Reproduction and sexual dimorphism in the viviparous lizard Sceloporus palaciosi (Squamata: Phrynosomatidae) from the Trans‐Mexican Volcanic Belt,Mexico

In this study, sexual dimorphism, reproductive cycles, litter size and offspring size of a population of the little‐known species Sceloporus palaciosi in central Mexico were analysed. Significant male‐biased sexual size dimorphism was recorded in snout–vent length (SVL), head length, head width, forearm length and tibia length. Both sexes showed asynchronous reproductive cycles, and males reached sexual maturity at a smaller SVL (33 mm) than females (37 mm). Testes volumes were small from January to February, testicular recrudescence began from March to June, and decreased in July, but increased again in August and September, followed by a second decrease from October to December. In females, vitellogenesis began from May until ovulation in December. Embryonic development extended from November to March, and a small number of females carried embryos through July. Mean litter size was 4.0 and was positively correlated with female SVL. The length of the reproductive period in S. palaciosi recorded in this study is longer than that recorded for other populations in other parts of this species range. Further studies are needed to clarify reproductive cycles in the other isolated populations of S. palaciosi, and then extended to other species and chromosome races in the Sceloporus grammicus complex.     

Laboratory and field experiments identify sources of variation in phenotypes and survival of hatchling lizards

To understand the process of natural selection, relationships between phenotype and fitness and sources of phenotypic variation must be known. We examined the importance of incubation moisture conditions, maternal yolk investment, and clutch (genotype) to phenotypic variation in hatchlings of the lizard Sceloporus undulatus . Eggs were distributed among two moisture treatments and a third treatment in which yolk was removed. After hatching, mass, snout–vent length, tail length, body shape, thermal preference, running speed, desiccation rate, and growth rate were measured for each hatchling in the laboratory. Hatchlings were then released at a field site in order to monitor growth and survival under natural conditions. Hatchlings from the dry and yolk-removed treatments were significantly smaller than those from the wet treatment. However, neither performance nor survival were affected by moisture or yolk removal. All phenotypes were affected by clutch. Clutches that produced relatively large hatchlings had higher survival than clutches that produced relatively small hatchlings. Furthermore, clutches that produced relatively slow growing individuals and fast runners had higher survival rates than clutches that produced relatively rapid growing individuals and slow runners. Our results emphasize the overriding importance of clutch (genotype) to variation in phenotypes and survival in hatchling S. undulatus . © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 105–124.     

The influence of competitor density on space use in juvenile striped plateau lizards (Sceloporus virgatus)

To investigate the role of competitor density in influencing space use patterns of juvenile striped plateau lizards (Sceloporus virgatus), a density manipulation experiment was conducted within large (800 m²) field enclosures treated with low (20) and high (60) densities of hatchlings. Enclosures were monitored for 10 months, after which the experiment was replicated. Home range and core area sizes after release of lizards and initial establishment in the fall were significantly reduced in the high-density treatments; fall home range shape, measured as the perimeter-to-area ratio, was significantly reduced in low-density treatments; no significant differences were detected between treatments in core area shape or overlap. During the spring/early summer activity season after the lizard's first winter, no significant differences between density treatments were detected for any of these variables, as enclosure densities had converged between treatments. Individuals in high-density enclosures had increased their space use as competitor density had declined. These results illustrate that competitor density has significant influence on space use by juvenile lizards.     

Losing the trait without losing the signal: Evolutionary shifts in communicative colour signalling

Colour signalling traits are often lost over evolutionary time, perhaps because they increase vulnerability to visual predators or lose relevance in terms of sexual selection. Here, we used spectrometric and phylogenetic comparative analyses to ask whether four independent losses of a sexually selected blue patch are spectrally similar, and whether these losses equate to a decrease in conspicuousness or to loss of a signal. We found that patches were lost in two distinct ways: either increasing reflectance primarily at very long or at very short wavelengths, and that species with additional colour elements (UV, green and pink) may be evolutionary intermediates. In addition, we found that patch spectral profiles of all species were closely aligned with visual receptors in the receiver's retina. We found that loss of the blue patch makes males less conspicuous in terms of chromatic conspicuousness, but more conspicuous in terms of achromatic contrast, and that sexual dimorphism often persists regardless of patch loss. Dorsal surfaces were considerably more cryptic than were ventral surfaces, and species in which male bellies were the most similar in conspicuousness to their dorsal surfaces were also the most sexually dimorphic. These results emphasize the consistent importance of sexual selection and its flexible impact on different signal components through evolutionary time.     

Overcoming obstacles: the effect of obstacles on locomotor performance and behaviour

Sprinting and jumping ability are key performance measures that have been widely studied in vertebrates. The vast majority of these studies, however, use methodologies that lack an ecological context by failing to consider the complex habitats in which many animals live. Because successfully navigating obstacles within complex habitats is critical for predator escape, running, climbing, and/or jumping performance are each likely to be exposed to selection. In the present study, we quantify how behavioural strategies and locomotor performance change with increasing obstacle height. Obstacle size had a significant influence on behaviour (e.g. obstacle crossing strategy, intermittent locomotion) and performance (e.g. sprint speed, jump distance). Jump frequency and distance increased with obstacle size, suggesting that it likely evolved because it is more efficient (i.e. it reduces the time and distance required to reach a target position). Jump angle, jump velocity, and approach velocity accounted for 58% of the variation in jump distance on the large obstacle, and 33% on the small obstacle. Although these variables have been shown to significantly influence jump distance in static jumps, they do not appear to be influential in running (dynamic) jumps onto a small obstacle. Because selection operates in simple and complex habitats, future studies should consider quantifying additional measures such as jumping or climbing with respect to the evolution of locomotion performance. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

CONSERVED SEX CHROMOSOMES ACROSS ADAPTIVELY RADIATED ANOLIS LIZARDS

Vertebrates possess diverse sex‐determining systems, which differ in evolutionary stability among particular groups. It has been suggested that poikilotherms possess more frequent turnovers of sex chromosomes than homoiotherms, whose effective thermoregulation can prevent the emergence of the sex reversals induced by environmental temperature. Squamate reptiles used to be regarded as a group with an extensive variability in sex determination; however, we document how the rather old radiation of lizards from the genus Anolis, known for exceptional ecomorphological variability, was connected with stability in sex chromosomes. We found that 18 tested species, representing most of the phylogenetic diversity of the genus, share the gene content of their X chromosomes. Furthermore, we discovered homologous sex chromosomes in species of two genera (Sceloporus and Petrosaurus) from the family Phrynosomatidae, serving here as an outgroup to Anolis. We can conclude that the origin of sex chromosomes within iguanas largely predates the Anolis radiation and that the sex chromosomes of iguanas remained conserved for a significant part of their evolutionary history. Next to therian mammals and birds, Anolis lizards therefore represent another adaptively radiated amniote clade with conserved sex chromosomes. We argue that the evolutionary stability of sex‐determining systems may reflect an advanced stage of differentiation of sex chromosomes rather than thermoregulation strategy.     

MITOCHONDRIAL DNA SEQUENCE-BASED PHYLOGENY AND THE EVOLUTION OF VIVIPARITY IN THE SCELOPORUS SCALARIS GROUP (REPTILIA,SQUAMATA)

The lizard genus Sceloporus contains both oviparous and viviparous species. The scalaris complex is the only monophyletic group within the genus that includes both reproductive modes, thus it is particularly well suited for studies of the evolution of viviparity. Approximately 874 nucleotides of mtDNA sequence data, collected from 38 specimens, comprising 25 populations of all five recognized species within the group, were used in a phylogenetic analysis of the origin of viviparity. Viviparity appears to have evolved twice in this group: once in S. goldmani, included in a clade formed by a northern group consisting of S. scalaris, S. chaneyi, and S. goldmani, and one more time in S. bicanthalis, included in the southern group formed by S. bicanthalis and S. aeneus. An oviparous population of S. bicanthalis nested within that viviparous clade, indicates that reversal from viviparity to oviparity may be possible. Degree of sequence divergence among several S. bicanthalis individuals pertaining to a population in which both parity modes occur, was no larger between oviparous and viviparous lizards than among viviparous lizards. This suggests that this population is a single species, and it may represent a transition from oviparity to viviparity or vice-versa.     

Variation in home range size along an elevational gradient in the iguanid lizard Sceloporus merriami

Summary Variation in home range size among three populations of the saxicolous iguanid lizard Sceloporus merriami was studied in relation to density, food availability, behavioral patterns, and daily activity periods. These populations, separated by a maximum distance of 50 km, occur along an elevational gradient which spans the elevational range of the species in west Texas. The population with the smallest home ranges had the highest density, the lowest food availability, and the most restricted daily activity period. Within populations, males consistently had larger home ranges than females. No significant between-year variation in home range size was noted within a given area or sex. Intraspecific variation in home range size is not explained by any single factor, such as food availability or density, but appears to be complexly determined by the interaction of energetic, breeding, and activity constraints.     

Thyroxine affects behavioral thermoregulation but not growth rate among populations of the western fence lizard (Sceloporus occidentalis)

We investigated whether thyroxine influences hatchling growth rate of the western fence lizard (Sceloporus occidentalis) throught its effects on thermoregulatory behaviors. We reared control and thyroxine-injected hatchlings from three populations of S. occidentalis that differ in growth rate in a thermal gradient. We also measured the daily changes in body temperature and activity level (proportion of time spent out of retreat sites) of control and thyroxine-injected lizards. Previous studies have shown that within and among population differences in growth rate of the western fence lizard are correlated with the maintenance of high activity levels (proportion of time spent outside of retreat sites) and high body temperatures throughout the day. Growth rate was not influenced by injections of thyroxine. However, injections of thyroxine did elevate average daily body temperature and daily activity. Although administration of thyroxine uniformly increased the probability of activity throughout the day, it did not appear to alter the daily changes in activity. Previous studies have shown that the slower-growing hatchlings from northern populations exhibit a decline in activity during the later part of the thermal cycle, whereas the faster growing southern hatchlings maintain the same level of high activity throughout the thermal cycle. The decline in activity of northern populations was not prevented by thyroxine injection used in our current study. Northern lizards receiving exogenous thyroxine were still less active later in the day compared to early in the day, even though activity level throughout the day was elevated. Thus, the effects of thyroxine on temperature regulation observed in our study (general increase in activity level) appear to be unrelated to those aspects of temperature regulation (e.g., daily changes in behavioral thermoregulation) that are correlated with among population differences in growth rate. We also raised hatchlings in a cycling thermal regime (forced thermal cycle of 34°C:15°C, 12L:12D) where behavioral thermoregulation is not possible. The growth rate of lizards forced to cycle between 34°C:15°C on a daily basis was significantly lower than those lizards allowed to behaviorally thermoregulate, further underscoring the importance of the circadian pattern of thermoregulation for growth.Abbreviations GR growth rate - MR metabolic rate - SMR standard metabolic rate - SVL snout-vent length - T₄ thyroxine - T _b body temperature - T _e environmental temperature     

DYNAMICS OF A NOVEL CHROMOSOMAL POLYMORPHISM WITHIN A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE SCELOPORUS GRAMMICUS COMPLEX (SAURIA,PHRYNOSOMATIDAE)

Several chromosome races of the mesquite lizard, Sceloporus grammicus complex, hybridize at localities in central Mexico. In most cases, the hybridizing populations are delineated by centric fissions at one or more of the macrochromosomes. One notable exception is the Tulancingo hybrid zone between the F5 and FM2 cytotypes. In addition to fission and/or inversion differences at chromosomes 1, 3, 4, and 6, these races differ by a complex rearrangement of chromosome 2, which carries the nucleolus-organizer region in this species. The meiotic consequences of heterozygosity at this chromosome were examined in males to assess the potential for this chromosome to contribute to the dynamics of the hybrid zone. Chromosomal analysis revealed several putative F₁ hybrids and confirmed the production of nonparental chromosomal morphologies through recombination. Pachytene analysis revealed meiotic pairing difficulties associated with chromosome 2 in males heterozygous for the parental chromosomal morphologies. Significant aneuploidy is expected because of random disjunction of the chromosome-2 elements. As a result, these males likely suffer reduced fertiliity and fitness. In contrast, males heterozygous for recombinant chromosomal morphologies displayed low levels of meiotic irregularities and presumably exhibit higher fertility than individuals heterozygous for parental morphologies. It is hypothesized that the recombinant phenotypes facilitate gene flow between the F5 and FM2 cytotypes.