Variation in stomach temperature as indicator of meal size in harbour seal, Phoca vitulina
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To understand the role of the marine mammals as components in foodwebs, it is important to know the composition frequency and size of their meals. Marine mammals have a core body temperature of about 37°C, and feed mainly on poikilothennic animals. Food intake will therefor lead to a coating of the stomach, followed by an increased metabolism. Up till now there has been no direct method for measuring the amount of food ingested by free ranging animals. Here we describe the use of a method for remote registration of changes in stomach temperature to quantify meal sizes in captive harbour seals (Phoca vitulina). This might serve as a calibration of technology that may also be applied to free ranging seals. The animals were fed a range of meal sizes at various temperatures and individual fish sizes. The time taken to regain initial stomach temperature from minimum temperature (recovery time) was significantly related to both meal size and meal temperature. The time taken from initial temperature to initial temperature again was regained (total time) was also significantly related to both meal size and meal temperature. Recovery time is in this experiment considered the best available indicator of meal size when meal temperature is known.