Opportunistic Torpor Down Under

Seasonal torpor is well known in northern latitudes. Placental mammals, including some rodents and many bat species, enter torpor to conserve energy when ambient temperatures fall and food becomes scarce. Prolonged torpor is characterized by highly reduced body temperature and metabolic rates. Torpor is less common in mammals from the southern hemisphere. In fact, only one marsupial is known to undergo a seasonal hibernation, the mountain pygmy-possum (Burramys parvus).

Now, James Turner and his colleagues from the University of New England in Australia report that seasonal torpor may be more common in marsupials than previously thought. They studied wild western pygmy-possums (
Cercartetus concinnus), a small marsupial closely related to the mountain pygymy possum. These small possums (Figure 1) live in cooler temperate forests and semi-arid areas of southern Australia. By using implanted radio transmitters capable of measuring the animal’s body temperature, the research team was able to track the location and body temperature of seven possums over two winters.

Michael Pennay
Figure 1. A photo of a closely related eastern pygmy-possum (Cercatetus nanus). (From Michael Pennay/Flickr)

These tiny marsupials used some form of torpor over 60% of the days in winter. Western pygmy-possums used both short duration torpor lasting less than 24 hours interspersed with longer torpor bouts lasting over 24 hours (Figure 2). Interestingly, on any given winter day some individuals exhibited short torpor bouts while others showed prolonged torpor (Figure 3). Such highly flexible torpor patterns are uncommon in mammals. These marsupials are nocturnal and entered short torpor bouts just before dawn. Individuals that entered longer duration bouts entered into torpor just after sunset and stayed in torpor for up to 186 hours.

marsupialtorpor2
Figure 2. A plot of the body temperature (closed circles and black line) and ambient temperature (grey line) of two western pygmy-possums over a 6-week period. Short torpor bouts are illustrated by animal CcQ (top). Longer duration torpor was used by animal CcT (middle), with one section enlarged (bottom) to reveal a bout lasting over 7 days. (From Turner et al., 2012)


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Figure 3. Histogram for torpor bout duration (n =4). The dashed line indicates the division between short (<24 h) and prolonged (>24 h) torpor bouts. (From Turner et al., 2012)

The authors suggest that pygmy-possums may have “employed short torpor bouts as a response to a negative energy balance accrued while active during the previous night, whereas prolonged torpor was entered into on evenings where cold Ta indicated that activity might be too energetically expensive”.

Southern Australian winters are relatively mild and usually allow pygmy-possums to forage on warmer days. When temperatures drop for a day or more, the possums conserve energy by entering torpor and re-warm when ambient temperatures rise again.


References

Turner, J., Warnecke, L., K├Ârtner, G., & Geiser, F. (2011). Opportunistic hibernation by a free-ranging marsupial Journal of Zoology DOI: 10.1111/j.1469-7998.2011.00877.x