Chemical stimuli and species recognition in Liolaemus lizards

Chemical signaling glands are unlinked to species diversification in lizards

Sexual selection has long been thought to increase species diversification. Sexually selected traits, such as sexual signals that contribute to reproductive isolation, were thought to promote diversification. However, studies exploring links between sexually selected traits and species diversification have thus far primarily focused on visual or acoustic signals. Many animals often employ chemical signals (i.e., pheromones) for sexual communications, but large-scale analyses on the role of chemical communications in driving species diversification have been missing. Here, for the first time, we investigate whether traits associated with chemical communications-the presence of follicular epidermal glands-promote diversification across 6,672 lizard species. In most analyses, we found no strong association between the presence of follicular epidermal glands and species diversification rates, either across all lizard species or at lower phylogenetic scales. Previous studies suggest that follicular gland secretions act as species recognition signals that prevent hybridization during speciation in lizards. However, we show that geographic range overlap was no different in sibling species pairs with and without follicular epidermal glands. Together, these results imply that either follicular epidermal glands do not primarily function in sexual communications or sexually selected traits in general (here chemical communication) have a limited effect on species diversification. In our additional analysis accounting for sex-specific differences in glands, we again found no detectable effect of follicular epidermal glands on species diversification rates. Thus, our study challenges the general role of sexually selected traits in broad-scale species diversification patterns.

Response of the weeping lizard to distress calls: the effect of witnessing predation

Escaping from predation saves life, but costs energy and time. The "threat-sensitive predator-avoidance" hypothesis proposes that prey may modulate their antipredator responses, and thus the associated costs, in accordance with the magnitude of predation risk. This process requires that prey accurately assess this risk by decoding available information from various sources. For example, distress calls are uttered by prey when a predator traps them and can serve as public information on predation risk. Such is the case for the weeping lizard whose distress calls trigger immobility in conspecifics. Here, we tested whether this antipredator response of the weeping lizard is modulated by witnessing predation. We exposed lizards to distress calls alone or paired with models of a prey (conspecific), a predator (snake), or a predatory event (a snake subjugating the conspecific). Data show that the sole presence of the predator or prey paired with distress calls seems not to modulate the antipredator responses. Contrarily, witnessing a predatory event associated with calls intensified antipredator responses; lizards reduced their activity for longer and avoided proximity to the stimuli, which may decrease predation risk by reducing the likelihood of being detected by the predator. We conclude that the weeping lizard can use multisensorial public information to assess predation risk and modulate its antipredator responses.

Composition of a chemical signalling trait varies with phylogeny and precipitation across an Australian lizard radiation

The environment presents challenges to the transmission and detection of animal signalling systems, resulting in selective pressures that can drive signal divergence amongst populations in disparate environments. For chemical signals, climate is a potentially important selective force because factors such as temperature and moisture influence the persistence and detection of chemicals. We investigated an Australian lizard radiation (Heteronotia) to explore relationships between a sexually dimorphic chemical signalling trait (epidermal pore secretions) and two key climate variables: temperature and precipitation. We reconstructed the phylogeny of Heteronotia with exon capture phylogenomics, estimated phylogenetic signal in amongst-lineage chemical variation and assessed how chemical composition relates to temperature and precipitation using multivariate phylogenetic regressions. High estimates of phylogenetic signal indicate that the composition of epidermal pore secretions varies amongst lineages in a manner consistent with Brownian motion, although there are deviations to this, with stark divergences coinciding with two phylogenetic splits. Accounting for phylogenetic non-independence, we found that amongst-lineage chemical variation is associated with geographic variation in precipitation but not temperature. This contrasts somewhat with previous lizard studies, which have generally found an association between temperature and chemical composition. Our results suggest that geographic variation in precipitation can affect the evolution of chemical signalling traits, possibly influencing patterns of divergence amongst lineages and species.

Evolutionary and biogeographical support for species-specific proteins in lizard chemical signals

The species-specific components of animal signals can facilitate species recognition and reduce the risks of mismatching and interbreeding. Nonetheless, empirical evidence for species-specific components in chemical signals is scarce and mostly limited to insect pheromones. Based on the proteinaceous femoral gland secretions of 36 lizard species (Lacertidae), we examine the species-specific component potential of proteins in lizard chemical signals. By quantitative comparison of the one-dimensional electrophoretic patterns of the protein fraction from femoral gland secretions, we first reveal that the protein composition is species specific, accounting for a large part of the observed raw variation and allowing us to discriminate species on this basis. Secondly, we find increased protein pattern divergence in sympatric, closely related species. Thirdly, lizard protein profiles show a low phylogenetic signal, a recent and steep increase in relative disparity and a high rate of evolutionary change compared with non-specifically signal traits (i.e. body size and shape). Together, these findings provide support for the species specificity of proteins in the chemical signals of a vertebrate lineage.

An integrative approach to address species limits in the southernmost members of the Liolaemus kingii group (Squamata: Liolaemini)

Recent conceptual and methodological advances have enabled an increasing number of studies to address the problem of species delimitation in a comprehensive manner. This is of particular interest in cases of species whose divergence times are recent and/or effective population sizes are large, where the conclusions obtained from a single source of evidence may lead to erroneous estimations of true species numbers or incorrect assignment of individuals to species. Iguanian lizards of the Liolaemus kingii group (13 species) comprise an important component of the endemic fauna of Patagonia. The southernmost species of this group (namely L. baguali, L. escarchadosi, L. sarmientoi, and L. tari) show widely overlapping distributions across southern Patagonia, also, their phylogenetic relationships are ambiguous and species boundaries have not been explicitly tested. Here we use a comprehensive approach to assess species limits through the use of molecular and morphological information (mitochondrial cytb, nuclear sequences collected by ddRADseq, and linear, meristic and landmark-based morphometrics). We found support for the current taxonomy given that the different analyses recognized the nominal species (4 entities), also a candidate species was supported by mitochondrial and morphological data. In addition, we detected signs of admixture between some of the species. Our results indicate that the L. kingii group can serve as a model system in studies of diversification accompanied by hybridization in nature, which in turn might have been promoted by past climatic oscillations and generalist morphologies. We emphasize the importance of using multiple lines of evidence in order to solve evolutionary stories, and minimizing potential erroneous results that may arise when relying on a single source of information.

Does Liolaemus lemniscatus eavesdrop on the distress calls of the sympatric weeping lizard?

For a prey, its best ticket to stay alive is to get early and accurate information on predation risk and so, escape from predation at low cost. Some prey species have evolved the ability to eavesdrop signals intended for others, which contain information on predation risk. This is the case for the vocalizations produced by prey species when interacting with predators. Although primarily studied in birds and mammals, eavesdropping on vocal signals has been recorded in some lizard species. Here, we explored whether the lizard Liolaemus lemniscatus eavesdrops on the distress calls of its sympatric species, the Weeping lizard (L. chiliensis). Individuals of the Weeping lizard respond to these calls by displaying antipredator behaviours (i.e., reduced movement), and individuals of L. lemniscatus may potentially display similar defences if they decode the information contained in these calls. Our playback experiments showed that individuals of L. lemniscatus responded to the sound stimuli (distress calls and white noise), reducing their activity, but they did not discriminate between these two stimuli, suggesting that L. lemniscatus does not eavesdrop on the distress calls of its sympatric lizard species. We discuss some hypotheses to explain the lack of eavesdropping by L. lemniscatus on the Weeping lizard distress calls.

Hemipenes eversion behavior: a new form of communication in two Liolaemus lizards (Iguania: Liolaemidae)

Males of several animals have intromittent organs and may use these in a communicative context during sexual or intrasexual interactions. In some lizards, hemipenes eversion behavior have been observed, and the aim of this study is to find out whether this behavior is functionally significant under a communicative approach. Here, we investigated the eversion of hemipenes in the Light Blue Lizard (Liolaemus coeruleus Cei and Ortiz-Zapata, 1983) and in the Valley Lizard (Liolaemus quilmes Etheridge, 1993) by filming the response of male focal lizards in different experimental settings: (i) an agonistic context, i.e., with a conspecific male, (ii) a sexual context, i.e., with a conspecific female, and (iii) a control treatment, i.e., without a treatment lizard. In both species, focal lizards showed this behavior only in an agonistic context, with interspecific differences as follows. Liolaemus coeruleus has longer times until eversion and dragging of hemipenes; however, it has shorter time of eversion and exposition of the hemipenes. Liolaemus quilmes has the opposite pattern compared with L. coeruleus. These indicate that eversion of the hemipenes can act as a visual display and as a signal of aggressive behavior towards conspecific rival males. The present study offers a new behavioral perspective on the use of masculine genitalia in lizards.

The Macroecology of Chemical Communication in Lizards: Do Climatic Factors Drive the Evolution of Signalling Glands?

Chemical communication plays a pivotal role in shaping sexual and ecological interactions among animals. In lizards, fundamental mechanisms of sexual selection such as female mate choice have rarely been shown to be influenced by quantitative phenotypic traits (e.g., ornaments), while chemical signals have been found to potentially influence multiple forms of sexual and social interactions, including mate choice and territoriality. Chemical signals in lizards are secreted by glands primarily located on the edge of the cloacae (precloacal glands, PG) and thighs (femoral glands), and whose interspecific and interclade number ranges from 0 to > 100. However, elucidating the factors underlying the evolution of such remarkable variation remains an elusive endeavour. Competing hypotheses suggest a dominant role for phylogenetic conservatism (i.e., species within clades share similar numbers of glands) or for natural selection (i.e., their adaptive diversification results in deviating numbers of glands from ancestors). Using the prolific Liolaemus lizard radiation from South America (where PG vary from 0 to 14), we present one of the largest-scale tests of both hypotheses to date. Based on climatic and phylogenetic modelling, we show a clear role for both phylogenetic inertia and adaptation underlying gland variation: (i) solar radiation, net primary productivity, topographic heterogeneity and precipitation range have a significant effect on PG variation, (ii) humid and cold environments tend to concentrate species with a higher number of glands, (iii) there is a strong phylogenetic signal that tends to conserve the number of PG within clades. Collectively, our study confirms that the inertia of niche conservatism can be broken down by the need of species facing different selection regimes to adjust their glands to suit the demands of their specific environments.

Comparing the antipredator behaviour of two sympatric, but not syntopic, Liolaemus lizards

The microhabitat preferences of prey animals can modulate how they perceive predation risk, and therefore, their antipredator behaviour. We tested under standardized conditions how microhabitat preferences of two Liolaemus lizards affected their responses when confronted with two types of ambush predators (raptor vs. snake), under two levels of predation risk (low vs. high). These lizard species are sympatric, but not syntopic; L. chiliensis basks on bushes, a complex microhabitat that may provide protection against visual predators, while L. nitidus prefers open microhabitats, basking on the top of large bare rocks, highly exposed to visual predators. If microhabitat complexity modulates the antipredator response, L. chiliensis may perceive lower predation risk, exhibiting lower intensity of antipredator responses than L. nitidus. Both species reduced their activity after being exposed to both predators, but lizards differed in the assessment of predation risk; L. nitidus reduced its activity independently of the predation risk experienced, while L. chiliensis only reduced its activity in the high-risk condition. The microhabitat preferences shaped during the evolution of these species seem to modulate their perception of predation risk, which may cause interspecific differences in the associated costs of their antipredator responses.

Headbob displays signal sex, social context and species identity in a Liolaemus lizard

Animal communication has a key role in animals and identifying the signals’ function is crucial. Most lizards communicate with each other through visual signals with headbob displays, which are up-and-down movements of the head or the anterior part of the body. In the present work, I described and analysed the headbob displays of Liolaemus pacha lizards in their natural habitat. Specifically, the objectives were to describe the form of headbobs, to analyse their structure and to compare between sexes and social contexts. Adult lizards were video-recorded, registering the sex and the social context, classified as broadcast, same-sex and female-male interactions. The form and structure of sequences and headbobs were obtained. To evaluate the effect of sex and social context on the structure of headbob sequences and on headbob bouts, generalized linear mixed models were made. Intersexual differences were found in headbob display frequency and in the structure of headbob sequences. Lizards in same-sex context made sequences with more bouts, shorter intervals, headbob bouts of longer duration and higher amplitude than broadcast and female-male context. Presence of concurring behaviour such as lateral compression, gular expansion, and back arching occurred simultaneously with headbobs in same-sex context. Liolaemus pacha made four different headbob bout forms, and males were characterised by using bouts A and B, whereas females used bouts D more frequent. Sex and social context influenced only the structure of bouts A and B. The results showed that bouts A and B might be multi-component signals and non-redundant.

Macroevolutionary diversification of glands for chemical communication in squamate reptiles

Chemical communication plays a central role in social, sexual and ecological interactions among animals. However, the macroevolutionary diversification of traits responsible for chemical signaling remains fundamentally unknown. Most research investigating evolutionary diversification of glands responsible for the production of chemical signals has focused on arthropods, while its study among vertebrates remains neglected. Using a global-scale dataset covering > 80% (7,904 species) of the living diversity of lizards and snakes (squamates), we investigate rates, trajectories and phylogenetic patterns of diversification of their follicular glands for chemical communication. We observed these glands in 13.66% of species, that their expression has varying phylogenetic signal among lineages, and that the crown squamate ancestor lacked follicular glands, which therefore originated and diversified subsequently during their evolutionary history. Additionally, our findings challenge the longstanding view that within squamates the Iguania are visually oriented while Scleroglossa are chemically-oriented, given that Iguania doubles Scleroglossa in the frequency of glands. Our phylogenetic analyses identified stabilizing selection as the best model describing follicular gland diversification, and revealed high rates of disparity. We provide the first global-scale analysis investigating the diversification of one of the main forms of communication among reptiles, presenting a macroevolutionary angle to questions traditionally explored at microevolutionary scale.

Testing the functionality of precloacal secretions from both sexes in the South American lizard, Liolaemus chiliensis

The behavior of lizards can be highly influenced by chemical senses. The most studied pheromonal sources in lizards has been the femoral and precloacal gland secretions, although studies have been focused on male secretions, probably because these glands are usually only present in males or are poorly developed in females when they are present. Here, we aimed to study inLiolaemuschiliensis, one of the fewLiolaemusspecies in which females have precloacal glands, if female precloacal secretions convey information. We recorded the response of both sexes to secretions from females and males, as well as to control (solvent). The lizards started to explore the secretions sooner than the control. Both sexes moved more when exposed to female secretions than to the control, and males, but not females, explored female secretions more than the other scents. These results suggest that volatile compounds of the secretions allow lizards to recognize the presence of conspecifics, and, at least for males, these trigger the exploration of non-volatile compounds of the secretions that may reveal the sex of the individual that deposited them. This is the first study that explores the response to female precloacal secretions inLiolaemus, and data indicate that the female secretions ofL. chiliensiscontain relevant information for social interactions.

Condition-dependent chemosignals in reproductive behavior of lizards

This article is part of a Special Issue "Chemosignals and Reproduction". Many lizards have diverse glands that produce chemosignals used in intraspecific communication and that can have reproductive consequences. For example, information in chemosignals of male lizards can be used in intrasexual competition to identify and assess the fighting potential or dominance status of rival males either indirectly through territorial scent-marks or during agonistic encounters. Moreover, females of several lizard species "prefer" to establish or spend more time on areas scent-marked by males with compounds signaling a better health or body condition or a higher genetic compatibility, which can have consequences for their mating success and inter-sexual selection processes. We review here recent studies that suggest that the information content of chemosignals of lizards may be reliable because several physiological and endocrine processes would regulate the proportions of chemical compounds available for gland secretions. Because chemosignals are produced by the organism or come from the diet, they should reflect physiological changes, such as different hormonal levels (e.g. testosterone or corticosterone) or different health states (e.g. parasitic infections, immune response), and reflect the quality of the diet of an individual. More importantly, some compounds that may function as chemosignals also have other important functions in the organism (e.g. as antioxidants or regulating the immune system), so there could be trade-offs between allocating these compounds to attending physiological needs or to produce costly sexual "chemical ornaments". All these factors may contribute to maintain chemosignals as condition-dependent sexual signals, which can inform conspecifics on the characteristics and state of the sender and allow making behavioral decisions with reproductive consequences. To understand the evolution of chemical secretions of lizards as sexual signals and their relevance in reproduction, future studies should examine what information the signals are carrying, the physiological processes that can maintain the reliability of the message and how diverse behavioral responses to chemosignals may influence reproductive success.

Evolution and role of the follicular epidermal gland system in non-ophidian squamates

Many lizard and amphisbaenian lineages possess follicular glands in the dermis of the inner thighs and/or the area anterior to the cloaca. These tubular glands produce a holocrine secretion that finds its way to the external world through pore-bearing scales (femoral and/or preanal pores). Secretions are composed of proteins and many lipophilic compounds that may function as chemosignals in lizard and amphisbaenian communication. In recent years, we have begun to develop an understanding of the adaptive significance of these secretions, and they are currently thought to play an important role in a variety of processes in these animals. While it appears that epidermal gland secretions function in intra- and interspecific recognition and territoriality, research has focused largely on their role in mate assessment. Despite these recent studies, our knowledge on the true role of the chemicals found in epidermal secretions remains poorly studied, and there are many possible avenues for future research on this topic. Here, we review the literature on the follicular epidermal glands of non-ophidian squamates and provide a first taxon-wide overview of their distribution.

Arginine vasotocin, steroid hormones and social behavior in the green anole lizard (Anolis carolinensis)

Arginine vasotocin (AVT) is a potent regulator of social behavior in many species, but little is known about its role in reptilian behavior. Here we examine the effect of exogenous AVT on aggressive responding and courtship behavior in the green anole lizard (Anolis carolinensis). Aggressive behavior was stimulated in two ways: (1) mirror presentation (no relative status formed) and (2) size-matched pairs (where a social status is achieved). To elicit courtship behavior, a novel female was introduced into the home cage of a male. Regardless of the behavior condition, male anoles were injected i.p. with either reptile Ringer solution (vehicle) or AVT prior to testing. Animals treated with AVT performed fewer aggressive display bouts during mirror presentation but AVT treatment did not affect the overall number of aggressive display bouts within size-matched pairs. Male courtship behavior was not affected by AVT; however, untreated females displayed more frequently when paired with an AVT-treated male than a vehicle-injected control, suggesting that AVT-treated males were more attractive to females. Regardless of behavior condition, AVT injections led to increases in circulating corticosterone. Overall, we found that AVT tended to reduce aggressive behavior as has been reported for other territorial species. AVT did not perceptibly alter male courtship but did increase the display behavior of untreated females paired with treated males. Our study supports a role for AVT in the regulation of reptile social behavior.

Social behavior, chemical communication, and adult neurogenesis: studies of scent mark function in Podarcis wall lizards

Lacertid lizards have been hailed as a model system for the study of reptilian chemical communication. However, results obtained with the genus Podarcis, a diverse group of wall lizards with complex systematics, challenge emerging paradigms and caution against hasty generalizations. Here we review the available evidence on the role of chemical stimuli in male-female and male-male interactions in Iberian Podarcis. Males of several species can discriminate between chemicals left on substrates by females of their own or a different species, suggesting that differences in female chemical cues may underlie species recognition in this group. Females, on the other hand, do not respond differentially to conspecific and congeneric male scent marks. Males of Podarcis liolepis use scent marks to recognize rivals individually, evaluate their competitive ability (i.e., body size), and assess the threat posed by each individual rival neighbor. In contrast, females do not exhibit a preference for territories scent marked by larger (i.e., more competitive) males, which suggests a limited role for male scent marks in pre-copulatory mate choice. This behavioral sex difference is consistent with detailed neuro-ethological evidence showing that chemosensory brain areas in P. liolepis are sexually dimorphic. The accessory olfactory bulbs are larger (both in absolute and relative terms) in males than in females, probably as a result of sex-specific rates of adult neurogenesis. In both sexes, cell proliferation undergoes seasonal cycles that may have evolved to satisfy increased chemosensory demands at particular times of the year. Overall, and against recent generalizations, these results suggest that male scent marks have been shaped mainly by strong intrasexual selection.