Understanding how divergent mating strategies influence reproductive investment is crucial for elucidating the evolution of sexual traits. Here, we compare postcopulatory traits in two sympatric solifuge species with contrasting mating tactics: the coercive Oltacola chacoensis and the persuasive Titanopuga salinarum. We examined deferent duct morphology, sperm quantity, sperm quality, and their relationships with male body size. Oltacola chacoensis exhibited larger deferent ducts than T. salinarum. In both species, larger males presented more voluminous deferent ducts. Remarkably, both species stored comparable numbers of sperm and maintained consistently high sperm viability. Notably, T. salinarum produced seminal secretions positively associated with sperm number, whereas O. chacoensis lacked these secretions. These findings indicate alternative ejaculate allocation patterns aligned with distinct mating tactics: O. chacoensis might prioritize ejaculate volume and rapid transfer in coercive conditions, whereas T. salinarum invests in secretion-rich ejaculates, possibly associated with sperm performance during prolonged and persuasive courtship. The absence of differences in the number of sperm despite variation in deferent duct size suggests that ejaculate investment might vary in composition rather than quantity alone. This study provides new insights into the interplay between mating strategies and postcopulatory traits, reinforcing the relevance of solifuges as promising models for investigating reproductive diversification.

Seasonal reproductive dynamics and senescence have profound impacts on male fertility, yet these processes remain understudied in scorpions. This study investigates how reproductive parameters—such as testicular mass, sperm quantity, and viability—change over the course of the reproductive season in Urophonius achalensis males. We found that early-season males exhibited higher sperm quality and testicular mass compared to their older counterparts, suggesting that senescence, rather than reproductive effort, drives the decline in sperm viability. Interestingly, isolated males accumulated more sperm without a reduction in viability, highlighting potential mechanisms that preserve sperm quality despite senescence. Body condition also played a key role, with males in better condition showing higher sperm counts but facing a sharper decline in sperm viability as the season progressed, suggesting trade-offs between early reproductive success and later-life reproductive costs. Our findings offer novel insights into the reproductive strategies of scorpions, emphasizing the interplay between senescence, environmental stressors, and reproductive investment. This work provides new insights into the reproductive biology of scorpions, with broader implications for understanding the role of senescence and mating systems in shaping reproductive strategies.

Intraspecific sexual communication, crucial in reproductive interactions, often involves the interchange of signals like vibrations, visual signals, and chemical compounds like sex chemical cues. Within the arachnids, solifuges ("camel spiders") are an understudied group in terms of the detection of chemical compounds during mate searching and sexual interactions. In solifugids, the malleoli, chemoreceptor structures located on the ventral side of legs IV, are putative structures capable of capturing chemical signals. Our study focused on chemical male perception of female cues in Titanopuga salinarum (Ammotrechidae), examining associated behavioral modifications and the role of malleoli. We analyzed alterations in the motion pattern (activity pulse) and specific behaviors performed by males in association with female chemical stimuli. Using experimental arenas, we exposed males to female chemical cues stimuli similar to those available at the field. Stimuli comprised females (F) and cuticular extracts (CE), with corresponding controls. Males with intact malleoli (N = 26 with F, N = 20 with CE) were compared to those with malleoli removed (N = 21 with F, N = 17 with CE). We found partial evidence that males detect chemical cues of females deposited on the substrate. We observed no differences in the duration and number of the activity pulses spent in the zones with and without stimuli in any of the groups analyzed. However, the males exhibit significant changes in locomotion patterns in response to female chemical cues, suggesting a behavioral response to these stimuli. This finding suggests that the males of this species detect the female chemical cues and modify their behavior, and can quickly gather the necessary olfactory information where the stimulus is located. In addition, we found a possible condition-dependent regime for the detection of female cues by males, which would be expected from life-history characteristics of T. salinarum. Our findings prompt discussion from a sexual selection perspective, suggesting the importance of chemical communication in intraspecific interactions in this elusive but fascinating animal model.

Neofunctionalization of duplicated gene copies is thought to be an important process underlying the origin of evolutionary novelty and provides an elegant mechanism for the origin of new phenotypic traits. One putative case where a new gene copy has been linked to a novel morphological trait is the origin of the arachnid patella, a taxonomically restricted leg segment. In spiders, the origin of this segment has been linked to the origin of the paralog dachshund-2, suggesting that a new gene facilitated the expression of a new trait. However, various arachnid groups that possess patellae do not have a copy of dachshund-2, disfavoring the direct link between gene origin and trait origin. We investigated the developmental genetic basis for patellar patterning in the harvestman Phalangium opilio, which lacks dachshund-2. Here, we show that the harvestman patella is established by a novel expression domain of the transcription factor extradenticle. Leveraging this definition of patellar identity, we surveyed targeted groups across chelicerate phylogeny to assess when this trait evolved. We show that a patellar homolog is present in Pycnogonida (sea spiders) and various arachnid orders, suggesting a single origin of the patella in the ancestor of Chelicerata. A potential loss of the patella is observed in Ixodida. Our results suggest that the modification of an ancient gene, rather than the neofunctionalization of a new gene copy, underlies the origin of the patella. Broadly, this work underscores the value of comparative data and broad taxonomic sampling when testing hypotheses in evolutionary developmental biology.

Despite having >1200 described species and despite their nearly worldwide distribution and prevalence in many xeric ecosystems, relative to many other arachnid groups, we know little about the natural history and behaviour of animals in the order Solifugae (camel spiders, sun spiders, sun scorpions, etc.). Here, we review the current solifuge literature through the lens of conceptual research areas in ecology, evolution, and behaviour and propose ways in which solifuges can contribute to research in specific subfields, as follows: (i) ecology: community and trophic dynamics; connecting food webs; habitat specialization; and biodiversity and conservation; (ii) evolution: speciation and diversification; activity cycles and associated traits; adaptations for speed; and living in extreme environments; and (iii) behaviour and sensory systems: sleep, quiescence, and diapause; sensory systems and sensory ecology; learning and cognition; and mating systems, sexual selection, and sexual conflict. This resource can provide a starting point for identifying research programmes that will simultaneously contribute basic natural history information about this under-studied group and provide a broader understanding of fundamental concepts and theories across the life sciences. We hope that scientists will take this review as a challenge to develop creative ways of leveraging the unique features of solifuges to advance scientific knowledge and understanding.

The spermatozoa of scorpions are often bundled together, forming a type of sperm conjugation known as a sperm package. Sperm packages may be found inside the testes and seminal vesicles but vanish in the female atrium, leaving free spermatozoa. Previous studies, based on a limited number of taxa, suggested a diversity of sperm package morphology across the order Scorpiones C.L. Koch, 1850. However, the sperm packages of most scorpion taxa remained unknown. The present study provides the first systematic survey of sperm package morphology across the order, covering 89 exemplar species in 66 genera and 19 families representing all suprafamilial ranks, with a more detailed investigation of the family Bothriuridae Simon, 1880. Whereas all exemplar species of scorpions exhibit sperm packages, Buthida Soleglad and Fet, 2003, including Chaerilidae Pocock, 1893, and most Buthidae C.L. Koch, 1837, present unorganized sperm or loosely organized bundles. Although the details vary, three main types of sperm packages may be recognized in all other families: single folded; straight; and multiple folded. Subtypes may be identified according to general shape and folding patterns, mainly among sperm packages that are folded multiple times. Single-folded sperm packages are the most common type observed in the order. Although most sperm packages lack a covering, a conspicuous secretion sheath may be evident, e.g., in some Chactidae Pocock, 1893. Sperm packages vary in length from 112-354 mu m and bent sperm packages are not necessarily longer than straight sperm packages. Four exemplar species of Bothriuridae reveal that variation in sperm count within a single sperm package is consistent with the count derived in spermatogenesis. The diversity of sperm packages suggests a path from free spermatozoa, via bent sperm packages, to other forms. Sperm packages may aid in the transport, cooperation, competition, and survival of spermatozoa. The diverse morphology, function, and evolution of sperm packages merit further investigation.

Urbanization causes the loss of large amounts of habitat and produces significant changes in environmental conditions with consequences in the individual’s behavior, morphology, and physiology in natural populations. The urbanization can impact in the individual’s development instability (DI) that may be measured with the fluctuating asymmetry (FA) level making it an environmental quality bioindicator. In addition, the FA of each trait should be interpreted considering the trait functional importance, morphogenesis and history of selection pressures. In this paper, we evaluated the impact of two decades of increasing urbanization levels over a population of the pampean scorpion Bothriurus bonariensis. We analyze the FA level in somatic and genital traits in adult individuals of both sexes, collected every ~ 10 years (1997, 2008, 2018). Interestingly, in this study the somatic traits (pedipalp chelae) would be under directional selection and are shaped throughout the individual’s growth. In contrast, the genital traits (male’s hemispermatophore) are involved in the sperm transfer process and could be under stabilizing selection. Also, hemispermatophores are affected for less time by environmental stressors since they are formed once the individual has reached sexual maturity. Our results showed a significant increase in the FA levels in somatic traits of males and females, according to the increase of urbanization along years. In contrast, genital traits showed no changes in the level of FA in females, and only a few male genital traits presented an increase in the FA level. This work shows evidence that the measured scorpion traits (under different selective pressures and morphogenesis) might present differences in their susceptibility to environmental stress. We discuss these results in a context of DI of individuals caused by urbanization. We support the idea that the level of FA can be useful as an index to evaluate the effects of environmental stress on invertebrate populations. Furthermore, our paper presents the scorpions as a good model that could be used as an indicator group for studying the impact of the anthropogenic disturbance factors on natural ecosystems.

The exploration of new and diverse animal groups in the study of sexual selection is both necessary and important to help better understand broad patterns and test sexual selection hypotheses regarding the evolutionary origins and maintenance of reproductive tactics and associated traits. Solifuges are, in this matter, an exceptional group and very little explored from the sexual selection point of view. At first glance, mating is apparently quite simple and conserved within this arachnid order, but solifuge reproductive behavior is unique among arachnids and more diverse than previously thought. In particular, these voracious animals appear to exhibit high sexual conflict, as males need to avoid being eaten by their aggressive female partners and mating encounters in some species involve periods of apparently male-induced female inactivity during sperm transfer. The extent to which reproductive encounters are coercive versus collaborative, however, remains largely unknown. In this review, we begin with a historical perspective of sexual behavior research in solifuges. We then discuss precopulatory mating patterns, the role of the female and male during mating, sexual dimorphism, and the influence of sexual selection during different stages of mating. In addition, we explore cases of sexual cannibalism and provide an updated analysis of how postcopulatory sexual selection may be acting on these amazing arachnids. This review shows that there is much to be done in this extraordinary group of animals.