Mortality of lambs in free-ranging domestic sheep (Ovis aries) in northern Norway

Abstract

Domestic lamb mortality on an open forested and alpine summer range in the municipality of Målselv in northern Norway was investigated. Two hundred and fifty-three lambs in four of the area's six flocks were randomly fitted with mortality transmitters and monitored throughout the summer grazing season from June to September. Total losses in the six flocks were 183 (22.9%) lambs and 20 (4.2%) ewes. The carcasses of 69 lambs, 36 of which had radio-collars, and six ewes were recovered. Of the collared lambs, eight (22%) died as a result of disease and 27 (75%) were killed by predators. Implicated predators included red fox Vulpes vulpes, lynx Lynx lynx, golden eagle Aquila chrysaetos and wolverine Gulo gulo. Diseased animals included those diagnosed with pasteurellosis, disrupted intestinal functions and advanced coccidiosis. The remains of one lamb were too decomposed for necropsy, though there was no evidence that it had been attacked by a predator. Factors associated with losses were identified using logistic regression. Age of lamb at time of release on the summer range and age of dam were statistically associated with lamb losses (P<0.05). Mortality was higher among older lambs and those of year-old ewes. Parameters commonly associated with lamb size and growth were not statistically associated with mortality. The negative association between lamb mortality and age of dam may be related to the quality of maternal care provided by ewes of different ages, particularly vigilance in the habitat of predators. Similarly, the positive association between mortality and age of lamb may be related to changing lamb behaviour, specifically increased distance from ewe, as lambs grow older and more independent. Predators, when present, are a major cause of mortality in free-ranging lambs, as in most wild ungulates.