multiple times, but that's not going to
increase the number of times they actually produce offspring, for most animals.
And they are limited by resources then, again there's a high investment for
producing these offspring.
With males a lot of that isn't true,
now there is typically very high variation in male mating success.
There's some males who will produce many, many,
many offspring with many different females.
And there are a lot of males who will just produce no offspring.
So there's quite a bit of this variation in male mating success.
I'll show you an example of this in just a couple of minutes.
Typically with males more matings, in this case meaning more matings with
different females, will typically lead to far more offspring.
And finally they are limited by access to females.
So there's a lot of this competition for access to females.
Now, what is the relationship between mate number and offspring number.
Let me show you one example.