Galimberti, F., A. Fabiani and S. Sanvito (2002)
Measures of breeding inequality: a case study in southern elephant seals.
Canadian Journal of Zoology 80: 1240-1249.
Inequality in distribution of resources is a key aspect of evolutionary biology,
in particular in relation to distribution of mates an copulations.
Notwithstanding its important role, inequality is not easily defined, and its
measurement is complicated by theoretical and methodological issues. Although
the formal treatment of inequality has been mostly limited to the evolution of
lek mating system, a methodologically correct approach to measurement of
inequality has a general validity for the study of any kind of mating system. In
this paper, we analyze inequality in a large set of southern elephant seals
harems. The observed distribution of fertilizations was significantly different
from both the expected distribution with equal shares of resources, and the
expected distribution with equal propensities to get resources. We calculate and
compare various measures of inequality, observing a wide variation, in
particular among unbounded and bounded indices. We check the effect of choosing
a specific measure of inequality by considering the effect of two aspects of
harem socionomy, the number of females in the harem (i.e., the total amount of
resources to be shared) and the number of males associated to the harem (i.e.,
the number of competitors). The choice of a specific measure of inequality had a
strong impact on the results obtained, and should be considered a critical step
in every study of functional and evolutionary correlates of inequality. Not
bounded indices showed a strong relationship with both harem size and number of
males, while no effect was evident in the analysis of bounded indices. This
demonstrates that, in this species, the despotism of the mating system remains
high even in large harems and with many competitors, i.e., with the worst
conditions for monopolization.