Climatic Variation and Age Ratios in Bighorn Sheep and Mountain Goats in the Greater Yellowstone Area

Scientific Disciplines
Biological Sciences - Terrestrial
Keywords
Montana State University
Montana
conservation biology
greater yellowstone area
bighorn sheep
mountain goat
recruitment
recruitment rates
mountain goats
Natural Resource Conservation Service
Natural Resource Conservation Service Snotel
climatic conditions
Authors
Years
Volumes
Volume 18, No. 1-4

Climatic variation and age ratios in bighorn sheep 
And mountain goats in the greater yellowstone 
Area
Carson J. Butler *, Ecology Department, Montana State University, Bozeman, Montana 59717  
Robert A. Garrott, Ecology Department, Montana State University, Bozeman, Montana 59717   
Using management data regularly collected by state and federal agencies, we indexed 
recruitment rates of bighorn sheep and mountain goats in the Greater Yellowstone Area 
(GYA) by calculating young:adult ratios.  Annual and long term regional climatic conditions 
were indexed using data from Natural Resource Conservation Service Snotel sensors across 
the GYA.  Linear regression models were used to assess hypotheses that recruitment rates 
in bighorn sheep and mountain goats in the GYA were associated with annual and regional 
variation in climatic conditions.  The initial dataset consisted of 685 bighorn sheep lamb:ewe 
ratios from 21 herds since 1960 and 184 mountain goat kid:adult ratios from 18 herds since 
1966.  After censoring data, 369 bighorn sheep records remained, which were split into three 
seasonal subsets, and 123 mountain goat records remained in a single dataset.  Findings 
suggest that recruitment rates in bighorn sheep and mountain goats were associated with 
annual variation in both pre-birth and post-birth climatic conditions, interacting with long 
term regional climate conditions.  Additionally, strong interactions were found between 
precipitation during the birthing season and winter severity.  Collectively, these findings 
suggest that recruitment in bighorn sheep and mountain goat populations in the GYA may be 
sensitive to changes in future climate conditions and that the response may vary regionally 
across the GYA.