How are endogenous circadian rhythms adapted to life in different environments?

The circadian system provides an important mechanism for regulating many physiological and behavioral processes in synchrony with the natural day. Tropical habitats present extreme environmental conditions for seasonal processes and Arctic habitats present extreme conditions for circadian systems. How does the circadian system cope with the continuous daylight of the high Arctic summer, when day and night changes are basically abolished?

We have studied the seasonal changes in the circulating melatonin pattern of Lapland Longspurs (Calcarius lapponicus) in a temperate zone habitat (Washington State) and their high Arctic breeding site during continuous daylight (Alaska), in collaboration with T.J. Van't Hof (Wright State University and Max-Planck Institute, Germany, L.M. Romero (Tufts University) and J. Brawn. The hormone melatonin is one of the most important components of the circadian system in birds, yet it is very responsive to light and its secretion can be suppressed by light exposure at night. Can birds that breed in the Arctic tundra maintain an intact circadian system under continuous daylight or do these conditions adversely affect the function of this important physiological system? Our data show that Lapland Longspurs can maintain significant, but dramatically suppressed diel cycles in plasma melatonin concentrations and also in testosterone. These data suggest that even under polar day conditions 24-hr rhythms can persist. The maintenance of diurnal hormone rhythms could provide a physiological basis to reports of rhythmic behavior in many birds during the Arctic summer.

In the future, we will also investigate melatonin patterns in tropical birds to gain a better understanding of their circadian organization. Furthermore, we will study the circadian system of free-living birds at the Stony-Ford field station to unravel the mechanisms behind seasonal changes in circadian organization.

©Mike Danzenbaker

© Marie Read

[back to top]