Platypus (Ornithorhynchus anatinus) Fact Sheet: Physical Characteristics

Body measurements

Data sources: Grant (2015)

Notes:

• *This species exhibits sexual dimorphism. Males are about 25% larger than females (Grant 2015)—12-16% longer and 35-40% heavier (Bino et al. 2015).
• Considerable size variation occurs within some populations (Grant 2015). Also, body size increases with latitude, from smaller in northern Australia to larger in southern Australia (Furlan et al. 2011).

Body

• Streamlined body shape (Grant 2015)
• Legs splay to the side (not positioned underneath body) (Grant 2015)
  • Similar to echidnas
• Cloaca in both sexes (Grant 2015)
  • Body opening that serves as exit point for digestive, urinary, and reproductive tracts
• Also see “Introduction to the egg-laying mammals” in Taxonomy and Nomenclature

Fur

• Coarse guard hairs protect dense, fine underfur (Carter 1965; Grant and Dawson 1978; Grant 2015, except as noted)
  • 600-900 hairs per mm²
  • Fur provides insulation against cold water while swimming
    • Layer of air trapped next to skin—body surface stays dry
    • Better insulation than fur of polar bears and beavers (Nicol 2017)
  • Molted annually (Crandall 1964)
• Biofluorescence
  • Fur appears green or cyan under UV light (Anich et al. 2020)
    • Possibly adaptive for being active at dusk and at night
      • Might reduce detection by predators; more research needed

Coloration

• Upper body
  • Dark brown to reddish-brown (Grant 2015)
• Under body
  • Pale brown to silvery-cream (Grant 2015)
• Bill
  • Dark gray (Grant 2015)

Head, bill, and mouth

• Eyes and ears sit in groove behind bill (Grant 2015)
  • Groove folds shut while diving, closing the eyes and ears
    • Also see Diving
  • Eyes small (Grant 2007)
    • Nictitating membrane present (Booth and Connolly 2008)
      • Cleans and lubricates the eye (Grant 2007)
  • No external ear (Grant 2015)
• Bill (Grant 2015)
  • Flexible
  • Hairless
  • Skin moist
    • See Electroreception, below
  • Nostrils near tip of upper bill
  • Elongated jawbones support bill
• Mouth (Grant 2015)
  • No teeth (as adults)
  • Grooves along sides of bill used for filtering food from water
  • Pads made of keratin used to grind food
    • Replaced continuously
  • Cheek pouches
    • Located near keratinous pads
    • Hold food items while on dives
  • See Booth and Connolly (2008), Figure 6.3 [p. 106] for illustration

Limbs

• Short and robust (Grant 2015)
• Forelimbs and shoulders have large muscles attached (Grant 2015)
  • Added strength for swimming and digging
• In adult male, venomous spur present on hind ankles (Grant 2015)
  • Also see “Spurs and venom” in Sexual Dimorphism
  • Also see “Spurs and venom” in Other Physical and Physiological Characteristics

Feet

• Hairless (Grant 2015)
• Forefeet
  • Webbing conspicuous (Grant 2015)
  • Nails broad (Grant 2015)
• Rear feet (Grant 2015)
  • Webbing partial
  • Claws sharp
    • Used for grooming and anchoring body while digging burrow

Tail

• Broad and flat (Grant 2015)
• Hair more coarse and less dense than on body (Grant 2015)
• Tail stores body fat (Temple-Smith 1973; Grant 2015)
  • Up to 40% of a platypus’ total body fat
  • Often thinner after winter and during lactation (in females)
  • Tail volume used as an indicator of body condition (health/energy reserves); however, see Macgregor et al. (2016) for discussion and new indices

Body size

• Males about 25% larger than females (Grant 2015)
  • 12-15% longer (Bino et al. 2015)
  • 35-40% heavier (Bino et al. 2015)
• Juvenile males larger than females (Grant 2007)

Spurs and venom

• Spurs only found in males (Whittington and Belov 2014)
  • As juveniles, spur contained within sheath (Williams et al. 2013)
    • Shed between ~9-12 months of age
  • Venom production begins in adulthood (Whittington and Belov 2014)
    • Large amounts produced only during breeding season (June-August) (Wong et al. 2012)
• Juvenile females have non-functional spur sheath (Whittington et al. 2010; Williams et al. 2013; Whittington and Belov 2014)
  • Lost by adulthood
• Also see “Spurs and venom” in Other Physical and Physiological Characteristics
• Also see "Limbs" in General Appearance

Vision

• Less dependent on vision than other mammals (Asahara et al. 2016)
• Eyesight considered acute (Grant 2007)
• Not thought to use vision to forage (Grant 2007, except as noted)
  • Typically hunt at night (in the dark)
  • Sediment clouds water (Asahara et al. 2016)
• Eyes relatively high on sides of head (Grant 2007)
  • Likely have little binocular vision
• Cones present but extent of color vision not yet investigated (Grant 2007)

Hearing

• Acute (Booth and Connolly 2008)
• Unknown to what extent this sense is used underwater (Grant 2007)
  • Ear canals closed while diving but some sound waves may resonate through air spaces in the ear canal
• Most sensitive at about 5 kHz (Gates et al. 1974)
  • Range: 500 Hz to 20 kHz (Grant 2007)

Smell and taste

(Grant 2007)

• Use of sense of smell unknown
  • Produce “musky” odor and scent mark objects
  • Detection of odors unknown
• Grooves on back of tongue and Jacobson’s organ may be involved in taste and smell

Senses of the bill

• Mechanoreception
  • About 46,500 push-rod receptors (Manger and Pettigrew 1996; Czech-Damal et al. 2013)
    • Most dense on border of upper bill
  • Detect physical disturbance of water (Czech-Damal et al. 2013)
• Electroreception (Manger and Pettigrew 1996; Czech-Damal et al. 2013, and as noted)
  • Mucous gland receptors used to detect prey
    • Array of about 40,000 receptors
    • Act as “an antenna” to detect prey direction and possibly prey distance (Fjallbrant et al. 1998)
    • Mucous conducts electric signals
    • Similar to electroreceptors in fish
      • Platypus differs in that electric fields directly excite nerve fibers (Moyal 2001)
      • No chemical mediator, as in certain fish
  • Serous gland receptors (secondary receptor)
    • About 13,500 receptors
    • Function unknown
  • Receptor sensitivity
    • In the wild, likely perceive fields of 50 μv/cm, potentially lower
    • Sensitivity changes depending on whether the platypus is in or out of water (Czech-Damal et al. 2013)
      • Receptor openings automatically relax or constrict

Thermoregulation

• Body temperature
  • Relatively low, about 32°C (89.6°F) (Smyth 1973; Grant and Dawson 1978)
• Heat stress
  • Caused by ambient temperatures above 25°C (77°F) (Smyth 1973; Grant 2015)
  • Limited ability to cool themselves when out of the water (Brice 2009; Klamt et al. 2011)
    • Immerse body in cool water
    • Refuge in burrow
    • Limited ability to sweat
    • Cannot pant or use saliva spreading (as in kangaroos)
  • Climate change likely to increase heat stress in this species (Klamt et al. 2011)
• Cold stress
  • Adaptations to prevent heat loss (Grant 2015; Nicol 2017, except as noted)
    • Insulating fur and body tissues
    • Increased metabolism
      • Can forage for hours in frigid water
    • Blood vessels around pelvis reduce heat loss via countercurrent heat exchange (Booth and Connolly 2008)
    • Adopt body postures that limit exposure of body parts with little or no hair (feet, bill) to cold water (Smyth 1973)
    • Body size generally increases with latitude (larger body size in southern Australia/Tasmania)

Spurs and venom

• Present in males (Grant 2015)
  • Also see Sexual Dimorphism in Physical Characteristics
• Function
  • Thought to be used in fights between adult males during breeding (Temple-Smith 1973; Grant 2015)
    • Only mammal known to seasonally produce venom (Wong et al. 2012)
      • Many other venomous animals use venom to capture prey or for digestion (Whittington and Belov 2007)
• Spur structure (Whittington and Belov 2014)
  • Size: about 1.5 cm (0.59 in) long
  • Hollow, cone-shaped sheath
    • Made of keratin
    • Supported by bone
  • Connects, via a duct, to a venom gland near the pelvis
• Venom
  • Clear liquid (Koh et al. 2009)
  • Contains unique proteins and peptides (Koh et al. 2009; Whittington and Belov 2009)
    • At least five compounds unique to the platypus (Wong et al. 2012)
• Also see Limbs in General Appearance

Brain

• Large relative to body size (Nicol 2017)
• Forebrain well developed (Booth and Connolly 2008)
• Isocortex (neocortex) smooth and thick (Ashwell and Hardman 2012; Nicol 2017)
  • Differs from short-beaked echidna (folded and thin)

Platypuses rely on their electrosensitive bills to hunt.

While diving, a groove on each side of their head containing their eyes and ears closes shut. This protects these sensitive organs while diving but also renders the platypus blind and mostly deaf while underwater.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Image location: Taronga Zoo, Sydney

The platypus has a unique, flexible bill for catching and grinding up prey. Incredibly thick fur allows the platypus to swim for hours in cold water—as cold as 0°C (32°F) in Tasmania.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Image location: Taronga Zoo, Sydney