Further studies on the evolution of land and freshwater nemerteans: generic relationships among the paramonostiliferous taxa

The costs and benefits of fighting in bovids are high in terms of injury and reproductive success, respectively. The breakage of a horn would curtail reproductive success permanently. Therefore, the horns of bovids should include sufficient material so that they are strong enough to be unlikely to break in fighting but without being too heavy to carry around. The forces developed during fighting were measured in a computerized analysis of film of blackbuck and bighorn sheep. All possible modes of failure were investigated using a mechanical analysis to see how the horns are most likely to fail.
The maximum possible force developed during fighting is 3400 N for the bighorn sheep and 456 N for the blackbuck. Bending stress, shear stress, deflection, strain energy and critical crack lengths were calculated for the horns of these two bovids. Horns are most likely to fail in bending as indicated by safety factors. Most of the force is taken in compression due to the curvature of horns. Shear stresses are insignificant and deflections are negligible during the most forceful encounters.
The safety factor in bending of the horn of the bighorn sheep is greater (10) than that of the horn of the blackbuck (3.4) because the forces are probably more variable and unpredictable in the fighting of the former.
All of the energy of fighting is absorbed by the body musculature because horns store less than 1% of the energy produced in fighting as strain energy when they bend. Cracks and scratches mustbe more than 60% of the transverse basal dimensions of horns in order for there to be catastrophic failure at the maximum stresses developed during fighting.
The horns of the blackbuck and the bighorn sheep appear to be minimum weight structures given the variability of the forces acting on them and are unlikely to break in fighting given the forces calculated from the analyses of films.     

Size-specific defensive structures in a horned weevil confirm a classic battle plan: avoid fights with larger opponents

A rigid sheath that extends deep into the prothorax of male Parisoschoenus expositus serves to receive the horns of opponents in battles over females. The male cannot use his own horn unless he receives his opponent's horn in his sheath. The length of a male's sheath is always approximately equal to the length of his own horns, so his body design is appropriate only for horn-locking battles with males whose horns are equal to or shorter than his own. Horn length and sheath length are dimorphic with respect to each other and to an indicator of body size. The switch points between morphs are very nearly the same for several relationships, so the different aspects of fighting morphology are tightly coordinated.     

Horn Length Is the Determining Factor in the Outcomes of Escalated Fights Among Male Japanese Horned Beetles, <Emphasis Type="Italic">Allomyrina dichotoma</Emphasis> 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.     

Males with short horns spent more time mating in the Japanese horned beetle Allomyrina dichotoma

We examined the relationships between male body and horn sizes and mating duration in the Japanese horned beetle, Allomyrina dichotoma. Smaller males possessing shorter horns spent more time for copulation with a female and mounting the female without copulation. The results of multiple regression analyses indicate that the horn length is a determining factor for the time spent by the males during mating. A previous study has documented that the horn length of male A. dichotoma primarily determined the outcomes of aggressive male–male interactions; hence, predicts access to females. Therefore, instead of fighting for females, males possessing short horns may maximize their fertilization success by mating longer with the few females they have access to.     

笼养褐马鸡冬季的社群等级

本文用BBS方法（Batchelder-Bershad-Simpson Scaling Method)对笼养褐马鸡的社群等级进行了排序,发现其等级制度为近单线式。成鸟的等级明显高于亚成体,成年雄鸟的等级明显高于成年雌鸟,成年个体的等级与体重呈显著正相关,与面部红斑大小及耳簇羽长呈极显著正相关,全体14只鸟的体重与体长及耳簇羽长呈显著正相关,与面部红斑大小呈极显著正相关。体重可以认为是直接与个体的争斗实力相关的参数,而面部红斑大小,耳簇羽长则可能是个体争斗实力的外在信号,在体重相近的雌性个体中,面部红斑和耳簇羽对社群等级有很大的影响。褐马鸡的争斗炫耀可能是以面部为中心的。     

Male horn dimorphism in the scarab beetle, Onthophagus taurus: do alternative reproductive tactics favour alternative phenotypes?

In a variety of organisms morphological variation is discrete rather than continuous. Discrete variation within a sex has attracted particular interest as it is thought to reflect the existence of alternative adaptations to a heterogeneous selection environment. The beetle Onthophagus taurus shows a dimorphism for male horns: males that exceed a critical body size develop a pair of long, curved horns on their heads, while smaller males remain hornless. In this study we report on the alternative reproductive tactics used by males with these two morphologies, and present experimental and behavioural data suggesting that these alternative tactics selectively favour discretely different male phenotypes. Horned males aggressively defended tunnel entrances containing breeding females. Fights involved the use of horns, and males with longer horns were more likely to win fights. In contrast, hornless males employed nonaggressive sneaking behaviours when faced with competitively superior males. Sneaking behaviours appeared to require high degrees of manoeuvrability inside tunnels to access and mate with females despite the presence of a guarding male. Comparisons of running performances of males with identical body sizes but different horn lengths suggest that the possession of horns reduces male agility inside tunnels. Thus, horn possession confers a clear advantage to males using fighting behaviours to access females, whereas hornlessness may be favoured in males that rely primarily on sneaking behaviours. Combined, the two alternative reproductive tactics used by male O. taurus appear to favour opposite horn phenotypes, which may explain the paucity of intermediate morphologies in natural populations of O. taurus. Copyright 2000 The Association for the Study of Animal Behaviour.     

Chemoreception and the Assessment of Fighting Abilities in the Lizard Liolaemus monticola

When an individual faces the risk of a conflict, its ability to make ‘correct’ decisions is crucial to its fitness. Research on decision making has focused mainly on visual and acoustic signals, while chemical signals have received much less attention, despite their relevance for many species. Chemosignals can be detected in the absence of the signaller and, in the context of fighting risk, this property confers the advantage that the receiver can avoid agonistic interactions or, if they are unavoidable, that it can prepare itself for the conflict. I studied the behaviour of males of the lizard Liolaemus monticola in the laboratory when they were confronted with chemosignals of a potential opponent. During this ‘pre‐confrontation’ stage, I tested the following predictions: (1) lizards can derive precise information from chemosignals of conspecifics, and use this to respond with precision to the perceived risk and (2) the best predictor of the receiver behaviour, and therefore the best predictor of the risk involved in the fight, is the relative fighting ability of opponents. As a measure of fighting ability, I used body size. ‘Intruders’ were placed in the terrarium of unfamiliar ‘residents’ during the absence of the latter, and their behaviours were recorded. Simple regressions were performed between the different behavioural variables and with the body sizes of intruder and resident, and with the relative difference in body sizes of opponents. The latter was the best predictor of intruder behaviour: it was negatively correlated with behaviours associated with activity (i.e. motion time), chemoexploration (i.e. number of tongue flicks) and behaviours associated with social interactions (i.e. head bobs). These results suggest that males can process information from chemosignals and decisions made during the ‘pre‐confrontation’ stage are based on the assessment of the relative fighting abilities (i.e. relative body size) of opponents.     

Variation in cross‐sectional horn shape within and among rhinoceros beetle species

Sexual selection has equipped male rhinoceros beetles with large horns on their head and prothorax to aid in battle over access to females. Horns are used to pry and dislodge opponents from resource sites that attract females, so an optimal horn should be able both to withstand the high stresses imposed during fights, and to resist deflection in response to these loads. We examined the cross‐sectional morphology of horns using micro‐computed tomography scanning to determine how horn structure changes with horn length to withstand the different fighting loads. Specifically, we measured the second moment of area of horns within and among rhinoceros beetle species to assess whether changes in cross‐sectional morphology accompany changes in body size in order to maintain high strength and stiffness during fights. We find that the second moment of area of horns increases with body size both intra‐specifically and inter‐specifically, and that these relationships closely fit those predicted if horns have been selected to be strong and stiff fighting structures. Our results therefore support the hypothesis that rhinoceros beetle horns are structurally adapted for combat.     

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.     

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.     

Is the parallel walk between competing male fallow deer, Dama dama, a lateral display of individual quality?

During competitive encounters protagonists are expected to use signals of individual quality particularly if there is a risk of injury or death. Lateral presentation of body profile, by which information regarding phenotypic characteristics associated with individual quality are displayed, may represent such a strategy. During aggressive interactions, male fallow deer frequently engage in parallel walking which is assumed to represent a mutual display of quality, as mediated by exposure of the maximal profile of the body or antlers. We examined the context and role of the parallel walk during competitive encounters to investigate whether there was evidence that dyads of competing males were assessing differences in phenotypic characteristics. There was no evidence to support the hypotheses that the parallel walk is a lateral display of body size or weaponry or that its use is associated with a reduced level of escalated or risky behaviours during fighting. Total time spent fighting was not shorter when a parallel walk was present than when there was no parallel walk. The parallel walk was highly associated with fighting and it was more likely to be initiated by the subsequent loser. Furthermore, parallel walking frequently followed bouts of fighting and as such may represent a strategy that permits an animal the opportunity to decide whether to continue fighting. Parallel walking was also associated with a failure to resolve contests in favour of one animal indicating that it may be a means of withdrawing from further fighting without incurring a loss in dominance status. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

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.     

Sound Characteristics and Outcome of Contests in Male Croaking Gouramis (Teleostei)

A correlative study using similar-sized males of the croaking gourami Trichopsis vittata was carried out to investigate whether sound characteristics influenced winning and if relative fighting ability was assessed by acoustic signals. Pair-wise contests between males were decided using lateral displays (LD) and vocalization in 26 cases, whereas 66 fights escalated to the frontal display (FD) phase. Physical fighting (mouth wrestling) and injuries were rarely observed in this species. Winners were generally larger than their opponents, and this effect was more pronounced in non-escalated than in escalated contests. Sounds of fight winners had a higher sound pressure level and also a lower dominant frequency. Neither number of acoustic signals nor duration of lateral and frontal displays were predictors of contest outcome. Acoustic measures were highly correlated to body weight. These results indicate that traits correlated with RHP (such as sound pressure level and dominant frequency) were predictors of the outcome, while traits not correlated with size (such as number and duration of displays) did not influence winning. In accordance with the main prediction of assessment models, the contest duration (cost) increased with the decrease in asymmetry of body length as well as sound pressure level. No such relationships were found for weight and dominant frequencies in LD- and FD-contests. The present study indicates that morphological and sound characteristics influence winning in fish. Moreover, the results suggest that croaking gouramis settle conflicts without damaging combats by assessing asymmetries in different components of RHP such as body weight and length, which may reliably be signalled by acoustic and visual assessment signals.     

Parallel Lasers for Remote Measurements of Morphological Traits

Abstract: Animal ecology research could benefit from the measurement of individual morphological traits. In bovids, male horn size often correlates with annual reproductive success, is sensitive to resource abundance, and could be a predictor of survival. However, live captures are costly, involve some risk of injury or substantial disturbance to the animals, and are impossible in many situations. To remotely measure horn growth of free-ranging Alpine ibex (Capra ibex), I designed an aluminum frame that holds parallel laser pointers and a digital camera. I took digital pictures of ibex horns and calculated horn growth based on the fixed distance between the 2 laser points. This simple and accurate technique could benefit many ecological studies that require linear measurements, such as shoulder height, body length, leg length, or fin length. It could also help measure body features (e.g., fur or skin patterns, scars), increasing the reliability of individual photographic identification.     

Quantitative genetics and sex-specific selection on sexually dimorphic traits in bighorn sheep

Sexual conflict at loci influencing traits shared between the sexes occurs when sex-specific selection pressures are antagonistic relative to the genetic correlation between the sexes. To assess whether there is sexual conflict over shared traits, we estimated heritability and intersexual genetic correlations for highly sexually dimorphic traits (horn volume and body mass) in a wild population of bighorn sheep (Ovis canadensis) and quantified sex-specific selection using estimates of longevity and lifetime reproductive success. Body mass and horn volume showed significant additive genetic variance in both sexes, and intersexual genetic correlations were 0.24+/-0.28 for horn volume and 0.63+/-0.30 for body mass. For horn volume, selection coefficients did not significantly differ from zero in either sex. For body weight, selection coefficients were positive in females but did not differ from zero in males. The absence of detectable sexually antagonistic selection suggests that currently there are no sexual conflicts at loci influencing horn volume and body mass.     

Testing game theory models: fighting ability and decision rules in chameleon contests

Game theory models of animal contests make many non-mutually exclusive predictions, complicating empirical tests. These predictions regard the relationship between contest parameters and fighting ability, for which body size is usually used as a proxy. However, in many systems, body size may be a limited proxy since multiple traits and contextual factors such as experience influence fighting ability. Using contests between male Cape dwarf chameleons, Bradypodion pumilum, I test alternative game theory models of extended contests. I show how the most likely candidate model can be identified through a process of elimination, based on tests of key predictions. In addition, I present a measure of fighting ability based on multiple traits that allows ability to change as experience changes. In dwarf chameleons, persistence is based on loser thresholds rather than assessment of relative ability, ruling out the sequential assessment model. Winners and losers do not match behaviours in early parts of the contest, arguing against all types of war of attrition models. Although the cumulative assessment model remained as the most likely candidate model, not all specific predictions of this model were upheld.     

Fracture toughness of horns and a reinterpretation of the horning behaviour of bovids

The keratinous horns of bovids are used in intraspecific combat to gain access to females in oestrus. Horn sheath keratin is a composite material consisting of stiff protein fibres and a pliant protein matrix. Unlike antlers, horns are permanent structures which are likely to accumulate damage during fighting. Therefore, horn sheath keratin should be resistant to fracture (tough) and insensitive to surface defects (scratches and cracks) which may weaken horns by acting as stress concentrators.
The effect of water on the toughness and notch-sensitivity of horn sheath keratin was investigated in three-point bending and tensile tests. Several measures of toughness were made on dry (0% water content), fresh (20%) and wet (40%) horn keratin, including total work of fracture, Gurney & Hunt work of fracture, critical strain energy release rate and critical stress intensity factor.
The mean total work of fracture of fresh horn is about 40 kJ/m² which is relatively much greater than most biological and synthetic materials. Most of the work of fracture is due to plastic yielding of the matrix (50–75%); the rest is due to crack-tip specific fracture mechanisms such as fibre pull-out and Cook Gordon crack-stopping. Dehydration reduces the total work of fracture of horn keratin by preventing the yielding of the matrix.
The strength of fresh and wet horn is insensitive to notches, but dry horn is very notch-sensitive. Therefore, bovids must avoid dehydration of their horns due to the desiccating effect of the environment. The 'horning' behaviour of bovids may be a maintenance activity which ensures that the horn sheath is adequately hydrated to remain tough and notch-insensitive.     

Functional directional asymmetry in fallow deer (Dama dama) antlers

The right antlers of fallow deer are significantly more developed than the left antlers with respect to the lengths of the beam, trez tine and palm, as well as antler weight and number of points. The cross-section at the base is also more elliptical in the right antler. The size difference is correlated with the more intense use of right antlers in fighting, and with the greater breadth of the areas of insertion of the neck muscles in the skull's nuchal region.     

Structure and function of the large pronotal horn of the sand‐living anthicid beetle Mecynotarsus tenuipes

Large insect horns function as antipredator armaments, digging implements and intraspecific combat weapons. The sand‐living anthicid beetle Mecynotarsus tenuipes possesses a large horn on the pronotum. Allometric relationships between body size and horn size did not show either a slope of more than 1 or sexual dimorphism, suggesting another function of the horn other than sexual selection via combat. Behavioral observation of individuals using a microvideo camera indicated that the horn is used to dig and move forward in loose sand. Only the horned M. tenuipes could dig into sand, in contrast to the hornless anthicid beetles Stricticollis valgipes and Clavicollis fugiens, which could not dig. When moving in sand, M. tenuipes joins its pronotal horn and head to form a conical shape, with which it pierces into the sand. Then, it opens its horn and head outward to create a space in the sand for forward motion. Although it can dig deeply into sand by repeating these behaviors sequentially, digging speed tends to slow with depth, probably because the weight of the substrate increases.     

Mouth morphology, testes size and body size in male Tilapia zillii: implications for fighting and assessment

In Tilapia zillii , the outcome of fights is closely related to difference in testis weight between opponents rather than differences in body weight. Testes size was predictable statistically on the basis of the size of the mouth in relation to body size, suggesting that among other functions mouth morphology may have evolved to facilitate assessment during fighting.