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Palaeognathae

Palaeognathae is a monophyletic clade of birds within the subclass Neornithes, serving as the sister group to the more diverse Neognathae and representing one of the two primary lineages of modern birds.^[1] This group encompasses approximately 60 extant species distributed across five orders: Struthioniformes (ostriches), Rheiformes (rheas), Casuariiformes (cassowaries and emus), Apterygiformes (kiwis), and Tinamiformes (tinamous).^[1] ^[2] Characterized by a distinctive primitive palate featuring enlarged basipterygoid processes, fused pterygoids and palatines, a grooved rhamphotheca, a single quadrate articular facet, and open ilioischiadic foramina, palaeognaths include both flightless forms known as ratites and the volant tinamous.^[1] Phylogenetically, palaeognaths originated likely before the Cretaceous-Paleogene extinction event around 66 million years ago, with their diversification tied to the breakup of the Gondwanan supercontinent and subsequent ecological opportunities in the Paleogene.^[1] Molecular and morphological analyses confirm the monophyly of Palaeognathae, though traditional ratites are paraphyletic, with tinamous nested within them; ostriches often appear as the basalmost lineage, followed by a clade including rheas, tinamous, kiwis, and extinct groups like moas and elephant birds.^[1] ^[2] Flightlessness has evolved independently at least six times within the clade, resulting in reduced wings, strong cursorial legs, and a flat or reduced sternal keel in ratites, while tinamous retain the ability to fly short distances despite their terrestrial habits.^[1] The fossil record of palaeognaths extends from the Paleocene, with early taxa like the Lithornithidae from Europe and North America representing small, volant ancestors, and later forms showing gigantism and regional endemism, such as the extinct Dinornithiformes (moas) in New Zealand and Aepyornithiformes (elephant birds) in Madagascar.^[1] Extant palaeognaths exhibit diverse ecologies, from the large, fast-running ostriches of African savannas to the nocturnal, insectivorous kiwis of New Zealand forests, and the quail-like tinamous of South American grasslands.^[1] ^[3] Many species face conservation challenges due to habitat loss and introduced predators, with kiwis and some tinamous classified as vulnerable or endangered.^[3]

Taxonomy and classification

Historical classifications

In the early 19th century, ratites were classified in separate taxonomic groups rather than as a unified assemblage. Johann Karl Illiger, in his 1811 Prodromus systematis mammalium et avium, established the order Struthiones specifically for ostriches (Struthio), while rheas were placed in the distinct order Cursores and other flightless forms like cassowaries in Proceri, reflecting a focus on geographic distribution and superficial morphology over shared traits.^[4] By the late 19th century, more detailed anatomical studies began to highlight potential affinities among ratites. Max Fürbringer's comprehensive 1888 work, Untersuchungen zur Morphologie und Systematik der Vögel, emphasized skeletal and muscular characters, noting the diverse morphology of the palaeognathous palate across ratite groups and suggesting it as a primitive trait potentially linking them, though he leaned toward polyphyletic origins due to variations in palate structure and limb adaptations.^[5] The early 20th century saw the formal proposal of Palaeognathae as a higher taxon encompassing both ratites and tinamous, based on shared primitive features like the complex, groove-patterned bony palate. Percy R. Lowe, in his 1920s publications such as the 1928 Ibis paper on ostrich phylogeny, advocated for Palaeognathae as a subclass, arguing that the palate's configuration—featuring fused pterygoids and enlarged basipterygoid processes—indicated a common ancestral stock diverging early from other birds. Alexander Wetmore, in his 1930 systematic classification, endorsed this by placing ratites and tinamous within the superorder Palaeognathae but as separate orders (Ratitae and Tinamiformes), citing palatal and cranial similarities despite differences in sternum keeling and flight ability; however, critics like Joel Asaph Allen rejected full inclusion of tinamous, emphasizing the palate's subtler variations and the flighted nature of tinamous as evidence against close ratite affinity.^[6]^[7]^[8] Pre-molecular era debates persisted into the mid-20th century, particularly regarding internal ratite relationships, such as whether kiwis (Apteryx) and emus (Dromaius) formed a distinct clade. Studies in the 1960s on eggshell microstructure, including those by W.J. Schmidt, revealed shared features like a thick mammillary layer and columnar crystals in kiwi and emu shells, supporting a potential sister-group relationship between them and contrasting with ostrich and rhea eggshells, though these interpretations fueled ongoing controversies over convergence versus homology in flightless adaptations.^[9]^[10] These morphological frameworks laid the groundwork for later cladistic analyses.

Modern phylogeny

Genomic studies have firmly established the monophyly of Palaeognathae as the sister group to Neognathae within the clade Neornithes, representing one of the two basal lineages of modern birds. This positioning is supported by whole-genome analyses of 48 avian species, which resolved early branches in the avian tree of life using over 400 million base pairs of aligned sequence data.^[11] Recent time-calibrated phylogenies, incorporating updated fossil calibrations and expanded taxon sampling, continue to affirm this topology, with crown Palaeognathae diverging around 68–62 million years ago in the Late Cretaceous to Early Paleogene.^[12]^[13] Within Palaeognathae, the phylogeny shows ostriches (Struthioniformes) as the basalmost lineage, followed by rheas (Rheiformes), with tinamous (Tinamiformes, comprising approximately 47 species across two subfamilies) sister to the clade of kiwis (Apterygiformes), emus (Casuariiformes), and cassowaries (Casuariiformes).^[14] This nesting of tinamous within the otherwise flightless ratites renders the traditional Ratitae paraphyletic. Ratites include the extant orders Struthioniformes (ostrich, Struthio spp.), Rheiformes (rheas, Rhea spp.), Casuariiformes (cassowaries, Casuarius spp., and emu, Dromaius novaehollandiae), and Apterygiformes (kiwis, Apteryx spp.), as well as extinct groups such as Aepyornithiformes (elephant birds) and Dinornithiformes (moas).^[14] Molecular phylogenies recover a resolved topology where the ostrich is sister to all other palaeognaths, followed by the rhea as the next successive outgroup, with tinamous sister to the clade comprising kiwis sister to emus and cassowaries.^[14] This structure is congruent across concatenated and coalescent species-tree methods using nuclear and mitochondrial markers.^[14] Debates persist regarding the exact branching order within ratites, particularly the position of kiwis relative to emus and cassowaries, stemming from earlier Bayesian analyses that suggested alternative affinities based on incomplete sampling or conflicting morphological data.^[15] However, comprehensive phylogenomic datasets have largely resolved these uncertainties, supporting the kiwi-(emu + cassowary) clade through whole-genome alignments that account for incomplete lineage sorting, with tinamous as their sister group.^[14] A recent molecular phylogeny of tinamous, incorporating all 47 recognized species, confirms the division into the forest-dwelling Tinaminae (e.g., Crypturellus, Tinamus, Nothocercus) and open-habitat Nothurinae (e.g., Nothoprocta, Nothura, Rhynchotus), with Neotropical diversification dated to 31–40 million years ago in the late Eocene to early Oligocene.^[16] Key synapomorphies uniting Palaeognathae include a reduced or absent sternal keel in ratites, adapted for flightlessness, contrasted with the retained, well-developed keel in tinamous that supports limited flight capability; molecular evidence further demonstrates that ratites form a paraphyletic group with tinamous nested within them.^[17]^[18]

Morphology and physiology

Size and body form

Palaeognathae display remarkable variation in body size and form among extant species, spanning from the largest living birds to some of the smallest flight-capable forms. The ostrich (Struthio camelus), the tallest and heaviest, stands up to 2.75 m high and can weigh 156 kg, primarily in males which are larger than females.^[19] At the opposite extreme, kiwis (Apteryx spp.) are among the smallest, with the great spotted kiwi reaching a maximum length of 55 cm and weight of 3.3 kg in females, while the little spotted kiwi measures about 40 cm and weighs up to 1.9 kg.^[20] Tinamous, the only flying palaeognaths, are generally small ground-dwellers averaging 20–50 cm in length and 40 g–1,250 g in mass, exemplified by the dwarf tinamou at 14.5 cm and 43 g or the great tinamou at 46 cm and up to 1.25 kg.^[21] This size gradient highlights adaptations in flightless ratites toward larger bodies for terrestrial life, contrasting with the compact, agile builds of flying tinamous. Body plans in palaeognathae reflect ecological niches, with flightless forms like ostriches and rheas featuring elongated necks—up to 1 m in ostriches—for foraging and vigilance, paired with robust, upright postures.^[19] In contrast, kiwis and tinamous possess more compact, rounded bodies suited to dense undergrowth navigation, with short legs and minimal tails.^[20] Sexual size dimorphism is prevalent, with females exceeding males in mass for most ratites including emus (females ~37 kg vs. males ~32 kg), cassowaries (females ~46 kg vs. males ~32 kg), and kiwis (e.g., females ~2.7 kg vs. males ~2.3 kg in A. australis), as well as in tinamous; however, males are larger in ostriches (~115 kg vs. ~100 kg) and rheas (~28 kg vs. ~22 kg).^[22] Distinct proportions further differentiate palaeognathae, such as the relatively large brain size in kiwis compared to body mass, supporting advanced olfaction and nocturnality despite their small eyes—the smallest relative to body size among birds.^[23] Flightless ratites exhibit reduced wing proportions, with vestigial limbs often hidden beneath dense feathers and measuring mere centimeters, emphasizing terrestrial specialization over aerial capabilities.^[24] Fossil evidence from stem-palaeognaths like lithornithids reveals ancestral medium-sized flying forms, with body masses estimated at 1.5–2.8 kg based on skeletal and phylogenetic analyses.^[25]

Skeletal and flight adaptations

The palaeognathous palate, a defining feature of the group, is characterized by a broad, flat vomer and robust pterygoids that articulate with the palatine, retaining primitive avian traits such as an unossified or reduced vomer and fused elements that differ from the more ossified, narrower palates in neognathous birds.^[26] This structure, historically viewed as plesiomorphic for crown-group birds (Neornithes), may instead represent a synapomorphy unique to Palaeognathae, as supported by comparative analyses of cranial fossils.^[26]^[6] The sternum in ratites exhibits a flat or greatly reduced carina (keel), an adaptation correlated with the loss of flight capability, as the keel normally anchors the large pectoral muscles required for wing-powered locomotion in flying birds.^[18] In contrast, tinamous retain a keeled sternum, albeit smaller and less pronounced than in most neognaths, enabling short bursts of flight for escape or navigation.^[18] Adaptations in the wing and pectoral girdle further distinguish ratites from tinamous. In ratites, the humerus is reduced in size and robusticity, while the scapula and coracoid are fused into a single element, minimizing the flight apparatus and redirecting skeletal support toward terrestrial locomotion.^[26] Tinamous, however, possess a functional yet asymmetric pectoral girdle with a more developed, albeit simplified, coracoid and scapula, supporting limited aerial capabilities despite overall reduction compared to neognaths.^[26] Recent paleoneurological studies of stem palaeognaths, such as the endocast from Lithornis vulturinus, reveal braincase morphology with relatively large olfactory bulbs and regions, a plesiomorphic condition that exceeds the reduced olfactory proportions typical in Neognathae, indicating retention of ancestral sensory priorities in early palaeognath evolution.^[27] Hindlimb adaptations in Palaeognathae emphasize terrestrial prowess, with elongated femora and tibiotarsi providing leverage for rapid running, as seen in ostriches that achieve speeds over 70 km/h.^[18] Emus reach speeds up to 50 km/h. In species such as cassowaries, the feet feature three stout toes equipped with powerful claws, enhancing stability and defensive capabilities on forested floors.^[28]

Locomotion

Palaeognathae exhibit a range of locomotion strategies adapted to their environments, from the flightless, cursorial running of ratites to the short-distance flight of tinamous. Ratites, including ostriches, emus, rheas, cassowaries, and kiwis, are primarily bipedal runners, relying on powerful hindlimbs for terrestrial movement.^[29] Ostriches (Struthio camelus), the largest ratites, achieve sustained running speeds of 50-60 km/h and short bursts up to 70 km/h, enabled by their elongated legs and efficient stride mechanics.^[30] These birds also employ powerful kicks for defense, delivering forward slashes with their strong legs that can injure predators.^[31] In contrast, kiwis (Apteryx spp.) are nocturnal walkers, moving slowly and deliberately through undergrowth at night using their short legs for foraging and navigation.^[32] Tinamous, the only flying palaeognaths, are predominantly ground-dwelling but capable of short, explosive flights when disturbed. Their takeoffs involve rapid, noisy wing beats producing a characteristic whirring sound, low and direct, seldom exceeding 10-20 meters in height, reflecting their small wings and high wing loading.^[33] Recent phylogenetic analyses of stem-palaeognaths indicate that ancestral members possessed flight capabilities akin to those of modern tinamous, with flightlessness in ratites evolving convergently across multiple lineages due to insular or terrestrial adaptations.^[34] This supports the hypothesis of multiple independent losses of flight within the group, rather than a single ancestral event.^[35] Energy efficiency in ratite locomotion is enhanced by high stride lengths, particularly in rheas and ostriches, where long legs allow strides of 3.5-7 meters, minimizing the cost of transport during sustained running.^[30] Ratites use their vestigial wings minimally, primarily for balance and stability during high-speed maneuvers rather than propulsion.^[36] These adaptations, supported by robust pelvic limb skeletons, optimize bipedal gait for endurance over long distances.^[37]

Evolutionary history

Fossil record

The fossil record of Palaeognathae begins in the Late Cretaceous with early neornithine forms that exhibit traits toward flightlessness. Patagopteryx deferrariisi, discovered in the Anacleto Formation of Patagonia, Argentina, dates to approximately 80 million years ago (Ma) and represents an early flightless bird. This small bird, measuring about 50 cm in height, exhibits skeletal features such as reduced wings and robust legs, indicating secondary flightlessness derived from flying ancestors within the broader avian radiation.^[38] In the Paleogene, the record expands with volant forms of the Lithornithidae, small flying palaeognaths that bridge stem and crown groups. Lithornis species, such as L. vulturinus from the early Eocene Fur Formation in Denmark (around 55 Ma), preserve nearly complete skeletons showing well-developed flight adaptations, including strong keeled sterna and long wings. Similar fossils from the Green River Formation in North America further document this group's distribution across Laurasia during the early Eocene, suggesting early palaeognaths were ecologically diverse and capable of powered flight before the evolution of modern ratites. Ostrich relatives first appear in the Eocene, with stem-struthionids like Palaeotis from Middle Eocene deposits in central Europe (around 45 Ma); later diversification includes fossils from Miocene deposits in Asia, such as Mongolia and northwest China.^[39]^[40]^[41]^[42] Later fossils reveal the evolution of gigantic extinct ratites, underscoring the group's capacity for extreme body size increases. Aepyornis, an elephant bird from Pleistocene to Holocene deposits in Madagascar, reached weights of up to 450 kg and heights exceeding 3 m, with eggshell fragments and skeletal remains providing evidence of its herbivorous lifestyle in island ecosystems. Similarly, moas of the genus Dinornis from New Zealand, known from Holocene subfossil bones in caves and swamps, attained masses up to 250 kg, representing the largest terrestrial birds until their rapid extinction around 600 years ago due to human hunting. These giants exemplify late Cenozoic adaptive peaks in isolated Gondwanan settings.^[43]^[44] Recent analyses incorporating internal fossil constraints, including tip-dating methods on phylogenomic datasets, have refined divergence estimates for crown Palaeognathae to approximately 66–80 Ma, placing the group's origin near or before the Cretaceous-Paleogene (K-Pg) extinction and emphasizing the role of Mesozoic fossils in calibrating avian timelines. However, the overall record remains fragmentary, particularly in the Southern Hemisphere, where preservation biases limit insights into Gondwanan evolution; notable exceptions include ratite tarsometatarsi from the Paleogene La Meseta Formation on Seymour Island, Antarctica, which suggest early southern presence and vicariant origins tied to continental breakup. These Antarctic discoveries, though rare, imply a broader prehistoric distribution for palaeognaths across polar Gondwana.^[45]^[46]

Origin and diversification

The origin of Palaeognathae traces back to the divergence from Neognathae approximately 110 million years ago (Mya) during the Early Cretaceous, likely within the ancient supercontinent of Gondwana, where ancestral lineages of modern birds began to differentiate. These early palaeognaths are inferred to have been small, volant birds adapted to forested environments, which facilitated their survival across the Cretaceous-Paleogene (K-Pg) boundary mass extinction event around 66 Mya. Post-extinction, the absence of large predatory dinosaurs and enantiornithine birds created ecological opportunities for these surviving lineages to persist and begin radiating in the Paleogene. Recent phylogenomic studies confirm ostriches as the basal extant lineage, with tinamous nested within ratites, supporting a primarily Gondwanan origin but with debates on Laurasian contributions for northern forms like lithornithids.^[47] Diversification within Palaeognathae accelerated in the Cenozoic, with flightlessness evolving independently at least four to six times within the clade between approximately 40 and 60 Mya, driven by reduced predation pressure and the availability of open habitats following the K-Pg event.^[48] The tinamous, the sole volant palaeognath order, underwent a radiation in South America around 40 Mya, adapting to diverse Neotropical ecosystems while retaining flight capabilities. Time-calibrated phylogenies from recent genomic analyses place the ostrich (Struthio) as the basal extant lineage, diverging around 50-60 Mya, followed by splits such as that between kiwis and emus/cassowaries approximately 30-40 Mya. These timelines highlight how vicariance from Gondwanan fragmentation and subsequent dispersals shaped clade distributions, with later extinction events—such as the human-driven loss of moas in New Zealand about 600 years ago—further sculpting modern diversity. Key evolutionary drivers included the post-K-Pg decline in aerial and terrestrial predators, enabling terrestrial adaptations like flight reduction and, in isolated island settings, convergent gigantism as seen in moas and extinct elephant birds. Recent phylogenetic reconstructions incorporating fossil evidence support a pan-palaeognath clade that includes volant Paleogene forms like lithornithids, confirming that flightlessness represents a derived trait rather than a primitive condition for the group.

Biogeography

Current distribution

The extant species of Palaeognathae exhibit a highly disjunct distribution across the Southern Hemisphere and Neotropics, reflecting their ancient Gondwanan origins but with no overlap between major lineages. Ratites, the flightless members, are confined to specific southern landmasses: the ostrich (Struthio camelus) is native to sub-Saharan Africa, spanning countries including Angola, Botswana, Ethiopia, Kenya, Namibia, South Africa, Tanzania, and Zambia, among others.^[49] The greater rhea (Rhea americana) and lesser rhea (Rhea pennata) are endemic to South America, with the greater rhea ranging from northeast Brazil through eastern Bolivia, Paraguay, Uruguay, and northern Argentina, while the lesser rhea occupies southern and western regions including Patagonia in Argentina, Bolivia, Chile, and Peru.^[50]^[51] The emu (Dromaius novaehollandiae) is widespread across mainland Australia, from coastal areas to inland regions.^[52] Cassowaries (Casuarius spp.), comprising three species, are native to New Guinea (in Indonesia and Papua New Guinea) and northeastern Australia (Queensland), particularly in lowland rainforests.^[53] Kiwis (Apteryx spp.), with five species, are strictly endemic to New Zealand, primarily on the North and South Islands and adjacent offshore islands.^[54] Tinamous, the volant palaeognaths, are exclusively Neotropical and represent the most diverse group with 47 species distributed from southern Mexico through Central America to southern South America, reaching Patagonia in species like the Darwin's nothura (Nothura darwinii). In 2025, the dwarf tinamou (Taoniscus nanus) was newly recorded in Bolivia, extending its known range approximately 1,500 km westward from central Brazil.^[55]^[56] They occupy a broad latitudinal range, from sea level to high elevations up to 4,500 m, in varied open and forested habitats.^[57] Unlike ratites, tinamous show no current presence in North America, despite fossil evidence of stem-palaeognath relatives like lithornithids from the Paleocene and Eocene of that continent.^[58] Introduced populations of ratites occur outside their native ranges, primarily due to farming and escapes. Ostriches have established feral groups in Australia and are commercially farmed worldwide for meat, feathers, and hides, with significant operations in the United States, China, and Europe.^[49]^[59] Emus are raised on farms globally, including in the United States and Europe, and have introduced wild populations on Kangaroo Island (South Australia) and Maria Island (Tasmania). Rheas have feral populations in northern Germany from escaped farm birds, with occasional escapes reported in parts of Asia and Europe.^[60] Cassowaries and kiwis have no established introduced populations outside their native ranges, though kiwis have been translocated within New Zealand for conservation. Tinamous lack any documented introduced populations.^[53] Range fragmentation is pronounced in several lineages, particularly kiwis, whose populations are isolated across fragmented forest remnants on New Zealand's islands, with each of the five species occupying discrete, often small areas due to habitat loss and predation.^[61] Tinamous exhibit some disjunct distributions, such as the black tinamou (Tinamus osgoodi) in isolated Andean pockets, but overall maintain more continuous ranges compared to ratites.^[62] As of November 2025, while no widespread range expansions have been recorded for palaeognath species, isolated extensions such as that of the dwarf tinamou in Bolivia highlight potential undocumented distributions. Climate change poses emerging threats to southern distributions, including intensified dry seasons in Neotropical tinamou habitats and shifting suitable areas for Australian and South American ratites.^[63]^[64]

Origin hypotheses

The Gondwana vicariance hypothesis posits that the ancestors of modern palaeognaths diverged as the supercontinent Gondwana fragmented between approximately 80 and 100 million years ago, leading to the isolation of lineages on separate landmasses such as ostriches in Africa, rheas in South America, emus and cassowaries in Australasia, and kiwis in New Zealand.^[65] This model is supported by morphological phylogenetic analyses that align divergence patterns with known geological events of continental drift.^[65] However, it has been challenged by molecular dating evidence indicating more recent divergences that postdate the full breakup of Gondwana.^[25] In contrast, the Tertiary radiation and dispersal hypothesis proposes that palaeognaths underwent multiple overland or trans-oceanic dispersals after the Cretaceous-Paleogene boundary, around 30 to 50 million years ago, rather than being strictly confined by vicariance.^[66] For instance, ostriches are thought to have dispersed from Asia to Africa via land bridges during the Miocene.^[66] Recent phylogenomic and biomechanical studies from 2025 emphasize the flight capabilities of stem-palaeognaths, supporting multiple independent losses of flight and facilitating dispersals across southern continents and islands.^[34] Hybrid models integrate elements of both vicariance and dispersal, with tinamous as ancient Neotropical relicts nested within the palaeognath clade, while kiwis arrived in New Zealand through overwater rafting of a volant ancestor during the Miocene.^[67]^[68] These scenarios account for the non-monophyletic distribution of ratites by positing initial Gondwanan splits followed by subsequent colonizations.^[66] Key evidence includes molecular clock estimates showing crown-group divergences as young as 40 million years ago for some lineages, such as basal tinamou splits between 31 and 40 million years ago, which conflict with the older fossil minima of around 80 million years ago implied by vicariance.^[16]^[25] Critiques of pure vicariance highlight the presence of Northern Hemisphere fossils, such as the Eocene Lithornis from Europe and North America, which suggest an early northern origin or dispersal pathway incompatible with a strictly southern Gondwanan radiation.^[25] This disjunct modern distribution across Africa, South America, and Australasia underscores the ongoing debate between these mechanisms.^[66]

Ecology and behavior

Habitat and diet

Palaeognathae exhibit diverse habitat preferences shaped by their phylogenetic branches, with ratites generally occupying open or forested environments across southern continents. Ostriches (Struthio camelus) thrive in arid savannas, grasslands, and scrub forests of Africa, favoring drier sandy areas where they form territorial flocks during breeding seasons.^[69] Rheas (Rhea spp.) inhabit open pampas, grasslands, and sparse woodlands in South America, adapting to both temperate and arid conditions.^[70] Emus (Dromaius novaehollandiae) range across a broad spectrum of Australian habitats, from arid shrublands to coastal woodlands, showing high environmental tolerance.^[70] In contrast, cassowaries (Casuarius spp.) and kiwis (Apteryx spp.) prefer dense rainforest understories and temperate forests in New Guinea, Australia, and New Zealand, respectively, where vegetation cover supports their ground-dwelling lifestyles.^[70] Tinamous, the volant members of Palaeognathae, predominantly occupy the understory of Neotropical tropical woodlands, rainforests, scrublands, and grasslands from Mexico to southern South America, spanning elevations up to 5,000 meters.^[71] They favor dense cover for concealment, spending most time on the ground but occasionally roosting in low trees, and adapt to varied conditions including arid steppes and forest edges.^[71] Diets among ratites are largely herbivorous or frugivorous, centered on seeds, leaves, fruits, and grasses, though many incorporate insects seasonally for protein. Ostriches primarily graze on forbs, new grasses, and seeds, with occasional invertebrates like locusts.^[69] Emus consume green plants, fruits, and insects, avoiding dry herbage.^[70] Cassowaries are obligate frugivores, relying on fallen fruits from over 100 plant species while dispersing seeds intact.^[70] Kiwis, uniquely among ratites, maintain an insectivorous diet dominated by earthworms (40-45%) and other invertebrates (40-45%), supplemented by 10-15% plant matter.^[70] Tinamous are omnivorous ground-foragers, feeding on fruits, seeds, roots, arthropods (including insects and spiders), and small vertebrates like lizards and frogs.^[71] Foraging strategies reflect locomotor adaptations, with most palaeognaths pecking or probing the ground. Ostriches use their long necks to selectively peck seed heads and flowers in open areas, often foraging in groups that enhance detection of resources.^[69] Kiwis forage nocturnally by probing soil with their bills, aided by vibrissae-like bristle feathers around the gape that form a sensory "net" to detect prey in leaf litter.^[72] Tinamous dig shallowly with their bills for buried items, exploiting understory vegetation for opportunistic meals.^[71] Key dietary adaptations include the use of gizzard stones (gastroliths) across ratites to grind tough plant material in their muscular stomachs, compensating for weak bills and facilitating digestion of fibrous foods; ostriches, for instance, retain digesta for 21-76 hours with stone assistance.^[70] Seasonal shifts occur, as in greater rheas (Rhea americana), where females increase protein intake (e.g., via insects) during breeding to support egg production, with captive studies showing improved reproductive success on high-protein diets.^[73] Recent studies highlight climate change impacts on tinamou ecology in the Neotropics, where altered rainfall seasonality and warming may reduce fruit availability, potentially favoring adaptable species like the elegant crested tinamou (Eudromia elegans) in semiarid regions while stressing forest-dependent frugivores.^[74] Projections indicate high vulnerability for specialized Neotropical frugivores, including tinamous, due to niche constraints and shifting plant phenology.^[75]

Reproduction

Palaeognathae exhibit diverse mating systems adapted to their ecological niches. Among ratites, ostriches and rheas display polygyny, with a single male mating with multiple females that contribute eggs to a communal nest; emus show a flexible system combining monogamy, polyandry, and promiscuity; cassowaries exhibit polyandry, with females mating with multiple males; while kiwis maintain monogamous pairs that remain together for life, with both partners contributing to territory defense but the male taking primary reproductive roles.^[76]^[77]^[78]^[79] In contrast, tinamous display polygynandry, with females laying eggs in the nests of several males, while males form sequential pair bonds with multiple females during the breeding season.^[80] These systems reflect evolutionary pressures for maximizing reproductive output in flightless or semi-flightless species, often involving seasonal breeding triggered by photoperiod or rainfall. Eggs in Palaeognathae are notably large relative to body size, featuring thick, pigmented shells that provide protection and camouflage; for instance, ostrich eggs weigh up to 1.5 kg and measure about 15 cm in length.^[81] Ratites typically lay in simple ground scrapes or shallow depressions that may be covered with vegetation for concealment, while tinamous use unlined scrapes often hidden under leaf litter.^[76] Clutch sizes vary widely, from 5–45 eggs in emus to 8–56 in rheas, with communal laying allowing multiple females to contribute to a single nest in polygynous species.^[76] A distinctive trait in tinamous is egg mimicry, where the glossy, iridescent shells in shades of blue, green, or purple resemble surrounding foliage to deter predators.^[82] Incubation is predominantly male-only in ratites and tinamous, lasting 30–56 days depending on the species; ostriches require 42 days, primarily by the male but with some female assistance, while emus and rheas rely exclusively on males who do not eat or drink during this period.^[76] In tinamous, males incubate clutches of 4–12 eggs for about 16–22 days, often remaining on the nest for extended bouts up to 47 hours.^[83] Kiwis have the longest incubation period among birds, 70–90 days, mostly performed by the male in a burrow nest, with females occasionally relieving briefly.^[79] This extended duration in kiwis supports the development of their large, nutrient-rich eggs, which are laid singly or occasionally as pairs annually. Chicks in Palaeognathae are precocial, hatching covered in downy plumage and capable of following parents shortly after emerging from the egg, typically within hours to two days.^[76] Parental care post-hatching involves leading and protecting the young, with males providing most guidance in ratites and tinamous; however, chick mortality is high in species like emus and cassowaries due to predation and environmental stressors.^[84] A unique aspect of kiwi reproduction is their strategy of producing only one large egg per clutch each year, which enhances offspring survival in nutrient-poor habitats despite the low reproductive rate.^[79]

Human interactions

Conservation status

The conservation status of Palaeognathae varies widely across its taxa, with many species facing significant threats from human activities, while others remain relatively secure. Kiwis (Apteryx spp.) are classified as Endangered or Vulnerable on the IUCN Red List, with a total population estimated at approximately 70,000 individuals as of 2023, primarily due to predation by introduced mammals; recent conservation efforts have increased numbers in managed areas by around 7,000 since 2020. Cassowaries (Casuarius spp.) are listed as Least Concern overall, though populations are decreasing due to ongoing habitat loss from deforestation and fragmentation in their tropical rainforest ranges.^[53]^[85] Ostriches (Struthio spp.), including the common ostrich, are categorized as Least Concern overall, though the Somali ostrich is Vulnerable; the common ostrich population is estimated at 300,000–900,000 mature individuals (as of 2016, decreasing trend), with the Somali ostrich smaller and declining, facing localized poaching pressures.^[49]^[86] Emus (Dromaius novaehollandiae) are also Least Concern, with an estimated population of around 700,000 in Australia, where they contend with invasive predators and habitat degradation. Rheas (Rhea spp.) are Near Threatened, particularly the greater rhea, owing to hunting and agricultural expansion in South America.^[50] Tinamous (Tinamidae), comprising over 40 species, are mostly Least Concern but include several Vulnerable or Endangered forms like the dwarf tinamou; their populations are generally stable yet understudied, with recent assessments indicating declines in some due to deforestation. Major threats to Palaeognathae include habitat fragmentation, which severely impacts kiwis in New Zealand's forests by isolating populations and exacerbating predation risks. Hunting poses a direct peril to rheas across pampas and grasslands in South America, where they are targeted for meat and feathers, contributing to population fragmentation.^[50] Invasive predators, such as cats, stoats, and rats, threaten emus and other ground-nesting species in Australia by preying on eggs and chicks, though emus' large size offers some resilience. For tinamous, ongoing deforestation in Neotropical regions has led to habitat loss, with 2025 monitoring efforts revealing accelerated declines in several species amid expanding agriculture.^[87] Conservation efforts have yielded notable successes, particularly for kiwis through programs like Operation Nest Egg, which involves collecting eggs from wild nests, hatching them in controlled environments, and releasing juveniles into predator-free areas to boost survival rates to over 65%.^[88] This initiative, led by the New Zealand Department of Conservation and partners, has contributed to population increases in subspecies like the rowi kiwi, with recent 2025 reports showing stabilized numbers in key sites such as Ōkārito Forest. Reintroduction programs for rheas in Argentina and Brazil have restored populations in protected grasslands, using captive-bred individuals to counter hunting losses and habitat recovery. Tinamou conservation focuses on monitoring and habitat protection in the Amazon and Andean regions, with 2025 IUCN updates emphasizing the need for expanded surveys to address understudied declines from deforestation.^[87] Historical extinctions underscore the vulnerability of Palaeognathae to human impacts, as seen with the moas (Dinornithiformes), which were driven to extinction in New Zealand around the 1400s AD primarily through overhunting by Polynesian settlers.^[89] Similarly, the elephant birds (Aepyornithidae) of Madagascar became extinct around 1,000 years ago (circa 1000 CE) due to overhunting and habitat alteration following human settlement, serving as a cautionary analogy for current threats to surviving ratites like kiwis and cassowaries.^[90]^[91] These cases highlight the importance of proactive measures to prevent further losses in this ancient avian clade.

Cultural significance

Palaeognathae species hold diverse roles in human societies, particularly among indigenous communities where they feature in myths, rituals, and traditional practices. In Māori culture of New Zealand, the kiwi is revered as a taonga (treasured possession) under the protection of Tāne Mahuta, the forest god, with its feathers historically used in cloaks symbolizing status and heritage.^[92] The extinct moa also appears in Māori whakataukī (proverbs) and oral traditions, reflecting its former ecological dominance and cultural memory as a significant food source and emblem of the pre-human landscape.^[93] Among Aboriginal Australians, the emu embodies creator spirits in Dreamtime stories, often depicted as a flying ancestor that shaped the land, and plays a central role in male initiation ceremonies tied to spiritual and kinship networks.^[94] In various African indigenous groups, ostrich feathers symbolize purity, bravery, and spiritual connection; for instance, the Maasai incorporate them into headdresses during rituals to denote social standing and courage, while in Yoruba traditions, they facilitate divination and divine communion.^[95]^[96] Economically, ostrich farming represents a key human interaction with Palaeognathae, centered in South Africa where it supplies meat, leather, and feathers to global markets, with the industry valued for its sustainable protein and durable products.^[97] Emu farming, prominent in Australia and expanding internationally, focuses on oil extraction from back fat, which is utilized in cosmetics and pharmaceuticals for its anti-inflammatory and moisturizing properties, drawing from traditional Aboriginal uses for joint treatments.^[98] As scientific and cultural icons, tinamous serve as valued game birds in Neotropical indigenous communities, integral to folklore and subsistence hunting, with species like the great tinamou featuring in stories of forest spirits and providing feathers for ceremonial adornments.^[99] The extinct moas inspire paleoart reconstructions and bolster tourism in New Zealand, where museum displays of their skeletons and habitats draw visitors to sites like Auckland War Memorial Museum, fostering public appreciation of prehistoric biodiversity.^[100] In modern contexts, ostriches and other ratites appear in zoos worldwide, educating visitors on avian diversity, as seen in exhibits highlighting their dinosaurian traits.^[101] Recent 2025 museum displays, such as the Louisville Zoo's addition of a southern cassowary to its Wallaroo Walkabout, emphasize ratite evolution and conservation through immersive setups.^[102] Human interactions also involve conflicts, including illegal trade in cassowary eggs across New Guinea, where tens of thousands are harvested annually from wild nests, threatening local populations despite cultural taboos against overexploitation.^[103] Ethical debates surround ratite racing events, criticized by animal welfare advocates for causing stress, spinal injuries, and unnatural handling of ostriches, leading some venues to phase out rides in favor of observational tourism.^[104]^[105]

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