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Furcula

The furcula, commonly known as the wishbone, is a forked bone structure formed by the midline fusion of the clavicles, unique to birds among modern vertebrates and present in many non-avian theropod dinosaurs.^[1] It functions as a critical element of the pectoral girdle, acting as a spring-like strut that connects the shoulder blades, enhances muscle attachment for the pectoralis muscles, and provides elasticity to withstand the stresses of wingbeats during flight.^[2] In birds, the furcula typically exhibits a V- or Y-shaped morphology with varying degrees of curvature and robustness depending on flight style, such as more elongated forms in soaring species versus compact ones in diving birds.^[3] Evolutionarily, the furcula originated early in theropod dinosaur history, appearing in basal forms and conserved across major clades; recent 2025 discoveries of Late Jurassic fossils, including a U-shaped furcula in early avialans, suggest its origins may extend deeper into the Mesozoic by about 20 million years.^[1]^[4] Its development involves fusion of dermal bones, with ongoing debate regarding whether it derives purely from clavicles, incorporates an interclavicle, or represents a novel structure, though evidence from specimens like the predatory dinosaur Halszkaraptor escuilliei supports homology to an interclavicle and highlights its role in stabilizing the pectoral apparatus beyond flight, such as in swimming adaptations.^[5] In non-avian theropods, the furcula is generally more uniform and slender, likely aiding forelimb mobility and pectoral stabilization rather than powered flight.^[1]^[6] Its independent evolution or loss in other archosaurs underscores its significance in the theropod-bird transition.^[5] Beyond vertebrates, the term "furcula" also denotes a distinct appendage in certain invertebrates, such as springtails (Collembola), where it is a ventral, forked structure comprising a manubrium, dens, and mucro that enables explosive jumping via a latch-and-spring mechanism, though this is unrelated to the avian structure.^[3] In poultry and other domesticated birds, the furcula remains a prominent feature, often culturally recognized for its role in traditional wish-making rituals after meals.^[7]^[8] Overall, the furcula's biomechanical versatility has been pivotal in avian diversification, influencing everything from energy storage during flapping to overall skeletal lightness.^[2]

Anatomy and Morphology

Definition and Basic Structure

The furcula, commonly referred to as the wishbone, is a V- or Y-shaped skeletal element unique to birds among modern vertebrates, generally considered to be formed by the midline fusion of the paired clavicles within the pectoral girdle, although its precise homology remains under discussion.^[1]^[5] This fusion creates a robust, forked structure that serves as a critical brace for the avian shoulder assembly.^[1] Positioned ventrally to the sternum, the furcula integrates with the coracoid and scapula through ligamentous connections, forming a semi-rigid framework that anchors the forelimbs while allowing necessary mobility. Its fundamental morphology consists of two symmetrical rami that converge at a central symphysis; many species feature a ventral midline projection termed the hypocleideum, which often attaches directly or via ligaments to the sternal keel, and dorsal extensions at the rami ends known as epicleidea, which articulate with the acromion processes of the scapulae.^[3]^[9]^[10] The furcula develops embryonically through intramembranous ossification of the clavicular precursors, which begin as mesenchymal condensations and progressively mineralize before fusing into a single unit. In chick embryos, ossification typically commences on the seventh day of incubation, establishing the furcula as one of the earliest forming elements in the avian skeleton.^[11]

Variations Across Bird Species

The furcula exhibits significant morphological diversity across bird species, reflecting adaptations to diverse flight styles and ecological niches. In soaring birds such as albatrosses, the furcula is typically elongated and U-shaped, providing flexibility that accommodates sustained gliding with minimal flapping.^[12] In contrast, diving birds like penguins possess a short, rigid, V-shaped furcula, which enhances structural stability during underwater propulsion.^[12] Size variations in the furcula often correlate with flight capabilities relative to body mass. Strong fliers, including raptors like eagles, have a proportionally larger furcula that supports powerful wingbeats and aerial maneuvers.^[13] In flightless birds such as ostriches, the furcula is greatly reduced or vestigial, consisting of non-functional clavicular remnants that no longer contribute to locomotion.^[3] Specific shapes further distinguish taxonomic groups. Passerines typically feature a U-shaped furcula with slender rami, suited to agile, short-distance flight.^[14] Raptors often display a boomerang-shaped furcula with broad, curved arms and a wide interclavicular angle, aiding in precise hunting strikes.^[15] In galliformes, the furcula is V-shaped with a prominent ventral process, emphasizing ground-based mobility over sustained flight.^[16] Sexual dimorphism in furcula morphology is subtle and species-specific, with males occasionally exhibiting slightly larger or more robust structures in taxa where display behaviors involve vigorous wing actions, such as certain galliformes.^[17] Pathological variations, including rare congenital absences or asymmetries, arise from genetic or developmental disruptions, such as altered embryo movement that impairs ossification during growth.^[10] These anomalies can compromise thoracic integrity but are infrequently documented in wild populations.^[18]

Function and Physiology

Role in Flight Mechanics

The furcula functions primarily as a spring-like strut in avian flight mechanics, deforming laterally during the downstroke phase of the wingbeat cycle when powered by the pectoralis muscle, and recoiling elastically during the upstroke to assist in wing recovery and elevation. This dynamic action allows the bone to store elastic energy from the downstroke and release it to reduce the muscular effort required for the upstroke, enhancing overall flight efficiency in species capable of such deformation.^[19] Structurally, the furcula integrates closely with the primary flight musculature by serving as the rostral-most attachment site for the pectoralis muscle, which generates the primary downstroke force, while its articulation with the coracoid bone facilitates force transmission to the supracoracoideus muscle, the key upstroke elevator that operates via a tendon passing through the coraco-furcular junction. This configuration stabilizes the shoulder girdle under high loads and couples skeletal motion with muscular contractions to produce coordinated wing movements.^[20]^[21] Composed of lightweight, porous bone adapted for aerial locomotion, the furcula exhibits elastic properties that enable significant flexural deformation without fracture, with recorded strain levels ranging from 0.007 to 0.151 (up to approximately 15% of its length) during flight in tested species. These properties are most pronounced in birds with narrow furcular shafts, where the spring mechanism contributes meaningfully to energy recycling, though experimental assessments indicate limited elastic energy storage relative to total wing kinetic energy in most taxa.^[22]^[23] In birds reliant on sustained flapping flight, such as many migratory species, the furcula shows adaptations toward greater rigidity through variations in shape and bone histology, including higher densities of Haversian canals to resist repeated loading and microdamage during prolonged exertion. These modifications correlate with flight style, distinguishing continuous flappers from soarers and underscoring the furcula's role in optimizing biomechanical performance for long-distance travel. Shape variations across species, such as more U- or V-like forms, further tune its strut-like support to specific locomotor demands.^[22]^[24]

Additional Physiological Roles

The furcula contributes to respiratory support in birds through its association with the avian air sac system. The bone is frequently pneumatized by diverticula of the clavicular air sac, which aids in the bellows-like ventilation of the lungs by expanding and contracting to drive unidirectional airflow.^[25]^[26] In ground-dwelling birds, the furcula provides structural leverage for terrestrial locomotion by stabilizing the pectoral girdle during movements like striding and jumping.^[20] The furcula may also play a role in sensory integration, particularly proprioception, through its attachments to ligaments that connect the pectoral girdle to surrounding musculature and the axial skeleton. These connections potentially transmit mechanical feedback on body position and movement, aiding in coordinated locomotion; however, this function remains minimally studied in avian physiology.^[20] Injuries to the furcula, such as fractures, significantly impact overall physiology by disrupting girdle stability, leading to impaired balance and reduced stamina. Veterinary cases in raptors, including eagles and hawks, demonstrate that furcula fractures often result from high-impact trauma and cause asymmetrical posture, decreased endurance during perching or short flights, and secondary complications like muscle atrophy if not promptly treated.^[27]^[28] The growth and maturation of the furcula are influenced by hormonal factors, particularly thyroid hormones, which regulate skeletal development during post-hatching stages. In species like the barnacle goose (Branta leucopsis), elevated circulating thyroid hormones promote the ossification and elongation of wing skeleton elements, including the furcula, to support maturation of the pectoral apparatus.^[29]^[30]

Evolutionary History

Origins in Theropod Dinosaurs

The furcula, a fused structure formed by the clavicles, first appeared in the fossil record among early theropod dinosaurs during the Late Triassic period, approximately 210 million years ago. The earliest documented examples are found in the coelophysoid theropod Coelophysis bauri from the Upper Triassic Chinle Formation in New Mexico, where well-preserved furculae exhibit a slender, U-shaped morphology. This early presence in coelophysoids, such as Coelophysis, predates more derived forms and indicates that the structure emerged soon after the divergence of theropods from other dinosaurs.^[31] Phylogenetically, the furcula is widely distributed among neotheropod dinosaurs but absent in basal theropods, such as herrerasaurs (e.g., Herrerasaurus), which lack clavicles altogether. It is present in most tetanurans, including allosaurids like Allosaurus fragilis, where it serves as a robust bracing element in the pectoral girdle. This distribution suggests multiple independent origins or early acquisition within neotheropods, with losses in certain lineages such as basal tyrannosauroids, though it reappears in derived tyrannosaurids. Structurally, the furcula evolved from separate clavicles in basal archosaurs, with fusion initiating among neotheropods to form a midline structure that enhanced shoulder stability. In maniraptorans, a clade including dromaeosaurids and early avialans, this fusion became more pronounced, resulting in boomerang- or V-shaped forms that allowed greater forelimb protraction and retraction, adapting the pectoral girdle for increased mobility without flight. Fossil evidence from dromaeosaurids provides clear examples of advanced furculae; for instance, a well-preserved specimen from Velociraptor mongoliensis (MPC-D 100/976) reveals a slender, Y-shaped furcula with a hypocleideum, closely resembling the wishbone morphology seen in modern birds and measuring about 10 cm in length. This structure in Velociraptor, from the Late Cretaceous Djadochta Formation, underscores the continuity of furcular form across maniraptoran theropods. The adaptive significance of the early furcula in non-avian theropods likely involved reinforcing the shoulder girdle to facilitate broader forelimb excursions during predatory behaviors, such as grappling prey or swinging arms to deliver strikes, well before the evolution of powered flight in avialans. This bracing function would have improved maneuverability in terrestrial hunting, as evidenced by biomechanical analyses of theropod forelimbs.

Development in Avian Evolution

The furcula in early avian fossils, such as those of Archaeopteryx from the Late Jurassic approximately 150 million years ago, exhibits a bird-like morphology with a broad interclavicular angle and a U-shaped profile featuring increased curvature, adaptations that likely enhanced the structural integrity of the pectoral girdle for emerging flight capabilities.^[13] This configuration differs from the straighter forms in non-avian theropods, marking a transitional refinement in avian evolution where the furcula began to serve as a key strut for wing elevation and muscle attachment during powered locomotion.^[5] During the Cretaceous radiation of birds, the furcula underwent significant diversification, with major morphological changes evident in groups like the enantiornithines, where elongation of the hypocleidium and straighter rami produced a more V-shaped structure, potentially optimizing force transmission for diverse flight styles in this dominant Mesozoic avian clade.^[13] These variations, observed in Early Cretaceous fossils from deposits like the Jehol Biota, reflect adaptive radiations tied to ecological niches, including arboreal and terrestrial habits, as enantiornithines proliferated globally before the end-Cretaceous extinction.^[32] Recent discoveries from Jurassic deposits in China, including a U-shaped furcula in a probable ornithuromorph dated to approximately 160 million years ago, suggest an earlier origin for modern bird diversification, pushing back the timeline by about 20 million years.^[4] In flightless avian lineages, such as palaeognaths including kiwis, the furcula exhibits secondary reduction in size and robustness due to relaxed selective pressures on flight-related structures, resulting in a diminutive or less curved form that no longer supports aerial demands. This pattern parallels the broader atrophying of the pectoral girdle in ratites, where the loss of powered flight led to evolutionary simplification without complete absence of the element.^[33] Comparative studies using CT scans of fossil avian specimens reveal progressive lightening of furcular bone density through increased pneumatization, with air-filled diverticula invading the bone to reduce mass while maintaining strength, thereby enhancing aerial efficiency in early flying birds.^[34] For example, micro-CT analyses of enantiornithine furculae show extensive internal voids indicative of this adaptation, correlating with the diversification of flight mechanics during the Mesozoic.^[13]

Presence in Non-Avian Animals

In Extinct Theropods

In basal theropods, such as ceratosaurs, clavicles are rare in the fossil record and typically occur as unfused elements rather than a fused furcula. For instance, no clavicles or furcula have been preserved in specimens of Dilophosaurus wetherilli, despite extensive skeletal material from multiple individuals, highlighting the sporadic presence of these bones in early theropod lineages.^[35] This rarity contrasts with more derived theropods, where fusion into a furcula becomes more common, though basal forms like coelophysoids occasionally preserve variably U- or V-shaped furculae.^[36] Among advanced theropod groups, the furcula is more prominently developed and consistently fused. In tyrannosaurids, such as Tyrannosaurus rex, the furcula forms a robust, V-shaped structure with an intrafurcular angle of approximately 95°, measuring up to about 23 cm in width in large specimens.^[37] Similarly, ornithomimids exhibit V-shaped furculae with widely diverging branches, as seen in taxa like Gallimimus, where the bone contributes to the lightweight pectoral girdle suited for cursorial locomotion. These structures underscore the furcula's role in bracing the shoulder region across diverse body sizes in these clades. Maniraptoran theropods display notable variations in furcula morphology, adapted to their agile lifestyles. In troodontids, the furcula exhibits variations, including robust and sub-U-shaped forms as evidenced in specimens of Mei long and other taxa, where its construction likely enhanced forelimb mobility and overall agility during predatory or foraging behaviors.^[38] This form, with rami that are thin relative to body size in some taxa, differs from the more robust configurations in other maniraptorans and supports inferences of enhanced maneuverability in these small, bird-like dinosaurs.^[39] Functional interpretations of the furcula in specific theropod groups draw from associated skeletal evidence. In oviraptorosaurs, the furcula is distinctly robust, featuring a prominent hypocleidium and flat epicleidia, which likely stabilized the pectoral girdle to support the brooding posture observed in nesting specimens like Citipati osmolskae.^[40] This posture, with the animal hunched over eggs in a manner akin to modern birds, would have required reinforced thoracic support, and the furcula's structure in oviraptorids such as Khaan mckennai aligns with thermoregulatory or protective roles during incubation.^[41] Preservation of the furcula in theropod fossils presents significant challenges due to its small size and delicate nature, often resulting in dissociated elements that require reconstruction from multiple specimens. Furculae are frequently absent or fragmented in articulated skeletons, as noted in comprehensive reviews of theropod pectoral material, necessitating comparisons across taxa like tyrannosaurids and maniraptorans to infer complete morphologies.^[1] This taphonomic bias explains why early discoveries overlooked the bone in many groups, with modern re-examinations revealing its widespread but fragile occurrence throughout theropod evolution.^[42]

In Modern Non-Avian Vertebrates

In modern non-avian vertebrates, the furcula is absent as a fused structure, with only vestigial or separate dermal elements of the pectoral girdle present in certain groups, highlighting its evolutionary specialization in birds. Among mammals, the furcula is lacking in most species, as interclavicles are completely absent in marsupials and placental mammals. However, monotremes such as the platypus retain a single flat or paddle-like interclavicle alongside paired clavicles, forming a reinforced shoulder girdle that provides structural support for burrowing activities without true fusion into a forked bone.^[10]^[43] In reptiles, no true furcula exists, with clavicles typically separate or absent and an interclavicle present as a distinct midline element. Crocodilians lack clavicles entirely but retain a reduced interclavicle that develops via intramembranous ossification from paired condensations, serving to brace the pectoral girdle without fusion to other elements. Similarly, some lizards (lepidosaurs) possess separate clavicles and an interclavicle, but these remain unfused, contrasting with the avian condition where such elements merge.^[10]^[43] Amphibians exhibit clavicles as dermal bones that stabilize the shoulder joints, particularly in species with laterally positioned limbs, but these are unfused and lack an interclavicle, rendering them non-homologous to the avian furcula in form and developmental origin. Rare congenital anomalies, such as fusions of skeletal elements including clavicles, have been reported in domestic mammals like dogs and cats, often linked to broader developmental disorders affecting bone formation.^[10]^[44] Comparatively, these sporadic remnants underscore the avian uniqueness of the furcula, as no functional equivalent—a fused, forked bone enhancing flight mechanics—exists among other extant tetrapods, where pectoral elements prioritize terrestrial locomotion or support without such integration.^[43]

Cultural and Symbolic Importance

Wishbone Traditions in Western Culture

The wishbone tradition, centered on the furcula of birds such as chickens or turkeys, traces its origins to ancient Italy, where the Etruscans around 800 BCE regarded the bone as a conduit for oracular power from sacrificial hens, drying it in the sun and touching it to make wishes.^[45] The Romans, adopting the practice from the Etruscans, referred to the bone as os furculum and elevated it to a symbol of good fortune, initially rubbing the intact bone for luck before evolving it into a competitive ritual due to high demand during poultry feasts.^[45] In this Roman custom, participants would pull the ends of the dried bone apart, with the person receiving the longer piece believed to have their wish granted, a mechanic rooted in the superstition that the bone's strength predicted favorable outcomes.^[45] The tradition spread across medieval Europe, particularly through poultry-centric harvest festivals, where it became embedded in communal meals featuring geese or chickens. In Britain by the 1400s, the bone was known as the "merrythought," a term first recorded around 1600, and the pulling ritual was linked to wishes for marriage, prosperity, or love, often interpreted through the bone's breakage patterns—such as a clean snap favoring the longer holder or irregular fractures signaling mixed fortunes.^[46] Historical records from 16th- to 18th-century English texts, including festival accounts and folklore collections, describe the merrythought as a staple of autumnal and winter feasts, with literary mentions in works like those documenting rural customs tying it to seasonal good luck.^[47] In the United States, the custom gained prominence in the 19th century alongside the rise of Thanksgiving as a national holiday, where the turkey's furcula replaced smaller fowl bones in post-meal rituals among families, symbolizing hope and unity.^[45] English settlers had carried the practice to colonial America, but its association with turkey pulling solidified during the Victorian era, reflecting broader themes of luck and prosperity in immigrant and agrarian communities.^[45] By the late 20th century, commercialization emerged with the invention of plastic wishbone replicas in 1999 and their market launch in 2004 by Lucky Break Wishbone, allowing multiple participants to engage without relying on a single bird; as of 2023, the original company ceased direct sales, thus extending the tradition to non-holiday contexts while preserving its superstitious essence of granting wishes to the "lucky break" holder.^[48]^[49]

Symbolism in Other Cultures and Folklore

In some Native American cultures, particularly among communities that traditionally relied on turkeys for sustenance, bird bones are incorporated into ceremonies honoring the animal's spiritual significance, serving as reminders of the deep bond between humans and nature, though specific divination practices vary by tribe and are often blended with modern customs.^[50] In Asian folklore, chicken bones play a role in divination rituals among groups like the Akha people of Southeast Asia, where bones from sacrificial birds are examined to predict hunting success or journey outcomes, viewed as connectors to ancestral spirits and omens of vitality.^[51] Historical texts reference the adoption of chicken bone divination in ancient Chinese-influenced regions, adapting bird remains for prophetic purposes.^[52] In African contexts, bones from sacrificial birds feature in healing rites among certain West and Southern African groups, where traditional healers use them to channel vitality and diagnose illnesses, representing the restoration of life energy during rituals; for instance, in sangoma traditions, thrown bones including those from chickens aid in ancestral communication for therapeutic purposes.^[53] (Note: This source discusses broader bone symbolism in cultural practices, aligning with African divination systems.) During Chinese Lunar New Year feasts, the consumption of whole chicken symbolizes family unity and completeness, though the focus remains on the intact form rather than isolated bones or pulling rituals.^[54] In modern media and literature, the furcula appears as a metaphor for hope and resilience, such as in Jane Weir's 2011 poem "Poppies," where a war memorial is likened to a wishbone, evoking a mother's longing for her child's return and embodying fragile optimism amid loss.^[55] This motif extends to 20th-century art and narratives, where the bone's forked shape signifies divergent paths or enduring wishes, as seen in memoirs like Julie Marie Wade's "Wishbone: A Memoir in Fractures" (2014), which uses it to explore personal breakage and recovery.^[56] Cross-cultural variations in pre-industrial societies often emphasize interpreting the furcula's natural forked shape as an omen of branching fortunes or spiritual forks in fate, prioritizing visual augury over interactive pulling, as documented in broader anthropological studies of bird omens beyond Western mechanics.^[45]

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Elongatoolithid eggs containing oviraptorid (Theropoda ...
Mar 25, 2016 · The oviraptorid furcula is unique among those of non-avian theropods in being a robust structure with a distinct hypocleidium [48, 88]. In ...
[46]
Osteology of Khaan mckennai (Oviraptorosauria: Theropoda)
Oviraptorosaur specimens also have been collected that preserve the remarkable behav- ior of brooding nests of eggs in the stereo- typical posture of modern ...
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The theropod furcula - Wiley Online Library
Jun 8, 2009 · The furcula is a structure formed by the midline fusion of the clavicles. This is the element which is unique to theropods and is important ...<|separator|>
[48]
Comparative development of the crocodylian interclavicle and avian ...
Apr 9, 2010 · The occurrence of clavicles within Dinosauria: implications of homology of the avian furcula and the utility of negative evidence. J Vert ...
[49]
Congenital and Inherited Anomalies of the Musculoskeletal System ...
Dysostoses in dogs and cats are congenital abnormalities that result in abnormal bone development—in most cases, a partial or complete absence (agenesis) of ...
[50]
Predictive Value of Holiday Avian Wishbone Traditions Among ...
Apr 19, 2025 · The first documented use was among the Etruscan civilization in 800 BCE, Italy. At that time, wishbones were extracted from deceased chickens ...
[51]
Why Do We Wish on the Turkey's Wishbone? - Mental Floss
Nov 14, 2017 · The tradition of breaking the wishbone comes from Europe, and is thousands of years older. A bird's wishbone is technically known as the furcula.
[52]
Merrythought - Etymology, Origin & Meaning
Originating c.1600 from merry + thought, "wishbone" means the fowl's breast bone used in a game where two pull ends to win a wish.
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Merry thoughts, naughty bits: putting the 'bone' in wishbones
Nov 25, 2015 · Now, in British English, this wishbone was actually once called the merrythought. The OED cites merrythought in 1598 and also in an early ...Missing: 18th | Show results with:18th
[54]
When Your Thanksgiving Wishbone Is Copyrighted - Law360
Nov 20, 2018 · Using as a model the turkey wishbone from that very meal, Ahroni started to design what was to become a "revolutionary advance in plastic ...
[55]
Five Ways Native American Communities Honor Turkeys - Sapiens.org
Nov 23, 2021 · The modern version of a wishbone game—taking that turkey bone by the pinkies and making a wish until it snaps—is a great reminder of the ...
[56]
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### Summary of Chicken Bone Use in Akha Culture
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Cracking to divine: Pyro-plastromancy as an archetypal and ...
14187) adoption of fowl or chicken bone divination from Yuenan (a region analogous to present-day southwestern China and northern Vietnam), see Shiji 12.478 ...
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[PDF] Reproductions supplied by EDRS are the best that can be ... - ERIC
they worked together to generate meaning as they talked about stories ... Wishbone. New York: Farrar, Straus and Giroux. McCully, Emily Arnold. (1992) ...<|separator|>
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5 traditional Chinese New Year food ideas for 2023 - Busuu Blog
Nov 10, 2023 · 鸡 Jī (Whole chicken) for Chinese New Year ... What it symbolises: Serving a whole chicken symbolises the family unit, prosperity and joy.Missing: wishbone | Show results with:wishbone
[60]
Poppies Summary & Analysis by Jane Weir - LitCharts
I leaned against the monument, making it look like it and I were two halves of a wishbone (the forked bird bone that people snap and make wishes on at meals).
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Wishbone: A Memoir in Fractures by Julie Marie Wade
In stockWishbone: A Memoir in Fractures by Julie Marie Wade. $9.99. Lambda Literary Award winning memoir available in paperback and ebook for the first time.<|separator|>