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.