Now the question you might wanna ask is, why do the cattle
tolerate these birds hanging on them that are opening their wounds?
Well I asked my class this back in 2012, but here's a set of some of the responses.
On the negative side, somebody suggested that the hosts don't
actually tolerate the oxpeckers, but they just are not able to get them off.
That is a possibility.
There's a neutral side that maybe it's not actually that big a deal to the cattle.
So it's kind of like when a fly's buzzing in my face and after awhile,
I just let it pester me cuz it won't go away.
This suggests that maybe the oxpeckers affect on the cattle was fairly small,
even though they're getting potentially nutrients themselves.
There's the mutual advantage side that somehow they're strengthening
the cattle's immunity [COUGH] possibly by increasing antibodies to fight antigens.
That would definitely be an adaptationist's explanation.
And there were some crazy out there explanations people suggested,
like maybe they like the company, or the oxpeckers generate mini force fields.
That's probably not it.
[COUGH] When we look at the research results for why this was the case, so
looking at the research results for why the hosts tolerate these, it turns out
in fact that some of these hosts try to get them off, but are unable to do so.
Just like that one student had suggested the previous slide.
That, in the case of rhinos they tried quite hard,
but were only able to get them off half the time when they were at wounds.
So this figure down here shows attempts to remove the oxpecker from at wounds.
So successful versus unsuccessful, so
only about half of them were successful at getting them off the wounds.
In contrast, when they are on the ears, they are able to flick them off fairly
easily, or other places on their body they're able to flick them off easily.
So it is not a mutualistic thing as was previously assumed,
even though that was the adaptationist's explanation.
So let's look at applying optimality theory to studying adaptive
feeding behavior.
But let's try to be careful not to be overly adaptationist.
So what considerations go into optimal feeding?
Well if you want to feed, and
again you wanna do it as efficiently as possible, what you wanna do is you want to
maximize energy while minimizing consumption of energy and time.
So high energy per low unit of time is the best.
So on the plus side, you're getting calories from food, that's your energy.
On the minus side, you're expending energy to get the food, so
energy may be involved in searching, handling, eating or digesting.
And on the minus, there's also time involved in getting food,
which also involves, of course, burning more energy.
But there's time involved in searching, handling, and digesting.
So if we were to put together a very simple formula for
optimal foraging, we want to maximize this quantity.
We want to maximize calories obtained from food minus calories expended getting food,
divided by time to get it.
Again, we want the prey with the highest caloric content, we want to spend
the minimal energy and time getting this prey, so this is the idea behind it.
So let's look at this in the context of whelk-eating crows,
another set of birds eating shelled animals.
[LAUGH] In this case, these are crows in British Columbia that pick up whelks on
average about 4 centimeters long rarely less than that.
They fly up into the air with these whelks and they drop them.
They fly almost exactly 5 meters and then drop them try to shatter the shell and
get the snail meat outside.
Now sometimes it takes a couple of flights to do it.
Is this adaptive?
There's a couple of different things these could do.
They could pick different sized whelks, they could fly different heights,
they could fly more times.
So let's look and see how this might be adaptive.
So break it up into parameters.
First, let's look at the height of drop.
Obviously, it takes energy to fly high, so
you wanna minimize the energy you're taking for flying.
But you want a high probability the shell will break.
You don't wanna have to fly 40 times.
You wanna fly like once or twice and be able to break it.
So people looked this in the context of the underlying physics, and it turns out,
in fact, 5 meters height is optimal for that sized shell to shatter.
So you can look here on the x-axis, the height in meters.
So this is from 0 to 15 meters in the air.
On the y-axis is number of drops required to break the whelk shell.
And we note when you're at 5 meters,
you're pretty much at a level point there where it doesn't seem like if you go much
higher you have a much higher probability of breaking the whelk shell.
So that's at a very optimal point.
There's very little increase in probability in shell
breaking if you go any higher.