The Allometry of Giant Flightless Birds

by Dickison, Michael R.

Abstract (Summary)
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 di?er from that Aves as a whole, probably due to between-taxa

e?ects. 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 e?ect 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.

Bibliographical Information:

Advisor:Roth, V. Louise; Nijhout, H. Frederick; McShea, Daniel W.; Rosenberg, Alex; Vogel, Steven

School:Duke University

School Location:USA - North Carolina

Source Type:Master's Thesis

Keywords:allometric equations ratites kiwis


Date of Publication:05/10/2007

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