Liver size reveals social status in the African cichlid Neolamprologus pulcher
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Wild groups (n = 167) of the cooperatively breeding Lake Tanganyika cichlid, Neolamprologus pulcher, were used to investigate how social status and sex influence liver investment. In contrast to expectations, males and females (controlling for body size) had similar liver investment and subordinates (both sexes) had relatively larger livers compared with dominants. Three hypotheses were considered for why social status results in liver size disparity: liver mass might reflect status-dependent differences in (1) energy expenditure, (2) energy storage and (3) energy acquisition. First, dominants performed more energetically costly behaviours (e.g. social policing and care) compared with subordinates, supporting the notion that energy expenditure drives liver investment. Moreover, dominants in large groups (with many subordinates to monitor) and those holding multiple territories (with large areas to patrol), tended to have smaller livers. Second, subordinates did not appear to use the liver as a strategic energy storage organ. In laboratory and field experiments, subordinates ascending in rank had similar or larger livers during periods of rapid growth compared with non-ascending controls. Third, although subordinates fed more frequently than dominants, a negative relationship was found between feeding rates and liver size. Hence, these results contrast with previous liver studies and suggest that liver investment patterns were linked to status-driven differences in energy expenditure but not to energy intake or storage in N. pulcher.
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