In the egg shells of Rhea sp. there are grooves on the surface and it is into these grooves that one or more pore mouths open.
Five egg shells of Rhea americana and four of Rhea darwinii were studied in detail in respect of the numbers of pore mouths and the number, length and orientation of the grooves. It was found that R. americana had nearly twice as many pore mouths and grooves as had R. darwinii. Their distribution in both species showed more and longer grooves and more pore mouths in the equatorial regions than in the polar regions of the shell. The grooves were also found to be approximately longitudinally orientated in the equatorial regions but this orientation became less marked towards the polar regions. Not surprisingly longer grooves tended to have more pore mouths than did shorter grooves, but, in addition, broader grooves were associated with greater length too.