Reproductive Rate and Calf Body Mass in a North-Boreal Reindeer Herd: Effects of NAO and Snow Conditions

Abstract

Aikio, P. & Kojola, I. 2014: Reproductive rate and calf body mass in a north-boreal reindeer herd: effects of NAO and snow conditions. -Ann. Zool. Fennici 51: 507-514.Population demographics of northern ungulates are influenced by density and weather conditions. For caribou and reindeer (Rangifer tarandus) that dig in the snow for winter food, snow accumulation and duration of the snow layer are suggested to be influential. Indices based on large-scale alterations of atmospheric pressure such as the North Atlantic Oscillation (NAO) are connected to climate change and may predict ecological processes better than does local weather. Some empirical evidence of the effects of climate variation is available for Arctic and alpine populations but practically none for sedentary, boreal reindeer. We examined the effects of population density, winter and spring NAO, snow accumulation, and the timing of snowmelt on reproductive success (here the calf/female ratio) and on mean summer body mass of calves in a north-boreal, free-ranging herd of semi-domesticated reindeer in northeastern Finland. In this herd, day of snowmelt explained variation in the calf/female ratio and in the midsummer body mass of calves, more so than NAO, winter snow depth and population density.

Early‐life conditions determine the between‐individual heterogeneity in plasticity of calving date in reindeer

“…As previously reported (see Aikio & Kojola, ; Kumpula & Colpaert, ; Paoli et al, ), the early spring period appeared a key period for the reproductive success of reindeer as females with a higher physical condition in spring before their first calving season will have an earlier first calving date and thereafter earlier calving dates throughout their lifetime (Figure c). A better physical condition during the last trimester of pregnancy certainly contributed to advance the date at which the foetus is mature, resulting in an earlier birth (Rowell & Shipka, ).…”

Maternal carryover, winter severity, and brown bear abundance relate to elk demographics

“…Increased winter severity, in the form of increased snowfall [21], snow depth [13], and decreased temperatures [71] can have a negative effect on elk survival across a broad range of climatic conditions, which in turn leads to decreased abundance after harsh winters [72]. Though severe winters and later, colder springs can be detrimental to ungulate recruitment [14,73,74], our findings on early calf recruitment did not support the winter severity or spring bottleneck hypotheses, indicating that winter severity has an impact on…”

“…Increased winter severity, in the form of increased snowfall [ 21 ], snow depth [ 13 ], and decreased temperatures [ 71 ] can have a negative effect on elk survival across a broad range of climatic conditions, which in turn leads to decreased abundance after harsh winters [ 72 ]. Though severe winters and later, colder springs can be detrimental to ungulate recruitment [ 14 , 73 , 74 ], our findings on early calf recruitment did not support the winter severity or spring bottleneck hypotheses, indicating that winter severity has an impact on overwinter survival, rather than successful parturition and survival of calves to two months.. Winter may have less impact on early calf parturition for northern ungulates in coastal areas with milder winters and warmer summers than typical mainland subpolar and continental climates [ 6 , 75 ]. For example, caribou early calf recruitment was best predicted by higher availability and quality of forage on summer range than winter ranges on the Kenai Peninsula, Alaska [ 76 ], indicating support for the maternal carryover hypothesis rather than the spring bottleneck or winter severity hypotheses.…”

“…Winter severity can reduce maternal ungulate body condition and juvenile survival through increased thermoregulatory costs from cold temperatures [ 11 ], and increased difficulty of movement or reduced forage availability due to increased snowfall [ 12 , 13 ]. Spring weather also can affect ungulate population dynamics [ 14 ] because later, colder, and drier springs limit vegetation growth and availability [ 15 ]. Reduced forage quantity and quality can cause a spring nutritional “bottleneck” that may cause ungulate energetic costs to exceed reserves, reducing survival and reproductive success [ 16 , 17 ].…”