David Vieco-Galvez

and 4 more

The structure of Apteryx’s eggshell has generated much debate over the decades because it does not fit well with most allometric predictions. Apteryx eggshells are unusually thin and have been reported to be 60% less porous than expected. It has been suggested that these adaptations are compensations for a very long incubation period. Most studies so far have been carried out in what has been reported as Apteryx australis, and using infertile eggs or eggs laid in captivity. However, A. australis once comprised all kiwi with brown plumage, now separated into three distinct species: Brown Kiwi (A.mantelli), Rowi (A.rowi), and Tokoeka (A.australis). These three species use different habitats and live at different latitudes and altitudes. In addition, captive eggs are much smaller than wild laid eggs. These confounding factors make necessary to revise the assumptions made for Apteryx in the past. In this study, we analysed the physical characteristics of the Apteryx eggshells making a comparison between the three species of brown coloured kiwi and for some of the analysis we included some specimens of Roroa (A. haastii, Great Spotted Kiwi). We found that shell characteristics are different between the different species studied. The pore area of Apteryx eggshells was higher than previously suggested, and the water vapour conductance was much closer to what is expected for an egg that size. We found several new features such as triangular mineral particles composing the cuticle, only reported for a cretaceous Theropod, and the presence of plugs and caps on the eggshell pores. We suggest that the characteristics of the eggshells of the different species relate to the mating system of each species in addition to environmental variables, particularly pluviosity. We also suggest that the erosion of the cuticle during incubation is an adaptation to a long incubation period in a burrow.