Male Horn Length Research Articles

Habitat loss reduces abundance and body size of forest-dwelling dung beetles in an Amazonian urban landscape

The loss of forest cover in urban landscapes alters the dynamics of spatial and food resources, challenging the maintenance of forest species, which may have their condition compromised. Dung beetles are sensitive to changes in vegetation structure and land use caused by human activities, processes that are intrinsically related to the establishment and development of cities. The aim of this study was to evaluate the effect of landscape structure on the abundance and morphological characteristics of two species of dung beetle (Dichotomius boreus and Dichotomius quadrilobatus) that inhabit forested areas in urbanized landscapes in the Amazon region. We carried out the study in 38 landscapes located in six urban regions in the central region of the Amazon. We evaluated the effect of landscape structure, at the site and city scales, on beetle abundance, individual body size, and relative horn length of males. At the local scale, landscapes with greater forest cover showed greater abundance of dung beetles, as well as greater lengths of D. boreus horns. Cities with a greater amount of forest cover had larger individuals than those with less forest cover. We conclude that forested areas in urban landscapes are a key habitat for the maintenance of dung beetle populations with a strong relationship between the amount of forest cover in the landscape. The maintenance of healthy and abundant populations of beetles in urban Amazonian landscapes guarantees the persistence of ecosystem services provided by these organisms in urban ecosystems.

Genetic rescue from protected areas is modulated by migration, hunting rate, and timing of harvest.

In terrestrial and marine ecosystems, migrants from protected areas may buffer the risk of harvest-induced evolutionary changes in exploited populations that face strong selective harvest pressures. Understanding the mechanisms favoring genetic rescue through migration could help ensure evolutionarily sustainable harvest outside protected areas and conserve genetic diversity inside those areas. We developed a stochastic individual-based metapopulation model to evaluate the potential for migration from protected areas to mitigate the evolutionary consequences of selective harvest. We parameterized the model with detailed data from individual monitoring of two populations of bighorn sheep subjected to trophy hunting. We tracked horn length through time in a large protected and a trophy-hunted populations connected through male breeding migrations. We quantified and compared declines in horn length and rescue potential under various combinations of migration rate, hunting rate in hunted areas and temporal overlap in timing of harvest and migrations, which affects the migrants' survival and chances to breed within exploited areas. Our simulations suggest that the effects of size-selective harvest on male horn length in hunted populations can be dampened or avoided if harvest pressure is low, migration rate is substantial, and migrants leaving protected areas have a low risk of being shot. Intense size-selective harvest impacts the phenotypic and genetic diversity in horn length, and population structure through changes in proportions of large-horned males, sex ratio and age structure. When hunting pressure is high and overlaps with male migrations, effects of selective removal also emerge in the protected population, so that instead of a genetic rescue of hunted populations, our model predicts undesirable effects inside protected areas. Our results stress the importance of a landscape approach to management, to promote genetic rescue from protected areas and limit ecological and evolutionary impacts of harvest on both harvested and protected populations.

Male mating behaviour affects growth of secondary sexual traits: a mechanism for rapid phenotypic change

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.

Social network position experiences more variable selection than weaponry in wild subpopulations of forked fungus beetles.

The phenotypic expression and fitness consequences of behaviours that are exhibited during social interactions are especially sensitive to their local social context. This context-dependence is expected to generate more variation in the sign and magnitude of selection on social behaviour than that experienced by static characters like morphology. Relatively few studies, however, have examined selection on behavioural traits in multiple populations. We estimated sexual selection in the wild to determine if the strength and form of selection on social phenotypes is more variable than that on morphology. We compared selection gradients on social network position, body size, and weaponry of male forked fungus beetles Bolitotherus cornutus as they influenced mating success across nine natural subpopulations. Male horn length consistently experienced positive sexual selection. However, the sign and magnitude of selection on individual measures of network centrality (strength and betweenness) differed significantly among subpopulations. Moreover, selection on social behaviours occurred at a local scale ('soft selection'), whereas selection on horn length occurred at the metapopulation scale ('hard selection'). These results indicate that an individual with a given social phenotype could experience different fitness consequences depending on the network it occupies. While individuals seem to be unable to escape the fitness effects of their morphology, they may have the potential to mediate the pressures of selection on behavioural phenotypes by moving among subpopulations or altering social connections within a network.

Does selection on horn length of males and females differ in protected and hunted populations of a weakly dimorphic ungulate?

Weaponry in ungulates may be costly to grow and maintain, and different selective pressures in males and females may lead to sex‐biased natural survival. Sexual differences in the relationship between weapon growth and survival may increase under anthropogenic selection through culling, for example because of trophy hunting. Selection on weaponry growth under different scenarios has been largely investigated in males of highly dimorphic ungulates, for which survival costs (either natural or hunting related) are thought to be greatest. Little is known, however, about the survival costs of weaponry in males and females of weakly dimorphic species. We collected information on horn length and age at death/shooting of 407 chamois Rupicapra rupicapra in a protected population and in two hunted populations with different hunting regimes, to explore sexual differences in the selection on early horn growth under contrasting selective pressures. We also investigated the variation of horn growth and body mass in yearling males (n = 688) and females (n = 539) culled in one of the hunted populations over 14 years. The relationship between horn growth and survival showed remarkable sexual differences under different evolutionary scenarios. Within the protected population, under natural selection, we found no significant trade‐off in either males or females. Under anthropogenic pressure, selection on early horn growth of culled individuals showed diametrically opposed sex‐biased patterns, depending on the culling regime and hunters’ preferences. Despite the selective bias between males and females in one of the hunted populations, we did not detect significant sex‐specific differences in the long‐term pattern of early growth. The relationship between early horn growth and natural survival in either sex might suggest stabilizing selection on horn size in chamois. Selection through culling can be strongly sex‐biased also in weakly dimorphic species, depending on hunters’ preferences and hunting regulations, and long‐term data are needed to reveal potential undesirable evolutionary consequences.

Environmental and evolutionary effects on horn growth of male bighorn sheep

The development of male secondary sexual characters such as antlers or horns has substantial biological and socio‐economic importance because in many species these traits affect male fitness positively through sexual selection and negatively through trophy hunting. Both environmental conditions and selective hunting can affect horn growth but their relative importance remains unexplored. We first examined how a large‐scale climate index, the Pacific Decadal Oscillation (PDO), local weather and population density influenced both absolute and relative annual horn growth from birth to three years of male bighorn sheep Ovis canadensis over 42 years. We then examined the relative influence of environmental conditions and evolution mainly driven by trophy hunting on male horn length at three years of age. Horn growth was positively influenced by low population density and warm spring temperature, suggesting that ongoing climate change should lead to larger horns. Seasonal values of PDO were highly correlated. Horn growth increased with PDO in spring or summer at low density, but was weak at high density regardless of PDO. The interaction between population density and PDO in spring or summer accounted for a similar proportion of the observed annual variation in horn growth (32% or 37%) as did the additive effects of spring temperature and density (34%). When environmental conditions deteriorated, males allocated relatively more resources to summer mass gain than to horn growth, suggesting a conservative strategy favoring maintenance of condition over allocation to secondary sexual characters. Population density explained 27% of the variation in horn length, while evolutionary effects explained 9% of the variance. Thus, our study underlines the importance of both evolution and phenotypic plasticity on the development of a secondary sexual trait.

Demographic drivers of age-dependent sexual selection.

Sexual selection has a critical role in evolution, and it is fundamental to identify what ecological factors drive its variation. Disentangling the ecological correlates of sexual selection over the long term, however, is challenging and has rarely been done in nature. We sought to assess how demographic changes influenced the intensity, direction and form of sexual selection and whether selective pressures varied with age. We tested whether breeder sex ratio, number of competitors and age structure influenced selection differentials on horn length of wild bighorn rams (Ovis canadensis) of different age classes on Ram Mountain, Alberta. We used 21years of data including a detailed pedigree, demographic parameters and repeated morphological measurements. Sexual selection on horn length of males of all ages was directional and positive. Selection intensity increased with the number of competitors, reflecting male-male encounter rate during the rut, but was independent of breeder sex ratio or age structure. This result can also be linked to changes in population size because the number of competitors was highly correlated to total number of sheep. This demographic effect likely arises from age-dependent mating tactics. Males aged 2-4years are weakly competitive and experienced stronger sexual selection as they accounted for a greater proportion of all males. Selection experienced by mature males appeared independent of demography. Our study provides a rare description of the demographic determinants of sexual selection in nature.

Sexual dimorphism and physiological correlates of horn length in a South African isopod crustacean

Although isopod crustaceans frequently show sexual dimorphism in body size, sexual differences in other non-reproductive (secondary sexual) traits have been rarely studied. Furthermore, little is known about the physiological correlates of variation in the expression of sexually dimorphic traits in animals generally. Here, we show that the unusual dorsal horns of the South African horned isopod Deto echinata exhibit significantly different allometric scaling relationships with body size in males versus females. Not only are the horns of adult males substantially longer than those of adult females but also their log–log scaling slope with body length is significantly steeper than that of females and juveniles. Furthermore, in males relative horn length is positively correlated with body condition, as estimated by body mass relative to body length, and with resting metabolic (CO2 release) rate, also adjusted to body length. However, although showing a weak positive trend, the relative horn length of males is not significantly correlated with body length adjusted active metabolic rate. It is also not significantly correlated with the relative rates of whole body or cuticular water loss rates or with two measures of activity level. We hypothesize that the sexual dimorphism of horn length in D. echinata is the result of sexual selection and that horn length is a reliable indicator of the ability of a male to acquire and process resources, which in turn may be critical to female mate choice and (or) male–male competition for mates. However, this hypothesis requires further testing based on direct observations of female mate choice and male–male behavioral interactions, which have not yet been carried out.

Intense selective hunting leads to artificial evolution in horn size.

The potential for selective harvests to induce rapid evolutionary change is an important question for conservation and evolutionary biology, with numerous biological, social and economic implications. We analyze 39 years of phenotypic data on horn size in bighorn sheep (Ovis canadensis) subject to intense trophy hunting for 23 years, after which harvests nearly ceased. Our analyses revealed a significant decline in genetic value for horn length of rams, consistent with an evolutionary response to artificial selection on this trait. The probability that the observed change in male horn length was due solely to drift is 9.9%. Female horn length and male horn base, traits genetically correlated to the trait under selection, showed weak declining trends. There was no temporal trend in genetic value for female horn base circumference, a trait not directly targeted by selective hunting and not genetically correlated with male horn length. The decline in genetic value for male horn length stopped, but was not reversed, when hunting pressure was drastically reduced. Our analysis provides support for the contention that selective hunting led to a reduction in horn length through evolutionary change. It also confirms that after artificial selection stops, recovery through natural selection is slow.

Sexual and Male Horn Dimorphism in Copris ochus (Coleoptera: Scarabaeidae)

Copris ochus (Coleoptera: Scarabaeidae), an endangered species, is the largest dung beetle in Japan. In C. ochus, males have a long head horn, while females lack this long horn (sexual dimorphism). Very large males of C. ochus have disproportionately longer head horns than small males, suggesting male horn dimorphism, although the dimorphism has not been investigated quantitatively. To clarify sexual and male horn dimorphism in C. ochus quantitatively, we examined the scaling relationship between body size (prothorax width) and head horn length in 94 females and 76 males. These beetles were captured during July 1978 from a natural population on Mt. Aso in southwestern Japan using a light trap. Although the horn length of the females and males scaled with prothorax width, the scaling relationship differed between the sexes, i.e., the relationship was linear in females and nonlinear in males. Statistical tests for dimorphism in male horn length showed a significant discontinuous relationship, thus indicating distinct sexual and male dimorphism in head horns. Long- and short-horned C. ochus males may have different reproductive behaviors, as described in other horned dung beetles.

THE INTENSITY OF SEXUAL SELECTION PREDICTS WEAPON SIZE IN MALE BOVIDS

As a classical example of a sexually selected trait, the horns of male bovids offer a prime opportunity to identify predictors of the intensity of sexual selection. Here I use the comparative method to quantify sexual and natural selection pressures behind interspecific variation in horn length. I show that male horn length depends on factors proposed to affect the mean mate number per mating male, correlating positively with group size and negatively with male territoriality. This suggests that whereas group size increases the opportunity for sexual selection, territoriality reduces it because territorial males are unable to follow and monopolize female groups as effectively as males in nonterritorial species. Sexual body size dimorphism also correlates positively with group size and negatively with territoriality, corroborating these factors as predictors of the intensity of sexual selection on males. Female horn length was unaffected by the factors related to mating system, suggesting that this trait is mainly under natural selection. Using female horn length as a proxy for forces of natural selection revealed a negative effect on male horn length. Thus where natural selection favors female horns, possibly as effective weapons against predators, a similar selection pressure on males might prevent them from evolving too elaborate horns through sexual selection. There was no correlation found between horn length and latitude, thus providing no support for the hypothesis that horns have a thermoregulatory function.

Horn Length Is the Determining Factor in the Outcomes of Escalated Fights Among Male Japanese Horned Beetles, Allomyrina dichotoma L. (Coleoptera: Scarabaeidae)

Male horn length in some horned beetles shows a sigmoidal relationship with body size. This has often been considered as the reflection of alternative reproductive tactics of males based on body size. Large males should possess long horns to acquire females through fights with other males using their horns, whereas small males do not require long horns because they usually avoid intermale fights and adopt alternative tactics such as sneaking. This may lead to a prediction that horn length is a reliable indicator of the fighting ability of the male. We examined the effects of both male horn length and body size of Allomyrina dichotoma on the outcomes of escalated fights. Results indicate that male horn length was more important than body size in predicting the outcomes of fight, and this may support the hypothesis that the evolution of the horn dimorphism in male horned beetles is the result of different reproductive tactics.

Allometric plasticity in a polyphenic beetle

Abstract 1. Environmental conditions, such as variation in nutrition, commonly contribute to morphological variation among individuals by affecting body size and the expression of certain morphological traits; however the scaling relationship between a morphological trait and body size over a range of body sizes is generally assumed not to change in response to environmental fluctuation (allometric plasticity), but instead to be constant and diagnostic for a particular trait and species or population. The work reported here examined diet‐induced allometric plasticity in the polyphenic beetle Onthophagus taurus Schreber (1759) (Coleoptera: Scarabaeidae).2. Male O. taurus vary in body size depending on larval nutrition. Only males above a critical body size threshold express fully developed horns; males smaller than this threshold develop only rudimentary horns or no horns at all.3. Field populations that naturally utilise two different resources for feeding larvae (horse dung vs. cow manure) exhibited significant differences in the average scaling relationship between body size and male horn length over the same range of body sizes. Males collected from cow manure populations expressed consistently longer horns for a given body size than males collected from horse dung populations.4. Males reared in the laboratory on horse dung or cow manure also exhibited significant differences in the average scaling relationship between body size and horn length. Differences between laboratory populations reared on horse dung or cow manure were of the same kind and magnitude as differences between field populations that utilise these different resources naturally.5. These findings suggest that between‐population differences in scaling relationships between horn length and body size can be the product of differences in the quality of resources available to developing larvae. Results are discussed in the context of onthophagine mating systems and recent insights in the developmental and endocrine control of horn polyphenisms.

Hormonal control of male horn length dimorphism in Onthophagus taurus (Coleoptera: Scarabaeidae): a second critical period of sensitivity to juvenile hormone

Male dung beetles ( Onthophagus taurus) facultatively produce a pair of horns that extend from the base of the head: males larger than a threshold body size develop long horns, whereas males that do not achieve this size develop only rudimentary horns or no horns at all. Using topical applications of methoprene, we identified a sensitive period during the feeding stage of third (final) instar larvae when application of methoprene shifted the threshold body size for horn expression. Male larvae that received methoprene at this time delayed horn production until they attained a larger threshold body size than acetone-treated control larvae. This new sensitive period occurs earlier than a sensitive period previously reported for male horn regulation, and it coincides with a morph-specific pulse of ecdysteroid secretion described for this species. It appears that male horn expression is influenced by endocrine events at two different periods of larval development. We incorporate these results into an expanded model for the endocrine regulation of male horn expression.

Early development, adult mass, and reproductive success in bighorn sheep

Despite considerable empirical and theoretical work on the individual and population consequences of early development, little is known about the correlations between early mass and adult size or lifetime reproductive success of free-ranging mammals. Using a 26-year study of bighorn sheep (Ovis canadensis), we examined how mass as a lamb and mass gain as a yearling affected adult mass for both sexes, horn length of males and lifetime reproductive success of females at different population densities. Mass as a 3-week-old lamb was either weakly or not correlated with adult mass, horn length of adult males, or the number of lambs weaned over a ewe’s lifetime. Weaning mass was correlated with most of these variables when the number of ewes in the population was taken into account. When weaning mass was controlled through partial correlation, mass as a yearling was correlated with adult mass of ewes but not with ewe reproductive success or with adult mass or horn length of rams. Lamb mass and number of ewes explained more of the variance in adult characteristics for males than for females. Our results suggest that mass gain during lactation, possibly but not necessarily related to the amount of maternal care received, affects adult mass and reproductive success. Females appear better able than males to compensate for poor early development, likely by postponing their first reproduction. Mass gain over several years and the number of ewes in the population strongly affect adult mass of both sexes and therefore can have profound effects on reproductive success of this long-lived species with a multi-year growth period.

Allometric relationships and character evolution inOnthophagus taurus(Coleoptera: Scarabaeidae)

Allometric relationships in primary sexual traits (male and female genitalia), secondary sexual traits (male horns and female carinae), and non-sex-related traits (external body traits, epipharynx traits) were studied in the dung beetle Onthophagus taurus. Model II regressions of log-transformed data were used to quantify relationships, with pronotum width as regressor and indicator of overall body size. Slopes (allometric values) for the different trait categories were significantly different, with secondary sexual traits showing the highest values (higher than 1.0), followed by external body traits (slightly lower than 1.00) and epipharynx traits (around 0.2). Primary sexual traits and body size were mostly uncorrelated and genital sizes were virtually constant. Allometries of secondary sexual traits were quite different in the two sexes: the relationship between male horn length and pronotum width was approximately sigmoidal, while that between female carina length and pronotum width was linear. External body traits had significantly higher allometric values in females than in males. Our results suggest that traits in the different categories are under different kinds of selection. Genital allometries can be explained on the basis of sexual selection by cryptic female choice or by the lock-and-key hypothesis. Among secondary sexual characters, male horn morphology seems to be mostly "environmentally" determined and sexual selection would affect only a component of the developmental mechanism of horn expression. External body characters are likely under natural selection, even though a few traits could be sex-related. Finally, in both sexes, internal epipharynx traits seem to be subject to the same selective pressure, probably because males and females use the same feeding niche. The constancy of genital sizes in O. taurus suggests that in developmental processes, more is invested in primary sexual traits (to produce genitalia of the proper size) than in secondary sexual or body traits. Males receiving low quantities of food may incur costs associated with a small horn or small body size, but do not incur costs associated with small genitalia. Females probably share the same developmental pattern.

Allometric relationships and character evolution in <i>Onthophagus taurus</i> (Coleoptera: Scarabaeidae)

Allometric relationships in primary sexual traits (male and female genitalia), secondary sexual traits (male horns and female carinae), and non-sex-related traits (external body traits, epipharynx traits) were studied in the dung beetle Onthophagus taurus. Model II regressions of log-transformed data were used to quantify relationships, with pronotum width as regressor and indicator of overall body size. Slopes (allometric values) for the different trait categories were significantly different, with secondary sexual traits showing the highest values (higher than 1.0), followed by external body traits (slightly lower than 1.00) and epipharynx traits (around 0.2). Primary sexual traits and body size were mostly uncorrelated and genital sizes were virtually constant. Allometries of secondary sexual traits were quite different in the two sexes: the relationship between male horn length and pronotum width was approximately sigmoidal, while that between female carina length and pronotum width was linear. Extern...

Proximate determination of male horn dimorphism in the beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

Abstract The existence of discrete phenotypic variation within one sex poses interesting questions regarding how such intrasexual polymorphisms are produced and modified during the course of evolution. Approaching these kinds of questions requires insights into the genetic architecture underlying a polymorphism and an understanding of the proximate mechanisms determining phenotype expression. Here we explore the genetic underpinnings and proximate factors influencing the expression of beetle horns – a dramatic sexually selected trait exhibiting intramale dimorphism in many species. Two relatively discrete male morphs are present in natural populations of the dung beetle Onthophagus taurus (Scarabaeidae, Onthophagini). Males exceeding a critical body size develop a pair of long, curved horns on their heads, while those smaller than this critical body size remain essentially hornless. We present results from laboratory breeding experiments designed to assess the relative importance of inherited and environmental factors as determinants of male morphology. Using father–son regressions, our findings demonstrate that horn length and body size of male progeny are not predicted from paternal morphology. Instead, natural variation in an environmental factor, the amount of food available to larvae, determined both the body sizes exhibited by males as adults and the presence or absence of horns. The nonlinear scaling relationship between the body size and horn length of males bred in the laboratory did not differ from the pattern of variation present in natural populations, suggesting that nutritional conditions account for variation in male morphology in natural populations as well. We discuss our results by extending ideas proposed to explain the evolution of conditional expression of alternative phenotypes in physically heterogeneous environments toward incorporating facultative expression of secondary sexual traits. We use this synthesis to begin characterizing the potential origin and subsequent evolution of facultative horn expression in onthophagine beetles.

Diet alters male horn allometry in the beetleOnthophagus acuminatus(Coleoptera: Scarabaeidae)

Darwin considered the horns of male beetles to be among the most striking examples of sexual selection. As with antlers in deer or elk, beetle horns scale positively with male body size, with the result that large males have disproportionately longer horns than small males. It is generally assumed that such scaling relationships ('static allometries') are insensitive to short-term changes in the environment, and for this reason they are regularly used as diagnostic attributes of populations or species. Here I report breeding experiments on horned beetles that demonstrate that the scaling relationship between male horn length and body size changes when larval nutrition changes. Males reared on a low-quality diet had longer horn lengths at any given body size than sibling males reared on a high-quality diet. Such 'allometry plasticity' may explain seasonal changes observed in this same scaling relationship in a natural population. These experiments demonstrate that scaling relationships of sexually selected traits can respond facultatively to variation in the environment, thereby revealing a new mechanism by which males regulate the production of exaggerated secondary sexual traits.

ARTIFICIAL SELECTION ON HORN LENGTH-BODY SIZE ALLOMETRY IN THE HORNED BEETLE ONTHOPHAGUS ACUMINATUS (COLEOPTERA: SCARABAEIDAE).

Males of the horned beetle Onthophagus acuminatus Har. (Coleoptera: Scarabaeidae) exhibit horn length dimorphism due to a sigmoidal allometric relationship between horn length and body size: the steep slope of the allometry around the inflection of the sigmoid curve separates males into two groups; those larger than this inflection possess long horns, and those smaller than this inflection have short horns or lack horns. I examined the genetic basis of the allometric relationship between horn length and body size by selecting males that produced unusually long horns, and males that produced unusually short horns, for their respective body sizes. After seven generations of selection, lines selected for relatively long horns had significantly longer horn lengths for a given body size than lines selected for relatively short horns, indicating a heritable component to variation in the allometry. The sigmoidal shape of the allometry was not affected by this selection regime. Rather, selected lines differed in the position of the allometry along the body size axis. One consequence of lateral shifts in this allometric relationship was that the body size separating horned from hornless males (the point of inflection of the sigmoid curve) differed between selection lines: lines in which males were selected for relatively long horns began horn production at smaller body sizes than lines selected for relatively short horns. These results suggest that populations can evolve in response to selection on male horn length through modification of the growth relationship between horn length and body size.