Osteopathy of the pectoral and pelvic limbs including pentadactyly in a young Kestrel (Falco t. tinnuncalus)

A five and a half weeks old female Kestrel exhibiting osteopathy of the pec toral and pelvic limbs, including symmetrical hyperdactyly, was investigate d in order to clarify the pattern of the involved anatomical alterations an d the possible causes of this developmental malformation. In the pectoral l imb it consisted of a triplication of the alular digit, in the pelvic limb of a duplication of digit I. The live young Kestrel was observed for a peri od of two weeks to ascertain that it was unable to fly or procure prey on i ts own. After its death radiographs were taken and, apart from an eidonomic inspection including the wing claws, a detailed macroscopic dissection of the musculature of the pectoral and pelvic limbs was carried out using the 'in-water-method'. Consecutive dissection steps were documented by a series of photographic slides. The relevant musculature, particularly that of the supernumerary digits, was recorded in proportional drawings. Subsequent to maceration of the limbs the isolated bones were reassembled according to t he radiographs and also documented by means of photographs and drawings. Th is anatomical approach produced a reliable reconstruction of the skeletomus cular apparatus of the hyperdactylous limb parts. The eidonomic inspection revealed that at least young Kestrels may have two (alular and major digit) or even three wing claws per side. The proximal skeletal elements of both pectoral and pelvic limb were more sturdily built than in a typical Kestrel of comparable age. The proximal elements of the pelvic limb, the tarsometa tarsus in particular, were shorter than in a typical Kestrel. In addition, the long axis of the tarsometatarsus was laterally bent in the transverse p lane so that its proximal articular surfaces were medially inclined. Duplic ation of the cutaneous and osseous elements in the foot was accompanied by a duplication of some of the muscular and/or tendinous elements supplying d igit I proper and the accessory digit I'. There were left-to-right asymmetr ies of the pedal musculature concerned. In contrast, the two accessory alul ar digits of each wing were almost completely devoid of musculature. Apart from atypical points of origin or insertion of the remaining distal muscula ture, left-to-right asymmetries and the two accessory alulae per wing, pres umably, affected aerodynamic properties and resulted in flightlessness. A juvenile Kestrel of similar age and without hyperdactyly was dissected fo r comparison, In addition, the external appearance of the carpometacarpal r egion of two female Silkies, an obligatory pentadactylous breed of domestic fowl, was inspected and the skeletal parts of their pectoral and pelvic li mbs compared with those of the hyperdactylous Kestrel. Our results and a li terature review suggest that the symmetrical hyperdactyly in the Kestrel be ars striking similarities to the hereditary hyperdactyly observed in certai n breeds of domestic fowl. In addition, there is a striking resemblance bet ween the hyperdactyly of the young Kestrel and certain forms of hyperdactyl y induced by molecular genetical experiments of other authors on chicks. Co mparison with these results taken from the literature suggest that the symm etrical hyperdactyly in the young Kestrel, including the alterations of the proximal skeletal elements, is caused by an unusually early expression of the Hoxd-11 gene group during embryological development. Most likely, this gene group is situated on the 2(nd) chromosome in birds just as it is in ma mmals.