Life for the cichlid fish Haplochromis burtoni,
native to Lake Tanganyika in East Africa, might appear carefree.
Nothing could be further from the truth, for these
ten-centimetre-long fish are perpetually subject to dramatic
upheaval.
But as Hans A. Hofmann and colleagues of Stanford
University, California describe in Proceedings of the National
Academy of Sciences1,
these fish have learned to roll with the punches, continually
switching their energies between growth and sex to cope best with
the prevailing circumstances. Fish in a fluctuating habitat undergo
more frequent changes in social status, and these changes affect
their growth rate
H.burtoni lives in shallow,
temporary ponds on the margins of the larger lake - a habitat always
subject to change. The males are territorial, defending a patch of
stones or a scrape in the gravel. But this is a temporary
topography, easily rearranged by currents, winds and the casual
catastrophes wrought by passing hippopotami.
In the wild, 20 to 40% of
the males are territorial - vigorous, solitary, brightly coloured
fighters. The remaining males are camouflaged, sexually repressed
and more sociable. But the distinction between territorial and
non-territorial males is less clear-cut than it might seem, because
males may change their social status, and do so more often in
frequently disturbed habitats.
Hofmann and colleagues simulated
disturbed and stable habitats in their laboratory fish tanks. They
didn't have any hippopotami, but a simple rearrangement of
terracotta flower-pots on the floor of the tank was enough to get
cichlid males in a frenzy of vicious fights, territorial acquisition
- and physiological change. Formerly territorial males faded and
were replaced by victors acquiring the bright colours of the
sexually active.
To the researchers' self-confessed
surprise, this social whirl had a pronounced influence on growth
rates. Fish that had just acquired territory grew much faster than
other males, a difference that was accentuated in fluctuating rather
than stable habitats. Fish that had just ceded territory grew the
slowest. In general, though, non-territorial fish grew faster than
territorial ones.
This finding suggests that fish tend to
build up their reserves while they are non-territorial, to be ready
when opportunity knocks. Bigger fish tend to be socially dominant,
getting territories when they become vacant through either the death
of the incumbent (birds and water snakes are very fond of bite-sized
cichlid snacks) or the rearrangement of the pondscape. But when a
fish acquires a territory, it ploughs its energy into defence and
reproduction rather than growth.
Hofmann,H. A., Benson, M. E. &
Fernald, R. D. Social status regulates growth rate:
Consequences for life-history strategies PNAS96, 14171 - 14176 1999. | Article | PubMed |
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