S. D. Albon

Red deer : behavior and ecology of two sexes

Red Deer: Behavior and Ecology of Two Sexes is the most extensive study yet available of reproduction in wild vertebrate. The authors synthesize data collected over ten years on a population of individually recognizable red deer, usually regarded as conspecific with the American elk. Their results reveal the extent of sex differences in behavior, reproduction, and ecology and make a substantial contribution to our understanding of sexual selection.

Noise and determinism in synchronized sheep dynamics

A major debate in ecology concerns the relative importance of intrinsic factors and extrinsic environmental variations in determining population size fluctuations. Spatial correlation of fluctuations in different populations caused by synchronous environmental shocks,, is a powerful tool for quantifying the impact of environmental variations on population dynamics,. However, interpretation of synchrony is often complicated by migration between populations,. Here we address this issue by using time series from sheep populations on two islands in the St Kilda archipelago. Fluctuations in the sizes of the two populations are remarkably synchronized over a 40-year period. A nonlinear time-series model shows that a high and frequent degree of environmental correlation is required to achieve this level of synchrony. The model indicates that if there were less environmental correlation, population dynamics would be much less synchronous than is observed. This is because of a threshold effect that is dependent on population size; the threshold magnifies random differences between populations. A refined model showsthat part of the required environmental synchronicity can be accounted for by large-scale weather variations. These results underline the importance of understanding the interaction between intrinsic and extrinsic influences on population dynamics.

Early development and population dynamics in red deer. I: Density-dependent effects on juvenile survival

(1) The paper investigates the effects of density-dependent changes in early development on population dynamics in red deer (Cervus elaphus) on Rhum. (2) The survival of calves through their first year of life was related to their birth weight and birth date. (3) As population density increased, the average date of parturition became later. Mean birth weight did not change, but at high density, its influence on over-winter calf survival intensified. (4) An extension of key factor analysis showed that these two effects were responsible for much of the increase in over-winter calf mortality at high population density.

Climate, Plant Phenology and Variation in Age of First Reproduction in a Temperate Herbivore

Density-independent weather effects can have important consequences for the demography of terrestrial herbivores because precipitation, temperature and insolation influence plant phenology, forage quality and biomass production, which in turn affects the habitat carrying capacity. Since forage digestibility influences intake and weight gain, life-history traits of young, growing animals are likely to reflect variation in the prevailing weather. This paper specifically investigates spatial and temporal variation in age at maturation in female red deer (Cervus elaphus) in Norway in relation to climate variables known to influence primary production. Our findings are corroborated by analysing differences in age at maturation in 21 cohorts of red deer on the Isle of Rum, Scotland. In Norway the majority of females ovulated as yearlings and calved for the first time as 2-year-olds. The proportion calving for the first time at two years varied from 0.23 to 0.67 between regions and fluctuated from 0.46 to 0.76 between cohorts. On Rum, where age of maturation was delayed at least a year, the proportion calving for the first time as 3-year-olds varied between cohorts from 0.0 to 0.89. In a subset of yearlings culled in Norway at the time of conception, the spatial and temporal differences in ovulation rates were related to the geographical and annual variation in body weight. 5. Both the spatial and temporal variation in the proportion of 2-year-olds calving in Norway, and cohort differences in the proportion calving as 3-year-olds on Rum, were negatively related to variation in May-June degree days 12 months earlier. Although primary production on the preferred herb-rich Agrostis-Festuca grasslands was positively correlated with temperature in May and June on Rum, the proportion of females calving as three years old, was negatively correlated with annual differences in May June primary production. We argue that retarded phenological development, during periods of cooler weather, enhances diet quality because leaf :stem ratios and digestibility of plant parts decline more slowly. Thus, weight gain during the early summer growth spurt should be rapid during cool May-June weather, increasing the probability of conception in the autumn. Since density-independent variation in food availability also influences fitness components which commonly have a more pronounced influence on population demography, for example offspring survival, we argue that our results highlight the potential importance of variation in weather on herbivore abundance.

Early development and population fluctuations in Soay sheep

In the Soay sheep population of Hirta (St. Kilda), high winter mortality occurs every 3-4 years following summers when population density exceeds 2.2 sheep ha −1 . During these die-offs, more than 50% of adults, 70% of yearlings and 90% of lambs die and population density falls by around 65%. This paper investigates the extent to which density-related changes in early growth might be responsible for fluctuations in over-winter survival and population size

Factors Affecting Calf Mortality in Red Deer (Cervus elaphus)

(1) The paper investigates the factors affecting calf survival between 1971 and 1976 in the red deer population of the North Block of the Isle of Rhum, Scotland. (2) On average, 18% of all calves born in the study area died before the end of September and a further 11 % died during the winter and early spring. (3) There was no overall difference in mortality between stag and hind calves. However, light-born hind calves were more likely to die than light-born stag calves whereas heavy-born stag calves were more likely to die than heavy-born hind calves. (4) Mortality during the first 6 months of life was higher among the offspring of young and old hinds than among those of 7-10-year-olds. (5) Overall mortality did not differ between calves born to milk and yeld hinds. However, medium weight calves born to yeld hinds were more likely to die than similar calves born to milk hinds. (6) Winter mortality was higher among the offspring of hinds using the part of the study area where population density was highest. (7) Very light calves were more likely to die in summer than heavier calves. (8) Late-born calves showed higher mortality than those born early or within the main birth period. (9) Changes in winter (but not summer) mortality were correlated with increasing population density of hinds in the study area.

POPULATION SUBSTRUCTURE, LOCAL DENSITY, AND CALF WINTER SURVIVAL IN RED DEER (CERVUS ELAPHUS)

Population substructure and the effects of scale have recently received much theoretical attention, but few studies have examined these factors in free-living populations of vertebrates. We used > 200 000 sightings of recognized females recorded over a con- tinuous 20-yr period to explore population substructure and spatial heterogeneity in red deer on the Isle of Rum, Scotland. We used hierarchical cluster analysis to group individuals together by their proximities in space, and we explored the influence of scale, considering scales ranging between the whole population and groups of one or two individuals. Inter- mediate scales were isolated as being the best at describing calf winter survival, the key factor in determining future population density. The most statistically explanatory scale isolated a population substructure related to vegetation, with higher local densities occurring around herb-rich Festuca-Agrostis grassland. Calves at high local density were most likely to die. Patterns of local population density varied between seasons in relation to food availability. High-resolution scales were the best descriptors of calf winter survival in summer; coarser scales were better in winter. In both summer and winter, local population density was more important than total population density in influencing calf winter survival. The effects on calf survival of local population density during the summer interacted significantly with calf sex and the mothers reproductive status. In this study, the technique of grouping animals by their proximity in space was more realistic and informative than discrete spatial divisions of the study area.

GENETIC-VARIATION AND JUVENILE SURVIVAL IN RED DEER

The survival of red deer (Cervus elaphus L.) calves to two years of age was examined in relation to electrophoretic variation in a population on the Scottish island of Rhum. Survival was analyzed using logistic analysis in which the “phenotypic” factors birth weight, birth date, subdivision of the study area, cohort, and sex, which affect the probability of a calfs survival, were taken into account. All three polymorphic loci examined, Mpi, Idh‐2, and Trf (each with two detected alleles) are significantly associated with juvenile survival. At Mpi, there is selection against one allele, f (or an allele at a linked locus), and there are indications that this effect is stronger in females than males. For Idh‐2, overall, the heterozygote class survives better than the two homozygotes, which survive equally well. However, again there is a difference between the sexes; female heterozygotes survive much better than homozygotes, whereas male homozygotes survive better than heterozygotes, and the difference in survival is smaller. Furthermore, there is an interaction involving Mpi, Idh‐2, and survival in which Mpif carriers that are also Idh‐2 homozygotes survive very badly compared with other Mpi‐Idh‐2 combinations, which all survive equally well. For Trf, the heterozygote class survives best, and there is also a difference in survival between the two homozygote classes. Genotype frequencies in the adult population are consistent with the results for calf survival, in that the Mpif frequency is lower in succeeding cohorts of surviving adults, whereas no significant gene frequency change is apparent for Idh‐2 or Trf.

Local extinction in a small and declining population : wild dogs in the Serengeti

Altered assumptions about how different ecological factors limit population numbers may lead to different conclusions about the causes of decline and ultimate extinction of a small population. Here, alternative hypotheses for the local disappearance of the Serengeti plains study population of African wild dogs (Lycaon pictus) are examined in light of observations of density dependence, deterministic decline and frequent rapid fluctuations in population number. After a population crash from 60 individuals in 1975 to 30 individuals in 1976, the Serengeti plains population fluctuated around a mean value of 22 individuals for 16 years before local extinction occurred. Variation in population numbers was extreme, with inter-annual reductions in population size of at least 40% occurring three times. Several explanations are consistent with the observed trends in population size including outbreaks of various epizootics and competition with other predators. Monte Carlo simulation, with parameters set to reflect observed fluctuations, demonstrate that population extinction was likely from chance factors alone. In small and declining populations, for which precise data and controls are unavailable, determining the cause(s) of extinction usually will be impossible.

COUNTERVAILING SELECTION IN DIFFERENT FITNESS COMPONENTS IN FEMALE RED DEER

In individually monitored red deer (Cervus elaphus) living in the North Block of the Isle of Rhum, Scotland, juvenile survival is related to the genotype at the enzyme loci Mpi and Idh‐2 (each with two alleles, f and s). To establish whether other fitness components also are related to genetic differences, we examined whether age at first breeding, fecundity, and adult survival of females were related to genotype at the same loci. Fertility in females shot outside the study area was also analyzed in relation to Mpi and Idh‐2 genotype. The analyses controlled for phenotypic and environmental factors affecting female reproductive performance. At Mpi, f‐carrying females in the study area bred earlier than ss individuals and tended to be more fecund. However, no association was found between Mpi genotype and adult survival. In culled females, Mpi f‐carriers were more likely to be pregnant than ss females. At Idh‐2, homozygous females in the study area started breeding earlier than heterozygous females. Idh‐2 fs and ss females were more fecund than ff females though this relationship was complicated by an interaction with spring temperature in the year of birth. When the population was at high density, adult survival of Idh‐2 ss females was better than survival of ff females, which was, in turn, better than survival of fs females. No association was found between Idh‐2 genotype and fertility in culled females. Overall, the associations found in female reproductive measures favor those genotypes that survive particularly badly over the first two years of life. This result supports the idea that countervailing selection in different fitness components (antagonistic pleiotropy) is a common and powerful force maintaining polymorphism in natural populations. It may also explain how fitness components can have large heritabilities while overall fitness may have a low heritability.