- it's the result of an intricate web of evolutionary relationships. Why'd I pick this topic? Because I came across Chet Raymo's musings on Sacculina, a barnacle that over time has become an internal parasite on crabs. Female Sacculina larvae settle on a crab's exoskeleton & injects a mass of cells that move to the crab's abdomen, whence they send a mass of tendrils throughout their host's body. If a male Sacculina larva comes along, it can enter via a pore opened (& subsequently sealed) by the female & provides its sperm to fertilise her eggs. The crab remains alive but its behaviour & physiology are hijacked by the barnacle - to the extent that both male & female crabs behave like gravid females, growing a broad abdomen containing not crab eggs but those of the parasite, & releasing them into the ocean when mature. Works well for the parasite, & enough non-parasitised crabs survive to procreate & ensure a future supply of hosts. (In fact, if that wasn't the case, Sacculina would eventually run out of hosts & would then itself become rare - nature is full of checks & balances.)
Now, people can regard this sort of thing as more than a tad distasteful. And they'd be in good company. Charles Darwin wrote (in a letter to his American friend & colleague, Asa Gray) that I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice. He went on to argue that this, & other examples of apparent cruelty in the world effectively argued against the existence of any form of designer (& used the term 'intelligent designer' in its modern sense). Richard Dawkins comments that [if] nature were kind, she would at least make the minor concession of anaesthetising caterpillars before they are eaten alive from within. But nature is neither kind nor unkind. She is neither against suffering, nor for it. Nature is not interested in suffering one way or the other unless it affects the survival of DNA.
In the natural world there's no 'right' way to do things. And indeed, that humans can consider such concepts is itself the product of our own evolutionary history, shaped by natural selection in just the same way as that parasitic barnacle and its unfortunate hosts.