Cats and humans, what domesticated whom?
The domestication of cats and its consequences have been a disaster for the human race. In antiquity cats and other domesticated animals lived largely outdoors, and did hunt vermin. Specifically, it is the requisition of cats indoors promotes a considerable mental health risk.
Domesticated cats are the ONLY definitive host to Toxoplasma Gondii, a protozoan that can form cysts in muscle tissue and has been shown to influence the behavior of intermediate hosts such as rodents. How does this work? Well, the parasitic alveolate can live-in warm-blooded animals and forms cysts in muscle and brain tissue. When a cat eats an infected rodent toxoplasma gondii can sexually reproduce within the intestine of the cat and it’s fecal matter contains eggs. If more warm-blooded animals (herd animals, rodents) come in contact with the fecal matter the parasite can survive and form cysts in the new intermediate hosts. These parasites undergo asexual reproduction in mammals and even find their way to the brain.
Rodents exposed to toxoplasma gondii change their behavior from finding cat urine aversive to being imprudently attracted to it. Also, their reaction times are slower, so can be easy prey for the definitive host. This observed behavior is well documented and has led to the hypothesis that parasites direct or control the behavior of intermediate hosts to optimize contact with the definitive host and continue sexual reproduction. This hypothesis seems uncanny, how can a little alveolate control the brain of a rodent, much less galaxy brained humans?
Evidence for Parasitic Control of Host Organism Behavior
Parasites have been shown to control the nervous system of insects rather readily. The kamikaze horsehair worm infects grasshoppers, crickets, and Praying Mantis and can accelerated cell division in the brain of these insects. Once the worms are fully mature the host jumps into a body of water, and drowns, and the worms exit the back door.
Another interesting phenomenon is the castrator barnacle, which enters into the claw of a crab and feminizes male crabs so that they lay parasitic “eggs” and tend to them just as a female crab would. The barnacle uses xenoestrogenic hormones to stop production of the fighting claws and grow a larger abdomen to host barnacle larvae. So there is plenty of evidence in the insect and crustacean world of parasites controlling host behavior. The behavior of infected rodents is easily observable, what is the mechanism behind it?
In the case of rodents,if the parasite were to actually influence the behavior in this directed manner than the cysts would target the neuronal circuitry underlying threat detection and undermine it. In this case I would predict that the toxoplasmosa would form cysts in the amygdala, perhaps specifically the basolateral amygdala, of rodents. A 2007 study in the Proceedings of the National Academy of Scientists demonstrated both the behavioral phenomenon and that the cysts were three times as likely to form in the amygdala as other parts of the brain. Other portions of the fear response circuit, the periacqueductal grey and the hypothalamus, also had more cysts than the rest of the brain. This means that the parasite must have a way, a mechanism for detecting or navigating specifically to these ventral areas of the brain. We don’t know how that is exactly but the distribution of the cysts or more likely on the underside of the brain where the fear circuit resides.
The Case for Toxoplasmosa Influence of Human Female Behavior
Although the mechanism is not known in humans, it has been shown that behavior in humans in altered with toxoplasmosis. For example, it Doubles Testosterone in women. Exposure to cats is also considered a risk factor for schizophrenia. People exposed to Toxoplasmosis are three times more likely to develop the mental disease. Unfortunately, all of the studies using brain histology have been done on rodents and little is known about the human brain with latent and active toxoplasmosis. It is often the case that scientific evidence only demonstrates explicitly what folk knowledge already knows implicitly, in this case that cat ladies are “crazy”. These ladies should donate their brains to science in the same fashion as football players in order to test whether the same circuitry is influenced by toxoplasmosa gondii.
From an evolutionary perspective it makes sense that women are more predisposed to cat hoarding, although the circuitry involved in programming women is more complex than in the rodents. Protozoans that are known to have an abortifacient effect on pregnant women and thus there is a likelihood that the circuitry underlying pair bonding a young woman would have with her newborns is hijacked and redirected toward nurturing cats, the definitive host of the toxoplasmosa. The desire to have a large brood of children ultimately becomes replaced with a large herd of cats, and the accumulation of cysts in the brain produces a positive feedback loop (or vicious cycle) in which impaired judgement begets more impaired judgement. Oxytocin is a key neurotransmitter in the formation of pair bonding and has an accumulation of receptor sites in ventral medial prefrontal cortex. The prefrontal cortex is, like the amygdala, located on the underside of the mammalian brain. Determining whether this brain region is disproportionally impaired in the cat lady is an interesting question, one that could be addressed by the cat lady brain donation program.
In the meantime, if you must own a cat keep it outside and not inside where you have to change litter boxes. The life you save may be your own.