ABSTRACT
Locomotory muscle temperature and swim velocity profiles of an adult Weddell seal were recorded over a 21h period. The highest temperatures occurred during a prolonged surface period (mean 37.3°C, S.D. 0.16°C). Muscle temperature averaged 36.8 and 36.6°C (S.D. 0.25°C, 0.19°C) during two dive bouts and showed no consistent fluctuations between dive and interdive surface intervals. Swim velocities were also constant, near 1.3 ms−1. These data indicate that past records of low aortic temperatures (35°C) during and after prolonged dives are not indicative of whole-body temperature changes, and that muscle temperature, even during dives as long as 45min, remains near 37°C.
INTRODUCTION
Aortic temperature reductions of 1–3°C have previously been recorded during or after extended dives in Weddell seals, Leptonychotes weddellii (Kooyman et al. 1980; Hill et al. 1987). Such changes appeared to be consistent with the 1°C temperature decreases reported for multiple tissues during 15min forced submersions of harbor seals, Phoca vitulina (Scholander et al. 1942). While hypometabolism has been considered to be a potential cause of such changes, increased conductive and convective heat loss, including alterations in skin and flipper blood flow, have also been suggested as possible mechanisms (Kooyman et al. 1980; Hill et al. 1987).
In view of these data, we now present the muscle temperature profile of a Weddell seal during diving. In addition, swim velocity is also reported, as an indicator of relative muscle work effort during diving.
MATERIALS AND METHODS
An adult, male seal Leptonychotes weddellii ( Bonner) (448kg) was captured near McMurdo Station, Antarctica, brought to an experimental hut, and weighed and anesthetized (1mgkg−1 intramuscular ketamine induction, and 1–3% isoflurane and oxygen for maintenance via a mask connected to a large-animal, semiclosed-circuit anesthesia machine). The seal was allowed entry into a dive hole below the hut 17h after the termination of anesthesia.
While the seal was anesthetized, an intramuscular temperature probe was inserted percutaneously into the longissimus dorsi-iliocostalis lumborum muscle complex. Measurements were recorded at 15s intervals by an attached microprocessor (MP). Depth and swim velocity were recorded at 15s intervals by the MP and an attached velocity meter, as previously described (Ponganis et al. 1990; Castellini et al. 1992b).
The temperature probe was a Yellow Springs Instruments thermistor, model 511, sensitive to 0.1°C, calibrated in a Lauda temperature bath. The factory-specified time constant of the thermistor (63% response time) was 0.2s. The probe calibration curve was generated by linear regression analysis.
Surface times and behavior between dives were recorded for correlation with the MP data. After the study, the seal was reanesthetized and the instruments removed without any apparent complications.
Data were stored, processed and prepared for analysis on Quattro Pro, Statistix and NCSS with the use of a customized Pascal program.
RESULTS
Fig. 1 contains the swim velocity and muscle temperature profiles during the study period. Entry into the water occurred at 19:01h. During the first dive bout, dive profiles from 23:00–02:00h were considered to be characteristic of a foraging pattern (Castellini et al. 1992a), in that dive depths, durations and surface intervals were uniform at about 100m, 10min and 2min, respectively. Probable exploratory dives occurred at 02:25, 06:15 and 09:30h. These dives were 27, 36 and 45min in duration and were less than 50m in depth. The prolonged surface period of 78min began at 04:10h. The MP was disconnected and removed from the seal while it slept in the water at 28min into a second prolonged surface period.
Since consistent temperature oscillations between dive and interdive surface intervals during dive bouts were not apparent (Fig. 1), all these temperatures were pooled and compared to the pooled temperatures recorded while the seal rested in the water (Table 1). The mean temperatures for these surface periods (SP) and dive periods (DP) were significantly different (Table 1, ANOVA, F ratio 979.9, P>F=0.0000). The range of mean temperatures was 0.7°C; Duncan’s and Newman–Keul’s range tests (P=0.05) revealed that SP1=DP1, while Fisher’s LSD test (P=0.05) showed that DP2=SP3. All other comparisons of mean surface period and dive period temperatures were not equal by these tests.
Overall, mean swim velocity was 1.3 ms−1 (S.D. 0.36 ms−1, N=3389). During the three long extended dives, mean velocity was 1.5 ms−1 (S.D. 0.18 ms−1, N=441).
DISCUSSION
During this 21h record, the highest temperatures occurred during the prolonged surface period (SP2) between dive bouts. This temperature increase was similar to that previously observed in the arterial blood of diving Weddell seals (Kooyman et al. 1980; Hill et al. 1987). Such a temperature elevation during the surface period could be due to several factors: decreases in convective heat loss (Gallivan and Ronald, 1979), absence of cold prey ingestion (Imms and Lighten, 1989) and food-induced thermogenesis (FIT) (Costa and Kooyman, 1984). Why such an increase did not occur during the recorded portion of the last surface period (SP3) is unclear; however, the predominance of exploratory dives prior to SP3, the lower aortic temperatures during such long dives (Kooyman et al. 1980; Hill et al. 1987) and possibly decreased FIT (due to decreased or no food intake during exploratory dives), may contribute to such a pattern.
These muscle temperature measurements near 37°C during diving also suggest that no single site temperature is a satisfactory indicator of whole body metabolism or whole body temperature, as has been proposed in earlier reports (Kooyman et al. 1980; Hill et al. 1987). Although aortic temperatures near 35°C have been reported for extended dives (Kooyman et al. 1980; Hill et al. 1987), such a temperature decrease was not seen in locomotory muscle during this study. It appears that propulsive muscle temperature is regulated at a constant temperature during diving.
ACKNOWLEDGEMENTS
We thank P. Thorson and S. Eckert for assistance in the field, and L. Winter for data analysis. This work was supported by NSF DPP 86-13729 and USPHS HL 17731.