Age-related effects on breeding phenology and success of Common Guillemots Uria aalge at a North Sea colony

Capsule: Common Guillemots Uria aalge show delayed breeding and marked age-related changes in reproductive success consistent with improved performance with experience.

Aims: To determine age of first breeding and age-related effects on breeding phenology and success of Common Guillemots.

Methods: Resighting data from a long-term colour-ringing study of Common Guillemot chicks were combined with observations of breeding phenology and success to follow the recruitment process, breeding phenology and success of 62 birds at a major North Sea colony over a 30-year period.

Results: The median age of first breeding of Common Guillemots was 6.6 years. There were no detectable costs of first breeding on return rates or the likelihood of breeding the next season but first time breeders bred later and less successfully. Age of first breeding and lifetime breeding success both varied among individuals but there was no clear optimal age of first breeding and early first breeding was not associated with higher lifetime breeding success.

Conclusions: Common Guillemots in the Isle of May population delayed breeding for 3–4 years beyond physiological maturity. The marked increase in breeding success with age was consistent with improved performance with experience rather than selection for higher quality individuals. Findings from this study will inform population models by providing improved estimates of age of first breeding and age-related changes in reproductive performance.     

Estimating variance components and heritabilities in the wild: a case study using the ‘animal model’ approach

Using a genealogy containing over 1800 dams and nearly 400 sires (estimated by genetic paternity techniques), combined with maximum likelihood procedures and an ‘animal model’, we have estimated the heritabilities, genetic correlations and variance components of three morphometric traits in the Soay sheep (Ovis aries) on St Kilda, Scotland. This approach allows heritabilities to be estimated in natural populations that violate the assumptions of offspring–parent regression methods. Maternal (or paternal) effects can also be estimated under natural conditions. We demonstrate that all the traits, body weight, hind leg length and incisor arcade breadth, have low but significant heritabilities. Body weight, the trait that experiences the strongest selection, had the lowest heritability but the highest additive genetic coefficient of variation. An evolutionary response to selection is predicted. When maternal effects were not taken into consideration heritabilities were over‐estimated, although this effect was only significant in female offspring.     

Repeated selection of morphometric traits in the Soay sheep on St Kilda

1. Long-term studies allow the outcomes of repeated selection events to be monitored. Here, we investigate phenotypic selection in successive winter mortality events in the Soay sheep of St Kilda, Scotland, between 1985 and 1996. Selection of three quantitative morphometric traits, body weight, hindleg length and incisor arcade breadth, was investigated in different sectors of the population.
2. Evidence from fitness differentials of positive directional selection for large size was repeatedly found in lambs and adult females. Selection in the opposing direction was only found in one year in lambs.
3. Selection gradients showed that in most years when significant selection occurred, body weight was the focus of direct selection, whereas selection of hindleg length and incisor breadth was indirect, arising from their correlation with body weight.
4. Selection was strongest in years of low over-winter survival and almost absent in years when survival was high. Intensity of selection was greatest in lambs, emphasizing the differences in selection pressure experienced by different sectors of the population, in addition to the temporal variation in selection pressure due to population density and environmental conditions.
5. Despite repeated positive selection of body weight, no evidence of a change in the population mean was found over the course of the study.     

Evidence for continued transmission of parasitic nematodes in reindeer during the Arctic winter.

Living in the high Arctic, the Svalbard reindeer (Rangifer tarandus platyrhynchus) and its trichostrongyle nematodes experience a long cold winter from October to late May/early June. Over this period, transmission would be expected to be low. However, in culled reindeer the abundance of infection increased from autumn to late winter, providing evidence for continued transmission within this period. To our knowledge this is the first time this has been demonstrated in a climate with temperatures consistently below 0 degrees C. In one winter (1996-1997), the average fraction of nematodes found as larvae in the abomasal mucosa increased from around 10% to 50% between October and March. This suggests that arrested development took place throughout the winter. We found no evidence for an efficient acquired immune response towards the nematodes. The abundance of infection did not tend to decrease with increasing host age after an earlier peak, but levelled off instead, as predicted by a simple immigration-death model. In the late winter when the nutritional plane is low, both adult reindeer and calves had high worm burdens at intensities that may affect their condition and fitness.     

Correlates of male mating success and female choice in a lek-breeding antelope

We investigated factors underlying variation in male matingsuccess in Uganda kob (Kobus kob thomasi), a lek-breeding antelope.We found that only heavy (and, possibly, relatively old) malesheld lek territories and that female choice was an importantdeterminant of nonrandom mating patterns at leks. Our measureof male mating success was closely related to the historicalpopularity of the territory that a male defended, and individualfemales showed consistent preferences for particular lek territories,despite changes in territory ownership. Male success increasedwith body weight and declined independently of territory effectsduring each bout of lek territory tenure. We also found someevidence that female kob copied one another's choice of matesbecause females arriving at a lek tended to join territoriesthat already had relatively large harems on them. When comparedacross leks, average male mating success increased with leksize. Our results suggest that female kob may use a suite ofmale- and territory-based cues in mate choice at leks and, asa result, mate with particularly large males. However, we wereunable to determine whether female kob gain any direct or indirectbenefits through mate choice at leks.     

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.     

Constraints on plastic responses to climate variation in red deer

Influences of climate on life history traits in natural populations are well documented. However, the implications of between-individual variation in phenotypic plasticity underlying observed trait-environment relationships are rarely considered due to the large, long-term datasets required for such analysis. Studies typically present correlations of annual trait means with climate or assume that individual phenotypic responses are constant. Here, we examine this additional level of variation and show that, in a red deer population on the Isle of Rum, Scotland, changes in climate generate changes in phenotype only amongst individuals who have experienced favourable ecological conditions. Examination of relationships between offspring birth weight and spring temperature within the lifetimes of individual females revealed that the tendency to respond to climate declined as the population density experienced early in life increased. The presence of such systematic variation in individual plasticity is rarely documented in the wild, and has important implications for our understanding of the environmental dependencies of traits under varying ecological conditions.     

Testing five hypotheses of sexual segregation in an arctic ungulate

1. Sexual segregation occurs in most species of sexually dimorphic ungulates. At least five not mutually exclusive hypotheses have been formulated to explain patterns of social, habitat and spatial segregation; the indirect competition hypothesis (H1), the nutritional needs hypothesis (H2), the activity budget hypothesis (H3), the weather sensitivity hypothesis (H4), and the predation risk hypothesis (H5). 2. Each hypothesis has a unique set of predictions with respect to the occurrence of segregation, and how seasonality, density dependence and reproductive status affect sexual segregation. 3. We tested this set of predictions in order to separate the hypotheses H1-H5 for patterns of sexual segregation of the Svalbard reindeer based on 9 years data on seasonal estimates of spatial, habitat and social (i.e. grouping with their own sex) segregation in combination with resource selection functions. 4. Our results do not support that one single mechanism causes segregation. The activity budget hypothesis, the nutritional needs hypothesis and the weather sensitivity hypothesis were all partially supported, while the predation risk hypothesis was discarded for Svalbard reindeer because predators have been absent for at least 5000 years. Several mechanisms are thus interacting to explain the full-year pattern of sexual segregation and the combination of mechanisms varies among species and populations.