Shoals of fish tend to follow a leader when they are at greater risk of being eaten by a predator, a new study suggests.
Researchers studied different groups of wild guppies — from rivers in the Northern Range mountains in Trinidad — in fish tanks in a laboratory.
Using computer tracking software, they monitored how each individual moved within the shoal.
In groups taken from places with a high number of predators, the guppies tended to follow the movements of a lead fish.
This gave the shoal — used by many fish as a form of defence — greater cohesion.
In contrast, guppies from the areas with fewer predators tended to make their own choices, resulting in a more "egalitarian" social structure but also a less tightly packed shoal.
Writing in the journal Science Advances, the researchers, from Bristol University, Glasgow University and the University of the West Indies at St Augustine, said: “The risk of predation is a major force in the evolution of morphological, behavioural, and life history traits.
“Stunning examples of collective movement, such as the dynamics of fish schools and bird flocks, are widely believed to be adaptations to avoid predation.
“Despite this, there is little understanding of the effect of predation on another major aspect of living in groups — how animals make decisions in a social context.”
The scientists studied more than 300 guppies taken from different rivers within about 18km of each other.
They were picked from places with different levels of predators, such as the pike cichlid.
The fish were placed in a specially designed maze and their movements were monitored. The researchers found the majority of decisions by a leading fish were followed, creating a high level of cohesion in the shoal.
And this was particularly true among fish that came from areas where there was more danger from predators.
However, it remains unclear if the fish were consistently led by a small number of individuals or if they simply trusted the judgement of whoever happened to be in front of them at the time.
“Given the recent burgeoning of research in animal ‘personalities’, it would be of widespread interest to determine whether individuals consistently differ in their propensity to initiate movements over longer time scales, so that particular individuals consistently emerge as those attempting to lead group movements,” the researchers wrote.