Weapon allometry varies with latitude in the New Zealand giraffe weevil

Animal body size commonly shows a relationship with latitude to the degree that this phenomenon is one of the few ‘rules’ discussed in evolutionary ecology: Bergmann's rule. Although exaggerated secondary sexual traits frequently exhibit interesting relationships with body size (allometries) and are expected to evolve rapidly in response to environmental variation, the way in which allometry might interact with latitude has not been addressed. We present data showing latitudinal variation in body size and weapon allometry for the New Zealand giraffe weevil (Lasiorhynchus barbicornis). Males display an extremely elongated rostrum used as a weapon during fights for access to females. Consistent with Bergmann's rule, mean body size increased with latitude. More interestingly, weapon allometry also varied with latitude, such that lower latitude populations exhibited steeper allometric slopes between weapon and body size. To our knowledge, this is the first study to document a latitudinal cline in weapon allometry and is therefore a novel contribution to the collective work on Bergmann's rule and secondary sexual trait variation.     

Heritability of male mandible length in the stag beetle Cyclommatus metallifer

Numerous coleopteran species express male‐specific “weapon traits” that often show size variations among males, even within a single population. Many empirical studies have demonstrated that environmental conditions during development affect absolute weapon size. However, relatively few studies in horned beetles support the hypothesis that the relationship between weapon size and body size, also referred to as a “scaling relationship” or “static allometry”, is largely determined by genetic factors. In this study, the heritability of absolute mandible length and static allometry between mandible length and body size were estimated in the stag beetle Cyclommatus metallifer. While no significant heritable variation was observed in absolute mandible length, high heritability (h² = 0.57 ± 0.25) was detected in the static allometry between mandible length and body size. This is the first report on the genetic effect on male mandible size in Lucanidae, suggesting that absolute mandible size is largely determined by environmental conditions while the static allometry between weapon size and body size is primarily determined by genetic factors.     

Factors Influencing the Breeding Activity in Male Masu Salmon, Oncorhynchus masou: The Relationship with Body Size

Synopsis Male body size has been emphasized as an important factor contributing to the breeding success of individuals. However, the operational sex ratio (OSR: ratio of mature females to males) during the breeding season significantly change due to differences in the breeding timing and period within and between males and females and may influence the selective advantage of the male large body size for mates. We examined the reproductive ecology of masu salmon Oncorhynchus masou, inhabiting Lake Toya, Hokkaido, northern Japan. Precipitation triggered the upstream migration, although males migrated into the river earlier than females. As a result, the OSR in the river changed markedly during the breeding season, as did the size structure of males in the river. Large males migrated into the river earlier than the smaller males. Differential male survival resulted in the different population size structure between the early and late periods. Under these conditions, we analyzed which males were more successful in releasing the more sperm throughout the breeding season by estimating the decrease in the sperm content in male carcasses. The body size and sperm-releasing success of the males were not related. With a weak male-biased OSR and synchronous timing of reproduction in females, males that had entered the river succeeded in releasing the sperm regardless of body size. Such a fluctuating advantage for different body sizes likely contributes to the maintenance of the size variation in the male salmon.     

It takes two: Seasonal variation in sexually dimorphic weaponry results from divergent changes in males and females

Sexually dimorphic weaponry often results from intrasexual selection, and weapon size can vary seasonally when costs of bearing the weapon exceed the benefits outside of the reproductive season. Weapons can also be favored in competition over nonreproductive resources such as food or shelter, and if such nonreproductive competition occurs year‐round, weapons may be less likely to vary seasonally. In snapping shrimp (Alpheus angulosus), both sexes have an enlarged snapping claw (a potentially deadly weapon), and males of many species have larger claws than females, although females are more aggressive. This contrasting sexual dimorphism (larger weaponry in males, higher aggression in females) raises the question of whether weaponry and aggression are favored by the same mechanisms in males and females. We used field data to determine whether either sex shows seasonal variation in claw size such as described above. We found sexual dimorphism increased during the reproductive season due to opposing changes in both male and female claw size. Males had larger claws during the reproductive season than during the nonreproductive season, a pattern consistent with sexual selection. Females, however, had larger claws during the nonreproductive season than during the reproductive season—a previously unknown pattern of variation in weapon size. The observed changes in female weapon size suggest a trade‐off between claw growth and reproduction in the reproductive season, with investment in claw growth primarily in the nonreproductive season. Sexually dimorphic weaponry in snapping shrimp, then, varies seasonally due to sex differences in seasonal patterns of investment in claw growth, suggesting claws may be advantageous for both sexes but in different contexts. Thus, understanding sexual dimorphisms through the lens of one sex yields an incomplete understanding of the factors favoring their evolution.     

Sexual size dimorphism in northern giant petrels: ecological correlates and scaling

Northern giant petrels ( Macronectes halli ) are among the largest and most sexually size dimorphic species of seabirds, with females being only 80% the mass of males. Both sexes scavenge on seal and penguin carrion in the sub-Antarctic ecosystem, but during the breeding season females also feed extensively on other marine food resources and show more pelagic habits than males. The outstanding sexual segregation in foraging and feeding ecology in northern giant petrels suggests that mechanisms maintaining sexual size dimorphism by ecological factors may be operating. I evaluated this possibility by examining ecological correlates with body size and by static allometry analyses. Fledging sex ratio in four consecutive years did not depart from parity. There was no assortative mating by size neither association between the male size with the breeding performance. By contrast, smaller females raised their chick in better condition. Moreover, bill size showed a size dimorphism beyond that expected by body size dimorphism, i.e. when controlling for body mass, males showed relatively longer bill than females. This trait did not deviate from isometry with respect to body size and its phenotypic variability was low, suggesting that the disproportionately large bill of males is related to their more scavenging life style compared to females. In general, the increase and maintenance of sexual size dimorphism in giant petrels is more consistent with an ecological causation rather than a result of sexual selection.     

Exaggerated Trait Allometry,Compensation and Trade-Offs in the New Zealand Giraffe Weevil (Lasiorhynchus barbicornis)

Sexual selection has driven the evolution of exaggerated traits among diverse animal taxa. The production of exaggerated traits can come at a cost to other traits through trade-offs when resources allocated to trait development are limited. Alternatively some traits can be selected for in parallel to support or compensate for the cost of bearing the exaggerated trait. Male giraffe weevils (Lasiorhynchus barbicornis) display an extremely elongated rostrum used as a weapon during contests for mates. Here we characterise the scaling relationship between rostrum and body size and show that males have a steep positive allometry, but that the slope is non-linear due to a relative reduction in rostrum length for the largest males, suggesting a limitation in resource allocation or a diminishing requirement for large males to invest increasingly into larger rostra. We also measured testes, wings, antennae, fore- and hind-tibia size and found no evidence of a trade-off between these traits and rostrum length when comparing phenotypic correlations. However, the relative length of wings, antennae, fore- and hind-tibia all increased with relative rostrum length suggesting these traits may be under correlational selection. Increased investment in wing and leg length is therefore likely to compensate for the costs of flying with, and wielding the exaggerated rostrum of larger male giraffe weevils. These results provide a first step in identifying the potential for trait compensation and trades-offs, but are phenotypic correlations only and should be interpreted with care in the absence of breeding experiments.     

Consequences of breeding system for body condition and survival throughout the annual cycle of tidal marsh sparrows

An individual's body condition and probability of survival can change throughout the annual cycle, based on the combined effects of many factors, including reproductive investment during breeding, colder temperatures during winter, and elevated risks during migration. We evaluated body condition and survival during breeding and non‐breeding periods in two closely related species with notably different reproductive systems. Male and female saltmarsh sparrows Ammodramus caudacutus represent extremes in parental care: males perform none, leaving females to do everything from build nests to care for fledglings. In contrast, male and female seaside sparrows A. maritimus have bi‐parental care and similar levels of reproductive investment, intermediate between male and female saltmarsh sparrows. Our results are consistent with the idea that females experience non‐lethal effects of reproduction, and that differences between the breeding season and winter affect condition. In both species, females had lower scaled mass index (SMI) values than males during both breeding and non‐breeding seasons, and female saltmarsh sparrows had lower SMI values than female seaside sparrows. Females carried more fat than males during the breeding season, and female, but not male, fat and muscle scores decreased over time, which is consistent with the adaptive mass hypothesis. In winter, all groups carried more fat and had higher muscle scores than when breeding, despite having lower SMI scores. Although we observed variation in body condition, within‐season survival was uniformly high in both seasons, suggesting that sex, species, season, body size, and body condition have little impacts on within season survival. Comparisons with previously‐published estimates of annual adult survival suggest that most mortality occurs during migration, even in these short‐distance migrants. The importance of considering multiple aspects of body condition, multiple seasons, and difficult‐to‐monitor events, such as migration, should not be ignored when thinking about the events and processes that cumulatively determine population dynamics.     

Effects of body size on sex‐related migration vary between two closely related gull species with similar size dimorphism

Studies of migration have revealed multiple trade‐offs with other life‐history traits that may underlie observed variation in migratory properties among ages and sexes. To assess whether, and to what extent, body size and/or sex‐specific differences in competition for resources (e.g. breeding territories or winter food) may shape variation in migration distance and timing of arrival in ecologically and phylogenetically related species, we combined over 30 000 sightings of individually marked, sexually mature males and females of Herring Gulls Larus argentatus and Lesser Black‐Backed Gulls Larus fuscus with biometric measurements and phenological observations at a mixed breeding colony. In L. argentatus, larger males migrated further from the breeding colony, whereas migration distance was independent of body size in adult females. In L. fuscus, no relationship between body size and migration distance was apparent in either sex. Mean arrival dates at the breeding colony did not vary with migration distances but differed between males and females of L. argentatus (but not L. fuscus). As allometry at least partly explains sexual segregation in migration behaviour in L. argentatus, but not in L. fuscus, we conclude that the effect of body size on sex‐related migratory strategies may vary between closely related, sympatric species despite similar size dimorphism.     

Exploring sexual differences in external morphology and limb muscles of Hylarana guentheri (Anura: Ranidae) during non-breeding season

Studying sexual dimorphism facilitates better understanding of the general intersexual divergence of the same species and gives insights into the impact of selective forces on each sex. The sexual dimorphism in anuran external morphology or limb muscles has been well studied in reproductive context, but less so outside the breeding season. Herein, the intersexual differences in 12 external morphological characteristics, 11 forelimb and 12 hindlimb muscles of Hylarana guentheri during non-breeding season were investigated. We found that the males possessed significantly greater head width and tympanum diameter, while the females had significantly larger body size and head length; there was no sexual differences in the examined limb muscles, except for the mass of flexor carpi radialis. The larger body size means greater fertility for the females, and the longer head may be related to the allometry and reduce resource competition. For the males, the wider head is likely correlated with prey size and male–male competition, and also, the larger tympanum and heavier flexor carpi radialis probably aid the productive success. This study provides the comprehensive morphological accounts about the sexual differences of H. guentheri during non-breeding season, which will contribute to clarify the sex-specific resource allocation and reproductive strategies of anurans.     

Morphometric study of the red shrimp Aristeus antennatus in the Eastern Mediterranean

The relative growth of body parts (abdomen, uropod, scaphocerite, telson, rostrum and the third pereiopod) of both sexes of red shrimp Aristeus antennatus was studied in the Greek and Italian Ionian Sea (Eastern Mediterranean), at a depth range from 546 to 1,047 m. Morphometric relationships in the sexes showed similar patterns of relative growth in the two study areas. Although, morphological characters (abdomen, carapace length, pereiopod, rostrum, scaphocerite, telson, uropod and weight) of female were significantly greater than males, in both areas the observed differences between sexes could also attributed to the seasonal mobility pattern of females and to their reproductive activity. A negative allometry of swimming morphological characters (uropod, scaphocerite, telson) and of abdomen (related to the rapid locomotion, reproduction) was found in both sexes and areas. In contrast to this, the growth the third pereiopod (involved in walking ability) in both sexes and areas was isometric. Males showed no correlation of rostrum length with size. Some morphological characters (pereiopod, scaphocerite, uropodal exopodite) were proportionally longer in juvenile females than in adult females, indicating that the allometry differs for this species during its life cycle. A clear increasing trend of females’ size, including all the morphological characters, was shown with decreasing temperature and increasing depth.     

Variation in the magnitude of sexual size dimorphism in nestling Coal Tits (Periparus ater)

The magnitude of sexual size dimorphism can be affected by sex differences in environmental sensitivity early in ontogeny that result in differential growth rates of male and female nestlings. Here, the larger sex might either be more sensitive because of higher food demands or less sensitive due to greater competitive ability. When environmental conditions deteriorate during the breeding season, this “environmental stress” hypothesis predicts differential seasonal declines in the performance of male and female offspring. Based on a sample of molecularly sexed Coal Tit (Periparus ater) nestlings from 2 years, we investigated sexual size dimorphism in body mass, condition (i.e. size-corrected mass), tarsus and wing length and whether its magnitude changed from early to late broods. Male offspring were heavier, larger (in terms of tarsus and wing length) and had higher size-corrected mass than their female nest mates (the same was evident in adult breeders). In 2002 (the year with the longer effective breeding season), body mass and condition declined with progressing hatching date and this effect was significantly more pronounced in male than in female nestlings. There was also a seasonal decline in male wing length, while female wing length remained relatively constant, which resulted in males having shorter wings than females in late broods. Tarsus length was unaffected by time of breeding, except that the difference between males and females was relatively smaller in late (i.e. second) broods in 2002. While these results are in accordance with the idea of an increased environmental sensitivity of the larger males, confounding effects of sex-differential hatching order cannot be ruled out. Dedicated to Doris Winkel.     

Lifetime individual plasticity in body condition of a migratory bird

Body condition is an important trait that reflects the capacity of individuals to acquire food or resist disease, eventually allowing successful reproduction and survival. We first quantified the effects of condition on life history and a secondary sexual trait using long‐term lifetime data on condition in a migratory bird species, the barn swallow, Hirundo rustica. Second, we quantified sources of individual variation in condition by separating the within‐individual from the between‐individual components of variation of environmental condition and age on body condition. Therefore, we were able to partition variation as a result of selection and phenotypic plasticity. Within‐individual variation in body condition increased in early life until middle age (i.e. 3–4 years of age) in the two sexes followed by only a slight decrease in body condition during senescence in males but not in females. After accounting for age‐dependent variation, condition could be partitioned into a within‐individual plastic response to environmental conditions during migration and a nonplastic response (i.e. a between‐individual difference) to environmental conditions experienced in the African winter quarters. Specifically, there was a within‐individual increase in body condition as environmental conditions during migration improved in both males and females, independent of age. There was a between‐individual effect of condition found in the winter quarter in body condition of females, but not in males, which was attributed to the disappearance of females in poor body condition from the study population as a result of the higher natal dispersal of low‐quality females compared to high‐quality ones during years with favourable environmental conditions in the African winter quarters. Males and females also tended to be in better body condition during the warmer springs upon arrival at the breeding grounds. There was a temporal decline in female body condition during 1991–2007, whereas no significant trend was detected in males. Therefore, both intrinsic (e.g. age and sex) and extrinsic factors (e.g. climate) affected body condition. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 420–434.     

Body condition variation in kestrel (<Emphasis Type="Italic">Falco tinnunculus</Emphasis>) nestlings in relation to breeding conditions

The body condition index (i.e., body mass corrected for age or size differences) is commonly used to investigate offspring condition in nestling birds. The body condition index reflects different parameters related to the general nutritional state of nestlings and may predict survival prospects. Since conditions experienced during the growth period can affect the fitness of nestlings in adulthood, we investigated proximate and ultimate factors underlying body condition index variation in kestrel (Falco tinnunculus) nestlings in a 9-year field study and we carried out two cross-fostering experiments to disentangle the origin (genetics plus maternal effects) and rearing (environment effect) components of body condition index variation. In total, we sampled 2,065 nestlings from 464 broods and used 121 nestlings from 24 broods in the cross-fostering experiments. We found that nestlings from larger broods had higher body condition index than nestlings from smaller ones, but this pattern did not emerge in two of the 9 years of study; nestlings born later in the breeding season had lower body condition index in some years but not in others; the decrease of body condition index over the breeding season emerged in all but three-chick broods; males and females did not differ neither in body condition index nor in the covariation between body mass and wing length, while this result was limited to one of the nine field study years; the annual mean value of body condition index did not covary with the total rainfall; both the origin and rearing components explained body condition index variation, but their relative contributions varied from a year to another. Overall, these results suggest that the brood size is not a good predictor of body condition index; the rule “nesting early in the season is better” is less general than previously thought; the body condition index may contain origin variance, whose expression may be modulated by environmental conditions.     

Seasonal changes in condition, reproduction and fecundity in the wild European rabbit (Oryctolagus cuniculus)

A total of 1248 rabbits ( Oryctolagus amiculus ) were shot on farmland in Cambridgeshire, UK over a period of three years. This provided pooled estimates of changes in reproductive and body condition and of fecundity during the annual cycle. Both males and females showed a significant annual cycle in reproductive condition as indicated by changes in the size of the gonads and accessory glands of reproduction. No females were pregnant during October, November and December, but males with active spermatogenesis were found during every month of the year. The distribution of pregnancies through the year showed that the reproductive season varied greatly between individuals. Peak fecundity occurred during April, May and June. On average, each female conceived 23.9 and suckled 17.2 young per breeding season. Proportionately more young died during the early stages of the reproductive season. The body weight of adult males, but not of females, varied seasonally. Body condition (kidney fat index) in adults was maximal at the start of the breeding season and minimal at the end.     

Genital and body allometry in two species of noctuid moths (Lepidoptera: Noctuidae)

One‐size‐fits‐all and related hypotheses predict that static allometry slopes for male genitalia will be consistently lower than 1.0 and lower than the slopes for most other body parts (somatic traits). We examined the allometry of genitalic and somatic morphological traits in males and females of two species of noctuid moths, Spodoptera exigua (Hübner, [1808]) and Helicoverpa armigera (Hübner, [1808]). The relationship between genitalic traits and body size was generally strongly negative‐allometric in males but with no significant differences from 1.00 in females of the two species examined. However, in females, the slope of genital traits was also lower than the slopes for somatic traits. The relationship between somatic traits and the body size indicator was approximately isometric in most cases in males, except in four traits in S. exigua, in which the slopes showed slight negative allometry, and the hind tibia in H. armigera, in which the slope had positive allometry. However, in females, some somatic traits showed isometric and some other showed negative allometry in both species. The coefficients of variation (CV) for all structures in the males were low, not exceeding 10%. Genitalic traits showed significantly lower CV than somatic traits in males. In females, somatic traits showed lower CV than genitalic traits but with no significant difference in the H. armigera. Our observations of strongly negative allometry for genitalic traits in males are consistent with stabilizing selection on genital size and we suggest that male performance in interactions with females is the source of selection on male genital allometry. The difference in the degree of phenotypic variation between genitalic and somatic traits in the two studied species is attributed to the different developmental‐genetic architectures of these traits. Female genitalia showed a similar trend to the males, although the difference between genital and somatic traits was not significant in females. This finding suggests that selection is acting differently on male and female genitalia. Positive allometry of hind tibia in H. armigera may be a result of secondary sexual function.     

The effects of rainfall on different components of seasonal fecundity in a tropical forest passerine

Seasonal fecundity is a composite metric that is determined by component parameters such as clutch size, nest survival and re‐nesting probability. Many of these component parameters are known to vary with environmental conditions, in particular rainfall prior to or during the breeding season. In some species, seasonal fecundity is positively related to rainfall, but little is known about which component parameters of seasonal fecundity respond most strongly to rainfall. We used intensive nest monitoring of a multi‐brooded tropical forest passerine, the Montserrat Oriole Icterus oberi, to examine the effects of rainfall during the pre‐breeding season on component parameters of annual fecundity. We monitored all nests of a total of 42 pairs over 5 years in which rainfall varied substantially. We then related clutch size, nest survival, onset and length of the breeding season, re‐nesting probability and re‐nesting interval to pre‐breeding season rainfall using generalized linear mixed models that accounted for random variation across sites and individual pairs, and incorporated other variables known to affect the response. Higher pre‐breeding season rainfall led to an increase in clutch size and a decrease in re‐nesting interval, but nest survival, re‐nesting probability and length of the breeding season were not affected by variation in rainfall. The onset of the breeding season was delayed in very dry years. We conclude that higher rainfall is likely to increase food availability and thus body condition of female Montserrat Orioles, leading to an increase in fecundity due to larger clutch sizes.     

Directional selection on body size but no apparent survival cost to being large in wild New Zealand giraffe weevils

When an individual's reproductive success relies on winning fights to secure mating opportunities, bearing larger weapons is advantageous. However, sexual selection can be extremely complex, and over an animal's life the opportunity to mate is influenced by numerous factors. We studied a wild population of giraffe weevils (Lasiorhynchus barbicornis) that exhibit enormous intra and intersexual size variation. Males bear an elongated rostrum used as a weapon in fights for mating opportunities. However, small males also employ sneaking behavior as an alternative reproductive tactic. We investigated sexual selection on size by tracking individual males and females daily over two 30‐day periods to measure long‐term mating success. We also assessed how survival and recapture probabilities vary with sex and size to determine whether there might be a survival cost associated with size. We found evidence for directional selection on size through higher mating success, but no apparent survival trade‐off. Instead, larger individuals mate more often and have a higher survival probability, suggesting an accumulation of benefits to bigger individuals. Furthermore, we found evidence of size assortative mating where males appear to selectively mate with bigger females. Larger and more competitive males secure matings with larger females more frequently than smaller males, which may further increase their fitness.     

Adaptive significance of environmental sex determination in an amphipod

Environmental sex determination (ESD) permits adaptive sex choice under patchy environmental conditions, where the environment affects sex-specific fitness and where offspring can predict their likely adult status by monitoring an appropriate environmental cue. For Gammarus duebeni, an amphipod with ESD, it has been proposed that this flexible sex determination system is adaptive because males gain more from large size. Under ESD, young which are born earlier in the season become mostly males and, experiencing longer to grow, are therefore larger at breeding than females which are born later in the season. In order to test the hypothesis that ESD is adaptive for this species we investigated the relationship between size and fitness for both males and females, in a population of G. duebeni known to have ESD. We measured size related pairing success and fecundity, and used these two measures to calculate the relative fitness gains achieved through an increase in size for either sex. The fitness of both males and females increased with size, but males gained more from an increase in size than did females, throughout the breeding season. The data support the adaptive explanation for the evolution and maintenance of ESD in this species.     

Cold tolerance,and not earlier arrival on breeding grounds,explains why males winter further north in an Arctic‐breeding songbird

Sex biases in distributions of migratory birds during the non‐breeding season are widespread; however, the proximate mechanisms contributing to broad‐scale sex‐ratio variation are not well understood. We analyzed a long‐term winter‐banding dataset in combination with spring migration data from individuals tracked by using geolocators to test three hypotheses for observed variation in sex‐ratios in wintering flocks of snow buntings Plectrophenax nivalis. We quantified relevant weather conditions in winter (temperature, snowfall and snow depth) at each banding site each year and measured body size and condition (fat scores) of individual birds (n > 5500). We also directly measured spring migration distance for 17 individuals by using light‐level geolocators. If the distribution pattern of birds in winter is related to interactions between individual body size and thermoregulation, then larger bodied birds (males) should be found in colder sites (body size hypothesis). Males may also winter closer to breeding grounds to reduce migration distance for early arrival at breeding sites (arrival timing hypothesis). Finally, males may be socially dominant over females, and thus exclude females from high‐quality wintering sites (social dominance hypothesis). We found support for the body size hypothesis, in that colder and snowier weather predicted both larger body size and higher proportions of males banded. Direct tracking revealed that males did not winter significantly closer to their breeding site, despite being slightly further north on average than females from the same breeding population. We found some evidence for social dominance, in that females tended to carry more fat than males, potentially indicating lower habitat quality for females. Global climatic warming may reduce temperature constraints on females and smaller‐bodied males, resulting in broad‐scale changes in distributional patterns. Whether this has repercussions for individual fitness, and therefore population demography, is an important area of future research.     

Sexual dimorphism and allometry in two seed beetles (Coleoptera: Bruchidae)

Male Callosobruchus chinensis (Coleoptera: Bruchidae) have elaborated, pectinate antennae, which are absent from conspecific females and both sexes of a congener, Callosobruchus maculatus. To begin to unravel the mechanisms producing this striking dimorphism, we examined which morphological traits best explain body size variation in bruchid beetles and quantified sexual dimorphism of antenna size through allometric analyses. Using principal component analyses, we found that elytron length and pronotum width were significantly correlated with the first principal component, which was interpreted as explaining variation in body size. Regressions of log‐transformed body size measures on log‐transformed antenna length revealed that males of both species had longer antennae than conspecific females for any given body size, although most of this effect was attributable to higher intercepts, rather than increased allometry, in males. Comparisons among heterospecific males revealed that C. maculatus males have noticeably longer antennae than C. chinensis males at large body sizes. Callosobruchus chinensis males, thus, appear to have increased the receptive area of their antennae by adding to the width of, rather than further elongating, their antennae. Finally, we found evidence for positive allometry between log‐transformed antenna length and log‐transformed antenna width in C. chinensis males. We discuss our results in the context of evidence supporting the presence of an additional, and potentially unique, sex pheromone in C. chinensis females.