Pores, gas exchange and water vapor conductance
Prior to this study the understanding was that the water vapour conductance of Apteryx was 65% lower than expected by allometric predictions because of low porosity in relation to egg mass, and as a compensation for a very long incubation period (Tullett 1984). The water vapour conductance we measured using shells was in accordance to that measured for ground burrowing species reported by (Portugal et al. 2014).The different eggshell regions showed different water vapour conductance indicating a difference either in thickness or pore density. In some eggs, it was possible to observe a cluster of pores concentrated in the most apical extreme of the blunt end; however since the eggshells used in the study came from hatched individuals in most cases this particular region had fractures and a neighbouring fragment was used, possible underestimating the actual number of pores of that region, nevertheless it has been noted that the air-cell of Apteryx is off centre (Rowe 1978). Significant differences were seen between the blunt end and the acute end for Brown Kiwi and Rowi in water vapour conductance, it is possible that this is also the case for Haast Tokoeka but because of the difficulty of sampling this was not observed. Differences in porosity in different eggshell regions have been reported for ducks and gulls (Portugal et al. 2010), but the purpose of this trait has not been discussed.
The water vapour conductance of the whole egg was almost twice as high than previously reported at 40.84 mg/day.torr compared to 26.00 and 23.71 mg/day.torr reported by Calder (1978) and Silyn-Roberts (1983) respectively. Furthermore, when compared with the values reported by Ar et al.,1974 for a variety of avian species, Apteryx mantellifitted perfectly into the expected relationship between egg mas and water vapour conductance, meaning that the egg loses water in the same proportion as any other avian egg. However, these calculations were made towards the end of the incubating process as opposed to the previous studies which used freshly laid eggs.
Calder (1978) stated that Kiwi’s eggshell porosity was 60% of the predicted value by Ar et al., (1974) equation. However, we found that pore density and pore radius were greater than previously measured, indicating that porosity should be higher, this in conjunction with plugged pores could explain what this was found in the past. In his study, Calder (1978) used eggshells from infertile eggs, where the cuticle would be intact and occlusions in place, this could explain why when the pores where counted, the porosity was under-estimated leading to exaggerated assumptions regarding the water vapour conductance ofApteryx. The eggshells we used belonged to successfully hatched eggs, meaning that if there was any abrasion or pore opening, pores would be more visible at this stage than in an infertile egg. This still needs to be tested by comparing the porosity of freshly laid eggs versus the porosity of successfully hatched eggs in the wild.
Our observations suggest that the cuticle becomes thinner through the incubation process. We suggest that this wearing results in more open pores with plugs, and occlusions disappearing and even some unperforated pores reaching the surface of the shell as the embryo develops allowing greater gas exchange and water vapour conductance. This would make sense for a species with a long incubation period in an underground nest, as capped pores could help reduce the risk of microbial contamination in early stages of development where oxygen is barely required. Prinzinger et al. (1995) reported that precocial embryos would drastically increase the oxygen consumption later in the incubation period, by this pointApteryx could have “polished” the eggshell allowing an increased gas exchange, by that time the defences remaining in the albumin would help protecting the embryo until hatching. The cuticle has been observed to help modulating to some extent the gas exchange in the domestic fowl (Peebles and Brake 1986); it would be then necessary to compare further incubated and unincubated Apteryx eggshells to determine if something similar occurs in these species.
A similar increase of water vapour conductance during incubation has been suggested for Adelie Penguins (Pygostelis adeliaei ) to cope with the extreme aridity of the Antarctic (Thompson and Goldie 1990); similarly, a reduction in pore thickness by mammillary and pore erosion has been suggested for the Mallee Fowl (Leipoa ocellata ) as a mechanism to increase water vapour conductance during a long incubation period in a high humidity and low oxygen environment (Booth and Seymour 1987). These observations suggest that water vapour conductance does not remain constant throughout the entire incubation process. Furthermore, some of these adaptations could give an advantage to birds to adjust water loss according to immediate ecological demands.
The triangular particles in the cuticle of Apteryx have not been reported for any other bird, but similar ones were observed in the fossil eggshells of Trigonoolithus amoae a theropod from the lower cretaceous period found in La Cantalera, Spain (Moreno-Azanza 2013). Since it has been suggested that many dinosaurs might have buried their eggs (Tanaka et al. 2015), and there is evidence of a nesting theropod (Oviraptor philoceratos ) which might have similar nesting behaviours to modern ratites (Norell et al. 1994), it could be possible to suggest that these triangular particles play some role related to the nest environment.
The eggshell characteristics found in this study suggest thatApteryx species are adapted to the environmental conditions where the eggs are incubated, and possibly the breeding strategies. Each characteristic may be explained by more than one factor suggesting that it is the synergy between the factors that shaped the eggshells of the different species.