ArticlePDF Available

Relative allocation to horn and body growth in bighorn rams varies with resource availability

Authors:

Abstract

Males may allocate a greater proportion of metabolic resources to maintenance than to the development of secondary sexual characters when food is scarce, to avoid compromising their probability of survival. We assessed the effects of resource availability on body mass and horn growth of bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, Canada over 30 years. The number of adult ewes in the population tripled during our study, and the average mass of yearling females decreased by 13%. We used the average mass of yearling females as an index of resource availability. Yearling female mass was negatively correlated with the body mass of rams of all ages, but it affected horn growth only during the first three years of life. Yearly horn growth was affected by a complex interaction of age, body mass, and resource availability. Among rams aged 2--4 years, the heaviest individuals had similar horn growth at high and at low resource availability, but as ram mass decreased, horn growth for a given body mass became progressively smaller with decreasing resource availability. For rams aged 5--9 years, horn growth was weakly but positively correlated with body mass, and rams grew slightly more horn for a given body mass as resource availability decreased. When food is limited, young rams may direct more resources to body growth than to horn growth, possibly trading long-term reproductive success for short-term survival. Although horn growth of older rams appeared to be greater at low than at high resource availability, we found no correlation between early and late growth in horn length for the same ram, suggesting that compensatory horn growth does not occur in our study population. Young rams with longer horns were more likely to be shot by sport hunters than those with shorter horns. Trophy hunting could select against rams with fast-growing horns. Copyright 2004.
... All hypotheses assume an acute effect of disease, based on preliminary evidence from the BR, whereby increments grown during the die-off event experience reduced growth. Hypothesis 1 assumes compensatory growth in increments grown in years following outbreak events (acute effect, compensatory growth; Figure 1B), though this is unlikely based on previous research (Festa-Bianchet et al. 2004). In hypothesis 2, normal growth resumes in years following the disease event (acute effect, normal growth; Figure 1C). ...
... The hypotheses address growth following the die-off event: B) compensatory growth occurs in increments grown in years following the die-off event, C) normal growth resumes in years following the die-off event, and D) increment growth is stunted in years following the die-off event. Research suggests that compensatory growth is unlikely (Festa-Bianchet et al. 2004). We demonstrate each visually using an individual exposed to disease in the second year of life (labeled 2 in A-D and d in E). ...
Article
Full-text available
Secondary sexual traits (e.g., horns and antlers) have ecological and evolutionary importance and are of management interest for game species. Yet, how these traits respond to emerging threats like infectious disease remains underexplored. Infectious pneumonia threatens bighorn sheep (Ovis canadensis) populations across North America and we hypothesized it may also reduce horn growth in male sheep. We assess the effect of pneumonia on horn size in male bighorn sheep using 12 herd datasets from across the western United States that had horn growth and disease data. Disease resulted in 12–35% reduction in increment (yearly) length and 3–13% reduction in total horn length in exposed individuals. The disease effect was prolonged when pathogens continued to circulate in sheep populations. Further, disease likely delays the age at which horns reach ¾‐curl and prevents achievement of full‐curl. This is further evidenced with 6 of the 12 herds experiencing an increase in average age at harvest following die‐off events. Management of bighorn sheep for horn size and for population maintenance has focused on factors including nutrition, environmental conditions, and genetic diversity. We demonstrate that disease plays an important role in horn size: pneumonia disease outbreak events significantly reduced horn growth in male bighorn sheep, and continued horn stunting occurred when chronically infected individuals remained present in the population.
... We first tested the effect of density on age-and sex-specific body mass and horn length. Density effects have been previously examined in detail for each species (Festa-Bianchet et al. 2004, Toïgo et al. 2007, Hamel et al. 2010, Garel et al. 2011, Douhard et al. 2018. We updated these analyses with additional years. ...
... This suggests that under poor nutritional conditions generated by high population density, resource allocation to body mass is prioritized possibly at the expense of horn growth. These results confirm the key role of body mass for reproduction in these capital breeder species (Gaillard et al. 2000, Bonenfant et al. 2009b, and also corroborate earlier results by Jorgenson et al. (1998) and Festa-Bianchet et al. (2004) who showed that the effects of density on horn length and body mass mostly affect young age classes. The absence of statistically significant differences in slope may reflect a lack of statistical power. ...
Article
Full-text available
Little is known about the effects of environmental variation on allometric relationships of condition‐dependent traits, especially in wild populations. We estimated sex‐specific static allometry between horn length and body mass in four populations of mountain ungulates that experienced periods of contrasting density over the course of the study. These species displayed contrasting sexual dimorphism in horn size; high dimorphism in Capra ibex and Ovis canadensis and low dimorphism in Rupicapra rupicapra and Oreamnos americanus. The effects of density on static allometric slopes were weak and inconsistent while allometric intercepts were generally lower at high density, especially in males from species with high sexual dimorphism in horn length. These results confirm that static allometric slopes are more canalized than allometric intercepts against environmental variation induced by changes in population density.
... We found that male and female chamois horn length was significantly affected by snow cover and food availability. The influence of these variables was consistent with the previous knowledge on horn growth in chamois (Chirichella et al., 2013) and, generally, in other bovids (Carvalho et al., 2020;Festa-Bianchet, 2004;Giacometti et al., 2002). However, the magnitude of such influence, assessed by using the measures predicted by the best model, was scarce. ...
Article
Animal weapons are one of the most studied morphological traits, particularly in Artiodactyla. Since in polygynous species males with larger weapons tend to be more successful in gaining access to females, researchers have traditionally focused on horn size. However, in species with limited horn size, weapon size has been assumed to have a reduced or null effect on life history traits. We examined the effect of intrinsic and extrinsic factors on the length of the second and third segments of Alpine chamois horns (Rupicapra rupicapra) in a population living in a poor environment. Our aim was to test how environmental conditions affected weapon growth and whether compensatory growth occurred. We showed that horn length was isometric to body size, although male horns grew more quickly. Ecological factors such as snow and forage availability affected weapon length, though mildly. No sign of compensatory growth was detected. We inferred that chamois mainly use horns as armament in intrasexual interactions. However, horn length was not a key element since horn growth remained isometric, at least under suboptimal ecological conditions. In species without extreme weapons, the handicap caused by longer horns is likely not compensated by an increase in individual fitness. Studies on species with weapons that are not extreme have analysed intrinsic and extrinsic factors affecting weapon growth. However, they have often failed to provide full understanding of the actual benefits associated with larger weapons in these species. In Alpine chamois, we found that some environmental factors significantly affected horn length, however the magnitude of such influence was negligible from biological point of view, the actual weapon size being only mildly influenced. The linearity of the relationship between the horns and body weight and the absence of compensatory growth indicate that these biometric measures are isometric during the first years of life. Our findings, on a monomorphic species without extreme weapons, suggest focusing not only on weapon size but also on their use, shape and the different drivers affecting their ontogenesis.
... Adult males possess impressively large horns that can exceed 1 m in length (Lüps et al. 2007). The production and also the carrying of horns are both assumed to be energetically costly (Festa-Bianchet et al. 2004;Toïgo et al. 2013). In line with that, annual horn growth varies positively with body mass (Bergeron et al. 2010), suggesting that horn growth is condition-dependent. ...
Article
Full-text available
Trade-offs between reproductive effort and subsequent growth in males are not well explored, despite their relevance in questions of individual energy allocation. Regarding the growth of sexual secondary characters in polygynous breeding male mammals, indeed, no conclusive studies exist. We investigated in male Alpine ibex ( Capra ibex ) the relationship between their behavioral reproductive effort, current horn size, and subsequent horn growth. While controlling for age, no evidence was found for male behavioral reproductive effort during the rut being affected by their horn size. On the other hand, reproductive effort significantly decreased age-specific horn growth during the following summer. Our study provides evidence that growth of secondary sexual characters is traded against behavioral investments in reproduction in a male mammal. It bears important implications for the understanding of energy allocation between various life-history components and the evolutionary ecology of secondary sexual characters.
... Nubian ibex in the Sinai occupy a very arid environment, with low precipitation, low vegetation production, and consequently limited availability of early-phenology high quality forage as compared to other, more mesic, ibex habitats. In more mesic habitats, ibex allocate fewer resources to horn growth during drought years (Festa-Bianchet et al. 2004). This suggests that horn development in Nubian ibex in the Sinai may be below the species potential and may peak at an earlier age because of nutritional stress or lesser longevity as compared to other ibex. ...
Article
Documenting patterns of horn growth and horn-age relationships of Nubian ibex ( Capra nubiana ) can contribute to a more comprehensive understanding of their natural history, horn development in ibex in general, and future conservation of the species. Our specific objectives included (1) documenting age-horn growth patterns; (2) contrasting horn growth patterns of Nubian ibex with other ibex species; and (3) determining whether horn development accurately reflects age of Nubian ibex in Sinai, Egypt. As expected, all male and female horn measurements had significant relationships with age. Horn growth in males started plateauing at ca. age 7–8, whereas female horn growth started plateauing at ca. age 4–6. The extremely arid environment of Nubian ibex in the Sinai may account for the slowing of horn growth at a younger age than seen in populations of some other ibex species. We found a significant relationship between the number of horn ridges and age, indicating that counting horn ridges provides a viable method of aging males to within ±1 y. Thus counting horn ridges may be a useful and non-invasive method to determine age or age class, which can further our understanding of age structure, the natural history, and management of Nubian ibex populations.
Article
Full-text available
Energy stores and migration are important adaptations for animals in seasonal environments, but their roles may vary relative to an animal’s endogenous and exogenous environment. In partially migratory populations, migrants and residents experience different seasonal environments, thus the influence of energy stores on survival may differ relative to migratory tactic, with potential consequences to survival and fitness. Using data from Sierra Nevada bighorn sheep (Ovis canadensis sierrae; hereafter, Sierra bighorn), we tested the hypothesis that body fat (energy stores) buffers animals against their environment, but that buffering capacity differs across environments experienced by high‐elevation residents (using a single range year‐round), traditional migrants (making 1 round‐trip movement between high‐ and low‐elevation ranges during winter), and vacillating migrants (making ≥2 round trips between high‐ and low‐elevation ranges during winter). We predicted that: for animals with high levels of body fat, survival would be high regardless of migratory tactic; residents would require larger stores of body fat to survive than migrants; energy stores would be least influential to survival for vacillating migrants. High levels of body fat (≥14% for females and ≥19% for males) largely buffered animals against harsh environments (survival >0.90) regardless of migratory tactic. At lower levels of body fat, traditional migrants had higher survival than residents. Vacillating migrants exhibited nearly 100% survival with no detectable effect of body fat on survival. Collectively, these results support the hypothesis that body fat buffers animals against harsh environments, but that the buffering capacity differed relative to the environment, and highly flexible behaviors (i.e., vacillating migration) can allow animals to decouple survival from body fat. Our work reveals that synergies between physiological and behavioral adaptations of animals in highly seasonal environments carry potential fitness consequences for individuals and demographic consequences for populations. Interactions among these adaptations are key to understanding nutritional prerequisites for persistence in variable environments.
Chapter
Modern evolutionary theory provides a theoretical framework for functional analyses of animal behaviour. In order to investigate the adaptive value of individual behaviour patterns, it is necessary to operationalize fitness and to characterize the evolutionary mechanisms that influence it. In terms of the most important fitness components - survival and reproductive success - four basic problems that each individual must successfully master can be derived: finding food, avoiding being eaten, reproducing successfully and raising offspring. Traits determining the reproductive component of fitness are identical with some of the most important life history traits. In this chapter, I will therefore outline the most important variables, mechanisms, processes and relationships among behaviour, evolution and life histories to provide a basis for the subsequent chapters, which will focus on these four basic problems.
Chapter
Die moderne Evolutionstheorie liefert einen theoretischen Rahmen für funktionale Analysen einzelner Verhaltensweisen. Zur Untersuchung der Angepasstheit des Verhaltens ist es notwendig, Fitness zu operationalisieren und die evolutionären Mechanismen, die sie beeinflussen, näher zu charakterisieren. Aus den wichtigsten Fitnesskomponenten – Überleben und Fortpflanzungserfolg – lassen sich vier Grundprobleme ableiten, die jedes Individuum erfolgreich meistern muss: Fressen, Nicht-gefressen-Werden, Fortpflanzung und Jungenaufzucht. Merkmale, welche die Fortpflanzungskomponente der Fitness betreffen, sind identisch mit einigen der wichtigsten Merkmale der Life History (Lebensgeschichte). In diesem Kapitel skizziere ich die wichtigsten Variablen, Mechanismen, Prozesse und Zusammenhänge zwischen Verhalten, Evolution und Life Histories, um eine Grundlage für das Verständnis der nachfolgenden Kapitel zu schaffen, die sich inhaltlich an diesen vier Grundproblemen ausrichten.
Article
Full-text available
Biologists often must use incomplete information to make recommendations concerning harvest of large mammals. Consequently, those recommendations must draw on a firm understanding of the ecology of the species in question, along with selection of the most applicable population characteristics on which to base harvest-both essential components for prudent management. Density-dependent processes, which are ubiquitous among populations of large mammals, may be counterintuitive because of unexpected patterns in recruitment coincident with changes in population size. Misconceptions concerning population dynamics of ungulates also can occur when demo-graphics are based solely on correlations with environmental factors. Further, the concept of a harvestable surplus can be misleading for managing ungulate populations, because of the parabolic relationship between population size and number of recruits-harvest determines the surplus rather than vice versa. Understanding consequences of mortality, especially relative components of compensatory or additive mortality, also is necessary. Knowledge of the proximity of an ungulate population to ecological carrying capacity (K) is required to fully assess whether most mortality is compensatory or additive. We describe selected life-history traits and population characteristics of ungulates useful in parametrizing where populations are in relation to K, thereby allowing for a reasonable harvest despite some uncertainty in population size. We advocate an adaptive-management approach while monitoring those life-history traits to evaluate the suitability of a particular harvest strategy. ALCES VOL. 56: 15-38 (2020)
Article
Intra- and intersexual selection drives the evolution of secondary sexual traits, leading to increased body size, trait size and generally increased reproductive success in bearers with the largest, most attractive traits. Evolutionary change through natural selection is often thought of primarily in terms of genetic changes through mutations and adaptive selection. However, this view ignores the role of the plasticity in phenotypes and behaviour and its impact on accelerating or decelerating the expression of sexually selected traits. Here, we argue that sudden changes in selection pressures (e.g. predation pressure) may cause a cascade of behavioural responses, leading to a rapid change in the size of such traits. We propose that selective removal of individuals with the most prominent traits (such as large antlers or horns in male ungulates) induces behavioural changes in the surviving males, leading to a reduction in the growth of these traits (phenotypic expression). To test this idea, we used an individual-based simulation, parametrized with empirical data of male bighorn sheep, Ovis candensis. Our model shows that the expression (phenotype, not genotype) of the trait under selection (here horn size) can be negatively impacted, if the biggest, most dominant males in the population are removed. While the selective removal of prime males opens breeding opportunities for younger, smaller males, we predicted that it would come at the expense of growth and maintenance. As predicted, we observed a rapid decline in average male horn length in our model. We argue that this decline happens because smaller males, instead of allocating energy into growth, divert this energy towards participation in mating activities that are typically exclusively available to prime males. While our model deals with ecological life-history trade-offs, it cannot predict evolutionary outcomes. However, this nongenetic mechanism is important for the understanding of evolutionary processes because it describes how heritable traits can rapidly change because of behavioural plasticity, long before any genetic changes might be detectable.
Article
Full-text available
In large-herbivore populations, environmental variation and density dependence co-occur and have similar effects on various fitness components. Our review aims to quantify the temporal variability of fitness components and examine how that variability affects changes in population growth rates. Regardless of the source of variation, adult female survival shows little year-to-year variation [coefficient of variation (CV<10%)], fecundity of prime-aged females and yearling survival rates show moderate year-to-year variation (CV<20%), and juvenile survival and fecundity of young females show strong variation (CV>30%). Old females show senescence in both survival and reproduction. These patterns of variation are independent of differences in body mass, taxonomic group, and ecological conditions. Differences in levels of maternal care may fine-tune the temporal variation of early survival. The immature stage, despite a low relative impact on population growth rate compared with the adult stage, may be the critical component of population dynamics of large herbivores. Observed differences in temporal variation may be more important than estimated relative sensitivity or elasticity in determining the relative demographic impact of various fitness components.
Article
Full-text available
The factors affecting variation in age at first reproduction of bighorn ewes (Ovis canadensis) were investigated in two marked populations in Alberta. One population was studied for 20 years, the other for 11 years. As yearlings, females that lactated at 2 years of age were on average heavier and larger, and had longer horns than females that did not lactate at 2 years. However, there was wide overlap in body mass between early and late producers, and increases in body mass over the threshold for reproduction had little effect on the probability of early lambing. The body mass of females at 4 months of age explained less than half of the variance in female body mass at 1 year or at 15 months. In one population, the proportion of 2-year-old ewes lactating was not correlated with density and declined after a pneumonia epizootic. In the other population, the proportion of 2-year-old ewes lactating was higher during an experimental reduction of density, and dropped to near zero as density increased. There
Article
Full-text available
We studied long-term cohort effects on chest girth and horn length in a recently established population of alpine ibex (Capra ibex ibex). Environmental conditions of the year of birth affected chest girth and first-annual increment of horns of males but did not affect chest girth and horns of females. Females compensated for a slow horn growth during their Ist year of life, whereas males did not. Level of polygyny of the species and environmental conditions experienced by the population could account for the occurrence of long-term cohort effects in male growth and its absence in female growth. Abundance of food resources throughout the study period allowed females to show compensatory growth. However, evolutionary constraints on growth that may exist in males of polygynous species may have prevented males from showing compensatory growth.
Article
Full-text available
Horns of bovids are important social organs, their growth is often indicative of population characteristics and habitat quality, but Little is known of the factors affecting their growth in individuals. We studied horn growth of 135 (51 males, 84 females) marked mountain goats (Oreamnos americanus) in Alberta, Canada, for 9 years. In both sexes, horn growth was quadratic during the first 5 years of life and not significant after 5 years of age. Goats completed 93% of horn growth by 3 years of age. Horns of males grew more than those of females during the first 1.5 years of life. Horns of females grew more than those of males during the third year. Although males maintained longer horns than females because of their longer first increment, adult males had shorter horns than females for a given body size. Males had thicker horns than females at all ages, absolutely or relative to body size. Horn growth early in life was correlated negatively with later growth. Annual growth increments of horns of females
Article
The genetic correlation is a central parameter of quantitative genetics, providing a measure of the rate at which traits respond to indirect selection (i.e., selection that does not act upon the traits under study, but some other trait with which they have genes in common). In this paper, I review the pattern of variation among four combinations of traits: life history × life history (L × L), morphological × morphological (M × M), life history × morphological (L × M), and behavioral × behavioral (B × B). A few other combinations were investigated, but insufficient data were obtained for separate analysis. A total of 1798 correlations, distributed over 51 different animal and plant species, were analyzed. The analysis was conducted at two levels: first by dividing the data set solely by trait combination, and second by blocking the data by trait combination and species. Because selection will tend to fix alleles that show positive correlations with fitness traits faster than those that are negative and because the latter are expected to arise more frequently by mutation, correlations between life-history traits are predicted to be more often negative than those between morphological traits. This prediction was supported, with the ranking in decreasing proportion of negative correlations being: L × L > L × M > B × B > M × M. The mean magnitude of the genetic correlation shows little variation among morphological and life-history combinations, and the distribution of values is remarkably flat. However, the estimated standard errors and the coefficient of variation (SE/rG ) are large, making it difficult to separate biological factors influencing the pattern of dispersion from experimental error. Analysis of the phenotypic and genetic correlations suggest that for the combinations M × M and L × M, but not L × L or B × B, the phenotypic correlation is an adequate estimate of the genetic correlation.
Article
Longitudinal studies of survival are valuable because age-specific survival affects population dynamics and the evolution of several life history traits. We used capture-mark-recapture models to assess the relationship between survival and sex, age, population, year of study, disease, winter weather, and population density in two populations of bighorn sheep (Ovis canadensis) in Alberta, Canada. The Ram Mountain population, monitored for 20 yr, more than doubled in density; the Sheep River population, monitored for 13 yr, experienced a pneumonia epizootic. Yearling survival varied among years and was lower than that of older sheep of the same sex, except for yearling males at Ram Mountain. Yearling females at Ram Mountain were the only sex-age class exhibiting density dependence in survival. Senescence was evident for both sexes in both populations. Female survival from age 2 to age 7 was very high in both populations, but males aged 2 and 3 yr enjoyed better survival than males aged 4-6 yr. Our data support the suggestion that where hunters remove many males older than 5 yr of age, the natural mortality of males increases at 3-5 yr, possibly because young males suffer a mortality cost of participating in rutting activity. The decline in survival for sheep older than 7 yr was greater for males than for females. Survival was lower for males than for females, both among prime-aged sheep (0.896 vs. 0.939 at Sheep River; 0.837 vs. 0.945 at Ram Mountain) and among older sheep (0.777 vs. 0.859 at Sheep River; 0.624 vs. 0.850 at Ram Mountain), but not among yearlings. Survival of sheep aged 2-7 yr was not significantly different between the two populations. Winter weather did not affect survival. Survival of sheep 2 yr of age and older did not vary significantly between years, except at Sheep River where survival of prime-aged sheep of both sexes was lower in the year of the pneumonia epizootic. Studies of survival of mountain sheep based upon skull collections may have overestimated survival of young rams. Our results underline the need for accurate information on age-specific survival.
Conference Paper
Conservation science is often interested in predicting the future state of populations living in variable environments. Behavioral ecology is often interested in understanding individual behavior in terms of adaptive strategies for dealing with variable ecological, social, or phenotypic states. To the extent that individual behavior is strategic, predictable in different environments, and has population-level consequences, the study of behavior can advance the predictive goals of conservation. Mating system studies, via data on variation in individual reproductive success, have most frequently been applied to conservation in the estimation of effective population size and prediction of genetic trajectories of single populations. However, mating strategy may also influence dispersal, gene flow, and genetic trajectories at spatial scales involving many populations. Mating competition within populations and dispersal will often have opposing effects on effective population size. Future studies of mating systems might aid conservation (i) in evaluating the genetic effects of mating strategy at different spatial scales, and (ii) by providing, in place of point estimates, descriptions of the way in which variation in lifetime reproductive success and population dispersal pattern change in reaction to environmental variation. Such information may allow conservation science to provide better guidance regarding the appropriate spatial scale of habitat protection and restoration and effective strategies for reserve design and re-design.
Article
Presents data from a 12-yr study of the reproductive success of more than 1,000 red deer on the island of Rhum, Scotland in which the contrasting factors affecting reproductive success in males and females are shown to be the ultimate cause of most differences between the sexes. Competition between stags for harems is intense and males fight regularly for possession. The hind's breeding success is related to the number of offspring the hind can produce and rear, and depends mainly on the food resources available to her. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Article
Sexual dimorphism in Cantabrian chamois Rupicapra pyrenaica parva (Cabrera, 1911) horn length was recorded annually for the first five years of life. The horns of males were larger than those of females, though horn growth rate in the first two years (ie horn length in the first two years divided by horn length in the first five years) was greater in females than males. Sexual dimorphism in horn length of Cantabrian chamois adults was found to be the highest of genus according to available data. Males and females with faster horn growth in the first two years of life reduced their horn growth rate in the third and fourth year. Chamois' horns from the Eastern Cantabrian Mountains were smaller than those from Western Cantabrian Mountains, although at five years of age these differences were not evident. Total horn length and jaw length showed significant positive correlation (females: rp2 = 0.75, males: rp2 = 0.54, p < 0.05 in both cases). This relationship suggests that horn growth dynamics may be used as an estimation of body-size dynamics. Horn growth / jaw length correlation was higher in females than in males. Horn growth was also positively correlated with annual precipitation in the previous year, but not with precipitation in the current year. Winter horn growth was observed in the younger age classes. Post-winter horn growth was recorded in 55% of the animals in March. The use of this information to age Cantabrian chamois is described.