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In bears, body temperature drops during hibernation similar to moderate human hypothermia, yet they rarely face fatal outcomes during the winter. This suggests protective adaptations in bear electrophysiology. We studied seasonality in ursine ventricular repolarization by analyzing &gt;1\u2009year electrocardiogram (ECG) recordings from loop recorders implanted in 57 free\u2010ranging Eurasian brown bears. In sinus rhythm, bears exhibited significantly longer RR, QT, and T<jats:sub>peak<\/jats:sub>\u2010T<jats:sub>end<\/jats:sub> intervals (2441\u2009\u00b1\u2009470, 508\u2009\u00b1\u200950, and 53\u2009\u00b1\u20098\u2009ms, respectively) during hibernation than in the active period (649\u2009\u00b1\u2009323, 232\u2009\u00b1\u200939, and 29\u2009\u00b1\u20095\u2009ms, respectively). Optimal heart rate correction of QT interval (QT\/RR<jats:sup>0.435<\/jats:sup>) demonstrated significant prolongation during hibernation. QT and T<jats:sub>peak<\/jats:sub>\u2010T<jats:sub>end<\/jats:sub> intervals remained longer during hibernation than in the active period, even when comparing ECGs with similar RR intervals in the two periods. Ventricular fibrillation occurred in four bears shot during licensed hunting in summer, which led to death. In conclusion, seasonal variations in ventricular repolarization in bears appear, at least partially, independently of heart rate. Compared to humans, ventricular repolarization is slower but more homogeneous. These findings, combined with the absence of fatalities during hibernation, support the theory of protective electrophysiological adaptations in bears. Insights into the underlying mechanisms have biomimetic potential for human therapy.<\/jats:p>","DOI":"10.14814\/phy2.70531","type":"journal-article","created":{"date-parts":[[2025,8,30]],"date-time":"2025-08-30T06:55:01Z","timestamp":1756536901000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Seasonal variations in ventricular repolarization and tachyarrhythmias in hibernating brown bears (<i>Ursus arctos arctos<\/i>)"],"prefix":"10.14814","volume":"13","author":[{"ORCID":"https:\/\/orcid.org\/0009-0000-0267-2511","authenticated-orcid":false,"given":"Lucas Alexander","family":"Lindberg","sequence":"first","affiliation":[{"name":"Department of Biomedical Sciences University of Copenhagen  Copenhagen Denmark"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Boris","family":"Fuchs","sequence":"additional","affiliation":[{"name":"Department of Forestry and Wildlife Management, Faculty of Applied Ecology and Biotechnology Inland Norway University of Applied Sciences  Koppang Norway"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Alina Lynn","family":"Evans","sequence":"additional","affiliation":[{"name":"Department of Forestry and Wildlife Management, Faculty of Applied Ecology and Biotechnology Inland Norway University of Applied Sciences  Koppang Norway"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Timothy","family":"Laske","sequence":"additional","affiliation":[{"name":"Medtronic Inc.  Mounds View Minnesota USA"},{"name":"University of Minnesota  Minneapolis Minnesota USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4288-3310","authenticated-orcid":false,"given":"Anna","family":"Bj\u00f6rkenheim","sequence":"additional","affiliation":[{"name":"Department of Cardiology, School of Medical Sciences \u00d6rebro University  \u00d6rebro Sweden"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ole","family":"Fr\u00f6bert","sequence":"additional","affiliation":[{"name":"Department of Cardiology, School of Medical Sciences \u00d6rebro University  \u00d6rebro Sweden"},{"name":"Department of Clinical Medicine, Faculty of Health Aarhus University  Aarhus Denmark"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8121-4275","authenticated-orcid":false,"given":"Lisa Amalie","family":"Gottlieb","sequence":"additional","affiliation":[{"name":"Department of Biomedical 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