The Allometry of Giant Flightless Birds

Abstract

Despite our intuition, birds are no smaller than mammals when the constraints of a flying body plan are taken into account. Nevertheless, the largest mammals are ten times the mass of the largest birds. Allometric equations generated for anseriforms and ratites suggest mid-shaft femur circumference is the best measure to use in estimating avian body mass. The small sample size of extant ratites makes mass estimate extrapolation to larger extinct species inaccurate. The division of ratites into cursorial and graviportal groups is supported. Aepyornithids do not show atypical femoral shaft asymmetry. New and more accurate estimates of egg masses, and separate male and female body masses for sexually-dimorphic ratites are generated. Egg mass scaling exponents for individual bird orders differ from that Aves as a whole, probably due to between-taxa effects. Ratite egg mass does not scale with the same exponent as other avian orders, whether kiwi are included or excluded. Total clutch mass in ratites, however, scales similarly to egg mass in other birds, perhaps as a consequence of the extreme variation in ratite clutch size. Kiwi and elephant bird eggs are consistent with the allometric trend for ratites as a whole, taking clutch size into account. Thus kiwi egg mass is probably an adaptation for a precocial life history, not a side effect of their being a dwarfed descendant of a moa-sized ancestor. Relatively small body size in ancestral kiwis is consistent with a trans-oceanic dispersal to New Zealand in the Tertiary, as suggested by recent molecular trees. This implies multiple loss of flight in Tertiary ratite lineages, which is supported by biogeographic, molecular, paleontological, and osteological evidence, but which is not the currently prevailing hypothesis.