Biomedical subjects

Duncan J Irschick

Publications and source records attributed to Duncan J Irschick.

17 recordsLinked to original sources

PubMed2006

Microhabitat use, diet, and performance data on the Hispaniolan twig anole, Anolis sheplani: pushing the boundaries of morphospace.

Caribbean Anolis lizards are often cited as a textbook example of adaptive radiation. Similar morphologies (ecomorphs) have originated in similar ecological settings on different large islands in the West Indies. However, relatively little is known about one of the morphologically most specialized and divergent ecomorphs: the twig anoles. Here, we investigate aspects of morphology, dewlap size, locomotor and bite performance, structural habitat and diet of the poorly known twig anole, Anolis sheplani from Hispaniola. Few observations have previously been made of this species in its natural habitat, and few quantitative data on its natural history are available. A. sheplani is an extreme twig anole with respect to its morphology, performance capacities, and ecological niche. Males and females of this species do not differ from each other in body dimensions, performance or habitat use, but males do have a bigger dewlap than females. We present data for 25 individuals and compare them with data for other Greater Antillean anoles. It becomes apparent that twig anoles constitute a large component of the morphological, functional, and ecological diversity of Anolis lizards. Small twig anoles such as A. sheplani appear to be pushing the boundaries of morphospace and are thus crucial in our understanding of the evolution of phenotypic diversity.

Animals↗

PubMed2006

Whole-organism studies of adhesion in pad-bearing lizards: creative evolutionary solutions to functional problems.

Understanding the evolution of complex functional traits is a challenge for evolutionary physiology. Here we investigate the evolution of subdigital toepads in lizards, which have arisen independently at least three times, although with subtle anatomical differences. Some designs (anole, gecko) appear functionally equivalent, whereas other designs (skink) are inferior. The functional equivalence of geckos and anoles highlights the creative aspect of the evolutionary process in that these two groups have arrived at the same functional endpoint along very different trajectories. However, this functional equivalence does not result in equivalence for performance at whole-organism tasks (e.g., running uphill), as the evolution of behavior (e.g., toe-furling) has enabled geckos to be superior climbers than anoles. We also show that adaptive increases in the toepad size within a closely related lizard genus (Anolis) has resulted in concomitant evolution of enhanced clinging ability and increased perch heights. A third insight is that pad-bearing geckos are capable of carrying tremendous loads (up to 250% of body weight) up smooth surfaces, and that the toepad itself does not appear limiting. This comparative and whole-organism approach to lizard toepads underscores how organisms can evolve multiple solutions to evolutionary problems.

Adaptation, Physiological↗

PubMed2006

Effects of temperature on maximum acceleration, deceleration and power output during vertical running in geckos.

We studied performance and kinematics of the diurnal gekkonid lizard Phelsuma dubia while running vertically on a smooth surface at different temperatures. Trials were conducted at 5 degrees C intervals from 15 degrees C to 35 degrees C. High-speed video recordings and digitization were used to obtain measures of instantaneous velocity, acceleration, deceleration and mass-specific power output and maximal values for each were taken as performance measures. Kinematic variables were also obtained from high-speed video recordings and included stride length and duration, step (stance phase) length and duration, and duty factor. Maximal instantaneous velocity, acceleration and deceleration increased by a factor of approximately 1.7 between 15 degrees C and 25 degrees C, and less so (approximately 1.2x) between 25 degrees C and 35 degrees C. Mass-specific power output was more temperature-sensitive, increasing 2.5x up to 25 degrees C and a further 1.4x above that temperature. Stride length increased 1.5x over the entire temperature interval studied, while stride duration decreased by a factor of 1.9, suggesting that velocity is modulated by changes in both stride length and duration in P. dubia. Duty factor was not significantly influenced by temperature. Stride length was the only kinematic measure to be influenced by stride number, with second steps from a standstill being longer than first steps. We discuss the significance of velocity and acceleration being affected in a similar manner by temperature, and that speed is modulated by both changes in stride length and duration.

Animals↗

PubMed2006

Out on a limb: The differential effect of substrate diameter on acceleration capacity in Anolis lizards.

We investigated how substrate diameter affects acceleration performance in three Anolis lizard species (A. sagrei, A. carolinensis and A. valencienni), representing three different ecomorphs (trunk-ground, trunk-crown, and twig, respectively). We did so by measuring maximal acceleration capacity of the three species on a broad and narrow dowel. In addition to acceleration capacity, we quantified maximal sprint speed on both dowels. Both acceleration capacity and sprint speed are affected by substrate diameter, but the way in which they are, differs among species. Acceleration capacity in the trunk-ground anole, A. sagrei, was least affected by dowel diameter, whereas it was greatly reduced on the narrow dowel in the twig anole, A. valencienni. Sprint speed on the narrow dowel, however, was reduced to the greatest extent in the fastest running species, A. sagrei, whereas sprint speed was hardly affected by dowel diameter in the slow A. valencienni. The differential effect of dowel diameter on maximal acceleration capacity cannot be explained by differences in the timing of reaching maximal acceleration, but may be due to interspecific differences in the relative positioning of the limbs on the different dowels. The differential effect of dowel diameter on sprint speed, on the other hand, may be based on interspecific differences in the relative contribution of subsequent acceleratory bouts to maximal sprint speed on the broad and narrow dowel.

Acceleration↗

PubMed2006

The quick and the fast: the evolution of acceleration capacity in Anolis lizards.

Although of prime ecological relevance, acceleration capacity is a poorly understood locomotor performance trait in terrestrial vertebrates. No empirical data exist on which design characteristics determine acceleration capacity among species and whether these design traits influence other aspects of locomotor performance. In this study we explore how acceleration capacity and sprint speed have evolved in Anolis lizards. We investigate whether the same or different morphological traits (i.e., limb dimensions and muscle mass) correlate with both locomotor traits. Within our sample of Anolis lizards, relative sprint speed and acceleration capacity coevolved. However, whereas the variation in relative acceleration capacity is primarily explained by the variation in relative knee extensor muscle mass, the variation in relative sprint speed is correlated to the variation in relative femur, tibia, and metatarsus length as well as knee extensor muscle mass. The fact that the design features required to excel in either performance trait partly overlap might explain the positive correlation between the variation in relative sprint speed and acceleration capacity. Furthermore, our data show how similar levels of sprint performance can be achieved through different morphological traits (limb segment lengths and muscle mass) suggesting that redundant mapping has potentially played a role in mitigating trade-offs.

Acceleration↗

PubMed2005

Age-specific forced polymorphism: implications of ontogenetic changes in morphology for male mating tactics.

Age-specific forced polymorphism is the presence of two or more distinct phenotypes (here we consider only males) that occur in separate sexually mature age groups (e.g., horns in older males but not younger males). The life-stage morph maturation hypothesis posits that all younger males that possess a particular structure can transform into older males with a different structure, most likely via the influence of hormones. The life-stage morph selection hypothesis posits that polymorphism is due to intense selection resulting in a highly nonrandom sample of younger males surviving to become older males, thus leading to different mean phenotypes in different age groups. We conducted an extensive review of literature from the past 20 years (1983-2003) for cases of age-specific forced polymorphism. Overall, we found only a few cases that fit our criteria of age-specific forced polymorphism, and we argue that most (e.g., orangutans, elephant seals) have likely arisen via the life-stage morph maturation mechanism, but we also present several examples (e.g., green anole lizards) that appear to be candidates for life-stage morph selection. However, none of the reviewed studies provided enough information (e.g., age of morphs, growth patterns of the morphological structure) to definitively invoke either of the two mechanisms. We suggest that age-specific forced polymorphism is more common than reflected in this review and that future studies should gather demographic and laboratory data that will directly compare the life-stage morph maturation and life-stage morph selection hypotheses.

Aging↗

PubMed2005

Effects of temperature on maximum clinging ability in a diurnal gecko: evidence for a passive clinging mechanism?

The thermal dependence of performance of ectotherms, and particularly locomotor performance in lizards, has received much attention. However, only a single study has examined the effects of temperature on adhesive clinging ability in geckos, despite the importance of adhesion for many pad-bearing lizards and invertebrates. We set out to characterize the thermal response of clinging ability in the diurnal gecko, Phelsuma dubia in the temperature range of 15-35 degrees C. Our findings indicate that there is no significant trend in clinging ability for P. dubia with temperature and that there is high variation about the mean at all temperatures. These findings differ from other whole-organism studies of clinging performance and are suggestive of a passive clinging mechanism that is dominated by intermolecular van der Waals forces. These findings also suggest that clinging ability in this species is not under selective pressures resulting from thermal variation, and that P. dubia does not need to regulate body temperature closely to maximize clinging ability.

Adhesiveness↗

PubMed2005

Movement control strategies during jumping in a lizard (Anolis valencienni).

Whereas maximal performance is subjected to specific control criteria, sub-maximal movements theoretically allow for an infinite number of control strategies. Yet sub-maximal movements are predominant in the locomotor repertoire of most organisms and often little understood. Previous data on sub-maximal vertical jumping in humans has suggested that a movement effectiveness criterion might best explain the observed control strategy employed. Here we test the generality of this criterion in jumping by inducing lizards to jump both at a range of distances as well as a range of take-off angles. Our results show that while movement effectiveness appears to best explain jumping for different take-off angles, a 'push harder' strategy (i.e. mostly increasing the force output of the system), is used in the control of distance jumping. Thus, our data support the generality of the movement effectiveness criterion for vertical jumping, but not for distance jumping. Sub-maximal distance jumping in the lizard Anolis valencienni appears to be governed by a relatively simple control strategy that allows a rapid response. This accords well to the ecological circumstances in which long jumps are typically used (escape from predators).

Animals↗

PubMed2005

Comparisons of aquatic versus terrestrial predatory strikes in the pitviper, Agkistrodon piscivorus.

Recent studies comparing terrestrial versus aquatic locomotion in animals have shown that changes in kinematics, muscular activation patterns, and performance across media are often dramatic. Surprisingly, however, despite the importance of feeding to the survival of most animals, few studies have compared differences in feeding behaviour between media. The present study compares prey-capture behaviour, strike success, strike velocity and acceleration, and prey-capture kinematics in a semi-aquatic pitviper (Agkistrodon piscivorus) when capturing both terrestrial (mice) and aquatic (fish) prey in a standardized laboratory setting. Strike velocity and acceleration did not differ significantly between media, but instead were positively correlated with initial prey distance. By contrast, the kinematics of terrestrial and aquatic strikes differed significantly in several aspects: max gape angle during the retraction phase, angular velocity of mouth closing during the strike, and the initial head angle before the strike. Terrestrial strikes were associated with higher gape angles during the retraction phase, higher angular velocities of mouth closure, and a more inclined head angle at the onset of the strike. Finally, strike success differed significantly between strike types, with terrestrial strikes being considerably more successful than aquatic strikes. Strike success likely differed due to the relatively slow mouth-closing velocity of aquatic strikes.

Acceleration↗

PubMed2005

Have male and female genitalia coevolved? A phylogenetic analysis of genitalic morphology and sexual size dimorphism in web-building spiders (Araneae: Araneoidea).

Sexual size dimorphism (SSD) can strongly influence the evolution of reproductive strategies and life history. If SSD is extreme, and other characters (e.g., genitalic size) also increase with size, then functional conflicts may arise between the sexes. Spiders offer an excellent opportunity to investigate this issue because of their wide range of SSD. By using modern phylogenetic methods with 16 species of orb-weaving spiders, we provide strong evidence for the "positive genitalic divergence" model, implying that sexual genitalic dimorphism (SGD) increases as SSD increases. This pattern is supported by an evolutionary mismatch between the absolute sizes of male and female genitalia across species. Indeed, our findings reveal a dramatic reversal from male genitalia that are up to 87x larger than female genitalia in size-monomorphic species to female genitalia that are up to 2.8x larger in extremely size-dimorphic species. We infer that divergence in SGD could limit SSD both in spiders, and potentially in other taxa as well. Further, male and female body size, as well as male and female genitalia size, are decoupled evolutionarily. Finally, we show a negative scaling (hypoallometry) of male and female genitalic morphology within sexes. Evolutionary forces specific to each sex, such as larger female size (increased fecundity) or smaller male size (enhanced mate-searching ability), may be balanced by stabilizing selection on relative genitalic size.

Animals↗

PubMed2005

Locomotor compensation creates a mismatch between laboratory and field estimates of escape speed in lizards: a cautionary tale for performance-to-fitness studies.

A key assumption in evolutionary studies of locomotor adaptation is that standard laboratory measures of performance accurately reflect what animals do under natural circumstances. One widely examined measure of performance is maximum sprint speed, which is believed to be important for eluding predators, capturing prey, and defending territories. Previous studies linking maximum sprint speed to fitness have focused on laboratory measurements, and we suggest that such analyses may be appropriate for some species and intraspecific classes, but not others. We provide evidence for a general inverse relationship between maximum laboratory sprint speed and the percentage of maximum capacity that animals use when escaping from a threat in the field (the model of locomotor compensation). Further, absolute values of field escape speed and maximum laboratory speed are not significantly related when comparing across a diverse group of Anolis and lacertid lizards. We show that this pattern of locomotor compensation holds both within (i.e., among intraspecific classes) and among lizard species (with some exceptions). We propose a simple method of plotting field escape speed (y-axis) versus maximum laboratory speed (x-axis) among species and/or intraspecific classes that allows researchers to determine whether their study organisms are good candidates for relating laboratory performance to fitness. We suggest that species that reside directly on, or near the "best fitness line" (field escape speed = maximum laboratory speed) are most likely to bear fruit for such studies.

Analysis of Variance↗

PubMed2004

Performance capacity, fighting tactics and the evolution of life-stage male morphs in the green anole lizard (Anolis carolinensis).

The evolution of alternative male phenotypes is probably driven by male-male competition for access to reproductive females, but few studies have examined whether whole-organism performance capacities differ between male morphs, and if so whether any such differences affect fighting ability. We show how ontogenetic changes in performance and morphology have given rise to two distinct life-stage male morphs exhibiting different fighting tactics within the green anole lizard (Anolis carolinensis). Field studies show a bimodal distribution of adult males within a single population: larger 'heavyweight' males have relatively large heads and high bite forces for their size, whereas smaller 'lightweight' males have smaller heads and lower bite forces. In staged fights between size-matched heavyweight males, males with greater biting ability won more frequently, whereas in lightweight fights, males with greater jumping velocity and acceleration won more often. Because growth in reptiles is indeterminate, and the anole males examined are sexually mature, we propose that the heavyweight morph arose through selection against males with small heads and poor bite forces at the lightweight-heavyweight size transition. Our findings imply that one may not be able to predict male fighting success (and hence potential mating success) by examining aspects of male 'quality' at only one life stage.

Agonistic Behavior↗

PubMed2004

Overcoming an evolutionary conflict: removal of a reproductive organ greatly increases locomotor performance.

One potential consequence of sexual size dimorphism is conflict among characters. For example, a structure evolved for reproduction can impair performance during other activities (e.g., locomotion). Here we provide quantitative evidence for an animal overcoming an evolutionary conflict generated by differential scaling and sexual size dimorphism by obligatorily removing an undamaged reproductive organ, and thus dramatically enhancing its locomotor performance. The spider genus Tidarren (Araneae, Theridiidae) is interesting because, within several species presenting extreme sexual size dimorphism (males representing approximately 1% of the total mass of the female), males voluntarily remove one of their two disproportionately large pedipalps (modified copulatory organs; a single one represents approximately 10% of the body mass in an adult) before achieving sexual maturity. Whether the left or right pedipalp is removed appears to be random. Previous researchers have hypothesized that pedipalp removal might enhance locomotor performance, a prediction that has remained untested. We found that, for male Tidarren sisyphoides, maximum speed increased (44%) significantly and endurance increased (63%) significantly after pedipalp removal. Furthermore, spiders with one pedipalp moved approximately 300% greater distances before exhaustion and had a higher survival after exertion than those with two pedipalps. Removal of the pedipalp may have evolved in male Tidarren because of enhanced abilities to search for females (higher endurance and survival after exertion) and to out-compete rival males on the female's web (higher maximum speed). Our data also highlight how the evolution of conflicts can result in the evolution of a novel behavior.

Animals↗

PubMed2003

A biomechanical analysis of intra- and interspecific scaling of jumping and morphology in Caribbean Anolis lizards.

Scaling models predict how functional variables change as animals grow or increase in size evolutionarily. However, few experimental studies have found support for the predictions of these models. Here, we use a force plate to investigate the scaling of functional variables associated with jumping within (for three species) and across adults of 12 species of Anolis lizards. Both ontogenetically (with the exception of Anolis carolinensis) and across the 12 species examined, limb dimensions increased geometrically, making Anolis lizards an ideal study system to test the predictions of geometric scaling models. However, both the ontogenetic and interspecific scaling of functional variables deviated in several aspects from model predictions. Unexpectedly, the scaling of functional variables such as acceleration differed for different species. Whereas acceleration capacity increases with hindlimb length for A. carolinensis, no relationship was detected for the other two species. Interspecifically, the inclusion of two large species in our analysis appears to drive the absence of a correlation between acceleration capacity and hindlimb length across species. These data suggest that selection for enhanced jumping performance is relaxed in larger anoles and support the notion that no scaling model seems to be able to comprehensively predict changes in function with size across species; rather, natural selection seems to drive changes in the scaling relationships of some key variables such as force output or acceleration capacity.

Animals↗

PubMed2003

Effects of loading and size on maximum power output and gait characteristics in geckos.

Stride length, stride frequency and power output are all factors influencing locomotor performance. Here, we first test whether mass-specific power output limits climbing performance in two species of geckos (Hemidactylus garnoti and Gekko gecko) by adding external loads to their bodies. We then test whether body size has a negative effect on mass-specific power output. Finally, we test whether loading affects kinematics in both gecko species. Lizards were induced to run vertically on a smooth wooden surface with loads of 0-200% of body mass (BM) in H. garnoti and 0-100% BM in G. gecko. For each stride, we calculated angular and linear kinematics (e.g. trunk angle, stride length), performance (maximum speed) and mean mass-specific power output per stride. The addition of increasingly large loads caused an initial increase in maximum mass-specific power output in both species, but for H. garnoti, mass-specific power output remained constant at higher loads (150% and 200% BM), even though maximum velocity declined. This result, in combination with the fact that stride frequency showed no evidence of leveling off as speed increased in either species, suggests that power limits maximum speed. In addition, the large gecko (G. gecko) produced significantly less power than the smaller H. garnoti, despite the fact that both species ran at similar speeds. This difference disappeared, however, when we recalculated power output based on higher maximum speeds for unloaded G. gecko moving vertically obtained by other researchers. Finally, the addition of external loads did not affect speed modulation in either species: both G. gecko and H. garnoti increase speed primarily by increasing stride frequency, regardless of loading condition. For a given speed, both species take shorter but more strides with heavier loads, but for a given load, G. gecko attains similar speeds to H. garnoti by taking longer but fewer strides.

Analysis of Variance↗

PubMed2002

Molecular phylogenetic perspective on evolution of lizards of the Anolis grahami series.

We report the results of phylogenetic analyses of 1447 bases of mitochondrial DNA sequence for 21 populations representing seven species of the Anolis grahami series (A. conspersus, A. garmani, A. grahami, A. lineatopus, A. opalinus, A. reconditus, and A. valencienni), six of which occur on Jamaica. These data include 705 characters that are phylogenetically informative according to parsimony. A parsimony analysis of these data combined with previously published allozymic data yields a single most parsimonious tree with strong support for monophyly of the A. grahami series, the sister-group relationship between Anolis lineatopus and A. reconditus and a clade composed of Anolis garmani, A. grahami, and A. opalinus. Based on DNA data alone, A. conspersus is nested within A. grahami. Haplotypes sampled from geographic populations of A. grahami, A. lineatopus, and A. opalinus are highly divergent (approximately 12-15% sequence difference on average for each species) and show similar phylogeographic patterns, suggesting that each of these currently recognized species may be a complex of species. Anolis valencienni also shows high sequence divergence among haplotypes from different geographic populations (approximately 8% sequence difference) and may contain cryptic species. Divergence among haplotypes within A. garmani is substantially lower (approximately 3% sequence difference), and phylogeographic patterns are significantly different from those observed in A. grahami, A. lineatopus and A. opalinus.

Alleles↗

PubMed1999

Do Lizards Avoid Habitats in Which Performance Is Submaximal? The Relationship between Sprinting Capabilities and Structural Habitat Use in Caribbean Anoles.

Recent years have seen an increased emphasis on measuring ecologically relevant performance capabilities to understand associations between morphology and habitat use. Such studies presume that performance is invariant, but in eight Caribbean Anolis lizard species, we found that maximal sprinting ability depends on surface diameter. Moreover, these species differ in the degree to which sprint speed declines with decreasing surface diameter, defined as "sprint sensitivity" (high sprint [Formula: see text] declines in speed between broad and narrow dowels). The habitat constraint hypothesis postulates that Anolis lizards will avoid structural habitats in which their maximal sprinting capabilities are impaired. The habitat breadth hypothesis postulates that species whose performance is less affected by substrate will use a greater variety of habitats than species whose performance varies to a greater extent on surfaces of different diameters. Field observations quantified the proportion of time that lizards spent on different perch diameters. Both hypotheses were confirmed: species with high values of sprint sensitivity avoided using perches on which their maximal sprinting abilities are impaired, whereas species with low sprint sensitivity used such "submaximal" surfaces more frequently. Species with low sprint sensitivity used a broader range of structural habitats than species with high sprint sensitivity.

comparative↗