Observations of mixed-aged litters in brown bears

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We report on 3 cases of mixed-aged litters (young born in different years) in brown bears (Ursus arctos); in 1 instance the cub-of-the-year (hereafter called cubs) died in the den. Two cases occurred in Sweden after mothers were separated from their young during the breeding season. In one, the mother was separated from the accompanying cub for at least 12.5 hours and possibly up to 3.3 days, and later possibly separated for 4 days. In the other, the mother was separated from her yearling at least 3 times for 1-14, 1-6 and 1-6 days. She was with a male during the first separation. Specific events that produced the mixed-aged litter observed in Greater Yellowstone Ecosystem were unknown and our interpretation is based on estimates of ages of accompanying young from photographs. The observation of only 2 mixed-aged litters, after den emergence, from a sample of 406 observed cub litters accompanying radiomarked females confirms the rarity of this phenomenon. The mechanism apparently includes a short separation of mother and young, and, in the case of cubs, the mother must mate while lactating. Better understanding of the physiological mechanisms that allow mixed-age litters would help us in the debate about the occurrence of sexually selected infanticide in bears.

Publication type Article
Publication Subtype Journal Article
Title Observations of mixed-aged litters in brown bears
Series title Ursus
DOI 10.2192/07SC017R.1
Volume 19
Issue 1
Year Published 2008
Language English
Publisher International Association for Bear Research and Management
Contributing office(s) Northern Rocky Mountain Science Center
Description 7 p.
First page 73
Last page 79
Online Only (Y/N) N
Additional Online Files (Y/N) N
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