THE CRURAL SYSTEM

Male platypuses have a crural system consisting of a venom (crural) gland, connecting duct and spur - a system unique among vertebrate species. The spur is made of keratin (cornu calcaris) and is supported by an accessory ossification, the os calcaris, on the medial side of the tarsus (Fig.

1.1 Venom

Platypus venom contains 19 different fractions with at least 10 venom proteins, two enzymes and eight other polypep­tides, including defensin-like peptides, C-type natriuretic peptides, nerve growth factors and other uncharacterised protein and non-protein components (de Plater et al. 1995; Whittington et al. 2008; Koh et al. 2009; Wong et al. 2012). The effects of these components have not been fully char­acterised, but nerve growth factors are also found in snake venoms and C-type natriuretic peptides have been found in the venom of the South American pit viper (Whitting­ton et al. 2008). The defensin-like peptides have similar structures to crotamine-like peptides present in the venom of some sauropsid reptiles. Although they have evolved from similar antimicrobial peptides to those of sauropsids, these venom molecules have evolved independently in platypuses (Whittington et al. 2008; Macgregor 2015).

Fig. 28.1. Whole-body radiograph of a platypus (Ornithorhynchus anatinus). (a) Lateral view (adult female) and (b) ventrodorsal view (adult male), illustrating some unique anatomical features: streamlined body compressed dorsoventrally; bill and toothless jaws; lack of tympanic bullae; cervical ribs; reptilian pectoral girdle; and two pairs of pentadactyl limbs, with claws and webbing on the digits of the manus and pes. Note: one of the 4th pair of ribs is displaced as a result of trauma in the female and a transponder can be seen in both.

1.2 Envenomation

Platypus venom may be fatal to mice, dogs and other platypuses (Booth and Connolly 2008). The frequent finding of rake and stab marks on the tails of platypuses indicative of spurring injuries, suggests fatal envenoma­tion in platypuses may be rare (Grant 2007). Male platy­puses are considered dangerous and people are at risk of envenomation during handling attempts, with most accounts describing spurring to the hand or wrist. No human deaths have been reported and there are only two published reports detailing medical treatment of enveno- mated people (Fenner et al. 1992; Tonkin and Negrine 1994). Platypus spurs can inflict deep punctures and envenomation results in immediate onset of intense pain and swelling that may encompass the entire limb and take up to 3 wk to subside. Ongoing weakness and loss of function of the affected part may persist for up to 3 mo. Ischemia, tissue necrosis and regional lymphadenopathy have not been reported and there are no apparent sys­temic effects of envenomation. No antivenom is available. Treatment is symptomatic, with parenteral opiates or regional nerve blockade required for adequate pain relief (Fenner et al. 1992; Tonkin and Negrine 1994).

1.3 Age determination using spur morphology

Determining the age of platypuses is particularly useful in field research, species management and for the provision of age-appropriate husbandry in rescued individuals. The size and shape of the spur can be used to determine sex and estimate age (Booth and Connolly 2008; Williams et al. 2013). The morphology of spurs and associated struc­tures in males, and vestigial spur sheaths in females, varies with age. Juvenile females have a rudimentary spur sheath, only 1-4 mm in length, which disintegrates or falls off at

Fig. 28.2. Dorsoplantar radiograph of the pes showing the os calcaris (OsC) supporting the keratinous spur (cornu calcaris) (arrow) (Vogelnest and Allan 2015).

Fig.

approximately 9 mo of age, leaving a permanent hole or recess where the spur sheath was lost. Consequently, female platypuses of unknown age can only be classed as juveniles (9 mo; if the spur sheath is absent). In males, spur morphology can be used to distinguish between juveniles (first yr of life Fig. 28.3 a-c), sub-adults (second yr of life Fig. 28.3d), and adults (third yr of life or older Fig. 28.3e). Juvenile male platypuses are distin­guished by the presence of either a complete or disinte­grating fibrous spur sheath. The progression of exfoliation of the spur sheath varies considerably so that the amount of exfoliation can’t accurately be tied to number of months since emergence from the nesting burrow. Sub-adult and adult age classes are distinguished by the relative height of the fleshy epidermal collar, and whether this extends more (sub-adults) or less (adults) than about one-quarter from the base of the spur (Fig. 28.3d-e) (T Grant pers. comm.).

2.