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Struthioniformes

Struthioniformes is an order of flightless birds within the class Aves, commonly known as ratites due to their distinctive raft-like sternum that lacks a keel for flight muscle attachment.^[1] These birds are typically large, long-legged, and adapted for terrestrial life, with reduced wings and powerful legs enabling high speeds or strong kicks for defense.^[1] The order encompasses five extant families: Struthionidae (ostriches), Rheidae (rheas), Casuariidae (cassowaries), Dromaiidae (emus), and Apterygidae (kiwis), totaling 13 species distributed across the Southern Hemisphere.^[2]^[1] Ratites exhibit diverse morphologies suited to their environments, from the towering ostrich (Struthio camelus), the world's largest living bird reaching up to 2.7 meters in height and weighing over 150 kg, to the smaller, nocturnal kiwis (Apteryx spp.) of New Zealand, which are comparable in size to chickens but possess a keen sense of smell unusual among birds.^[3]^[1] Their feathers are often loose and fluffy rather than structured for flight, and many species display sexual dimorphism, with males typically larger and more vibrantly colored—such as the jet-black plumage and white wing markings of male ostriches or the bright blue necks of cassowaries (Casuarius spp.).^[4]^[1] Behaviorally, ratites are generally ground-dwelling omnivores or herbivores, with some like rheas (Rhea spp.) capable of running at speeds up to 60 km/h, while others like emus (Dromaius novaehollandiae) are known for nomadic foraging across arid landscapes.^[5]^[1] The evolutionary history of Struthioniformes traces back to the Paleogene period, with fossils indicating that ratites diverged from other birds early in avian evolution, possibly as far back as the late Cretaceous for lineages like ostriches (the basalmost ratites).^[6] This ancient group has faced significant extinctions, including the moas of New Zealand (formerly in Dinornithidae), driven by human activity, leaving modern species vulnerable to habitat loss and predation.^[1] Geographically, they show a Gondwanan distribution pattern: ostriches native to Africa (with introductions elsewhere), rheas in South America, emus in Australia, cassowaries in Australia and New Guinea, and kiwis endemic to New Zealand, reflecting continental drift influences on their isolation and diversification.^[2] Conservation efforts focus on these relict populations, as ratites play key ecological roles in seed dispersal and grassland maintenance.^[1]

Taxonomy and Systematics

Classification

Struthioniformes belongs to the infraclass Palaeognathae within the class Aves, phylum Chordata, and kingdom Animalia. This order is characterized by its position among the basal avian lineages, distinguished by features such as a flat sternum lacking a keel, which aligns with the palaeognathous condition shared with other flightless or semi-flightless birds.^[7] The sole extant family within Struthioniformes is Struthionidae, comprising the genus Struthio with two recognized species: the common ostrich (Struthio camelus) and the Somali ostrich (Struthio molybdophanes).^[8] The common ostrich, the largest living bird species, is divided into four subspecies based on geographic distribution and morphological variations in plumage and skin coloration: S. c. camelus (North African ostrich, found from Mauritania to Sudan and Eritrea), S. c. massaicus (Masai ostrich, ranging from Kenya to Tanzania), S. c. australis (South African ostrich, occurring in southern Angola to South Africa), and the extinct S. c. syriacus (Arabian ostrich, formerly in the Middle East until around 1941).^[9] The Somali ostrich is monotypic, lacking subspecies, and is distinguished by its blue-gray neck and legs, primarily inhabiting the Horn of Africa.^[10] In molecular phylogenies, Struthioniformes occupies a basal position within Palaeognathae, serving as the sister group to all other palaeognaths, including tinamous (Tinamiformes) and the remaining ratite lineages (rheas, cassowaries, emus, and kiwis).^[11] This placement indicates that the ratites—traditionally grouped by their flightless morphology—do not form a monophyletic clade exclusive of tinamous; instead, Palaeognathae is resolved as a single clade where flightlessness has arisen convergently multiple times.^[12] Historically, Struthioniformes encompassed all ratite groups, including emus (Dromaius), rheas (Rhea and Pterocnemia), cassowaries (Casuarius) and emus (Dromaius), and kiwis (Apteryx), under a single order based on shared anatomical traits like reduced wings and keelless sterna.^[1] However, genomic and mitochondrial analyses from the 2010s onward revealed polyphyly among these lineages, prompting their separation into distinct orders: Rheiformes for rheas, Casuariiformes for cassowaries and emus, and Apterygiformes for kiwis, while retaining ostriches in Struthioniformes.^[12]^[11] This reclassification reflects the influence of molecular data in resolving deep avian divergences, overturning morphology-driven groupings.^[13]

Extinct Taxa

The extinct taxa of Struthioniformes encompass several fossil families that represent stem-group representatives of the order, primarily from the Paleogene and Neogene periods. These groups exhibit a range of morphologies, from small, flightless forms to larger, cursorial species with crane-like features, and are distinguished from extant ostriches by potential partial flight capabilities in some lineages and more primitive hindlimb proportions. Palaeotididae, known from the Early to Middle Eocene of Europe (e.g., Germany and France), includes the genus Palaeotis with the type species P. weigelti. This small, flightless bird reached approximately 1 m in height, featuring gracile hindlimbs with a notched tibiotarsus and ostrich-like proportions but retaining palaeognathous cranial traits such as a narrow bill. Fossils from sites like the Messel Pit and Geiseltal Formation highlight its basal position within Struthioniformes.^[14] Geranoididae, documented from the Early Eocene to possibly early Oligocene in North America (e.g., Wyoming's Willwood Formation) and Europe, is exemplified by Geranoides jepseni. This mid-sized species, comparable in stature to the extant sandhill crane (Grus canadensis), possessed elongated tarsometatarsi and slender legs adapted for cursorial locomotion, with a deep extensor sulcus suggesting enhanced mobility. Some specimens indicate partial flight ability, differing from the fully flightless modern Struthionidae. Eogruidae, spanning the Middle Eocene to Miocene across Eurasia (primarily Central Asia, including Mongolia and Kazakhstan) and with limited North American records, includes genera such as Eogrus (e.g., E. huxleyi and E. aeola). These crane-sized birds were largely cursorial, with reduced inner toes and long hindlimbs, and reached heights of up to 1.2 m; while mostly flightless, early members may have retained limited aerial capabilities. The family is considered paraphyletic, forming a grade leading toward crown Struthioniformes.^[15] Ergilornithidae, a derived subclade often nested within or sister to Eogruidae, is recorded from the Oligocene to early Pliocene in Asia (e.g., Mongolia's Ergilin Dzo Formation), with key taxa including Ergilornis (e.g., E. gobiensis and E. rapidus). These larger forms, up to 1.5 m tall and weighing around 80 kg, showed advanced cursorial adaptations such as a vestigial or absent inner toe and robust tibiotarsi, marking them as close precursors to ostriches but with more pronounced crane-like bills. Fossils indicate fully flightless lifestyles.^[15] Systematic placement of these families as stem Struthioniformes is supported by shared traits like toe reduction and a single quadrate facet, positioning them basal to the crown-group Struthionidae without direct ancestry to extant species. Recent analyses affirm this affiliation, resolving prior debates linking them to Gruiformes through morphological convergence in cursorial features.^[15]^[16]

Evolutionary History

Origins and Fossil Record

The origins of Struthioniformes, the order comprising ratites, trace back to the divergence of Palaeognathae from other birds in the Late Cretaceous, approximately 80-110 million years ago, based on molecular clock estimates and fossil evidence.^[17] Early stem palaeognaths, such as Lithornis and Calciavis grandei from the Early Eocene Green River Formation in Wyoming (around 50 million years ago), represent volant ancestors closely related to modern ratites, including ostriches, kiwis, emus, and rheas.^[18] In Europe, fossils like Palaeotis weigelti from the Middle Eocene Messel Pit in Germany (approximately 48 million years old) indicate early flightless forms within the palaeognathous lineage.^[19] These Paleogene discoveries, primarily isolated bones and partial skeletons, suggest an initial diversification in the Northern Hemisphere among stem-group ratites, with flightlessness evolving independently multiple times across lineages.^[20] The fossil record remains sparse during the Paleogene but shows diversification in the Neogene for various ratite families. For ostriches (Struthionidae), the oldest definitive fossils of the genus Struthio appear in the Early Miocene of Africa, around 20-21 million years ago, exemplified by Struthio coppensi from Namibia.^[21] Other ratite lineages have earlier records in Gondwana: in South America, stem ratites like Patagopteryx from the Late Cretaceous (~80 mya) and Eocene Rhegipternis indicate early divergence of rheas (Rheidae).^[22] In Australia and New Zealand, Miocene fossils document the emergence of emus (Dromaiidae, e.g., Dromornis relatives), cassowaries (Casuariidae), and kiwis (Apterygidae), with kiwis showing nocturnal adaptations by the Pliocene.^[23] Extinct families like Eogruidae (Eocene to Miocene in Asia and North America) and Ergilornithidae represent basal ratite-like forms bridging early palaeognaths to modern Struthioniformes. Phylogenetic analyses integrate fossil and genomic data, supporting ostriches as basal to other palaeognaths, with non-ostrich ratites forming a clade linked to Gondwanan vicariance around 50-80 million years ago.^[17] This framework highlights a Northern Hemisphere origin for ostriches followed by southward dispersal, contrasting with the southern origins of other families.

Biogeography and Dispersal

Struthioniformes exhibit a dual biogeographic pattern: ostriches originated in Laurasia (Eurasia) during the Eocene, while other ratites diversified in Gondwana following continental breakup. Early stem lineages such as Palaeotididae (Palaeotis) are documented in European sites like the Geiseltal in Germany around 48 million years ago.^[23] Volant palaeognath ancestors of ostriches dispersed southward to Africa via Eocene land bridges connecting Eurasia and Africa, establishing the lineage by the late Paleogene.^[24] Crown-group Struthionidae appeared in Africa by the early Miocene (21-20 million years ago), as evidenced by Struthio coppensi from Namibia.^[23] During the Miocene, ostriches expanded into Eurasia and Asia via overland routes, with fossils like Struthio karatheodoris from the late Miocene Balkans (~8-7 million years ago) and Struthio linxiaensis from late Miocene China (~8-6 million years ago) indicating northward migration from Africa.^[25]^[26] A debated early presence in South America is suggested by Miocene stem ratites in Patagonia (e.g., Rhetornithidae), potentially linked to trans-Atlantic dispersal or independent evolution, though phylogenetic ties to ostriches remain uncertain.^[22] These dispersals were facilitated by Miocene climatic warming and savanna expansion.^[23] Holarctic ostrich lineages faced extinctions by the Pleistocene due to Neogene cooling, aridification, and glaciations fragmenting habitats in Eurasia and potential North American extensions (though no confirmed Pleistocene Struthio fossils exist there). Large species like Pachystruthio pannonicus persisted in Eurasia into the Early Pleistocene but vanished, with possible remnants in Mongolia as recent as 7,500 years ago. African ostrich populations endured in savanna environments. Late Pleistocene dispersals reached India, where ancient DNA from eggshells confirms Struthio camelus until at least 25,000 years ago, but these went extinct, likely due to human activity.^[27] In contrast, non-ostrich ratites followed Gondwanan vicariance: rheas isolated in South America post-80 mya; emus, cassowaries, and kiwis in Australasia after Australia-New Guinea separation ~50 mya, with kiwis further isolated in New Zealand. Today, the order's natural distribution reflects this: ostriches in sub-Saharan Africa (extinct elsewhere ~10,000 years ago), rheas in South America, emus in Australia, cassowaries in Australia-New Guinea, and kiwis in New Zealand. Introduced ostrich populations exist in Australia and the Middle East.^[8]^[28] The biogeography of Struthioniformes underscores overland migration for ostriches from Laurasian origins, rendering Gondwanan breakup irrelevant for them, while vicariance drove diversification in other ratite groups.^[24] Ostriches are basal to other palaeognaths, with multiple independent flight losses post-dispersal.^[24]

Physical Characteristics

Morphology and Anatomy

Struthioniformes, the order encompassing ratites such as ostriches, emus, cassowaries, rheas, and kiwis, exhibit a distinctive morphology adapted to terrestrial life, characterized by their large size, flightlessness, and robust bipedal structure.^[2] Among extant species, the ostrich (Struthio camelus) represents the largest, with males reaching heights of up to 2.7 meters at the crown and weights of 120–160 kg, while females are slightly smaller at 2.0–2.5 meters and 90–130 kg.^[29] Other ratites vary in scale: emus (Dromaius novaehollandiae) measure 1.5–1.9 meters and weigh 38–55 kg, rheas (Rhea spp.) 1.2–1.7 meters and 20–40 kg, cassowaries (Casuarius spp.) 1.5–1.8 meters and 25–70 kg, and kiwis (Apteryx spp.) the smallest at 0.3–0.6 meters and 1–4 kg.^[2] Their overall build is sturdy and elongated, featuring a long neck for foraging and vigilance, powerful hind legs for rapid locomotion, and a heavy body supported by strong pelvic girdles.^[30] Externally, ratites display loose, fluffy plumage due to non-interlocking barbules, lacking the aerodynamic structure of flying birds; in ostriches, males possess striking black-and-white feathers on the body, wings, and tail, while females and juveniles have drab brown plumage for camouflage.^[29] The neck and thighs are typically bare and featherless, vividly colored—red in breeding male ostriches and pinkish in females—to facilitate thermoregulation and display.^[29] Their feet are specialized for speed and stability: ostriches have a unique didactyl (two-toed) configuration with the third and fourth toes bearing weight, the larger third toe equipped with a nail for traction, whereas rheas, emus, and cassowaries are tridactyl (three-toed), and kiwis retain a vestigial fourth toe.^[2] Cassowaries notably feature a dagger-like inner claw on each foot for defense.^[30] Internally, the wings of Struthioniformes are vestigial and non-functional for flight, reduced to small, flap-like appendages with claw-like structures—two in ostriches, one in rheas, none visible in emus or cassowaries, and kiwis have tiny, hidden wings each ending in one small claw—primarily aiding balance or display.^[29]^[31] The sternum is flat and raft-like (ratite), devoid of the ventral keel that anchors flight muscles in volant birds, reflecting their evolutionary loss of aerial capability.^[30] Eyes are prominently large in diurnal species like ostriches, measuring up to 5 cm in diameter—the largest of any land animal—endowing them with exceptional visual acuity for detecting predators from afar.^[32] The digestive system includes a powerful, muscular gizzard for grinding coarse vegetation and ingested grit, particularly robust in herbivorous ostriches and rheas, which lack a crop but possess a thick-walled ventriculus lined with koilin.^[29] Kiwis, by contrast, have a thinner gizzard suited to their insectivorous diet.^[2] Sexual dimorphism in Struthioniformes is evident across species, most pronounced in ostriches where males exceed females in size and exhibit bold plumage contrasts, while juveniles closely resemble females in coloration and patterning for protection.^[30] In emus and cassowaries, females are larger than males, reversing the pattern seen in ostriches and rheas; kiwis show minimal external differences beyond size.^[2] Males generally possess an intromittent organ, such as the ostrich's phallus, which can extend up to 40 cm when erect, facilitating internal fertilization uncommon among birds.^[29]

Adaptations for Flightlessness

Struthioniformes, commonly known as ratites, exhibit profound skeletal modifications that facilitate a terrestrial lifestyle devoid of flight capabilities. The sternum lacks a keel, eliminating the attachment site for large pectoral flight muscles and reducing overall weight while prioritizing stability on the ground.^[33] Vertebrae in the thoracic region and synsacrum are extensively fused, forming a rigid framework that enhances structural integrity during high-speed locomotion and resists torsional forces.^[34] Hindlimbs are elongated with robust, solid bones—lacking the pneumatic chambers found in flying birds except in the femur—supporting powerful strides; for instance, ostriches can sprint at speeds up to 70 km/h, aided by these adaptations.^[35] Respiratory and muscular systems in ratites are optimized for endurance on land rather than aerial exertion. They retain the avian air-sac system with nine or ten sacs, enabling unidirectional airflow through the lungs for efficient oxygen extraction comparable to flying birds, though the fixed sternum does not participate in respiratory movement.^[2] Leg musculature is dominated by the gastrocnemius, providing explosive power for rapid acceleration and sustained running, while wing muscles are vestigial and repurposed for balance during movement or display functions such as wing-fluttering in courtship.^[36] Sensory adaptations emphasize ground-based predator avoidance over aerial navigation. Ratites possess acute vision, with large eyes offering wide-field detection suited to open habitats, and enhanced hearing for locating threats at a distance.^[2] Lacking well-developed vocal cords, they produce non-vocal sounds like hissing through forced exhalation and throat inflation, serving as defensive signals without relying on syrinx-based calls.^[37] The evolution of flightlessness in Struthioniformes occurred independently across lineages, reflecting convergent adaptations to island or continental isolation. Paedomorphic traits, such as reduced wing development resembling juvenile stages in volant birds, contribute to miniaturized forelimbs in species like emus via downregulated signaling pathways during growth.^[38] In contrast, ostriches display peramorphic changes, with extended development leading to more pronounced skeletal modifications for cursoriality.^[38]

Distribution and Habitat

Current Range

Struthioniformes, the order encompassing ratites such as ostriches, rheas, emus, cassowaries, and kiwis, exhibit a disjunct contemporary distribution primarily across the Southern Hemisphere, reflecting their Gondwanan origins but shaped by modern ecological constraints. Native populations are confined to Africa for ostriches, South America for rheas, Australia for emus, New Guinea and northern Australia for cassowaries, and New Zealand for kiwis, with no overlap between these regions due to geographic isolation.^[1]^[2] The common ostrich (Struthio camelus) is native to sub-Saharan Africa, ranging from Senegal in the west to South Africa in the south, though its distribution is fragmented by human activities such as agriculture and urbanization in areas like the Sahel, East African savannas, and the Karoo semi-deserts. Ostriches prefer open grasslands, savannas, and semi-arid scrublands, avoiding dense forests and extreme deserts while requiring seasonal access to water sources for survival. In contrast, greater rheas (Rhea americana) occupy eastern and southern South America, from northeastern Brazil through Bolivia, Paraguay, Uruguay, and Argentina, favoring open pampas, grasslands, and scrub; their range has expanded in some agricultural landscapes but remains tied to low-vegetation habitats. Emus (Dromaius novaehollandiae) are widespread across mainland Australia, inhabiting sclerophyll woodlands, grasslands, and coastal plains, with adaptability to varied arid and semi-arid conditions. Cassowaries (Casuarius spp.), including the southern cassowary (C. casuarius), are restricted to tropical rainforests in New Guinea, the Aru Islands, and northeastern Australia (Cape York Peninsula), where they rely on dense understory vegetation. Kiwis (Apteryx spp.) are endemic to New Zealand's forests, from lowland podocarps to montane beech woods, though habitat loss has pushed some into secondary scrub.^[8]^[28]^[39]^[40]^[41]^[42]^[43]^[44]^[1]^[45] Introduced populations of Struthioniformes are limited and mostly non-native to ostriches, which have been farmed and occasionally released in Australia since the 1890s, the Middle East (e.g., Israel and UAE for agriculture), and the USA, where feral groups persist in small numbers but fail to establish self-sustaining wild populations due to predation and habitat unsuitability. Rheas have feral introductions in Europe, including an established population in Germany exceeding 550 individuals as of 2020. No significant introduced populations exist for emus, cassowaries, or kiwis outside their native ranges.^[46]^[47]^[48]^[49] Wild population estimates for Struthioniformes total over 1 million individuals as of the early 2020s, dominated by emus (630,000–725,000 as of 2009) and ostriches (300,000–900,000 mature individuals as of 2021), with rheas of unknown size but declining, cassowaries at 20,000–50,000 as of 2018, and kiwis around 70,000 as of 2023. Ostrich numbers are declining in West Africa due to habitat fragmentation, while other taxa show stable or regionally variable trends.^[8]^[41]^[39]^[43]^[50]^[51]^[46]^[52]

Historical and Fossil Distributions

The fossil record of Struthioniformes reveals a once-widespread distribution across the Northern Hemisphere during the Eocene epoch, with stem-group representatives documented in Europe, North America, and Asia. In Europe, fossils such as those of Palaeotis weigelti have been recovered from middle Eocene sites in Germany, including the Messel and Geisel Valley localities, while earlier Paleocene-Eocene lithornithids like Remiornis heberti occur in France.^[23]^[53] In North America, lithornithid remains, including Lithornis celetius, come from early Eocene deposits in the Green River Formation of Wyoming, indicating early diversification in lacustrine settings.^[54] Asian records include eogruid stem struthionids, such as Eogrus aeola, from middle to late Eocene strata in Mongolia's Omnogovi Province and Khoer Dzan, marking the eastern extent of this early radiation.^[23] These Eocene forms inhabited subtropical to paratropical forest environments under hothouse climatic conditions, with associated fauna suggesting humid, vegetated landscapes.^[23] By the Miocene, Struthioniformes had expanded into Africa and persisted in Eurasia, reflecting adaptations to changing paleoenvironments. In Africa, a ratite eggshell fragment (DPC 14570) from the early Miocene (~17 Ma) Moghra Formation in Egypt's Qattara Depression represents one of the earliest confirmed records on the continent, characterized by aepyornithoid microstructure with linear pore arrangements, consistent with palaeognathous affinities.^[55] Eurasian evidence includes ostrich-like eggshells from the late Miocene Dhok Pathan Formation (~10.1 Ma) near Haritalyangar in the Siwalik Hills of northern India (bordering Pakistan), alongside body fossils like the tarsometatarsus of Struthio asiaticus from uncertain but likely Miocene Siwalik localities.^[56] These Miocene fossils occur in fluvio-marine and deltaic settings transitioning from forested to more open savanna-like habitats, driven by global cooling and aridification.^[55]^[56] During the Pleistocene, Struthioniformes inhabited Eurasian steppes, with giant forms like Pachystruthio in Europe (e.g., Hungary, Crimea) and Struthio anderssoni in northern China and Mongolia, before undergoing regional extinctions across the Holarctic around 10,000 years ago at the end of the Pleistocene.^[23]^[27] This wipeout affected populations from Europe to the Indian subcontinent, where eggshells dated 25,000–40,000 years B.P. indicate late persistence before disappearance, likely linked to climatic shifts and human expansion.^[27] Possible early links to South American ratites remain debated and unconfirmed, with no definitive Struthioniformes fossils identified there beyond native rheas.^[23] The overall fossil record spans over 50 sites worldwide, primarily documented through abundant eggshell remains, which have enabled isotopic analyses (δ¹³C and δ¹⁸O) revealing dietary shifts from C₃-dominated forests in the Miocene to C₄ grasslands post-7 Ma, correlating with habitat transitions to open plains amid Neogene aridification in Africa and Eurasia.^[57]^[6]

Behavior and Ecology

Reproduction and Breeding

Ratites exhibit diverse reproductive strategies across their families, often characterized by polygyny or monogamy with significant male parental investment. In Struthionidae (ostriches), species such as Struthio camelus and Struthio molybdophanes display a polygynous mating system where a dominant territorial male forms a harem of 2 to 7 females.^[3] This structure supports communal reproduction, with the male competing through dominance displays to maintain his group. Courtship involves elaborate rituals, including deep booming calls produced by inflating a neck pouch, as well as visual displays like bowing, wing flapping, tail shaking, and foot stomping to attract females.^[3]^[58] Nesting in ostriches occurs in a communal scrape, a shallow depression 2-3 meters in diameter and 30-60 cm deep, excavated by the male in sandy ground.^[59] Breeding is seasonal, typically May to August in African savannas, timed with rainfall for chick survival.^[60] The nest holds a single clutch of 40-60 eggs from multiple females, with the dominant female laying the first and largest (up to 7-10 eggs per female, others 2-6); these eggs weigh about 1.5 kg and measure 15 cm long, the largest of any bird.^[58]^[3] The ostrich's large body size supports such voluminous eggs with ample yolk for development.^[61] Incubation lasts 42 days, with females incubating by day (using brownish plumage for camouflage) and males at night (with darker feathers).^[58] Hatching produces precocial chicks, downy and chicken-sized, able to run within hours and follow parents.^[3] The male primarily broods and protects the chicks, leading them to food and water, with both parents contributing to guidance.^[62] Chick mortality is high, around 50% in the first year from predation, stress, and disease.^[63] The male protects the brood for 4-5 months until near-adult independence, after which groups may disperse or join flocks.^[62]^[58] This paternal care boosts survival in savanna environments.^[3] Similar polygynous systems occur in Rheidae (rheas) and Dromaiidae (emus), where males build nests, incubate eggs (35-42 days for rheas, 50-56 days for emus), and rear chicks alone or with females contributing minimally.^[64]^[1] Rhea nests hold 10-60 eggs, while emus produce 5-20 per female in a single nest. In Casuariidae (cassowaries), males also incubate (50-55 days) and care for precocial chicks for up to 9 months in polygynous or sequential polyandry setups. Apterygidae (kiwis) differ, being monogamous with pairs sharing incubation of a single large egg (70-85 days, up to 25% of female body weight) in a burrow, and biparental care for 3-5 months post-hatching.^[1] Breeding seasons vary by species and hemisphere, often aligning with resource availability.^[65]

Diet and Foraging

Struthioniformes species display omnivorous diets that vary by taxon and habitat, but most are primarily herbivorous, consuming grasses, succulents, leaves, seeds, fruits, and flowers, with opportunistic intake of invertebrates such as insects, as well as small vertebrates including lizards and rodents.^[66] For example, ostriches (Struthio spp.) selectively graze on forbs, new grasses, and succulents in arid savannas, while emus (Dromaius novaehollandiae) favor green plant material, seeds, and fruits in Australian woodlands, and rheas (Rhea spp.) consume mostly dicotyledonous plants and seeds in South American grasslands.^[67]^[68] Cassowaries (Casuarius spp.) are predominantly frugivorous, relying on fallen fruits from over 200 plant species in rainforest understories, whereas kiwis (Apteryx spp.) are largely insectivorous, feeding on earthworms and other invertebrates.^[66] Dietary composition shifts seasonally to optimize nutrition, with increased consumption of protein-rich animal matter during dry periods when plant quality declines; ostriches and emus, for instance, augment their intake of insects and small vertebrates to compensate for reduced availability of fresh vegetation.^[67] In wet seasons, herbage dominates, providing essential moisture and energy through higher digestibility.^[66] Foraging occurs primarily during daylight hours for most species, involving slow walking and pecking motions with the beak to select and pluck food items, often interspersed with vigilance to detect threats.^[66] Ostriches and rheas forage in loose groups, covering daily distances of 8–20 km in search of optimal patches, while emus exhibit nomadic patterns within home ranges of 5–10 km², adapting to seasonal resource availability.^[67]^[68] To facilitate digestion, individuals swallow gastroliths—smooth stones up to 10 cm in length—that accumulate in the muscular gizzard to grind tough plant material, with ostriches retaining numerous such stones totaling around 1 kg.^[66]^[69] Cassowaries forage solitarily or in small family units for about 35% of daylight, peaking in mornings and afternoons, and kiwis forage nocturnally using olfaction to locate buried prey.^[66] Near human settlements, species like ostriches occasionally scavenge farm crops or waste, supplementing natural foraging.^[67] The digestive system lacks a true crop, relying instead on temporary storage in the dilated esophagus before food enters the proventriculus for chemical breakdown and the gizzard for mechanical grinding aided by gastroliths.^[66] A long hindgut, including paired ceca and an extended colon, enables microbial fermentation of fibrous plant matter, yielding volatile fatty acids that supply 50–75% of energy needs in herbivorous taxa like ostriches and emus.^[67] These species tolerate high-fiber diets, digesting 35–45% of neutral detergent fiber through hindgut processes, far exceeding many other birds.^[66] Water requirements are met largely from moisture in vegetation, though ostriches can consume up to 18 liters from free sources when available, and emus endure short-term deprivation by drawing on metabolic water.^[66] In frugivorous cassowaries and insectivorous kiwis, the system is adapted for quicker passage and less fiber processing, with kiwis using smaller gastroliths to pulverize invertebrates.^[66] Struthioniformes exhibit varied social structures across species, with ostriches (Struthio spp.) and greater rheas (Rhea americana) being notably gregarious outside the breeding season, forming loose flocks typically ranging from 5 to 50 individuals to enhance foraging efficiency and predator detection.^[3] Within these flocks, dominance hierarchies are established primarily through aggressive displays, including males extending and swaying their necks while hissing or charging, and physical confrontations involving powerful kicks to assert territorial control.^[2] Post-breeding, family units emerge as dominant males lead communal crèches of chicks from multiple females, providing protection and guidance for several months until juveniles disperse.^[1] In contrast, emus (Dromaius novaehollandiae) are generally solitary or occur in pairs but aggregate into larger groups near abundant resources, while cassowaries (Casuarius spp.) and kiwis (Apteryx spp.) maintain largely solitary or monogamous pair bonds, with territorial behaviors dominating interactions.^[2] Communication among Struthioniformes relies heavily on non-vocal cues, as these flightless birds lack the syrinx structure for complex songs found in most avian orders. Ostrich males produce deep booming calls by inflating a specialized throat pouch to attract mates or signal presence, while both sexes emit hisses during threats or submission displays.^[3] Visual signals predominate, including wing flapping, neck postures, and feather ruffling—such as tail fluffing in ostriches to convey agitation or dominance—allowing rapid assessment of group dynamics over open terrain.^[1] Olfactory cues play a minimal role overall, though kiwis uniquely employ a keen sense of smell for navigation and mate location via nostrils at the bill tip.^[2] Predation poses significant threats to Struthioniformes, particularly in African savannas where ostriches face attacks from lions (Panthera leo), spotted hyenas (Crocuta crocuta), and cheetahs (Acinonyx jubatus), while rheas in South America contend with similar mammalian carnivores like pumas (Puma concolor).^[1] Anti-predator strategies emphasize evasion and confrontation: adults achieve sprint speeds up to 70 km/h using long, muscular legs, enabling escape across open habitats, and deliver forceful kicks—capable of exerting over 8,900 N of impact—that can injure or kill predators, supplemented by heightened vigilance in flocks where individuals alternate scanning for threats.^[70] Chicks experience high mortality from avian predators such as eagles (Aquila spp.), which target vulnerable young in the first weeks post-hatching.^[1] Human activities exacerbate predation risks for Struthioniformes through historical overhunting for meat, feathers, and eggs, which decimated wild populations, particularly ostriches in the 19th and early 20th centuries.^[1] In modern contexts, conflicts arise with farmers who view ostriches and emus as crop pests, leading to inadvertent poisoning, trapping, or culling during pest control efforts, while introduced predators on islands have further intensified threats to kiwis and cassowaries.^[33]

Conservation Status

Threats and Population Trends

Struthioniformes species face multiple anthropogenic and environmental threats that vary by family and region, with habitat degradation emerging as the most pervasive issue across the order. For ostriches (Struthionidae), agricultural expansion and livestock overgrazing have fragmented savanna and semi-arid habitats, particularly in the Sahel region where ecosystem degradation has accelerated due to desertification and land conversion.^[71]^[72] Poaching remains a significant pressure on ostrich populations, driven by demand for meat, eggs, skins, and feathers, contributing to rapid declines in northern and eastern African subspecies such as the Somali ostrich (Struthio molybdophanes).^[71]^[73] Predation by feral dogs and wild canids poses risks primarily to eggs and juveniles in fragmented landscapes, exacerbating vulnerability in areas with high human-wildlife conflict.^[74] In other families, habitat loss from deforestation threatens cassowaries (Casuariidae) in New Guinea's rainforests and kiwis (Apterygidae) in New Zealand's forests, while rheas (Rheidae) experience grassland conversion for agriculture in South America.^[75]^[76] Climate change intensifies these pressures by altering forage availability and reproductive success in Struthioniformes. Prolonged droughts in the Sahel reduce vegetation cover essential for ostrich foraging, leading to nutritional stress and population instability.^[72] Temperature extremes, projected to increase under future scenarios, impair ostrich fertility, with females laying up to 40% fewer eggs and males producing fewer viable sperm following fluctuations as small as 5°C.^[77] Models suggest potential southward range shifts for African bird species, including ostriches, by 2050 in response to warming and drying trends, though expansions into marginal grasslands may offset some losses.^[78] For kiwis and cassowaries, rising temperatures and habitat alteration could further isolate small populations, heightening extinction risks.^[75] Diseases and genetic factors compound threats, particularly in managed or isolated groups. Avian influenza outbreaks pose risks to ostriches in proximity to poultry farms, with potential for spillover from wild birds.^[79] Low genetic diversity in fragmented ostrich populations, resulting from historical bottlenecks and isolation, reduces resilience to environmental stressors and diseases.^[80] Similar issues affect kiwis, where small, isolated groups exhibit reduced heterozygosity, increasing susceptibility to pathogens and inbreeding depression.^[75] Population trends for Struthioniformes reflect regional disparities, with overall wild numbers estimated at over 1 million individuals as of 2025, dominated by ostriches and emus.^[8]^[41] The common ostrich (Struthio camelus) maintains stable to slightly declining populations of 300,000–900,000 mature individuals in eastern and southern Africa, classified as Least Concern by IUCN.^[8] In contrast, the Somali ostrich is Vulnerable, with inferred declines of 30–49% over three generations due to ongoing threats in the Horn of Africa and Sahel.^[71] The North African ostrich (S. c. camelus) has experienced severe reductions, with over 99% range loss since the early 20th century, rendering remaining wild groups critically small (fewer than 50 individuals) and dependent on reintroductions.^[72] Emus (Dromaiidae) number over 600,000 wild individuals in Australia and are Least Concern, showing stable trends. Rheas exhibit mixed statuses, with the greater rhea Near Threatened and declining due to habitat loss, while the lesser rhea is Least Concern globally but faces local threats and ongoing reductions in some regions.^[75] Cassowaries and kiwis face steeper declines: southern cassowaries are Least Concern globally but Endangered in parts of Australia, with populations contracting from fragmentation; kiwis are mostly Vulnerable or worse, with some subspecies declining by over 5% annually from predation and habitat issues, though total population estimated at 60,000–70,000 as of 2025.^[81]^[76]

Conservation Efforts

Conservation efforts for Struthioniformes emphasize habitat protection, regulated trade, and population restoration globally, tailored to the Gondwanan distribution of its families. Protected areas play a central role in safeguarding these flightless birds from human encroachment and predation. For instance, the Serengeti National Park in Tanzania serves as a critical refuge, maintaining viable ostrich populations through anti-poaching patrols and habitat management that supports their savanna preferences. Similarly, Etosha National Park in Namibia protects ostriches within its semi-arid ecosystems, where the park's waterholes and open plains facilitate natural foraging and breeding, contributing to stable local densities. These reserves, designated as UNESCO World Heritage sites, indirectly benefit Struthioniformes by preserving biodiversity hotspots amid surrounding land-use pressures. In South America, efforts for rheas include protected grasslands and anti-poaching measures in Argentina and Brazil to counter agricultural expansion, with community programs promoting sustainable ranching to reduce conflicts. In Australia, emus benefit from broad landscape management in national parks, while cassowaries are supported by rainforest restoration and corridor projects in the Wet Tropics World Heritage Area, including vehicle strike mitigation and habitat connectivity initiatives. For kiwis in New Zealand, intensive predator control through programs like Operation Nest Egg and 1080 baiting has stabilized or increased populations in managed areas, with over 1,000 chicks reared annually in captivity for release; as of 2025, conservation has averted extinction for subspecies like the rowi kiwi. International trade regulations further bolster conservation by curbing exploitation for feathers, meat, and skins. The common ostrich is included in CITES Appendix I for populations in countries such as Algeria, Burkina Faso, Chad, Mali, Mauritania, Morocco, Niger, Nigeria, and Senegal since 1983, prohibiting commercial trade and requiring permits for non-commercial movement to prevent overexploitation.^[82] This listing has effectively limited illegal trafficking, particularly for the North African subspecies (S. c. camelus), allowing some populations to stabilize. Domestic initiatives, including sustainable farming and reintroduction programs, address local threats while promoting economic alternatives. In South Africa, ostrich farming—centered in the Oudtshoorn region—produces over 90% of the world's ostrich products, reducing poaching pressure on wild stocks by supplying market demand for leather, meat, and feathers through regulated, welfare-compliant operations.^[83] These programs, supported by the South African Ostrich Business Chamber, have diverted economic incentives away from wild harvesting since the early 20th century. Reintroduction efforts complement this by restoring extirpated groups; for example, the Sahara Conservation Fund's North African Ostrich Recovery Project has translocated and released ostrich chicks from captive breeding in Zakouma National Park (Chad) to Ouadi Rimé-Ouadi Achim Faunal Reserve since 2020, supporting recovery of this critically small population.^[73] In Kenya, the Reteti Community Conservation Trust rescues and rehabilitates Somali ostrich (Struthio molybdophanes) chicks for release into community conservancies like Samburu, enhancing genetic diversity in northern ranges.^[84] Research and monitoring underpin these actions, with genetic analyses confirming subspecies boundaries to guide targeted interventions. A phylogeographic study using mitochondrial and nuclear DNA validated four distinct ostrich subspecies, informing IUCN assessments that classify the Somali ostrich as Vulnerable due to habitat fragmentation and hunting.^[85] Community-based education programs, such as those in Ethiopia's Afar region, raise awareness about the ecological role of ostriches and alternatives to bushmeat consumption, reducing incidental hunting through partnerships with local NGOs. Similar community involvement drives kiwi and cassowary conservation, with iwi (Māori) partnerships in New Zealand emphasizing cultural significance. Looking ahead, strategies focus on connectivity and adaptation amid climate variability. Habitat corridors linking fragmented reserves, like those proposed between Etosha and adjacent Namibian conservancies, aim to facilitate ostrich movement and gene flow. Climate-resilient breeding protocols, developed in ex-situ programs, select for drought-tolerant traits to counter aridification impacts. International cooperation, aligned with the African Union's Climate Change and Resilient Development Strategy (2022–2032), fosters cross-border monitoring and funding for ratite conservation, ensuring coordinated responses to transboundary threats, while global initiatives like the Convention on Biological Diversity support efforts for all families.^[86]

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