Cooperative behaviour to acquire food resources has been observed
in hunting carnivores and web-building social spiders.
Now researchers have found comparable behaviours in a fish
species. A tiny striped fish called Neolamprologus obscurus only
found in Lake Tanganyika in Zambia excavates stones to create shelter and
increase the abundance of food for all fish in the group.
Led by Hirokazu Tanaka of
the University of Bern in Switzerland and the Osaka City University in Japan,
this study is the first to document how team work in fish helps them to acquire
more food. The research is published in Springer’s journal Behavioral
Ecology and Sociobiology.
Neolamprologus obscurus is a highly sociable
species of cichlid found only in the southern reaches of Lake Tanyanika. These
zebra-striped fish feed mainly on shrimp and other invertebrates found along
the bottom of the lake. At night, shrimp move into the water column, but by
dawn they sink back to the lake bottom to hide in crevices and holes, including
the shelters that the fish have dug out under stones. Such excavation work is
always done as a group, as is subsequent maintenance efforts. Breeding fish
seldom leave these safe havens and are supported by up to ten helpers from
their family group. The helpers protect the brood, and constantly remove sand
and debris that fall into the cavities.
“The function of these excavated cavities is much like that of the
webs of social spiders, which live in groups and share the trapped prey among
group members,” explains Tanaka.
In this study, Tanaka and his colleagues wanted to find out if the
size of the cavities at the bottom of the lake relate to the abundance of food
available in the area, and if the presence of helpers influences the size.
Through hours of scuba diving in Lake Tanyanika, the researchers created
artificial cavities and examined the stomach contents of some of the fish. In
another experiment, the researchers removed helpers that were assisting
breeding fish. Within a week, enough sand had fallen into the cavities to
decidedly shrink these spaces. This effect was augmented when the helpers
removed were big.
One of the key findings was that the size of an excavated crevice
had an influence on the amount of shrimps that subsequently gathered in it.
When there were more helpers around, the space that could be created was bigger
and more shrimps could be gathered.
“Helpers in Neolamprologus obscurus extend and
maintain the excavated cavities, and by doing so, contribute to an increase in
food abundance inside the territory of breeding females,” explains Tanaka.
“Fish living in groups may be able to increase and maintain
considerably larger excavated cavities per capita compared to solitary living
fish. Consequently, group living enables Neolamprologus obscurus to
efficiently increase the prey abundance in their territory. This increases the
body condition and future reproductive success of breeders and/or helpers,”
adds Tanaka, who suggests that there is a clear benefit to group living for
this species of fish.
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