The Risks and Rewards of Relationships
I have never yet happened upon a trace of evidence that seemed to show that any one animal was ever made for another as much as it was made for itself.
- John Muir
Questions about the definitions of parasite vs. predator have been on my mind a lot. Predation is usually the end of an organism's life, and if it hasn't reproduced yet that is also the end of its gene line (unless it happens to be a clone of course). The predator typically benefits from the death of its prey, even as its greatest interests are for the prey's species to flourish. On the other hand, it is in the individual parasite's best interest that its particular host thrives (or at least doesn't die) for as long as possible so it can produce as many offspring as possible. Predators and parasites who are too successful may threaten their own progeny's future but individual parasites who are too successful may also threaten their own personal survival as well if they kill their host before finding a new one (many parasites can live without a host for a while too of course).
Then there are the parasitoid wasps who want their hosts to flourish as long as possible while the young larva finish consuming it. There the parasite almost evolves into a predator at the end. But for a while, the larva take care not to injure the more vital organs, which is something that horrified Darwin and he believed shouldn't reflect a design choice of his "beneficent and omnipotent God".
Obviously some parasites will also neuter their hosts. In doing so, they can potentially jeopardize their progeny's future too. I'm thinking of Sacculina which only gets away with such behavior over the long term because of the prevalence of uninfected crabs that manage to reproduce.
I also have to question the very core notion of parasitism. It's starting to look like some types of disease like asthma and IBS may be caused by a kind of "parasite deficiency". Giving people hookworms apparently cures some cases. The Australian media has been recently reporting this story about helminthic therapies for treating coeliac disease and it keeps showing up in my google saved searches. So I have to question the bigger definition of parasite in light of this idea of the co-evolution of our immune system in relationship to a parasite. It really sounds a little more mutualistic to me. We need a word where one party gets both harm and benefit from such a relationship.
I see a kind of continuum of relationships that is roughly:
mutualist > commensalist > parasite's host > prey
So for any individual of a species with regard to another species, the further to the right in the above schema the more dangerous your relationship. There are also many cases that involve more than two species and it blurs the lines. Also it leaves out species that perform agriculture (which don't fit in the above chart neatly at all). I really like the idea of lichens as colonies of fungus who are farming bacteria. If you raise something to eat it (or its byproducts) that seems to be a different thing from strict predation. There are some mutualistic aspects too. As individuals, sheep suffer a kind of predation when they are slaughtered for mutton, but because we also ensure reproduction they still benefit overall as a species from the relationship (at least, for some values of 'benefit' - this is where I drag out the link to Muir's excellent essay Wild Wool).
I'm going to just briefly emphasize one thing in the last paragraph again: the further to the right in the above schema the more dangerous your relationship. Some might think that surely there is no possible danger in a mutualistic association on the far left. I can envision two risks:
1) Your mutualistic neighbor/inhabitant gets sick, and that causes you to get sick.
2) Your mutualistic neighbor/inhabitant experiences mutations that make him less friendly (which wouldn't seem to favor his survival either in most cases).
On a recent desert trip that I've already written another tl;dr blog about, we also discussed the idea of what makes one organism more "evolved" than another. At a certain level, I think we can look at the genome and the potential it had in its past that might be resurrected if needed and the right circumstances come to bear. Sometimes evolution seems to go backward. When whales returned to the ocean, did they have to start over from scratch changing legs back to flippers? Probably not I suspect. So from a view of pure possibilities, the more complicated the genome and evolutionary past the more likely it is to return to one of those forms or maybe more importantly, to synthesize from those past adaptations something wholly new.
And it seems some parasites do have the ability to manipulate their host's DNA directly to make proteins they need for various tasks. In one sense, even better than having your own huge genetic legacy to work with is the ability to get at those genes in someone else's DNA. With the right universal translator why carry anything around at all? Maybe objections that viruses aren't alive because they don't have the means to reproduce independently are mistaken. After all, they do reproduce. They just don't carry around all the baggage they need to do it. I know I'm into The Selfish Gene's territory now, but have sadly yet to read it.
I believe that all relationships pose risks to all parties in the end. Any acceptance of a benefit is a risk of dependence and loss of said benefit. As I see it, the only known creature on earth that has really played it safe is Desulforudis audaxviator which is a one-organism ecosystem. Even "normal" relationships get turned upside down. If you get inside of something else as a parasite, you might be enslaved and eventually lose your own DNA and become just another organelle. Likewise, if you eat another organism as a predator, you may also consume parasites in it that kill you, in some cases stopping your gene line. Who had the last laugh there if the prey did reproduce? Some symbionts give their hosts defenses against other parasites as discussed in this blog. The enemy of my enemy is my friend right?
The cuckoo was part of my original inspiration for the desert conversation and this blog. It is a brood parasite that lays its eggs in another species' nest. There are some funny pictures showing tiny warblers feeding gigantic cuckoo chicks like the one in this interesting piece that explores why cuckoos happen to look like hawks. We get the word cuckold from this bird and the notion that someone may be tricked into raising another's children.
In its evolutionary past the cuckoo must have been another species that knew how to build nests and rear young directly. I imagine that if the host species all become extinct, the last cuckoos would be under a pressure that would create a kind of epigenetic effect and allow the recovery of that in at least some individuals. Or maybe it is too far gone for them and that's a fantasy without a long period of adjustments. Part of the reason I raise the issue is because habitat fragmentation is putting pressures on cuckoos and their hosts.
It is a trite platitude now somewhat further explored here that all relationships have the potential for risk and reward. In human affairs, when you open your heart up you become vulnerable to having it broken, but there's just no other way to exist in the world and experience the love we seem to need either. Sometimes, I think I might actually envy the life of D. audaxviator just a little.
Mike Lewinski
Denver, CO
November 12, 2009
