Estimation of sex assigned at birth using dental crown and cervical measurements in a modern global sample.

Despite developing prior to the appearance of secondary sexual characteristics of the skeleton, the permanent dentition exhibits sexual dimorphism. Therefore, teeth can serve as a means to estimate sex assigned at birth even in young individuals. This project takes a large global sample of maximum dimensions of the crown as well as measurements of the crown at the cervix to explore sexual dimorphism. Dimorphism is noted in teeth throughout the dental arcade, particularly in the canines. We provide sectioning points as well as the probability of correct classification (ranging from 50.9% to 81.3%) for each measurement to aid the practitioner in sex estimation from the dentition. This research provides a method to estimate sex without arbitrary population specifications. We argue for a global approach that incorporates more population variation to remove the need to estimate "ancestry," (which in actuality is translated to a social race category) and therefore does not force sexual dimorphism-related variation into these mutable and ambiguous categories. Further, this paper demonstrates the utility of the dentition as an additional indicator to aid with the estimation of sex assigned at birth in forensic anthropology. The goal of this research is to better understand the expression of sexual dimorphism across the skeleton in a global context.

Frontal sinus size in South African Later Stone Age Holocene Khoe-San.

Frontal size variation is comparatively poorly sampled among sub-Saharan African populations. This study assessed frontal sinus size in a sample of Khoe-San skeletal remains from South African Later Stone Age contexts. Volumes were determined from CT scans of 102 adult crania; individual sex could be estimated in 82 cases. Sinus volume is not sexually dimorphic in this sample. The lack of frontal sinus aplasia is concordant with the low incidences recorded for other sub-Saharan African and most other global populations save those that inhabit high latitudes. There is considerable variation in frontal sinus size among global populations, and the Khoe-San possess among the smallest. The Khoe-San have rather diminutive sinuses compared to sub-Saharan Bantu-speaking populations but resemble a northern African (Sudanese) population. Genetic studies indicate the earliest population divergence within Homo sapiens to have been between the Khoe-San and all other living groups, and that this likely occurred in Africa during the span of Marine Isotope Stages 8-6. There is scant information on frontal sinus development among Late Quaternary African fossils that are likely either closely related or attributable to Homo sapiens. Among these, the MIS 3 cranium from Hofmeyr, South Africa, exhibits distinct Khoe-San cranial affinities and despite its large size has a very small frontal sinus. This raises the possibility that the small frontal sinuses of the Holocene South African Khoe-San might be a feature retained from an earlier MIS 3 population.

Differential growth rate, water use efficiency and climate sensitivity between males and females of Ilex aquifolium in north-western Spain.

BACKGROUND AND AIMS: Dioecious plant species, i.e., those in which male and female functions are housed in different individuals, are particularly vulnerable to global environmental changes. For long-lived plant species, such as trees, long-term studies are imperative to understand how growth patterns and their sensitivity to climate variability differentially affect the sexes. METHODS: Here, we explore long-term intersexual differences in wood traits, namely radial growth rates, water use efficiency quantified as stable carbon isotope abundance of wood cellulose, and their climate sensitivity in Ilex aquifolium trees growing in a natural population in NW Spain. KEY RESULTS: We found that sex differences in secondary growth rates were variable over time, with males outperforming females in both radial growth rates and water use efficiency in recent decades. Summer water stress significantly reduced the growth of female trees in the following growing season, while the growth of male trees was primarily favoured by cloudy and rainy conditions the previous fall and winter combined with low cloud cover and warm conditions in summer. Sex-dependent lagged correlations between radial growth and water availability were found, with a strong association between tree growth and cumulative water availability in females at 30 months and in males at 10 months. CONCLUSIONS: Overall, our results point to greater vulnerability of female tress to increasing drought, which could lead to sex-ratio biases threatening population viability in the future.

Integrating microbial 16S rRNA sequencing and non-targeted metabolomics to reveal sexual dimorphism of the chicken cecal microbiome and serum metabolome.

Background: The gut microbiome plays a key role in the formation of livestock and poultry traits via serum metabolites, and empirical evidence has indicated these traits are sex-linked. Methods: We examined 106 chickens (54 male chickens and 52 female chickens) and analyzed cecal content samples and serum samples by 16S rRNA gene sequencing and non-targeted metabolomics, respectively. Results: The cecal microbiome of female chickens was more stable and more complex than that of the male chickens. Lactobacillus and Family XIII UCG-001 were enriched in male chickens, while Eubacterium_nodatum_group, Blautia, unclassified_Anaerovoraceae, Romboutsia, Lachnoclostridium, and norank_Muribaculaceae were enriched in female chickens. Thirty-seven differential metabolites were identified in positive mode and 13 in negative mode, showing sex differences. Sphingomyelin metabolites possessed the strongest association with cecal microbes, while 11ß-hydroxytestosterone showed a negative correlation with Blautia. Conclusion: These results support the role of sexual dimorphism of the cecal microbiome and metabolome and implicate specific gender factors associated with production performance in chickens.

Growth and allometric curves of southern brown howler monkey (Alouatta guariba): Insights on its ontogeny and conservation.

Ontogenetic sexual dimorphism is observed in different primate species, with ecological and evolutionary relationships explaining this pattern. Understanding the growth of the southern brown howler monkey elucidates not only the ecology and evolution but also contributes to conservation projects for this species. Throughout 20 years of the Centro de Pesquisas Biológicas de Indaial-Projeto Bugio, Brazil, we collected morphological data on 105 howlers of the Alouatta guariba species to identify the growth differences between ontogenetic categories and sexes and generate a growth curve to estimate the age of rescued individuals. Linear measurements were employed to obtain body length as well as the dimensions of the head and limbs. All individuals were also weighed to obtain body mass. We assessed growth rate and duration using allometric analysis based on the individuals' ages. We compared growth rate and duration among infant, juvenile, and adult howlers and between sexes. We provide growth curves for body size for both sexes using the Von Bertalanffy model. Infants have accelerated growth rate in comparison to the juveniles and adults, with no differences between sexes in establishing body length at this ontogenetic stage. Males have a prolonged development duration from the juvenile stage, reaching adulthood later than females, which explains the body length differences found in this species. Variables of head and limbs analyzed also showed differences in growth rate and duration, but not so consistently among ontogenetic stages. Mass was not a good variable to understand the growth differences of the animals, since many arrived feeble in the project and may have lost mass due to different circumstances in old age. Therefore, growth curves were obtained only for body length, allowing the estimation of the age of these animals when rescued from the wild to more effectively provide needed care in captivity.

RNA sequencing analysis of sexual dimorphism in Japanese quail.

Introduction: Japanese quail are of significant economic value, providing protein nutrition to humans through their reproductive activity; however, sexual dimorphism in this species remains relatively unexplored compared with other model species. Method: A total of 114 RNA sequencing datasets (18 and 96 samples for quail and chicken, respectively) were collected from existing studies to gain a comprehensive understanding of sexual dimorphism in quail. Cross-species integrated analyses were performed with transcriptome data from evolutionarily close chickens to identify sex-biased genes in the embryonic, adult brain, and gonadal tissues. Results: Our findings indicate that the expression patterns of genes involved in sex-determination mechanisms during embryonic development, as well as those of most sex-biased genes in the adult brain and gonads, are identical between quails and chickens. Similar to most birds with a ZW sex determination system, quails lacked global dosage compensation for the Z chromosome, resulting in directional outcomes that supported the hypothesis that sex is determined by the individual dosage of Z-chromosomal genes, including long non-coding RNAs located in the male hypermethylated region. Furthermore, genes, such as WNT4 and VIP, reversed their sex-biased patterns at different points in embryonic development and/or in different adult tissues, suggesting a potential hurdle in breeding and transgenic experiments involving avian sex-related traits. Discussion: The findings of this study are expected to enhance our understanding of sexual dimorphism in birds and subsequently facilitate insights into the field of breeding and transgenesis of sex-related traits that economically benefit humans.

From darkness to twilight: Morphological divergence between cave and surface-subterranean ecotone Niphargus species.

Subterranean and surface habitats are in stark contrast in several environmental factors. Therefore, adaptation to the subterranean environment typically impedes the (re)colonisation of surface habitats. The genus Niphargus includes amphipod crustaceans that primarily occupy subterranean habitats. All its species show typical adaptations to the subterranean environment. However, some Niphargus species occur in surface-subterranean ecotones. To understand whether (i) habitat-based phenotypic divergence is present between the cave and the ecotone species and (ii) similar phenotypes emerge independently in each ecotone, we studied morphological divergence between four cave and four ecotone Niphargus species based on 13 functional morphological traits. To account for different selection acting on the sexes, we included both males (N = 244) and females (N = 222). Nine out of 13 traits showed habitat-divergence. Traits related to feeding and crawling were shorter, while traits related to oxygenation were larger in ecotone species. Eleven out of 13 traits were sexually dimorphic. Traits related to oxygenation and crawling were larger in females, while the trait related to swimming was larger in males. We found that the extent of sexual dimorphism differs between the habitats in eight traits related to sensing, feeding, oxygenation and crawling. Additionally, we found that in certain traits related to sensing and oxygenation, habitat-related differences are only present in one sex, but not the other. We conclude that the detected differences between the cave and the ecotone species indicate divergent evolution, where similarities among the different species within habitat type indicate convergent evolution. The high degree of sexual dimorphism paired with differences in sexual dimorphism between the habitats in certain traits suggest that sexual and fecundity selections have comparable effects to environmental selection. Thus, studies of habitat-dependent adaptations investigating one sex only, or not considering sexual dimorphism, can lead to erroneous conclusions.

Evolutionary drivers of sex-specific parasite prevalence in wild birds.

Males and females often differ in ecology, behaviour and lifestyle, and these differences are expected to lead to sex differences in parasite susceptibility. However, neither the sex differences in parasite prevalence, nor their ecological and evolutionary drivers have been investigated across a broad range of taxa using phylogenetically corrected analyses. Using the most extensive dataset yet that includes 755 prevalence estimates from 151 wild bird species in a meta-analytic framework, here we compare sex differences in blood and gastrointestinal parasites. We show that despite sex differences in parasite infection being frequently reported in the literature, only Haemoproteus infections were more prevalent in females than in males. Notably, only seasonality was strongly associated with the sex-specific parasite prevalence of both Leucocytozoon and Haemoproteus, where birds showed greater female bias in prevalence during breeding periods compared to the non-breeding period. No other ecological or sexual selection variables were associated with sex-specific prevalence of parasite prevalence. We suggest that much of the variation in sex-biased prevalence could be idiosyncratic, and driven by local ecology and behavioural differences of the parasite and the host. Therefore, breeding ecology and sexual selection may only have a modest influence on sex-different parasite prevalence across wild birds.

Associations between sex, systemic iron and inflammatory status and subcortical brain iron.

Brain iron increases in several neurodegenerative diseases are associated with disease progression. However, the causes of increased brain iron remain unclear. This study investigates relationships between subcortical iron, systemic iron and inflammatory status. Brain magnetic resonance imaging (MRI) scans and blood plasma samples were collected from cognitively healthy females (n = 176, mean age = 61.4 ± 4.5 years, age range = 28-72 years) and males (n = 152, mean age = 62.0 ± 5.1 years, age range = 32-74 years). Regional brain iron was quantified using quantitative susceptibility mapping. To assess systemic iron, haematocrit, ferritin and soluble transferrin receptor were measured, and total body iron index was calculated. To assess systemic inflammation, C-reactive protein (CRP), neutrophil:lymphocyte ratio (NLR), macrophage colony-stimulating factor 1 (MCSF), interleukin 6 (IL6) and interleukin 1ß (IL1ß) were measured. We demonstrated that iron levels in the right hippocampus were higher in males compared with females, while iron in the right caudate was higher in females compared with males. There were no significant associations observed between subcortical iron levels and blood markers of iron and inflammatory status indicating that such blood measures are not markers of brain iron. These results suggest that brain iron may be regulated independently of blood iron and so directly targeting global iron change in the treatment of neurodegenerative disease may have differential impacts on blood and brain iron.

Sex-specific association of circulating Isthmin-1 with isolated post-challenge hyperglycemia.

Introduction: To explore the distribution of Isthmin-1 (ISM1) level and its association with isolated post-challenge hyperglycemia (IPH). Methods: A total of 522 participants without a history of diabetes were invited to attend a standard 75g 2-h oral glucose tolerance test (OGTT), and 71 subjects were further invited for a 3-h oral minimal model test. Insulin sensitivity and ß-cell function were evaluated using both HOMA and estimated from OGTT. Circulating ISM1 levels were determined by a commercially available ELISA kit. Results: A total of 76 (14.6%) participants were diagnosed as IPH, accounting for 61.3% of the newly diagnosed diabetes. ISM1 levels were significantly higher in men than in women (1.74 ng/mL versus 0.88 ng/mL). The inverse correlation between ISM1 and ß-cell function and IPH was only significant in men. After multivariate adjustment, per unit increment in ISM1 was associated with 0.68-fold (95% CI: 0.49-0.90) reduced odds ratio (OR) of IPH in men. Compared to men with the lowest ISM1 levels, the adjusted OR of IPH with the highest ISM1 levels decreased by 73% (95% CI: 0.11-0.61). Moreover, incorporation of ISM1 into the New Chinese Diabetes Risk Score (NCDRS) model yielded a substantial improvement in net reclassification improvement of 58% (95% CI: 27%-89%) and integrated discrimination improvement of 6.4% (95% CI: 2.7%-10.2%) for IPH. Conclusions: ISM1 was significantly and independently associated with IPH, and serves as a feasible biomarker for the early identification of men with high risk of IPH.

Assessment of Sexual Dimorphism Through Lateral Cephalogram in Children Aged 10-12 Years.

INTRODUCTION: Forensic dentistry integrates interdisciplinary scientific knowledge to produce accurate and reliable forensic statements. Anthropometry, essential since its introduction by Alphonse Bertillon in 1882, describes human body shapes and has significant forensic applications. This study focuses on sexual dimorphism, phenotypic differences between males and females, using lateral cephalometric measurements to determine sex in children aged 10-12 years from the Chengalpattu population. MATERIALS AND METHODS: The cross-sectional study included 80 participants (40 boys and 40 girls). Lateral cephalograms were analyzed using Ceph Ninja Pro software to obtain 15 cephalometric measurements. Statistical analysis using SPSS Software (Version 22.0) involved t-tests to identify significant differences (P<0.05). Discriminant function analysis assessed the predictive power of these variables. RESULTS: Seven variables showed significant differences between sexes. Discriminant models based on these variables determined sex with varying reliability. Ramus length was the most reliable predictor (81%), while maxillary length had the lowest reliability (62%). DISCUSSION: The study's findings align with existing literature, indicating the robustness of ramus length for sex determination. However, the low reliability of maxillary length contrasts with studies that found it useful for sex differentiation, suggesting variability across populations and age groups. Combining multiple cephalometric variables improved accuracy, consistent with previous research. CONCLUSION: Lateral cephalograms are effective for assessing sexual dimorphism in children. The study supports the forensic and clinical utility of cephalometric measurements and calls for further research with diverse populations and advanced imaging techniques to enhance method reliability and applicability.

Spotted Hyena skull size variation across geography favors the energetic equivalence rule over Bergmann's Rule.

Much historic work has focused on establishing geographical and ecological rules that broadly explain patterns in size variation. We examined geographic variation in Spotted Hyena skull size using geometric morphometrics and spatial statistics. We quantified size variation and sexual size dimorphism of the skull, and evaluated the influence of temperature, precipitation, land cover type, and population density on skull size. We found that female spotted hyenas are slightly larger on average than males. Our analysis of regional differences did not indicate geographic variation in sexual size dimorphism. Skull size of Spotted Hyenas varies with geography but does not adhere to Bergmann's Rule. The smallest individuals of both sexes occur between -5.00° and 10.00° latitude and east of 28.50° longitude, with larger individuals being found elsewhere. Although Spotted Hyena skull size co-varies in some views with such variables as habitat type and climate indicators, skull size in this species most strongly co-varies with population density. The highest population densities are associated with the smallest skull size, possibly reflecting a relationship between high population density and access to resources. These results suggest that geographic variation in Spotted Hyena skull size is better explained by the energetic equivalence rule than Bergmann's Rule.

Sex moderates the effect of anhedonia on parietal alpha asymmetry.

Anhedonia, a transdiagnostic symptom present in many neuropsychiatric disorders, differs in males and females. Parietal EEG alpha asymmetry is associated with reduced arousal and low positive emotionality, and is, therefore, a promising neurophysiologic biomarker of anhedonia. To date, however, no prior studies have determined whether this measure captures sex differences in anhedonic expression. This preliminary study (N = 36) investigated whether anhedonia severity is associated with EEG resting-state parietal alpha asymmetry in adults and whether sex moderates this relationship. Results showed that there was a significant moderating effect of sex such that, only for females, higher levels of anhedonia were associated with increased parietal alpha asymmetry. These findings suggest that parietal alpha asymmetry is a promising biomarker of anhedonia severity in female adults and reinforces the need to account for sex differences in future research.

Eco-geographic and sexual variation of the ribcage in Homo sapiens.

Up to now, Allen and Bergmann's rules have been studied in modern humans by analyzing differences in limb length, height, or body mass. However, there are no publications studying the effects of latitude in the 3D configuration of the ribcage. To assess this issue, we digitally reconstructed the ribcages of a balanced sample of 109 adult individuals of global distribution. Shape and size of the ribcage was quantified using geometric morphometrics. Our results show that the ribcage belonging to tropical individuals is smaller and slenderer compared to others living in higher latitudes, which is in line with Allen and Bergmann's rules and suggests an allometric relationship between size and shape. Although sexual dimorphism was observed in the whole sample, significant differences were only found in tropical populations. Our proposal is that, apart from potential sexual selection, avoiding heat loss might be the limiting factor for sexual dimorphism in cold-adapted populations.

The consequences of calcium: investigating intracortical reproductive signals in the American alligator for sex determination.

Identifying sex in extinct archosaurs has proven difficult due, in part, to low sample sizes, preservation biases, and methodology. While previous studies have largely focused on morphological traits, here we investigate intracortical signals of egg-shelling in extant alligators. Egg-shelling requires large mobilizations of calcium reserves. Aves utilize medullary tissue as a calcium reserve, whereas crocodylians mobilize calcium from cortical bone or osteoderms. If crocodylians derive calcium from bone cortices for egg-shelling, then egg-shelling events should be detectable in female crocodylian cortical bone. We examined mid-diaphyseal Alligator mississippiensis femoral bone cross-sections for signals of reproduction. Compaction and area of resorbed tissue were measured in femoral cross-sections from captive raised male (n = 10) and female (n = 29) A. mississippiensis of 26-27 years at age of death. This sample is more robust than previous studies, though reproductive history data is unknown. Femora from a small sample of wild caught male (n = 6) and female (n = 6) A. mississippiensis were also measured. Data were analyzed by pairwise t-tests between sex and captivity status. There was no significant difference in either compaction or resorbed tissue values between male and female alligators, regardless of habitat (wild or captive-raised). A reproductive signal was undetectable in this study and any quantifiable differences between sexes appears to be driven by size dimorphism. Cortical resorption rates in the femora of male and female alligators are reflective of normal aging processes and not indicative of egg-shelling during reproduction. Examination of younger alligators would clarify processes driving bone turnover during reproductively active years.

Radiomorphometric analysis of the metapodial bones in the Scottish fold cats.

Scottish Fold cats (Felis catus, Linnaeus 1758) are one of the most well-known and popular cat breeds in the world, characterized by their folded ears attached to the head. Very frequently, cats fall prey of different trauma and accidents that can cause bone fractures especially in the metapodial bones. The method of radiometry is used in veterinary practice to visualize and measure different parts of the animal skeleton. The aim of this study was to assess the linear parameters derived from radiographic images of the metacarpals and metatarsals in Scottish Fold cats and additionally detecting potential sexual dimorphism. Radiographic images of 24 adult Scottish Fold cats (12 male and 12 females) of different ages and weights were analysed. Six linear measurements of the metapodial bones were evaluated to investigate any differences between the sexes. The linear radiometric measurements of the five metacarpals (MC1-5) and the four metatarsals (MT2-5) bones were larger in male metapodial bones than that of female cats. The maximum length (Ml) of the MC1 and MC2 was statistically different between sex, respectively, (p = 0.001) and (p = 0.05). The others metacarpal bones were different in mostly all linear parameters but not statistically significant. The most significant differences between sexes were observed in the parameter of width proximal end (Wp) of MC1-3 (p = 0.001) and MC4 (p = 0.05). More statistical different was MT2 and less MT3. The linear parameter of Bd of the MT4 was the most different statistically between sex (p = 0.001). The results of the study will be useful in function of comparative anatomy, in veterinary clinical practice, in zoo archaeology and in the veterinary forensic investigation.

Temporal sex specific brain gene expression pattern during early rat embryonic development.

Background: The classical concept of brain sex differentiation suggests that steroid hormones released from the gonads program male and female brains differently. However, several studies indicate that steroid hormones are not the only determinant of brain sex differentiation and that genetic differences could also be involved. Methods: In this study, we have performed RNA sequencing of rat brains at embryonic days 12 (E12), E13, and E14. The aim was to identify differentially expressed genes between male and female rat brains during early development. Results: Analysis of genes expressed with the highest sex differences showed that Xist was highly expressed in females having XX genotype with an increasing expression over time. Analysis of genes expressed with the highest male expression identified three early genes, Sry2, Eif2s3y, and Ddx3y. Discussion: The observed sex-specific expression of genes at early development confirms that the rat brain is sexually dimorphic prior to gonadal action on the brain and identifies Sry2 and Eif2s3y as early genes contributing to male brain development.

A novel animal model of symptomatic neuroma for assessing neuropathic pain.

INTRODUCTION: Following amputation, peripheral nerves lack distal targets for regeneration, often resulting in symptomatic neuromas and debilitating neuropathic pain. Animal models can establish a practical method for symptomatic neuroma formation for better understanding of neuropathic pain pathophysiology through behavioral and histological assessments. We created a clinically translatable animal model of symptomatic neuroma to mimic neuropathic pain in patients and assess sexual differences in pain behaviors. METHODS: Twenty-two male and female rats were randomly assigned to one of two experimental groups: (1) neuroma surgery, or (2) sham surgery. For the neuroma experimental group, the tibial nerve was transected in the thigh, and the proximal segment was placed under the skin for mechanical testing at the site of neuroma. For the sham surgery, rats underwent tibial nerve isolation without transection. Behavioral testing consisted of neuroma-site pain, mechanical allodynia, cold allodynia, and thermal hyperalgesia at baseline, and then weekly over 8 weeks. RESULTS: Male and female neuroma rats demonstrated significantly higher neuroma-site pain response compared to sham groups starting at weeks 3 and 4, indicating symptomatic neuroma formation. Weekly assessment of mechanical and cold allodynia among neuroma groups showed a significant difference in pain behavior compared to sham groups (p < 0.001). Overall, males and females did not display significant differences in their pain responses. Histology revealed a characteristic neuroma bulb at week 8, including disorganized axons, fibrotic tissue, Schwann cell displacement, and immune cell infiltration. CONCLUSION: This novel animal model is a useful tool to investigate underlying mechanisms of neuroma formation and neuropathic pain.

The tiniest violin: the male of Loxoscelesvallenar (Araneae, Sicariidae).

In recent years, several endemic species of Loxosceles, violin spiders, have been described from the North-Central Chile biodiversity hotspot, some of which have ambiguous placement within the species groups of the genus. In a recent expedition to the Atacama region, we collected male specimens representing new records of two recently described species: Loxoscelesvicentei Taucare-Ríos, Brescovit & Villablanca, 2022 and Loxoscelesvallenar Brescovit, Taucare-Ríos, Magalhaes & Santos, 2017 (Araneae, Sicariidae). Males of the latter are hitherto unknown and are here described for the first time. Examination of the morphology of these species revealed characters such as an embolic keel and digitiform median receptacles, which suggest they do not belong in the laeta species group, but rather in the spadicea species group, which is briefly re-diagnosed. With carapace lengths smaller than 2 mm, the newly discovered males of L.vallenar are the tiniest members of the genus. In addition, males of this species bear strong macrosetae in the clypeus, a sexually dimorphic character not previously reported in Loxosceles.

Sexually dimorphic eye size in dragonfishes, a response to a bioluminescent signalling gap.

Deep-sea fishes must overcome extremely large nearest-neighbour distances and darkness to find mates. Sexual dimorphism in the size of luminescent structures in many deep-sea taxa, including dragonfishes (family Stomiidae), indicates reproductive behaviours may be mediated by visual signalling. This presents a paradox: if male photophores are larger, females may find males at shorter distances than males find females. Solutions to this gap may include females closing this gap or by males gathering more photons with a larger eye. We examine the eye size of two species of dragonfishes (Malacosteus niger and Phostomias guernei) for sexual dimorphism and employ a model of detection distance to evaluate the potential for such dimorphism to bridge the detection gap. This model incorporates the flux of sexually dimorphic postorbital photophores and eye lens size to predict detection distances. In both species, we found a significant visual detection gap in which females find males before males find females and that male lens size is larger, marking the second known case of size dimorphism in the actinopterygian visual system. Our results indicate the larger eye affords males a significant improvement in detection distance. We conclude that this dimorphic phenotype may have evolved to close the detection gap.