Northern Prairie Wildlife Research Center
Rory P. Wilson, Institut für Meereskunde, Düsternbrooker Weg 20, D-24105 Kiel, Germany
Determination of the behavior of marine endotherms has long been considered less advanced than comparable studies on terrestrial animals because the properties of seawater make transmission telemetry problematic. One solution to this has been in the development of logging systems which record particular parameters while animals are at sea, these parameters being accessed when the logging unit is recovered. Since data are not transmitted, power requirements can be extremely low, reducing device size, and allowing massive amounts of data to be collected. I will present information to show that, far from trailing behind classic transmission telemetry, the development of logging systems has enabled work on the behavior of marine endotherms to progress to a point where it is now at least as advanced as that in terrestrial studies.
The activity of marine endotherms can be determined by loggers as changes in environmental variables brought about by the animals during particular activities, e.g. changes in hydrostatic pressure or light intensity as a measure of diving or differential pressure for swimming (speed). In addition, a recently-developed 3D compass provides information on angular changes in body orientation and promises to give radical new insights into endotherm activity at sea.
Although animal position is conventionally determined by some kind of transmission mechanism (e.g. radio-telemetry triangulation or GPS), estimates of marine endotherm positions can also be made from logged variables. Examples of this are (1) determination of geographic location by consideration of daylength and time of local noon with respect to GMT and date (for which only light intensity need be logged) and (2) dead reckoning, where movements from a known start position are determined by vectors (derived by logged information on animal speed, heading and change in depth/height).
Since marine endotherms have a considerably higher body temperature than the medium in which they forage, prey ingestion can generally be determined by logging stomach temperature in ingested units because stomach temperature drops at the point of ingestion. Here, the degree of the drop and the length of time which it remains below pre-drop values often can be used as a measure of prey mass.
Finally, the combination of the various elements described above can enhance our understanding of the way marine endotherms utilize their environment to a degree considerably greater than the sum of all its parts because various recorded parameters can be put into context. It would be a fitting tribute to the value of logging systems in general if the technology, often developed for studying animals at sea, were applied more to the study of animals on land.