Interdependence of social structure and demography in a Southern elephant seal colony

LEWIS, M., FERRARI, M., PASCUAL, M., CAMPAGNA, C.

Ciudad del Cabo, Sudafrica

Conferencia; 17 Biennial Conference on the Biology of Marine Mammals Ciudad del Cabo, Sudáfrica; 2007

The Society fo Marine Mammalogy

Southern elephant seals breed at Península Valdés (PV) along 200 km of coastline. Annual pup counts at peak breeding season increased from 12,430 in 1995 to 14,350 in 2006. Two demographic subunits were identified in the North and South of PV, with different sex ratio and harem sizes. Births number increased in the South, but decreased in the North. We explored the relationship between the different trends and the social structure, starting from counts of females, males (dominant and bachelors), pups and weanlings in all the distribution area. We fitted a general population model with social structure to obtain a configuration of variables and estimate parameters that best match census data. The social structure was included by means of a family of harmonic mean type fertility functions F=(1+(hR)a) 1/a whose variables are the annual sex ratio R (mean number of males per female) and annual harem size h (mean number of females per harem). The product hR ranges from 1.95 to 2.87 in the North and from 4.38 to 5.41 in the South. The parameter a quantifies the influence of the social structure from strong influence (a » 0) to null influence, equivalent to a linear model with constant  fertility (a » -¥). The general model with social structure was significantly better than the linear model (c12 = 4.027, p = 0.045). The estimate of parameter a was -1.13, with fertility ranges from 0.71 to 0.79 in the North and from 0.86 to 0.88 in the South (15% lower in the North). The projected growth rates were lN = 0.99 in the North and lS = 1.02 in the South. Results indicate the relevance of considering social structure in population modeling of gregarious and polygynous specie, when individual hierarchies and social structure influence population parameters.