The marine copepod Calanus finmarchicus is known to overwinter at depth in a state analogous to diapause in terrestrial insects, but the cues which cause individuals to enter and leave this state are the subject of speculation. In this paper we use a simple demographic model, driven by environmental data from 4 distinct locations in the organism's eastern range, to test the plausibility of a number of candidate mechanisms. We start from Miller et al. 's (1991; Mar Ecol Prog Ser 72:79-91) hypothesis that diapause is controlled by photoperiod, and show that thls leads to unrealistic patterns of reproductive activity in northern waters. We extend the model to include the recently reported phenomenon (Hirche 1996a; Ophelia 44:lll-128) of delayed gonad maturation in females experiencing low food concentrations. This ylelds more plausible annual patterns, but falls to reproduce observed geographical variation in the timing of the yearly onset of reproductive activity. Our final model dispenses with photoperiodic control and assumes that diapause duration is controlled by the normal development processes operating at a reduced rate. This model reproduces all the qualitative features of the patterns of reproductive activity observed at our 4 test sites, thus demonstrating that photoperiod cueing is not a necessary prerequisite for the regular re-ascension of C. finmarchicus into surface waters.
This paper describes a model of the metabolic cost of swimming in pinnipeds and its application to other marine homeotherms. The model takes account of both hydrodynamic and thermal processes. The thermal component incorporates both free and forced convection and takes account of the effect of hair on free convection. Using data from the literature to evaluate all but two of the parameters, we apply the model to metabolic rate data on phocid seals, otariids (sea lions), penguins and minke whales. We show that the model is able to reproduce two unusual features of the data; namely, a very rapid increase in metabolic rate at low velocities and an overall rise in metabolic rate with velocity which is slower than the rise in hydrodynamic drag force. The work shows the metabolic costs of propulsion and thermoregulation in a swimming homeotherm to be interlinked and suggests differing costs of propulsion for different modes of swimming. This is potentially of ecological significance since the swimming speed that minimises the cost of transport for an animal will change with changes in water temperature.
In this paper we describe a detailed model of the thermal balance of a seal in air. We tested the model against the limited experimental information available on thermoregulation for harbour seals (Phoca vitulina) in air. Since a mother must meet both her own and her pups' energetic costs, we suggest that there may be an energetic advantage for harbour seals in Scotland if lactation is timed to coincide with the most favourable conditions for hauling out. To test this hypothesis, we used the harbour seals in the Moray Firth as our case study. The model does predict an energetic cost resulting from thermoregulation during haul-out for a mother and her pup in the Moray Firth. Taking the mother and pup as a unit, we estimate the minimum cost during lactation. This combined cost, which must be met by the female seal, is similar to the minimum metabolic rate during haul-out for the summer predicted from the model. In winter the predicted minimum metabolic rate exceeds the lactation cost, and an additional cost of thermoregulation results. The model predicts the most energetically favourable time for lactation to be June and July, and this is coincident with the timing of pupping in this seal population. We suggest that for harbour seals in Scotland, the timing of pupping may be influenced by the thermoregulation costs of haul-out. This provides indirect evidence that thermoregulation influences haul-out behaviour in this small phocid species.
In this paper we describe a detailed model of the thermal balance of a seal in air. We tested the model against the limited experimental information available on thermoregulation for harbour seals (Phoca vitulina) in air. Since a mother must meet both her own and her pups' energetic costs, we suggest that there may be an energetic advantage for harbour seals in Scotland if lactation is timed to coincide with the most favourable conditions for hauling out. To test this hypothesis, we used the harbour seals in the Moray Firth as our case study. The model does predict an energetic cost resulting from thermoregulation during haul-out for a mother and her pup in the Moray Firth. Taking the mother and pup as a unit, we estimate the minimum cost during lactation. This combined cost, which must be met by the female seal, is similar to the minimum metabolic rate during haul-out for the summer predicted from the model. In winter the predicted minimum metabolic rate exceeds the lactation cost, and an additional cost of thermoregulation results. The model predicts the most energetically favourable time for lactation to be June and July, and this is coincident with the timing of pupping in this seal population. We suggest that for harbour seals in Scotland, the timing of pupping may be influenced by the thermoregulation costs of haul-out. This provides indirect evidence that thermoregulation influences haul-out behaviour in this small phocid species
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