Rainbow fish that prefer to use one eye are less brave than those who use their eyes equally
Rainbowfish that use both sides of their brain equally to analyze information are braver than those that prefer one side. The discovery throws light on brain lateralization in ways that may be relevant for humans as well.
 
Most vertebrates are not even-handed in their use of the brain. The familiar aspect of this is preference for using a particular hand or foot, reflecting to some extent brain dominance, but the influence emerges in many other ways. Since the right eye is connected to the left side of the brain, and vice versa, in species where connections between the two sides of the brain are poor odd things can happen. Information that is only presented to one side gets processed in very different ways from if it was seen with the other eye.
 
This has produced some pretty amusing examples. Male Gouldian finches with a patch over their right eye lose all judgment and flirt indiscriminately rather than only with birds they would normally be attracted to.
 
Now researchers at Australia's Macquarie University have compared the willingness of rainbow fish that show strong lateralization to leave hiding places to those that use both sides equally, publishing in Behavioral Ecology and Sociobiology.
 
Associate Professor Culum Brown says he “used a standard mirror test to establish lateralization.” Fish were placed in a flume with mirrors on either side. The fish do not recognize their reflection. “We record the time they spend looking with one eye or the other,” says Brown. “This has been shown to match the position they take up within schools.”
 
Right lateralized fish were shyer than those with a left lateralization, “but only marginally” Brown says. While this may have something interesting to say about hemispheric dominance the major finding was that either form of lateralization left the fish less inclined to face the world than those that used both sides equally.
 
Brown concludes that when a single hemisphere is doing the processing, fear becomes heightened. “We've done some work with parrots and show that non-lateralised animals tend to be at a cognitive disadvantage,” says Brown, “They're slower.” He suspects that, “Fear is like a break, but if you are slow at processing fear then those breaks are not applied. If it is not having a mitigating effect you are more spontaneous, so explore more quickly.”
 
This raises the question of why non-lateralization exists. If it slows down thinking it would appear to be a disadvantage, one possibly compounded if restraints on cautious behavior are removed. However, Brown notes, “If you are not a part of a shoal and strongly lateralized and a predator comes at you from your bad side you are likely to be killed. You need to look at the advantages and disadvantages in a lot of different contexts.”
 
Brown also thinks there may be value in rarity. Just as left-handers gain an advantage in certain sports because their opponents are not used to playing them, fish that have no lateralization at all may gain an advantage by being the odd ones out. As a frequency-dependent benefit however, it is subject to the same pressures as others of this sort, including beards and host bird responses to cuckoos.
 
Nevertheless, the findings were unexpected because, in the paper's words, “Given the emotive nature of [boldness], there is good reason to suspect that it is also likely to be expressed in a single hemisphere,” One theory holds that one side of the brain is naturally bold while the other tends to caution, and that these sides can alter in dominance over periods of time.
This might have been expected to make the difference between fish lateralized one way or the other larger than those with no lateralization at all. 
 
In another somewhat counter-intuitive result Brown also found that fish born are raised in the wild were bolder than those that grew up in captivity, not exposed to predators. Presumably experience surviving predators causes fear to eventually wear off, compared to individuals for whom dangers are new.
 
Brown and his co-author Anne-Laurence Bibost argue, “Variation in cerebral lateralisation contributes to the persistence of individual differences in boldness scores in animal populations.” Brown says that in the rainbow fish lateralization accounts for 30% of the difference in boldness among individuals. “It's not necessarily 30% in other species,” Brown says, “But it is probably something. If it were not for lateralization we would be more alike.”
 
Lateralization is not always innate, however. Research at the Australian National University has shown that certain parasites can cause increased lateralization in coral reef fish, affecting their response to predators.
 
Brown was one of the researchers who stunned marine biologists in 2010 with the evidence of a tuskfish using a rock as an anvil, arguably the first published case of tool use in fish.
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