Variability in the distribution and composition of adipose tissue in wild arctic foxes (Alopex lagopus) on Svalbard



Adipose tissue was dissected completely from 35 adult and subadult arctic foxes collected between November 1991 and March 1992 in four different areas of Svalbard (latitude 78°5′ to 79° 50′ N). The gross mass, lipid, protein and collagen content and mean adipocyte volume were measured in adipose tissue from six superficial, four intra-abdominal, three intermuscular and two cardiac depots homologous to those of other terrestrial mammals. The total adipocyte complement was calculated from the mass of each depot and its site-specific adipocyte volume. The mean fatness was 14.81 ± 1.3% and sex differences were not significant. All depots except the epicardial enlarged with increasing fatness, but the superficial depots expanded more than the internal depots. The average partitioning of adipose tissue between intra-abdominal and superficial depots was consistent with predictions from allometric equations fitted to data from other Carnivora, but there was much unexplainable variation between individuals. The relative masses of the four intra-abdominal depots were also variable. The mean adipocyte complement was low compared to other continually or seasonally obese arctic mammals, only slightly larger than that predicted from allometric equations relating adipocyte complement to body mass in other carnivorous mammals, but there were large differences between specimens, with some having more than four times the expected number and others only half the expected number. The size of the adipocyte complement was unrelated to age, sex or fatness. Because of such variation in number and size of adipocytes, measurements of adipocyte volume from biopsies of adipose tissue would not provide an accurate estimate of fatness.

Almost all the adipose depots found in other terrestrial mammals were present. Site-specific differences in adipocyte volume and the lipid and protein content of adipose tissue were similar to those of other wild mammals and did not change with fatness. The collagen content was highest in superficial and lowest in intra-abdominal adipose depots. The differences of up to 60% in the collagen content of homologous depots of different foxes could not be explained by age or cytologically visible blood vessels and fascia but correlated with adipocyte complement.