Evolution of Poison: How Ladybugs Gained a Defensive Trait

[LeonhardEuler]

I was just thinking today that I remember hearing that some insects like ladybugs and monarch butterflies are poisonous to birds that eat them. I got around to wondering how such a trait could evolve. The thing is that, while I see how being poisonous is advantageous to ladybugs in general, I don't see how it helps poisonous ladybugs over non-poisonous ones. The way I'm thinking of it, you had all these pre-ladybugs crawling around that were not poisonous, and then one comes along that is poisonous. How does that help him? If he does not get eaten before having children, then the poison did nothing for him since it only hurts birds that eat him. If a bird does eat him before he has children then the bird will learn not to eat any ladybugs, but the game over for the poisonous bug, and we have to wait for another one to come along before ladybugs can become poisonous.

I thought of a few things that might solve the problem, but I'm not sure if they're true. For instance if the trait of being poisonous arose simultaneously with the trait of being brightly colored then birds would learn not to eat brightly colored bugs by eating a poisonous bug. This would mean that if the original poisonous bug happened to pass this trait on to a sizable number of other bugs before being eaten, then a few of them could be eaten and this would give an advantage to the rest. Another idea would be that the bird can somehow tell that a bug is going to taste bad by smelling it or something. Or maybee there is something I am misunderstanding about the way all of this works: I am relying on my memory of this and I don't recall the source. Any ideas?

 

[Monique]

Yes, you'd think it would need to be accompanied by a visual cue that says 'I'm poisonous', as you say: the bird would have no idea how to select the bug. Was it not that the ladybug excretes its 'poison', giving it a chance to survive when the bird notices its error. The color coding could then evolve after that.

 

[LeonhardEuler]

Yes, you'd think it would need to be accompanied by a visual cue that says 'I'm poisonous', as you say: the bird would have no idea how to select the bug. Was it not that the ladybug excretes its 'poison', giving it a chance to survive when the bird notices its error. The color coding could then evolve after that.

That could be. I only remember hearing this a long time ago and it seems plausible for the ladybug to excrete something. But I can't imagine a butterfly excreting anything. From what I remember the monarch butterflies get their poison from something they eat, so maybe they did not need some sort of complex mechanism to generate poison, but it happens normally. Does that sound reasonable?

 

[Evo]

That could be. I only remember hearing this a long time ago and it seems plausible for the ladybug to excrete something. But I can't imagine a butterfly excreting anything. From what I remember the monarch butterflies get their poison from something they eat, so maybe they did not need some sort of complex mechanism to generate poison, but it happens normally. Does that sound reasonable?

If frightened the ladybug will emit blood (yellow) that contains toxic alkaloids and smells very bad.

 

[DocToxyn]

That could be. I only remember hearing this a long time ago and it seems plausible for the ladybug to excrete something. But I can't imagine a butterfly excreting anything. From what I remember the monarch butterflies get their poison from something they eat, so maybe they did not need some sort of complex mechanism to generate poison, but it happens normally. Does that sound reasonable?

Monarch caterpillars eat primarily milkweed which has a lot of nasty tasting and toxic substances in it. These agents remain in the adult butterfly and provide their measure of protection. Also, Evo brings up a good point, the insect may not actually be consumed if it has some means of releasing the toxins. It my get picked up and mouthed and then released when the predator is exposed.

 

[LeonhardEuler]

Thank you Monique, Evo and DocToxyn for your responses. I suppose it does make sense if the ladybug can prevent itself from being eaten and the butterfly does not actually produce the toxins. It really is amazing how consistent the theory of evolution is with with such a wide range of observations.

 

[gerben]

I was just thinking today that I remember hearing that some insects like ladybugs and monarch butterflies are poisonous to birds that eat them. I got around to wondering how such a trait could evolve. The thing is that, while I see how being poisonous is advantageous to ladybugs in general, I don't see how it helps poisonous ladybugs over non-poisonous ones.

You should not think that every beneficial trait started out with one single individual having that trait. There are all kinds of groups with different and overlapping traits within each population. Every trait is one among many. Traits may at first only appear in certain subpopulations and in time become prevalent in the whole population because the ones that have that trait are more proficient in producing offspring.

Here is a possible scenario:
A certain individual due to some mutation produces a slight variant of a chemical that it uses in its metabolism, which has no disadvantages, and its descendants will also have this altered chemical. It turns out that the predators do not like the specimens that have this altered chemical; it does not fit with the predator’s digestive system (it may even make the predator sick). Therefore the predators learn to avoid the areas in which they frequently encounter these distasteful variants. The predators will prey at places where they find good tasting prey and avoid places where they find bad tasting ones. However the bad tasting ones will spread into the adjacent areas, in which because of the predation there is less competition with other members of their species. There they will also mate with the good tasting variants of their species and eventually they will outnumber them. The predators will have to stray further and further in order to avoid eating the bad tasting variants. In the end this may even culminate in the predators having to refrain from considering this species as a suitable prey at all.