Blubber thickness was measured using a precision
calliper. Blubber thickness of the ventral grooves
(site D, Fig. 1) was measured from the top of the
folds, as they were close together during measurements
of thermal conductivity. The accuracy of this
method for measuring thermal conductivity has been
found to be +4% (Kvadsheim et al., 1994).
Water and lipid content
About 15 g of the frozen blubber samples were cut
into tiny pieces using a scalpel. Water content was
determined by dehydration at 60°C until constant
weight was obtained. The water-free samples were
then frozen again before they were homogenized
by use of a mortar. The lipid content of the blubber
was determined by extracting the lipid of 1 g of dry
blubber with petroleum ether (mixture of pentane
and hexane) using a Soxtec System (HT 1043 Extraction
Unit and 1046 Service Unit, Tecator AB,
Hogan&, Sweden). The extracted lipid was weighed
and the results are presented as percentage of wet
weight. The lipid content was determined in duplicate
for all samples.
Calculations of average thermal conductivity k,
When calculating the sensible heat loss rate of
marine mammals it is the overall thermal conductivity
(k,) of the entire blubber layer that should be
put into the equations. We have therefore developed
a weighting system to calculate the overall thermal
conductivity of the blubber layer based on the conductivity
in the four measuring sites A-D in Fig. 1.
Folkow and Blix (1992) made detailed investigations
of the distribution of blubber thickness in minke
whales. Based on their data we calculated the fraction
of the body surface having a blubber thickness close
to the thickness of each of our four measuring sites.
The thermal conductivities of the sites A-D were
thereafter weighted according to these fractions,