Hallvard Gjøstein

Effects of age, density and sex ratio on reproductive effort in male reindeer (Rangifer tarandus)

In sexually dimorphic ungulates, male reproductive success depends on fighting with other males for access to females during a brief rutting season. Large body size is necessary for success in intrasexual competition, and af ew large-sized males are often able to monopolize access to female groups. Earlier studies have reported that reproductive effort increases with age until prime-age is reached, and one study that population density lowered effort in (older) males. No study has directly assessed whether there is within-age-class variation in effort resulting from varying levels of intra-male competition. It is reported here the weight loss during the rutting season of 54 individual male reindeer Rangifer tarandus coming from eight herds with varying density (3.3–6.0 deer/km 2 ) and sex ratio (4–28% males). In agreement with earlier studies, reproductive effort was lower for young (1- to 2-year-old) than for prime-aged (3- to 5-year-old) males both on an absolute and relative scale. Among 1-year-old males (n = 33), effort was lower as sex ratio became closer to even, but density during the rutting season had no effect. This suggests that yearling males take a more active role when prime-aged males are absent. In addition to the insight into male ungulate life history, understanding male rutting behaviour may also have implications for population dynamics.

Adaptive adjustment of offspring sex ratio and maternal reproductive effort in an iteroparous mammal

Large mammals in seasonal environments have a pattern of high-reproductive synchrony in spring, but how the timing of reproduction affects resource allocation decisions at different stages of the reproductive cycle remains largely unexplored. By manipulating the timing of conception in reindeer (Rangifer tarandus), we tested how the timing of conception affected sex ratio, gestation length and weight development of mother and offspring. Females that conceived at their first ovulation within the rut had a 60.5% probability of producing a male; in contrast, females that conceived a cycle later had a 31.3% probability of producing a male. Late conceiving females had gestation times that were 10 days shorter and the calves were 0.6 kg (9.2%) lighter at birth and 7.4 kg (14.7%) lighter in autumn. Over the year, female weight changes was similar between the groups suggesting reindeer follow a bet-hedging strategy; reducing the quality of this years offspring to ensure their own future reproduction and survival. Harvesting is often selective leading to skewed sex ratios and age structure, which may influence the timing of reproduction due to females hesitation to mate with young males. Whenever this hesitation is strong enough to increase the frequency of recycling, harvesting is likely to have profound life history consequences.

Social rank in female reindeer (Rangifer tarandus): effects of body mass, antler size and age

In polygynous mammals, fitness differences may reflect differences in phenotypic quality as well as experience. This study determines dominance hierarchy among female reindeer Rangifer tarandus from two experimental herds (consisting of c. 45 animals in each) during 2 consecutive years. The influence of body mass, antler size and age on social rank in the herds was investigated, first using simple regression analysis. The combined effect of body mass, age and antler size on female rank was further assessed using principal component analysis, as these three parameters were significantly correlated. The improved Laundau linearity index of c. 0.5 (Pr < 0.001) in both herds indicated that a substantial part of the hierarchies was explained by their linearity properties. Consistently, body mass, female age and antler size, as well as their combined effect (measured by the scores on the main axis, PC1), influenced social rank in all four groups. It was concluded that both body mass and age are good predictors of social rank in female reindeer, whereas antler size in comparison plays a less important role in herds with a ‘normal’ female age structure. This suggests that female antlers may have evolved in intersexual rather than intrasexual competition. The temporal variations in the importance of body mass and age, probably owing to variation in female age structure between the 2 years, calls for conservative interpretations of whether body mass or age is more important in determining social rank among female reindeer. This is confirmed by the PCA analyses, where all three variables contributed more or less equally to the first component, the size variable, which on average explained more of the variation in female rank than body mass and age, suggesting that phenotypic quality expressed as the combination of the three variables is a better predictor of social rank than the variables per se. Hence, general conclusions about social rank based on single studies including few animals may not be credible.

Reproductive effort in relation to maternal social rank in reindeer (Rangifer tarandus)

In polygynous mammals, high-quality females may increase their fitness by providing superior care to their offspring. Based on the agonistic interactions of female reindeer in an experimental herd during two consecutive years (1997 and 1998), we tested whether maternal social rank influenced: (1) winter body-mass change of females, (2) preparturition reproductive effort (measured as fecundity, the birth mass and the birth date of their calves), (3) preweaning maternal effort (measured as calves’ preweaning mortality, early preweaning and late preweaning growth rate and September body mass of calves), and (4) postweaning maternal effort (measured as calves’ body-mass change during their first winter). In the models, we included September females’ body mass as a covariate to separate the effects of maternal rank and body mass. We also tested whether the effect of social rank on maternal efforts was dependent on offspring sex. High-ranked females gained body mass whereas low-ranked females lost weight during the winter. Fecundity was higher and date of birth was earlier in high-ranked females than in subordinates, whereas no effect of females’ rank on birth mass of calves was found. Early preweaning growth rate and September body mass of calves increased with increasing females’ social rank, whereas late preweaning daily growth rate of calves was not influenced by females’ rank. Calves’ preweaning mortality was only influenced by year, which also explained most of the variance in the winter body-mass change of calves. The effects of females’ rank on the reproductive-efforts parameters studied were not specific to offspring sex. These findings suggest that females’ rank influences reproductive effort during the preparturition, as well as the preweaning, period, the effect being sex independent.

Sexual dimorphism and intercorhort variation in reindeer calf antler length is associated with density and weather

We analysed intercohort variability of live weight and antler length of 5,123 reindeer calves. We further assessed the influence of climate and density on the interannual variation in antler length, and discussed sex-specific resource allocation and response to climate variability. Antler length varied significantly among years and between sexes, with interaction between year and sex. Body weight and antler length were highly positively correlated, showed similar intercohort variability, and had a strong allometrical link, suggesting that antler length could be an equally reliable measure of calf condition as live weight. We found a relative measure of antler length (i.e. antler length corrected for the allometric effect of body mass) to be positively influenced by increasing density and May–June precipitation, and also decreasing May–June temperature. We attributed the effect of early summer weather to its influence on forage availability and quality as well as the level of parasitic insect harassment. Gender difference in both the allometric exponents and the interannual variability suggest that young males and females may have different tactics for relative resource allocation towards growth of antlers as compared to body mass. Because antlers are costly to produce, they may be an honest signal of individual quality for both sexes. However, we found gender-specific allometry, as female calves more than males appear to prioritize their antler growth over body mass, especially when resources are limited. Thus, our results suggest that environmental variation may influence the extent of sexual dimorphism in antler length.

VARIATION IN MALE REPRODUCTIVE SUCCESS IN A WILD POPULATION OF REINDEER

Abstract Reindeer (Rangifer tarandus) is considered one of the most polygynous ungulates, so high variance in male mating success is expected. We assessed the variance in male mating success in a wild reindeer population with natural adult sex ratio in order to test this assumption. We obtained samples from 321 calves from the same year class and from 206 adult males. These samples represented an estimated 84% of the potential sires in the population. By analyses of 23 microsatellite loci and use of a likelihood ratio approach, we assigned 49% and 90% of the calves a sire at 95% and 80% confidence interval, respectively. Approximately 50% of the assigned calves had 1 or no paternal half-sibs, and approximately two thirds of the males contributed genetically to the cohort. At 80% confidence interval, 2.9% of the males sired >7 offspring, and 20 calves were assigned to the most successful male. Mean and variance in the number of calves assigned by different male age classes increased with age, although several of the yearling males were more successful than some of the adults. Our findings did not support the traditional assumption that most reindeer calves were fathered by a small group of highly successful males, suggesting that alternative mating strategies may play a more prominent role in reindeer than previously expected. Within a sustainable management concept, our findings demonstrate the importance of maintaining a reasonable proportion of young and adult males in the population.

Effect of calf stimulation on milk ejection in reindeer (Rangifer tarandus)

The objective of this study was to establish methods for stimulating the milk ejection in reindeer kept for milking purpose. Calves were used to stimulate milk does’ let down. In experiment 1, five does were allowed olfactory, acoustic and visual contact with their calves during milking, whereas four does were milked in isolation. The treatment of the groups was alternated every day during the eight days experiment. Olfactory, acoustic and visual contact with the calf did not influence the doe’s milk yield. The milk yield varied significantly between individual females within treatment (P < 0.01). In experiment 2, the calves were allowed to suckle their mother for a short period (two seconds) prior to milking being initiated. The same alternate design as in experiment 1 with groups consisting of three and two animals respectively was used, and the experiment lasted four days. The pre-suckling stimulation significantly increased the milk ejection measured as milk yield (P < 0.05), and the residual milk after the treatment was negectible. Moreover, the milk ejection varied between individual females within treatment (P < 0.05). We conclude that it is possible to achieve a complete milk removal by machine milking after the does have been pre-stimulated by suckling of calves. Olfactory, acoustic and visual contact with calves during milking failed to influence the milk ejection in this study. However, the results have to be interpreted with caution due to limited sample size. Abstract in Norwegian / Sammendrag: Formalet med dette forsoket var a prove ut ulike metoder for a stimulere nedgivninga av melk hos rein. Kalvene ble tatt i bruk for a stimulere nedgivninga. I forsok 1 hadde simla lyd-, lukt og synskontakt med kalven mens melkingen pagikk. Vi benyttet et ”switch back design” der fem simler hadde kontakt med kalven under melkingen og fire ble melket uten kontakt. Behandlingen ble byttet om annenhver dag i de atte dagene forsoket varte. Lyd-, lukt og synskontakt med kalven under melking hadde ingen innvirkning pa melkemengden ved maskinmelking. Det var imidlertid individuell variasjon i hvor mye melk man oppnadde hos simlene (P < 0.01). I forsok 2 lot vi kalvene suge simlene en kort stund for simlene ble melket. Kalven ble sluppet inn til simla og sugingen ble avbrutt etter to sekunder. Deretter ble simla fort inn for maskinmelking. Forsoket varte i fire dager og vi benyttet samme ”switch back design” som i forsok 1, med grupper bestaende av henholdsvis to og tre dyr. Stimuleringa med suging hadde en signifikant innvirkning pa nedgivninga. (P < 0.05), og mengden gjenvaerende melk var minimal. Dessuten var det en signifikant individuell variasjon i melkemengden innen behandlingen (P < 0.05). Vi konkluderer med at det er mulig a oppna en fullstendig tomming av juret ved maskinmelking dersom simlene forst er blitt stimulert med suging av kalven. Lyd-, lukt- og synskontakt med kalven under melking hadde ingen innvirkning pa nedgivninga av melk i dette forsoket. Resultatene ma imidlertid tolkes med varsomhet siden det statistiske utvalget er begrenset.