Parasitism

One example of a biotic factor is whether or not an organism is infected by any parasites.

Parasites are organisms that live on or in another organism, and typically cause them harm by doing things like using up their food resources, or maybe even damaging tissues and organs. As the host is harmed by the parasites, while the parasites benefit, parasitism is an example of the exploitation type of interspecific relationship.

But can parasite do more than just use up resources? Possibly. This article is about a parasite called Toxoplasma gondii, which lives for at least part of its cycle inside cats, and is egested in cat poo.  It has been observed, in studies on rats, that rats infected with T. gondii behave differently from uninfected rats: among other things, they are less fearful of cats (although not of other predators) and actively seek out areas where cats might be, through scent signals from cat urine. This would, of course, mean a rat has a much greater chance of being eaten by a cat, and thus bring the parasite back into a cat to complete its life cycle.

Humans can become infected with this parasite through handling cat poo (such as cleaning litter boxes) and not cleaning up afterwards properly, or through eating unwashed vegetables. Evidence collected so far suggests that T. gondii can also change the way our brains function, though with different effects in males compared to females. Males may become more introverted, less caring about what others think of them, and more suspicious and likely to break rules. Infected females do the opposite. One interesting (and scary) possibility is that this parasite could be the cause of schizophrenia! Maybe other parasites can explain behavioural changes often seen with some diseases. Read the article for details.

 How Your Cat Is Making You Crazy

Just another way that environment can affect phenotype.