The number of pores per area of eggshells is not always a reliable indicator of Rheidae species

Abstract

From the end of the Pleistocene and up until the late Holocene, bones and abun- dant eggshell fragments testify to the hunting by the indigenous people of Rheidae in the Pampas and Chaco regions (greater rhea, Rhea americana), and in the Argentinian Patagonia (lesser/ Darwin´s rhea, R.pennata). The traditional method to set apart eggshell fragments from these two species consisted in counting the number pores on a given area to estimate their density. In this paper we evaluate the validity of this method with a new protocol to facilitate counting and assess its reliability on a large eggshell sample. As has been repeatedly proved, the greater rhea has a larger pore density than the lesser rhea. However, the variability of this density within each species, and even within the same egg, needs to be considered as this may lead to erroneous identification. More so when the number of pores per cm2 falls in the lowest range of the greater rhea or the highest range of the lesser rhea. In general, it is easier to misidentify a greater rheaeggshell fragment for that of the lesser rhea than the other way around. The possibility of misidentification also depends on the area of the shell that is being analyzed, since the original method did not apparently assess the density of pores in different areas of the same egg for each species. Although our results indicate that identification based on the original method is not as reliable as the one we propose here, a reappraisal of it with larger samples deriving from a larger specter of populations from both species would be recommendable.