Much of the research in this field is conducted in mice (and other small animals). And mice, of course, aren"t humans. But given the mindboggling complexities of establishing causality between microbial signals and changes in human thought and behaviour, animal studies have provided some intriguing insights into the strange interactions between bacteria and brain. Research, for example, shows that "germ-free" rats and mice (those without any microbiota after being reared in sterile conditions) are more prone to anxiousness, and less sociable than those with an intact microbiota. Germ-free mice, and those given antibiotics have also been found to be more hyperactive, prone to risky behaviour and less able to learn or remember. Antibiotics, which can reduce the microbiota in an animal, also reduce shoaling behaviour in zebrafish, while probiotics boost it.