Changes in food intake, mass, and fat accumulation in association with variations in thyroid hormone levels of harbour seals (Phoca vitulina)

Food intake, mass, fat, locomotor activity, and thyroid hormones were monitored in five captive harbour seals (Phoca vitulina) over an 18-month period. Food intake ranged from anorexia to 11% body weight, with changes in body mass reaching 59% in the males. Food intake was negatively related to changes in mass and blubber except during a 6-week period in the fall when the three variables were positively correlated. From the end of November until June, when food intake decreased progressively, mass and fat increased, but not in what was otherwise direct association. Changes in fat were more consistently negatively related to water temperature than were changes in mass. When the male seals were losing weight despite rising food consumption, free thyroxine (T4) levels were positively correlated with food intake and water temperature but negatively correlated with mass and fat changes; the reverse relationships held for ratios of tri-iodothyronine (T3) to free T4. Over the winter months, when mass and blubber were augmenting despite lowered food intake, free T4 levels were significantly lower, whereas T3 and its ratio to both free and total T4 were reduced when food intake and mass accumulation were directly related. In the female seal, these hormones were not clearly associated with any of the variables measured. Locomotor activity was positively related to food intake but not in any clear way to levels of thyroid hormones. These results are discussed in the context of the metabolic defense of fat levels for insulation in cooling water.