Survival

In ungulates, body mass is often positively correlated with juvenile survival, but little is known of whether body mass affects survival of other age-classes. We studied two marked populations of bighorn sheep (Ovis canadensis) in Alberta, Canada, to determine if body mass affected the survival of different sex-age classes. Chest girth at weaning was correlated ( P < 0.0001) with survival of bighorn lambs in the Sheep River population. In the Ram Mountain population, body mass in mid-September had a stronger effect upon survival than mass in early June, mass gain in summer, or mass loss in winter. Body mass at weaning was correlated with lamb survival ( P = 0.004). In both study areas, relationships between size and survival of lambs were similar for the two sexes. At Ram Mountain, survival of yearling and adult males seemed to be independent of body mass. Light yearling females were less likely to survive than heavy yearling females. Among females aged 3 -6 years, body mass had no effect on survival. Among females 7 years of age and older, mass in mid-September had a weak but significant ( P = 0.03) effect on survival. Females were slightly lighter in mid-September in their last year of life than in the rest of their adult life. Maternal expenditure is unlikely to affect the survival of prime-age ewes, but may have a detrimental effect on the survival of older ewes. RCsurnC : Chez les ongulks, la survie des juvkniles dkpend de la masse corporelle, mais les effets de la masse sur la survie chez les autres classes d' ige sont ma1 connus. Pour dkterminer si la masse corporelle a un effet sur la survie des diffkrentes classes de sexe et d'ige, nous avons ktudiC deux populations de Mouflons d'Amkrique (Ovis canadensis) marques en Alberta, Canada. La circonfkrence thoracique au sevrage ktait en corrklation ( P < 0,0001) avec la survie des agneaux dans la population de Sheep River. Chez la population de Ram Mountain, la masse corporelle h la mi-septembre a eu un effet plus marquk sur la survie que la masse corporelle au debut de juin, le gain estival ou la perte hivernale de masse. La masse au sevrage 6tait en corrklation avec la survie des agneaux ( P = 0,004). Aux deux sites de l'ktude, les relations entre la masse et la survie ktaient semblables pour les agneaux et pour les agnelles. A Ram Mountain, la survie des mouflons miles igks de 1 an et des miles adultes semblait etre indkpendante de la masse corporelle; par contre, parmi les jeunes femelles de 1 an, celles qui ktaient plus Ikgkres avaient moins de chance de survivre que celles qui ktaient plus lourdes. Chez les femelles igkes de 3 h 6 ans, la masse corporelle est restke sans effet sur la survie. Chez les femelles igkes de 7 ans et plus, la masse h la mi-septembre a eu un effet limit6 mais significatif ( P = 0,03) sur la survie. De plus, les femelles ktaient un peu plus lkgkres h la mi-septembre lors de leur dernikre annke de vie qu'au cours des autres annkes de leur vie adulte. Les soins maternels ne devraient pas affecter la survie des femelles igkes de 3 h 7 ans, mais ils pourraient avoir un effet nkgatif sur la survie des femelles plus igkes. Introduction size is often associated with increased survival (Murie and Boag 1984; Richner et al. 1993). For small birds, daily mass It is important to identify factors that affect survival because the accumulation can affect overnight survival, and small variasurvival of different sex -age classes plays a major role in the tions in mass are under very strong selective pressures (Gosler evolution of life-history strategies (Stearns 1992) and affects et al. 1995). In larger animals, differences in body mass changes in population size. Body mass affects survival in many among individuals could affect survival on a seasonal basis. taxonomic groups (Hutchings 1994; Sedinger et al. 1995; For large mammals, individuals in poor condition (usually Williams et al. 1993). In small mammals and birds, large associated with low mass) are generally at greater risk of Received July 1 1, 1996. M. Festa-Bianchet. Groupe de recherche en kcologie, nutrition et knergktique, Dkpartement de biologie, UniversitC de Sherbrooke, Sherbrooke, PQ J 1K 2R1, Canada, ' and UMR 5558, Laboratoire de Biomktrie-Gknktique et Biologie des Populations, Universitk Lyon I (Universitk Claude-Bernard) 43 Boulevard du 1 1 novembre 19 18, 69622 Villeurbane Ckdex, France. J.T. Jorgenson. Alberta Department of Environmental Protection, Natural Resources Service, Suite 201, 800 Railway Avenue, Canmore, AB T I P 1W1, Canada. C.H. BCrubC. Groupe de recherche en kcologie, nutrition et knergktique, Dkpartement de biologie, Universitk de Sherbrooke, Sherbrooke, PQ J 1 K 2R1, Canada. C. Portier. Groupe de recherche en kcologie, nutrition et knergktique, Dkpartement de biologie, Universitk de Sherbrooke, Sherbrooke, PQ J1K 2R1, Canada, and UMR 5558, Laboratoire de Biomktrie-Gknktique et Biologie des Populations, Unive~sitk Lyon I (Universitk Claude-Bernard), 43 Boulevard du 11 novembre 1918, 69622 Villeurbane Ckdex, France, and Institut d'Ecologie, Universitk de Paris VI (Pierre et Marie Curie), Centre National de le Recherche Scientifique, URA 258, 75252 Paris, France. W.D. Wishart. Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada. ' Address for correspondence. Can. J . Zool. 75: 1372 1379 (1997) O 1997 NRC Canada C an . J . Z oo l. D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y U ni ve rs ité d e Sh er br oo ke o n 04 /2 9/ 14 Fo r pe rs on al u se o nl y. Festa-Bianchet et al mortality, regardless of the cause of the poor condition (Crste and Huot 1993; Gulland 1992; Owen-Smith 1990). No studies, however, have examined how the effects of body mass on survival change with an individual's age. Low birth mass increases the risk of perinatal mortality in several species of ungulates (Cassinello and Alados 1996; Clutton-Brock et a1 . 1987b; Clutton-Brock et al. 1992; White et al. 1987), but little is known of the effects of body mass on survival for older ungulates, despite suggestions that energy reserves accumulated during summer should affect adult winter survival (Parker et al. 1996). In temperate environments, body mass of ungulates fluctuates seasonally; in some species, individuals can lose over 25% of their mass during winter (Festa-Bianchet et al. 1996; Leader-Williams and Ricketts 1982). In these species, survival may be affected by body mass seasonally. For example, mass at the end of summer should be correlated with the amount of lipid reserves that each individual has accumulated (Rumpler et al. 1987). These reserves should increase survival during winter, when forage is scarce and of low nutritional quality (Parker et al. 1996). If the main effect of mass on survival is through the use of summer-accumulated energy reserves, then late-summer mass should affect survival more than mass at the beginning of the summer, when high-quality forage is readily available. Mass at the end of summer may reflect the amount of resources available to an individual, but relative changes in body mass may affect survival independently of absolute mass. In particular, changes in body mass of full-grown adults may be closely associated with changes in lipid reserves (Parker et al. 1993). For example, in mule deer (Odocoileus hemionus), fawns that starved during winter lost almost twice as much mass as fawns that survived (Bartmann et al. 1992). It is rare to obtain repeated captures of the same individuals because wild ungulates are often difficult or costly to capture. In one of our bighorn sheep (Ovis canadensis) study areas, however, we recaptured most individuals several times each year (Festa-Bianchet et al. 1996). Here we analyze detailed long-term data on marked individuals to explore the relationships among mass, changes in mass, and survival of bighorn sheep of different sex and age classes. We expected that heavier sheep would have a higher survival rate than lighter sheep, and that the correlation between mass and survival would weaken with age, as adult sheep should mostly be individuals that had sufficient mass to survive the juvenile stage. Because bighorn sheep accumulate mass until they are at least 7 years old (Festa-Bianchet et al. 1996), however, we expected that mass would affect survival of young adults. Material and methods We studied two populations of bighorn sheep in Alberta, Canada. Both studies are based upon monitoring of marked individuals. In this paper we consider only individuals that either survived or died of natural causes; sheep that were removed or died through human actions were withdrawn from the sample in the year of their removal or death. In over 80% of cases of assumed death, marked animals simply disappeared. Natural causes of death tentatively determined from sheep remains included predation by coyotes (Canis latrans) (N = 8) or cougars (Felis concolor) (N = 29) (Ross et al. 1997), accidents (h' = 9), and disease (N = 37). We found no sheep that appeared to have starved. We know of only one male emigrant at Sheep River despite compulsory registration of males killed by hunters and aerial surveys by the Alberta Natural Resources Service of all sheep winter ranges near Sheep River every 2-4 years. We documented 21 cases of emigration (7 females and 14 males) at Ram Mountain, less than 1 per year of study. The probability of resighting a marked sheep that was alive in any one year was over 99% for females and over 95% for males (Jorgenson et al. 1997). Ram Mountain Ram Mountain (52"N, 115"W, elevation 1082-2173 m) is an isolated outcrop about 30 km from the main Rocky Mountain range. A mark recapture program began at Ram Mountain in 197 1. After 1975, over 90% of the sheep were marked in most years; females were marked with canvas collars and males with Allflex ear tags. Lambs were marked with numbered metal Ketchum tags with attached colored strips of Safeflag plastic. From late