Image courtesy of  http://www.healthsci.utas.edu.au/physiol/mono/Mainpage.html

Diving mammals cannot breathe underwater.  This presents a problem, as muscular activity and oxidation of glucose require oxygen.  Most mammals solve this problem in several ways: Reduce heart rate (bradycardia) and shunt blood to only the most critical organs (tissue hypoperfusion).  This conserves O2  by depriving noncritical tissues. Myoglobin has high oxygen affinity and can store O2  in muscles during dives.  This means hemoglobin is not the only source of O2 for the muscles, somewhat counterbalancing the O2 deprivation during tissue hypoperfusion. During dives, muscles use anaerobic respiration.

A consequence of these strategies is that the muscles build up an oxygen debt in the form of stored lactate.  After the dive, the animal must resume full circulation and metabolize the lactate (Schmidt-Neilson, 1997).

Although it is also a diving mammal, the platypus does not share some of these usual traits.  We already know that the platypus does not have long surface intervals between dives (dive:surface ratios range from 1:2 to 1:20) but that it does experience bradycardia and tissue hypoperfusion.  This suggests that it has found some means of avoiding anaerobic metabolism (Evans et al., 1994).

Let's look at some physiological data:
 
 

*A lower pyruvate inhibition ratio indicates more anaerobic respiration.
#A higher number indicates better capacity to buffer lactic acid produced in anaerobiosis.

• Platypus are less dependent on myoglobin for storing oxygen

• Platypus do not have the capacity for extensive anaerobic metabolism

• Because platypi have lower body temperatures and metabolic rates than eutherian divers, they consume less oxygen while submerged and can subsist with smaller myoglobin oxygen stores (Evans et al., 1994).

• Platypi inhale deeply before diving (Evans et al., 1994), unlike most diving eutherians (Schmidt-Neilson, 1997).  Oxygen stores in the lungs may be more important than myoglobin stores.

• While submerged, platypus may utilize high-energy stores that do not require oxidation, such as phosphocreatine (Evans et al., 1994).

The whole picture of platypus diving is still unclear, but these animals are certainly doing something different from most diving eutherians.