 several studies demonstrate that rats ( rattus novergicus ) infected with protozoan parasite toxoplasma gondii exhibit lesser fear to cat odors . 
 this is thought to increase transmission of the parasite to its definitive hosts , i.e. cats . 
 this is an example of extended phenotype where a gene of an organism allegedly creates a phenotype in another organism . 
 we examined a possible proximate mechanism for this phenotype , describing an epigenetic change in arginine vasopressin gene in medial amygdala of male rats . 
 exogenously mimicking medial amygdala dna hypomethylation resulted in reduction of fear to cat odors in uninfected animals , thus suggesting sufficiency . 
 systemic blockade of infection - induced dna hypomethylation countermanded infection - induced behavioral change , thus suggesting necessity . 
 this leads us to propose an epigenetic basis for this extended phenotype . 
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