Bateman gradients in field and laboratory studies: a cautionary tale

Since tools of molecular genetics became readily available,our understanding of bird mating systems has undergone a revolution.The majority of passerine species investigated are sociallymonogamous, but have been shown to be genetically polygamous.Data sets from natural populations of juncos suggest that multiplemating by females results in a sexual selection gradient assteep for females as for males (a result that does not supportBateman's predictions). However, in males, fitness is enhanceddirectly through fertilization success with multiple matings;in females fitness benefits may be enhanced immediately throughdirect access to food, protection against predators, or otherresources received from males, or they may be delayed throughimprovement in offspring quality (e.g., through good genes,or greater genetic compatibility between the female and theextra-pair male). But a steep sexual selection gradient forfemales can be difficult to interpret. If all females copulatewith multiple partners that are equally likely to fertilizeeggs, then females that produce larger clutch sizes, for anyreason, will appear to have copulated with more males. Thatis, multiple sires have a higher probability of detection inlarger clutches than in smaller ones, giving the impressionthat females that mate with multiple males increase their reproductivesuccess. Yet, in most studies in which there is a correlationbetween number of offspring produced by females and number ofextra-pair males, causation has not been clearly establishedand other factors may explain the results. Additional complicationsin understanding male and female reproductive strategies are:(1) Molecular studies cannot detect extra-pair copulations thatdid not result in fertilizations; yet if a female acquires foodor other resources from extra-pair males, such extra-pair matingsmay have significant effects on female fitness. Thus, molecularstudies provide only a conservative estimate of the number ofextra-pair copulations or "mates" that a female has. (2) Clutchsize affects the probability that any given male will be successfulin fertilizing a female's eggs. Specifically, at any given point,a male's chances of fertilizing at least one egg in the female'sclutch will be greater as clutch size increases. We predictthat in avian species with small clutch sizes, males may beselected to be choosy and avoid extra-pair copulations, whilefemales should be selected to be less discriminating. Moreover,if extra-pair males provide resources that increase female fitness,the females should seek extra-pair copulations, whether or notthe males are likely to fertilize any of her eggs. Laboratory studies with insects have yielded clearer evidenceof the causal relationship between multiple mating and increasedfemale fitness. We review studies on a tenebrionid beetle inwhich female fecundity increases directly with number of mates.In these experiments, the nutritive value of the spermatophoresdoes not fully explain the increase in female reproductive success.     

Locomotor performance of hatchling fence lizards (Sceloporus occidentalis): Quantitative genetics and morphometric correlates

Summary We examined heritabilities and correlations among measures of locomotor performance (speed, stamina) and among possible morphometric determinants of performance (hindlimb span, tail length) in families of hatchling lizards (Sceloporus occidentalis). We were particularly interested in determining whether these traits were heritable and thus might potentially respond genetically to selection. Moreover, we wished to determine whether speed and stamina are negatively genetically correlated, as suggested bya priori physiological and empirical considerations. All four traits appeared to be significantly heritable. Broadsense heritabilities were 0.33–0.36 for speed, 0.35–0.36 for stamina, 0.45–0.51 for hindlimb span, and 0.46–0.47 for tail length. Contrary to expectations, speed and stamina were not negatively genetically correlated. Hindlimb span and tail length, however, were negatively genetically correlated (but not phenotypically correlated). Hindlimb span and stamina were positively phenotypically correlated. Thus, for example, selection for longer hindlimb span could potentially result in shorter tails, contrary to evolutionary predictions based only on phenotypic correlations.     

Variation in morphology, gait characteristics and speed of locomotion in two populations of lizards

Locomotor behaviour varies between two subspecies of the Spanish wall lizard Podarcis hispanica. One subspecies inhabits the Columbretes islands, the other lives on the Spanish mainland. Size standardized voluntary speeds (as measured in unrestrained laboratory conditions) are lower in the island population (P. h. atrata) than in the mainland population (P. h. hispanica). Maximal running performance (when chased) is much higher in the mainland population than in the island population. High speed video recordings show that subspecies differ in gait characteristics: individuals from the mainland modulate running velocity primarily by modifying stride length, individuals from the island primarily by altering stride frequency. P. h. hispanica's strategy for modulating speed probably allows this mainland subspecies to attain higher maximal speeds than the island subspecies P. h. atrata. Theoretical considerations suggest that at high speeds, P. h. hispanica's running style is energetically more favourable, but this hypothesis awaits experimental verification. We suggest that the differences in locomotion efficiency between the subspecies result from differences in predation pressure between the mainland and the island. The mainland study site has a higher predator diversity and offers less hiding opportunities to the lizards.     

Body size evolution simultaneously creates and collapses species boundaries in a clade of scincid lizards

Speciation is generally viewed as an irreversible process, although habitat alterations can erase reproductive barriers if divergence between ecologically differentiated species is recent. Reversed speciation might also occur if geographical contact is established between species that have evolved the same reproductive isolating barrier in parallel. Here, we demonstrate a loss of intrinsic reproductive isolation in a clade of scincid lizards as a result of parallel body size evolution, which has allowed for gene flow where large-bodied lineages are in secondary contact. An mtDNA phylogeny confirms the monophyly of the Plestiodon skiltonianus species complex, but rejects that of two size-differentiated ecomorphs. Mate compatibility experiments show that the high degree of body size divergence imposes a strong reproductive barrier between the two morphs; however, the strength of the barrier is greatly diminished between parallel-evolved forms. Since two large-bodied lineages are in geographical contact in the Sierra Nevada Mountains of California, we were also able to test for postzygotic isolation under natural conditions. Analyses of amplified fragment length polymorphisms show that extensive gene exchange is occurring across the contact zone, resulting in an overall pattern consistent with isolation by distance. These results provide evidence of reversed speciation between clades that diverged from a common ancestor more than 12Myr ago.     

Interacting effects of predation risk and resource level on escape speed of amphibian larvae along a latitudinal gradient

Fast‐growing genotypes living in time‐constrained environments are often more prone to predation, suggesting that growth‐predation risk trade‐offs are important factors maintaining variation in growth along climatic gradients. However, the mechanisms underlying how fast growth increases predation‐mediated mortality are not well understood. Here, we investigated if slow‐growing, low‐latitude individuals have faster escape swimming speed than fast‐growing high‐latitude individuals using common frog (Rana temporaria) tadpoles from eight populations collected along a 1500 km latitudinal gradient. We measured escape speed in terms of burst and endurance speeds in tadpoles raised in the laboratory at two food levels and in the presence and absence of a predator (Aeshna dragonfly larvae). We did not find any latitudinal trend in escape speed performance. In low food treatments, burst speed was higher in tadpoles reared with predators but did not differ between high‐food treatments. Endurance speed, on the contrary, was lower in high‐food tadpoles reared with predators and did not differ between treatments at low food levels. Tadpoles reared with predators showed inducible morphology (increased relative body size and tail depth), which had positive effects on speed endurance at low but not at high food levels. Burst speed was positively affected by tail length and tail muscle size in the absence of predators. Our results suggest that escape speed does not trade‐off with fast growth along the latitudinal gradient in R. temporaria tadpoles. Instead, escape speed is a plastic trait and strongly influenced by the interaction between resource level and predation risk.     

Characterization of the Drosophila gene-switch system in aging studies: a cautionary tale

Genetic studies have shown that in many model organisms, single gene mutations can dramatically influence aging. Systems that allow researchers to control a gene's temporal and spatial expression pattern, known as inducible gene-expression systems, are a valuable asset for the study of the influence of single genes on aging. One inducible gene-expression system reported to allow temporal and tissue-specific control of gene expression in Drosophila is the Gene-Switch system. However, this system has not been extensively characterized in the context of aging research. This report uses six Gene-Switch strains to examine the tissue localization and amount of expression achievable in the major tissue types of the fly. The quantitative analysis of adult flies fed with inducer through life reveals that the levels of expression are influenced by both the inducer concentration and the age of the animal in a strain-specific manner. Furthermore, the relationship between inducer concentration and expression level is unique to each strain and, in some cases, to each gender. The analysis of the spatial expression patterns in several strains revealed expression in more tissue types than previously assumed. Finally, most Gene-Switch strains display expression in the absence of inducer during development and/or during adulthood. These findings have important implications that may reconcile contradictions reported in studies investigating the effects of dFOXO on longevity. This study is an important guide to the design and interpretation of aging studies based on the Gene-Switch system.     

Morphological correlates of burst speed and field movement patterns: the behavioural adjustment of locomotion in wall lizards (Podarcis muralis)

Locomotion of lizards has clear morphological determinants and is important for developing activities such as feeding, social interaction and predator avoidance. Thus, morphological variation is believed to have fitness consequences through affecting locomotor performance. This paper firstly evaluates the dependence of burst speed on morphology, and secondly examines the movement patterns of free-ranging undisturbed wall lizards ( Podarcis muralis ) engaged in several kinds of activity. Body size was the most important correlate of burst speed as performed at the optimal temperature for running in the laboratory. After removing size effects from performance and morphological traits, the length of some particular limb segments had positive influence on burst speed, but these effects were weak, each trait explaining less than 16% of variance in burst speed. Free-ranging P. muralis exhibited intermittent locomotion, with movement sequences interrupted by frequent short pauses. Field movement patterns greatly differed depending upon the kind of activity and were in most aspects independent of the size and sex of the animal. P. muralis involved in thermoregulation performed short and low-speed displacements; exploratory activities were characterized by frequent, slow and short movements. On the contrary, lizards involved in intraspecific pursuits and predator escape developed comparatively high speeds, although only exceptionally did they attain the size-specific burst speed predicted from the laboratory trials. Speed of escape increased with distance to the refuge and the animals are able to assess predation risks to modulate approach distance, speed and pauses, so maximum exertion is seldom required. The evolution of locomotor capacities exceeding routine needs is discussed in the context of the principle of 'excessive construction'.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 135–146.     

The tradeoff between catch-up growth and escape speed: variation between habitats in the cost of compensation

Compensatory growth is the faster-than-normal growth that some species exhibit after a period of resource deprivation. Using three-spined sticklebacks as model species we tested the impact of compensatory growth on subsequent escape performance in populations from diverse habitats. We found clear population differences in the rate of compensatory growth, and strong inter-habitat differences in the impact that catch-up growth had on burst swimming performance when measured weeks later. In pond populations growth compensation had little effect on burst swimming, whereas fish from stream populations that exhibited rapid catch-up growth subsequently had slower escape speeds. Those differences could be explained by the non-linear nature of the tradeoff curve, suggesting that habitat-specific selection pressures lead to differences in the importance of burst swimming performance.     

Field use of maximal sprint speed by collared lizards (Crotaphytus collaris): compensation and sexual selection

To understand how selection acts on performance capacity, the ecological role of the performance trait being measured must be determined. Knowing if and when an animal uses maximal performance capacity may give insight into what specific selective pressures may be acting on performance, because individuals are expected to use close to maximal capacity only in contexts important to survival or reproductive success. Furthermore, if an ecological context is important, poor performers are expected to compensate behaviorally. To understand the relative roles of natural and sexual selection on maximal sprint speed capacity we measured maximal sprint speed of collared lizards (Crotaphytus collaris) in the laboratory and field-realized sprint speed for the same individuals in three different contexts (foraging, escaping a predator, and responding to a rival intruder). Females used closer to maximal speed while escaping predators than in the other contexts. Adult males, on the other hand, used closer to maximal speed while responding to an unfamiliar male intruder tethered within their territory. Sprint speeds during foraging attempts were far below maximal capacity for all lizards. Yearlings appeared to compensate for having lower absolute maximal capacity by using a greater percentage of their maximal capacity while foraging and escaping predators than did adults of either sex. We also found evidence for compensation within age and sex classes, where slower individuals used a greater percentage of their maximal capacity than faster individuals. However, this was true only while foraging and escaping predators and not while responding to a rival. Collared lizards appeared to choose microhabitats near refugia such that maximal speed was not necessary to escape predators. Although natural selection for predator avoidance cannot be ruled out as a selective force acting on locomotor performance in collared lizards, intrasexual selection for territory maintenance may be more important for territorial males.     

Locomotor capacity and dominance in male lizards Lacerta monticola: a trade‐off between survival and reproductive success?

Trade-offs between reproduction and survival are important determinants of life-history characteristics of lizards. Organisms cannot increase the allocation of limited resources to reproduction without diverting a proportional amount of energy from another trait. Locomotor performance is an ecologically relevant trait that potentially influences survival by affecting the ability to escape from predators. Most studies have used female lizards as subjects because pregnancy is known to reduce their locomotor abilities, whereas little is known on costs of reproduction in males. In this study we suggest that in males of the lizard Lacerta monticola reproductive investment in morphological traits that confer dominance (i.e. head size) might lead to a low probability of survival by decreasing investment in other traits that affect locomotor performance (i.e. limb symmetry). We staged laboratory agonistic encounters between males and measured their morphology and burst speed on a race track to examine possible relationships between morphology, social dominance and locomotor capacity. Our results indicate that social dominance was positively related to relative head height, and that escape speed was negatively related to levels of fluctuating asymmetry in femur length, but also negatively related to relative head height. Males with greater relative head height also had more asymmetrical femurs, thus dominant males suffered a decrease in locomotor performance. Males with higher heads tend to dominate male–male interactions and hence may gain access to reproductive females, thus increasing their current reproduction success. However, this might occur at the expense of future survivorship mediated by a decrease in escape speed. Therefore, in male L. monticola there might be a trade-off between current reproductive success and survival.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002; 77 , 201–209.     

Extreme specialization to rocky habitats in Tropidurus lizards from Brazil: Trade‐offs between a fitted ecomorph and autoecology in a harsh environment

Ecomorphological theory indicates that different ecological requirements lead to different organismal designs. Given that species with equal requirements could not coexist, traits leading to more efficient use of resources may be selected to avoid competition among closely related syntopic species, generating specialized ecomorphs. We compared habitat use, diet, thermal biology and morphology among the syntopic Tropidurus semitaeniatus, T. helenae and T. hispidus in the Caatinga of Northeastern Brazil. Tropidurus semitaeniatus and T. helenae are flattened lizards specialized to rocks and rock crevices, whereas T. hispidus has a robust body and generalist habits. We aimed to test the hypothesis that morphological modifications observed in the flattened ecomorphs are related to modifications in diet and habitat use. Also, we hypothesized that specialization to habitat induces morphological modifications, which in turn may constrain lizard performance. Flattened species differed in habitat use, morphology and prey size when compared with the generalist ecomorph. Morphological modifications were related to specializations to rocky habitats and constrained the variety of prey items consumed. This phenotype also reduced their reproductive output when compared with a robust, generalist ecomorph.     

Cryptic diversity in vertebrates: molecular data double estimates of species diversity in a radiation of Australian lizards (Diplodactylus,Gekkota)

A major problem for biodiversity conservation and management is that a significant portion of species diversity remains undocumented (the ‘taxonomic impediment’). This problem is widely acknowledged to be dire among invertebrates and in developing countries; here, we demonstrate that it can be acute even in conspicuous animals (reptiles) and in developed nations (Australia). A survey of mtDNA, allozyme and chromosomal variation in the Australian gecko, genus Diplodactylus, increases overall species diversity estimates from 13 to 29. Four nominal species each actually represent multi-species complexes; three of these species complexes are not even monophyletic. The high proportion of cryptic species discovered emphasizes the importance of continuing detailed assessments of species diversity, even in apparently well-known taxa from industrialized countries.     

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.     

Costs of sexual traits: a mismatch between theoretical considerations and empirical evidence

Costs of sexual traits are of central importance to the theory of sexual selection. To qualify as a cost in line with theoretical models, empirical studies must demonstrate that sexual traits cause negative effects on one component of fitness of the trait bearer. Moreover, it must be demonstrated that the costs are differential such that negative effects on fitness are more severe for individuals in poor condition than for individuals in good condition. However, in the current literature, there is confusion over what qualifies as a cost, and costs are often anticipated based on findings of increased expenditure. Consequently, it seems that the generally accepted notion that sexual traits are costly is in fact based almost exclusively on indirect evidence and that direct empirical evidence is very scarce.     

Pupal period and adult size in Drosophila melanogaster: a cautionary tale of contrasting correlations between two sexually dimorphic traits

Sexual dimorphism (SD) is widespread, reflecting a resolution of genetic conflicts arising from sex-specific differences in selection. However, genetic correlations among traits may constrain the evolution of SD. Drosophila melanogaster exhibits SD for pupal period (males longer) and adult weight (females heavier). This negative inter-sex covariance between the traits contrasts with a significant intra-sex positive genetic correlation (r(g) = 0.95) estimated using lines selected for fast larval development. Path analysis indicated that within sexes the selection regime indirectly reduced adult weight which in turn reduced pupal period. A hypothesis is proposed for the evolution of SD whereby the trait 'pupal period' is divided into 'intrinsic' (correlated with body size) and 'ecological' (uncorrelated with body size) components, and (the larger) females eclose earlier than males size via a shortening of the ecological component, thus achieving the advantage of provisioning eggs prior to sexual maturity. This hypothesis avoids invoking successful 'incompatible antagonistic selection'.     

The use of long‐term monitoring data for studies of planktonic diversity: a cautionary tale from two Swiss lakes

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Sexual differences in locomotor performance in Tropidurus catalanensis lizards (Squamata: Tropiduridae) – body shape,size and limb musculature explain variation between males and females

Sexual dimorphism (SD) is the evolutionary outcome of selection acting differently on males and females. Several studies describe sexual differences in body size, although other morphological traits might be allometric between sexes and imply functional consequences. Here we test whether morphological differences between sexes in size and shape in the lizard Tropidurus catalanensis explain variation in performance of four locomotor traits. Our results show that males are larger than females and also exhibit longer limbs, longer muscles and larger muscle cross‐sectional areas, while females have longer trunks and more sharped anterior claws; males outperform females in all locomotor performances measured. Sexual differences in sprinting and climbing is related with body size, and climbing performance is also explained by limb lengths, by differences in lengths and cross‐sectional areas of specific muscles, and by interlimb distances. Between‐sex differences in exertion are also related to SD, despite associations with sharper posterior claws that are independent of sex. Grasping performance, however, is associated with some muscle and morphological parameters that are not sexually dimorphic. Together our results suggest that morphology might be under sexual selection in T. catalanensis, given that better locomotor performance likely favours male lizards in typical activities of this polygenic species, such as territory defence and female acquisition. Moreover, the longer trunks that characterize females may confer more space to accommodate eggs. On the other hand, territory defence by males probably increases their exposure to predators, resulting in a synergistic effect of sexual and natural selection in the evolution of SD in T. catalanensis.     

Intrapopulation variation in predator-avoidance performance of Galápagos lava lizards: The interaction of sexual and natural selection

Summary The interaction of sexual and natural selection in shaping variation in defensive behavior was explored via three steps. (1) Three predictions from the hypothesis that varying responses to predation account for intrapopulation variation in locomotory performance and wariness were tested. One measure of locomotory performance and two measures of wariness were compared between lava lizards (Tropidurus albemarlensis) inhabiting sparsely (1% cover) and heavily (33% cover) vegetated areas of Isla Plaza Sur in the Galapagos Archipelago. (2) Variation in morphology was examined to identify proximate mechanisms for differences seen in (1). (3) Levels of predation were compared between males and females to test a prediction from a model explaining the observed variation in defensive behavior. Male, but not female, lizards from sparsely vegetated areas were faster than those from heavily vegetated areas. Both male and female lizards from sparsely vegetated areas were significantly warier than those from heavily vegetated areas. Multiple regression, covariance, and residual analyses identify relatively longer hindlimbs of males as the proximate cause of their greater speed over that of females, rather than differences in body size, but neither body nor hindlimb size account for the microgeographic differences in speed of males. Significantly higher predation on males provided a positive test of the major prediction from our model of selective forces in which sexual selection for territory defense by males favors short approach distances (by minimizing time away from the territory caused by erroneous flight) and leads to natural selection for their increased hindlimb size and speed.