Parasite Can Alter Host's Fear.

Toxoplasma gondii, a brain parasite spread by cat feces, can target and alter its host's feeling of fear!

Rats usually have an innate fear of cat urine. The fear extends to rodents that have never seen a feline and those generations removed from ever meeting a cat. After they get infected with the brain parasite Toxoplasma gondii, however, rats become attracted to cat pee, increasing the chance they'll become cat food.

This much researchers knew. But a new study shows the parasite, which also infects more half the world's human population, seems to target a rat's fear of cat urine with almost surgical precision, leaving other kinds of fear alone. - via Spluch

Parasite 'turns women into sex kittens',/a.

"A COMMON parasite can increase a women's attractiveness to the opposite sex but also make men more stupid, an Australian researcher says."
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If one can postulate an earlier, more generic form of the toxoplasma parasite, it may be seen as one whose natural habitat inside the host is the brain. Such parasites need to get to their habitat by circulating through the bloodstream, but they can settle in their favored location by attaching preferentially to molecules that are characteristic of particular organs. (This is known to work for the cold viruses, for cholera, for toxic E. coli).

Understandably, having parasites in one's brain will make the host sluggish or erratic in its behavior. Such deficiencies would make those hosts more prone to be caught and eaten by predators (It is well accepted that predators primarily catch the weak and ill).

As is the case with most living things, any population of individuals will have some variation in their makeup. Some will be more preferential to colonize particular parts of the brain. Some will be more generalist, and colonize the entire brain. (Such variation of individuals within a population is seen every day. Your office or classroom is filled with humans, but not filled with identical humans.) These variations are often inherited and are also subject to mutation.

A mouse that is infected with a toxoplasma that infects its entire brain may act so erratically that its ability to avoid predators is impaired before the colonies of parasites have had a chance to mature. A cat that eats such a mouse may not pick up enough parasites to become sustainably infected (The need for a minimum infectious load is well known in infectious disease studies).

A mouse infected with a more picky toxoplasma might take longer to develop such impairments. Perhaps such parasites are of a variant that colonize parts of the brain that are not directly involved in motor control (so the mouse doesn't stagger about), but are involved in less important controls (such as response to certain smells). When the infection becomes great enough, the mouse will begin to stagger about, and will be caught by a cat. The cat will now receive a parasite population of viable size.

Within the population of toxoplasma that infect non-critical parts of the brain, there may emerge varieties that have other preferences in other brain regions that are distinguished by their unique chemical markers. Among these would be a population whose infection makes mice less able to respond appropriately to indicators of danger. These risk-taking mice may be the ones we see today.

So, how does this relate to sex kitten humans? Promiscuous behavior in females and aggressive behavior in males may be two sides of the same coin of risky behavior patterns. Males and females with the same types of brain damage may express this in the form of different kinds of behavior.

So, the foundations of evolutionary theory are: A population has within itself a wide range of heritable variations. Some of these variations may confer a reproductive advantage over the rest of the population. Under conditions of relatively scarce resources, such advantages will result in the more advantageous variants becoming more prevalent with in the population. Further variation within this population will lead to refinement of the relevant trait.

Science is hard work, and not a game for quitters. Even a highly simplified speculation on the evolution of this trait takes a lot of words to describe. Each of the steps is known to us as a result of decades of research by thousands of full-time scientists, and to fully appreciate the smaller details may require a lifetime's dedication to study. It is not something summed up in a sarchastic one-liner.
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