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What could be a cuter end-of-year topic than polar bear circadian rhythms? Unlike some arctic animals, female polar bears have rhythms even in constant light conditions of arctic summer and winter. https://doi.org/10.1177/0748730419900877
The authors have radio collar activity data from 122 female polar bears collected over 8 years! (No single bear has 8 years of data). The data clearly demonstrate that a large proportion of bears have year-round, 24 h rhythms.
Looking closer, a larger proportion of bears are rhythmic in summer "LL" conditions (~90%), while in winter "DD" conditions, only about 60% were highly rhythmic. Photoperiod influences the rhythm period, with longer 𝛕 as days lengthened.
The acrophase (time of peak activity) was constant most of the year, except in seal-pupping season! Bears seemed to have very flexible activity patterns, with some becoming nocturnal, during this peak hunting season.
"Aschoff's rule" would suggest that because period increased on longer summer days that polar bears might be nocturnal. But their acrophase is consistently at about noon except in pupping season and masking is unlikely as they retain rhythms in LD and DD. So probably diurnal.
How are polar bear clocks keeping time? The authors suggest it could be a very wide entrainment range, sensing of the angle of the sun (as ice habitat allows unobstructed vistas) or sensing of spectral characteristics of light.
The different ways arctic animals handle huge changes in day length are so cool! Though of course, for polar bears the flexible changes in activity may be crucial to surviving in a rapidly melting environmental niche.
