Apteryx, the kiwi, is the smallest member of the ratite birds, a group that includes the largest extant birds (the ostrich, emu, rhea, and cassowaries) as well as extinct birds (the moas and elephant birds). None of the species allocated to Ratitae are able to fly. Some, such as the ostrich and emu, are cursorial (runners); however, the kiwis and cassowaries typically live in densely forested areas. Although morphologically diverse, the ratites are connected by a number of synapomorphies including a fused coracoid and scapula, the trochlea of metatarsi II of the tarsometatarsus is not plantarly deflected and the distal end does not reach farther distally than the distal end of the trochlea of metatarsi IV, the musculus flexor hallucis longus to the hallux is either weak or absent, and the oil gland is only minutely tufted or naked (Mayr and Clarke, 2003).
The ratites belong to the more inclusive clade, Palaeognathae, a taxon that also includes the tinamous. Previously, the palaeognaths were grouped together based on the morphology of their palate (‘dromaeognathous’ or, more commonly referred to as the ‘palaeognathous’ palate; Huxley, 1867; Pycraft, 1900). The features that comprise the palaeognathous palate are a broad vomer that unites rostrally with the broad maxillopalatine plates and caudally with both the palatines and the pterygiods, the lack of an articulation between the pterygoids and the parasphenoidal rostrum, basipterygoid processes that project from the body of the basisphenoid/parasphenoid complex rather than the parasphenoidal rostrum, the baspterygoid processes articulate with the pterygoids near the distal ends of the pterygoids, and there is only a single head to the quadrate (Huxley, 1867). In addition to these palatal characters, other morphological features also unite Palaeognathae: a marked furrow just rostral of the nasal opening on the upper beak, rostral extension of the mesethmoid beyond the naso-frontal hinge, two strong grooves on the ventral surface of the mandibular symphysis, a flat dorsal surface of the mandibular symphysis, three to four costal processes on the sternum, and a greatly reduced or no hallux (Mayr and Clarke, 2003). Monophyly of Palaeognathae, although historically disputed, is supported in recent morphological, molecular, and total evidence analyses (Cracraft, 1974; Bledsoe, 1988; Härlid et al., 1997, 1998, 1999; Cooper et al., 2000; Livezey and Zusi, 2001; Mayr and Clarke, 2003).
There are three species within the taxon Apteryx, A. australis (brown kiwi), A. owenii (little spotted kiwi), and A. haastii (great spotted kiwi). All three of these species are found on the islands of New Zealand. All kiwis are nocturnal, and they locate their food (mainly plants and small insects) by probing through the forest floor with their long beak, which is innervated by an unusually large number of small rami of the trigeminal nerve. Apteryx lays an incredibly large egg for its body size; a female can lay an egg that is up to 25% its body weight. The incubation period is also extremely long, lasting approximately 90 days. When the chick hatches it is completely precocial; the hatchling is able to take care of itself with little or no parental assistance (Davies, 2002).
Literature
Baumel, J. J., A. S. King, J. E. Breazile, H. E. Evans, and J. C. Vanden Berge (eds.). 1993. Handbook of Avian Anatomy: Nomina Anatomica Avium, Second Edition. Publication of the Nuttall Ornithological Club, number 23. Nuttall Ornithological Club, Cambridge, Massachusetts, 779 pp.
Bledsoe, A. H. 1988. A phylogenetic analysis of postcranial skeletal characters of the ratite birds. Annals of Carnegie Museum 57:73-90.
Cooper, A., C. Lalueza-Fox, S. Anderson, A. Rambaut, J. Austin, and R. Ward. 2000. Complete mitochondrial genome sequences of two extinct moas clarify ratite evolution. Nature 409:704-707.
Cracraft, J. 1974. Phylogeny and evolution of the ratite birds. Ibis 116:494-521.
Davies, S. J. J. F. 2002. Ratites and Tinamous. Oxford University Press, Oxford, England, 310 pp.
Härlid, A., A. Janke, and U. Arnason. 1997. The mtDNA sequence of the ostrich and the divergence between paleognathous and neognathous birds. Molecular Biology and Evolution 14:754-761.
Härlid, A., A. Janke, and U. Arnason. 1998. The complete mitochondrial genome of Rhea americana and early avian divergences. Journal of Molecular Evolution 46:669-679.
Härlid, A., A. Janke, and U. Arnason. 1999. Analyses of mitochondrial DNA nest ratite birds within the Neognathae: supporting a neotenous origin of ratite morphological characters. Proceedings of the Royal Society of London, B, 266:305-309.
Huxley, T. H. 1867. On the classification of birds; and on the taxonomic value of the modifications of certain of the cranial bones observable in that class. Proceedings of the Zoological Society of London 1867:415-472.
Livezey, B. C., and R. L. Zusi. 2001. Higher-order phylogenetics of modern Aves based on comparative anatomy. Netherlands Journal of Zoology 51:179-205.
Mayr, G., and J. Clarke. 2003. The deep divergences of neornithine birds: a phylogenetic analysis of morphological characters. Cladistics 19:527-553.
Parker, T. J. 1891. Observations on the anatomy and development of Apteryx. Philosphical Transactions of the Royal Society of London B, 182:25-134.
Parker, W. K. 1866. On the structure and development of the skull in the ostrich tribe. Philosophical Transactions of the Royal Society of London B, 156:113-183.
Pycraft, W. P. 1900. On the morphology and phylogeny of the Palaeognathae (Ratitae and Crypturi) and Neognathae (Carinatae). Transactions of the Zoological Society of London 15:149-290.
Saiff, E. I. 1982. The middle ear of the skull of the kiwi. Emu 82:75-79.
Links
Apteryx haastii on The Animal Diversity Web (The University of Michigan Museum of Zoology)
Apteryx owenii on The Animal Diversity Web (The University of Michigan Museum of Zoology)
Apteryx australis on The Animal Diversity Web (The University of Michigan Museum of Zoology)
Kiwi Recovery Programme website
Kiwi on the New Zealand Birds website
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