Costly carotenoids: a trade‐off between predation and infection risk?

Carotenoid reserves in copepods seem costly in terms of predation risk because they make individuals conspicuous. However, carotenoids also seem to play an important role in immune defence as free radical scavengers. To test whether predation risk influences carotenoid levels and whether changes in carotenoid levels are related to changes in immune defence, I examined individual changes in large carotenoid and other lipid droplets upon exposure to predation risk and subsequent exposure to parasites in the copepod Macrocyclops albidus. Copepods reduced carotenoid reserves upon exposure to predators, through which they potentially avoided the costs of being conspicuous under predation risk. Thus, the size of carotenoid reserves is a plastic trait. Such a decrease in carotenoid reserves may also have a negative impact on the copepods' immune system as individuals that decreased their reserves suffered higher parasite prevalence upon exposure to the cestode Schistocephalus solidus. These results suggest that carotenoid reserves may be individually optimized to trade-off each individual's unique costs (predation risk) and benefits (immune defence) of having these reserves.     

Is local adaptation in Mimulus guttatus caused by trade‐offs at individual loci?

Local adaptation is considered to be the result of fitness trade‐offs for particular phenotypes across different habitats. However, it is unclear whether such phenotypic trade‐offs exist at the level of individual genetic loci. Local adaptation could arise from trade‐offs of alternative alleles at individual loci or by complementary sets of loci with different fitness effects of alleles in one habitat but selective neutrality in the alternative habitat. To evaluate the genome‐wide basis of local adaptation, we performed a field‐based quantitative trait locus (QTL) mapping experiment on recombinant inbred lines (RILs) created from coastal perennial and inland annual races of the yellow monkeyflower (Mimulus guttatus) grown reciprocally in native parental habitats. Overall, we detected 19 QTLs affecting one or more of 16 traits measured in two environments, most of small effect. We identified 15 additional QTL effects at two previously identified candidate QTLs [DIV ERGENCE (DIV)]. Significant QTL by environment interactions were detected at the DIV loci, which was largely attributable to genotypic differences at a single field site. We found no detectable evidence for trade‐offs for any one component of fitness, although DIV2 showed a trade‐off involving different fitness traits between sites, suggesting that local adaptation is largely controlled by non‐overlapping loci. This is surprising for an outcrosser, implying that reduced gene flow prevents the evolution of individuals adapted to multiple environments. We also determined that native genotypes were not uniformly adaptive, possibly reflecting fixed mutational load in one of the populations.     

Evolutionary trade‐offs in locomotor capacities in lacertid lizards: are splendid sprinters clumsy climbers?

We tested the hypothesis that an evolutionary trade‐off exists between the capacity to run on level terrain and the ability to climb inclined structures in lacertid lizards. Biomechanical and physiological models of lizard locomotor performance suggest that the morphological design requirements of a ground‐dwelling vs. scansorial life style are difficult to reconcile. This conflict is thought to preclude simultaneous evolution of maximal locomotor performance on level and inclined terrain. This notion has been corroborated by comparative studies on lizard species from other groups (Anolis, Chamaeleo, Sceloporus), but is not supported by our data on 13 species from the family Lacertidae. We found no indication of a negative association between maximal sprint speed of lizards over a level racetrack (indicative of ground‐dwelling locomotor performance), on an inclined stony surface (indicative of climbing performance over rock faces) and inclined mesh surface (indicative of clambering performance among vegetation). Moreover, morphological characteristics associated with fast sprinting capacities (e.g. long hind limbs) apparently enhance, rather than hinder climbing and clambering performance. We conclude that in our sample of lacertid lizards, the evolution of fast sprinting capacity on level terrain has not inflicted major restrictions on climbing and clambering performance.     

Are parental care trade‐offs in shorebirds driven by parental investment or sexual selection?

Sexual selection, mating systems and parental behaviour are closely linked, although the exact nature of their relationship is controversial. The parental investment hypothesis (PIH) states that parental care disparity drives sexual selection intensity, because the sex providing less care competes for the sex that provides more. In contrast, the sexual selection hypothesis (SSH) asserts that more intense sexual selection on males leads to reduced male parental investment. We tested these hypotheses using directional phylogenetic comparative methods in shorebirds, which have an unusually diverse array of breeding systems. Changes in parental care and sexual selection intensity were tightly correlated, and we carried out three sets of analyses focusing on changes in male behaviour, female behaviour and in either sex. The results from the analyses were consistent with both PIH and SSH, although the patterns in male transition were sensitive to model values. We propose two explanations for these results. First, phylogenetic transitions may be idiosyncratic so that they depend on the ecological circumstances of individual species. Second, transitions in social traits, such as breeding systems, may be rapid and take place in ecological time, so directional phylogenetic methods that work through longer time scales may not infer accurately the timing and direction of all changes.     

Selection on defensive traits in a sterile caste – caste evolution: a mechanism to overcome life‐history trade‐offs?

SUMMARY During development and evolution individuals generally face a trade-off between the development of weapons and gonads. In termites, characterized by reproductive division of labor, a caste evolved—the soldiers—which is completely sterile and which might be released from developmental trade-offs between weapons and testes. These soldiers are exclusively dedicated to defense. First, we investigated whether defensive traits are under selection in sterile termite soldiers using allometric analyses. In soldiers of the genus Cryptotermes phragmotic traits such as a sculptured and foreshortened head evolve rapidly but were also lost twice. Second, we compared the scaling relationships of these weapons with those in solitary insects facing a trade-off between weapons and gonads. Defensive traits consistently had lower slopes than nondefensive traits which supports the existence of stabilizing selection on soldier phragmotic traits in order to plug galleries. Moreover, soldier head widths were colony specific and correlated with the minimum gallery diameter of a colony. This can proximately be explained by soldiers developing from different instars. The scaling relationships of these termite soldiers contrast strikingly with those of weapons of solitary insects, which are generally exaggerated (i.e., overscaling) male traits. These differences may provide important insights into trait evolution. Trade-offs constraining the development of individuals may have been uncoupled in termites by evolving different castes, each specialized for one function. When individuals in social insect are "released" from developmental constraints through the evolution of castes, this certainly contributed to the ecological and evolutionary success of social insects.     

Do parasitic common goldeneye Bucephala clangula females choose nests on the basis of host traits or nest site traits?

Conspecific brood parasitism (CBP) is an important alternative breeding strategy for gaining reproductive output in birds. While interactions between hosts and parasites and consequences of CBP to breeding success of both parties have been studied a lot, the roles of host characteristics and nest site characteristics in CBP have received less attention. We studied the relative importance of host‐related traits, such as female condition and breeding experience, and nest‐site‐related factors, such as overall nest site preference and occupation rate, in explaining the occurrence of CBP in a common goldeneye Bucephala clangula population. We used spatially and temporally extensive data sets, analysed the data with generalized linear mixed models that allowed us to account for the non‐independency of individual nesting attempts across females and nesting sites, and used an information theoretic approach in model selection and inference. About half of the nests were parasitized annually during the seven year study period. The occurrence of CBP decreased with advancement of the breeding season but late nests were also frequently parasitized. We found that the occurrence of CBP was better explained by nest‐site characteristics than host traits, implying that parasitic females target a given nest based on factors related to the nest site itself rather than on the host. Our results suggest that more attention should be paid to factors associated with nest site attractiveness and quality when studying laying decisions of parasites and the occurrence of CBP in general.     

A phallus for free? Quantitative genetics of sexual trade‐offs in the snail Bulinus truncatus

Resource allocation is thought to play a key role in the coexistence of different sexual morphs within hermaphroditic species. Indeed, most models assume that sexual functions are subject to a balance between reproductive advantage and energetic cost. Various types of cost (e.g. organ construction, maintenance and utilization) and levels of trade-off (physiological and genetic) may be considered. We here examine physiological and genetic costs of phallus construction and maintenance in Bulinus truncatus, a snail species in which aphallic individuals (without phallus) coexist with regular hermaphrodites. We use a quantitative genetic design involving 37 inbred lines (four populations) known to produce different proportions of aphallics, to test for the existence of genetic and nongenetic correlations between aphally and a range of life-history traits over the totality of the life cycle. Our results show that aphallic and euphallic individuals of the same line do not show consistent differences in either growth, fecundity (including offspring survival), or longevity. Furthermore, none of these traits is genetically correlated across lines with the frequency of the aphallic morph. We conclude that the cost of the construction and maintenance of the phallus must be very low in this species. Future studies should investigate the cost associated with using the phallus (i.e. male outcrossing behaviour) to explain the maintenance of high frequencies of aphallic individuals in natural populations.     

To be or not to be? Mating success and survival trade‐offs when switching between alternative reproductive tactics

Hormones underlie the decision of assuming a territorial or a nonterritorial role, with territorial individuals usually having higher hormonal levels than nonterritorial individuals. As a territorial status is linked to higher mating opportunities, it is unclear why animals do not keep high hormonal levels and one explanation is that this would imply survival costs. We have tested this using males of the territorial damselfly Argia emma in the field. We increased juvenile hormone (JH) levels using methoprene in both territorial and nonterritorial males and predicted that: (i) males will keep (the case of territorial males) or become (the case of nonterritorial males) territorial after hormonal increase, and (ii) there will be an increase in mating success for nonterritorial males only and an impaired survival for both male tactics. Hormonally treated males remained or became territorial but had their survival impaired compared with control groups. Also, hormonally treated, ex‐nonterritorial males increased their mating success compared with the other control, nonterritorial males. The reduced survival can be explained proximally by the energy devoted either to the enhanced aggression showed during territory defence or immune function (as detected previously in damselflies). Although nonterritorial males may increase their mating success by switching to a territorial tactic, they are possibly unable to do it naturally as JH is dietary dependent and usually nonterritorial animals are in poorer condition than territorial animals.     

When do trade‐offs occur? The roles of energy constraints and trait flexibility in bushcricket populations

In many animal species, the expression of sexually selected traits is negatively correlated with traits associated with survival such as immune function, a relationship termed a ‘trade‐off’. But an alternative in which sexually selected traits are positively correlated with survival traits is also widespread. We propose that the nature of intertrait relationships is largely determined by overall energy expenditure, energy availability and trait flexibility, with trade‐offs expected when individuals are subject to energy constraints. We tested this hypothesis in Ephippiger diurnus, a European bushcricket in which males are distinguished by two prominent sexually selected traits, acoustic calls and a large spermatophore transferred to the female at mating, and where immune function may be critical in survival. Ephippiger diurnus are distributed as small, isolated populations that are differentiated genetically and behaviourally. We analysed songs, spermatophores and the immune function in male individuals from eight populations spanning a range of song types. As predicted, we only found trade‐offs in those populations that expended more energy on song and were less flexible in their ability to adjust that expenditure. Ultimately, energy constraints and resulting trade‐offs may limit the evolution of song exaggeration in E. diurnus populations broadcasting long calls comprised of multiple ‘syllables’.     

Optimality modeling and fitness trade‐offs: when should plants become pollinator specialists?

The assumption that flowers readily evolve specializations for pollination by particular animals has been central to a standard view of pollinator-mediated adaptive divergence in angiosperms. Stebbins' Most Effective Pollinator Principle (MEPP) formalized this assumption in proposing that a plant should always evolve specializations to its most effective pollinator. I argue that the MEPP and its corollaries are unsupported by basic models of phenotypic selection which predict that a plant should evolve greater specialization to a particular pollinator when the marginal fitness gain exceeds the marginal fitness loss from becoming less adapted to all other pollinators. Differences in pollinator effectiveness are neither necessary nor sufficient to predict specialization. Differences in effectiveness certainly can foster floral specialization to the most effective pollinator in some cases, but when adaptation to a relatively ineffective pollinator requires little loss in the fitness contribution of a more effective pollinator, plants may exhibit striking specializations for the less effective pollinator. Recognizing that the effectiveness of pollinators is not tightly coupled to their importance in selecting for phenotypic novelty may resolve the mismatch between floral features that appear to represent clear evolutionary responses to specific pollinators and patterns of flower visitation that often seem generalized. To shed light on agents of selection and the adaptive value of floral traits I argue that we must go beyond measures of pollinator effectiveness to investigate pollinator-mediated fitness trade-offs over a range of floral phenotypes.     

Trade‐offs in oviposition choice? Food‐dependent performance and defence against predators of a herbivorous sawfly

The sawfly Athalia rosae L. (Hymenoptera: Tenthredinidae) is a feeding specialist on plant species of the Brassicaceae, which are characterised by secondary metabolites, called glucosinolates. The larvae can take up the respective glucosinolates of their hosts and concentrate them in their haemolymph to protect themselves against predators. Oviposition preferences of naïve females were tested for three species, Sinapis alba L., Brassica nigra (L.) Koch, and Barbarea stricta Andrz., and were related to larval performance patterns. Larvae were reared on either one of these plants and it was investigated how host‐plant quality influences both the developmental times and growth of larvae (bottom‐up) and the defence efficiency against predators (top‐down). Innately, almost all adult females avoided B. stricta for oviposition and clearly preferred B. nigra over S. alba. On average, larvae developed best on B. nigra. Female larvae reached similar final body masses on all host‐plant species, but males reared on S. alba were slightly lighter. The developmental time of larvae reared on B. stricta was significantly longer than on the other two plants. However, larvae reared on B. stricta were best protected against the predatory wasp Polistes dominulus Christ (Hymenoptera: Vespidae). The wasps rejected these larvae most often, while they attacked larvae reared on S. alba most frequently. Thus, larvae feeding on B. stricta theoretically run a higher risk of predation due to a prolonged developmental time, but in practice they are better protected against predators. Overall, oviposition preferences of A. rosae seem to be more influenced by bottom‐up effects on larval performance than by top‐down effects.     

Do non‐additive effects on decomposition in litter‐mix experiments result from differences in resource quality between litters?

Differences in resource quality between litter species have been postulated to explain why litter-mixtures may decompose at a different rate to that which would be predicted from single species litters (termed 'non-additive effects'). In particular, positive, non-additive effects of litter-mixing on decomposition have been explained by differences in initial nitrogen concentration between litter species. This interpretation is confounded because litter species that differ in nitrogen content also differ by a number of other resource quality attributes. Thus, to investigate whether initial nitrogen concentration does account for positive, non-additive effects of litter-mixing, we mixed grass litters that differed in initial nitrogen concentration but not species or structural plant part identity, and then followed mass loss from the litter-mixes over time. We used the litterbag technique and three grass species for which a gradient of four distinct initial nitrogen concentrations had been generated. We produced all no- to four-mix compositions of litter qualities for each species. Litter from different species was never mixed.
Contrary to what would be predicted, we found that when litters of the same species but with different initial nitrogen concentrations were mixed, that negative, non-additive effects on decomposition were generally observed. In addition, we found that once mixed, increasing litter quality richness from two to four mixtures had no significant, non-additive effect on decomposition. Litter quality composition explained little of the experimental variation when compared to litter quality richness, and different compositions generally behaved in the same manner. Our findings challenge the commonly held assumption that differences in nitrogen concentration between plant species are responsible for positive, non-additive effects of litter-mixing on decomposition.     

Is there a trade‐off between horn growth and survival in adult female chamois?

Life‐history theory predicts trade‐offs in energy allocation between different life‐history traits when resources are limited, i.e. certain traits should be negatively correlated. However, individuals differ in their ability to acquire resources, which can lead to positive correlations between traits at the population level. Here, we investigated the consequences of the allocation in horn growth and body mass on survival in a bovid (Rupicapra rupicapra) with capture‐mark re‐sighting data on 161 females. In female ungulates, body mass often covaries positively with demographic performance and the few studies on horn size suggest that this trait could be a signal of individual quality. Thus, we expected to measure positive correlations between the allocation in these traits and female survival. However, body mass was not correlated to female survival and there was only a negative, though marginal, effect of horn growth. Hence, it seems that the allocation in growth is not an indicator of female quality. Future studies could investigate the importance of growth on female reproduction to evaluate its effect on lifetime reproductive success. Moreover, it is important to confirm in other populations our result that suggests a cost of the allocation in horn growth to better understand the presence of horns in female bovids. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 516–521.     

Time matching between grooming partners: Do methodological distinctions between short versus long‐term reciprocation matter?

Primatologists have long focused on grooming exchanges to examine aspects of social relationships, co‐operation, and social cognition. One particular interest is the extent to which reciprocating grooming partners time match, and the time frame over which they do so. Conclusions about time matching vary across species. Generally, researchers focus on the duration of pauses between grooming episodes that involve a switch in partner roles and choose a cut‐off point to distinguish short from longer‐term reciprocation. Problematically, researchers have made inconsistent choices about cut‐offs. Such methodological variations are potentially concerning, as it is unclear whether inconsistent conclusions about short‐term time matching are attributable to species/ecological differences, or are due in part to methodological inconsistency. We ask whether various criteria for separating short versus long‐term reciprocation influence conclusions about short‐term time matching using data from free‐ranging rhesus ( Macaca mulatta) and captive‐crested macaques ( Macaca nigra). We compare several commonly used cut‐offs to ones generated by the currently preferred approach—survival analysis. Crested macaques displayed a mild degree of time matching regardless of the cutoff used. For rhesus macaques, whereas most cut‐offs yielded similar degrees of time matching as the one derived from survival analysis, very short ones significantly underestimated both the degree of time matching and the influence of rank distance on time matching. Although researchers may have some flexibility in their choice of cut‐offs, we suggest that they employ caution by using survival analysis when possible, and when not possible, by avoiding very short time windows.     

Do abandoned nests of leaf‐cutting ants enhance plant recruitment in the Atlantic Forest?

The role played by abandoned nests of leaf‐cutting ants (Atta spp.) as a small‐scale disturbance regime that affects plant recruitment, species coexistence and forest regeneration remains poorly investigated. Here we examine whether abandoned nests of Atta cephalotes serve as regeneration niches and operate as particular plant recruitment habitats, favouring forest regeneration after ant activities cease and leading to the establishment of taxonomically/ecologically distinct plant assemblages. Soil properties, canopy openness, light availability and regenerating plant assemblages were evaluated across 18 nests and adjacent control plots in a large remnant of Atlantic Forest in north‐east Brazil from December 2004 to December 2005. Surprisingly, nests and control plots exhibited very similar light environments irrespective of nest age, but nest soils exhibited substantial reductions in carbon content (1.45 ± 0.24 vs. 1.79 ± 0.13%) and organic matter (2.50 ± 0.41 vs. 3.08 ± 0.23%), and proved to be much more resistant to penetration (30.57 ± 6.08 vs. 39.48 ± 7.53 mm). Functional signature of regenerating plant assemblages exhibited little variation across both habitat types, as they were dominated by pioneer, small‐seeded and vertebrate‐dispersed species. However, abandoned nests exhibited less dense, impoverished and more homogeneous regenerating plant assemblages at local and landscape scale; they clearly lacked nest‐dependent plant species and represented floristic subsets of the flora inhabiting the undisturbed forest. This recruitment bottleneck was transient in the long term because nest‐related effects ameliorated in older nests. Our results suggest that, unlike treefall gaps, abandoned nests represent temporary (relatively long‐lasting) islands of unsuitable substrate that reduce plant recruitment, retard forest regeneration, and fail in providing a special regeneration niche able to promote species coexistence and plant diversity.     

Do interspecific interactions between females drive shifts in habitat use? A test using the lizards Anolis carolinensis and A. sagrei

An important goal in evolutionary ecology is to understand how and why coexisting closely related species partition habitat among themselves. Although studies of interspecific interactions typically focus on males, interactions between females may also play an important role in shaping habitat use within multi‐species communities. The green anole (Anolis carolinensis) exhibits a wide range of habitat use in south‐eastern Louisiana, but its observed habitat use is restricted and altered in areas where it occurs with the introduced Anolis sagrei. We staged interactions between these two species in the laboratory to test the hypothesis that A. sagrei dominate A. carolinensis in contests over shared habitat. We examined whether species identity, bite force, dewlap size, and body size affected the outcome of interspecific interactions between both males and females, and tested the prediction that bite force and size would be the most important determinants of interaction outcomes in both sexes. In male interspecific interactions, we found that individuals with relatively larger dewlaps tended to score higher on aggressive behaviours regardless of species identity, and that interactions consisted of signalling and rarely escalated to physical combat. However, we found that A. sagrei females achieved higher aggressive scores than A. carolinensis females in almost all cases, lending support to the notion that female interspecific behaviour is probably more important than male behaviour in driving changes in habitat use. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 843–851.     

Do large‐seeded herbs have a small range size? The seed mass–distribution range trade‐off hypothesis

We aimed to introduce and test the “seed mass–distribution range trade‐off” hypothesis, that is, that range size is negatively related to seed mass due to the generally better dispersal ability of smaller seeds. Studying the effects of environmental factors on the seed mass and range size of species, we also aimed to identify habitats where species may be at risk and need extra conservation effort to avoid local extinctions. We collected data for seed mass, global range size, and indicators for environmental factors of the habitat for 1,600 species of the Pannonian Ecoregion (Central Europe) from the literature. We tested the relationship between species’ seed mass, range size, and indicator values for soil moisture, light intensity, and nutrient supply. We found that seed mass is negatively correlated with range size; thus, a seed mass–distribution range trade‐off was validated based on the studied large species pool. We found increasing seed mass with decreasing light intensity and increasing nutrient availability, but decreasing seed mass with increasing soil moisture. Range size increased with increasing soil moisture and nutrient supply, but decreased with increasing light intensity. Our results supported the hypothesis that there is a trade‐off between seed mass and distribution range. We found that species of habitats characterized by low soil moisture and nutrient values but high light intensity values have small range size. This emphasizes that species of dry, infertile habitats, such as dry grasslands, could be more vulnerable to habitat fragmentation or degradation than species of wet and fertile habitats. The remarkably high number of species and the use of global distribution range in our study support our understanding of global biogeographic processes and patterns that are essential in defining conservation priorities.     

Combined effects of shade and drought on tulip poplar seedlings: trade‐off in tolerance or facilitation?

It has been hypothesised that many natural plant distribution patterns can be explained by a trade-off between shade and drought tolerance which would make plants more sensitive to shade under dry conditions. On the other hand, shading by nurse plants is often observed to enhance growth and survival of smaller plants in dry areas. This paper describes an experiment designed to address the interactive effects of drought and shade on growth and physiology of tulip poplar seedlings ( Liriodendron tulipifera ) grown in shade houses under different combinations of irradiance (1%, 5%, 12%, 17%, and 27% of ambient PAR) and soil water content (5–9%, 11–15%, and>20%). The results show no evidence that higher PAR levels compensate for the negative effects of drought on photosynthesis and growth. Rather, rates of estimated daily shoot carbon gain decreased with PAR under dry conditions. Daily xylem sapflow patterns indicated that this was associated with a strong reduction in stomatal conductance in plants growing in dry soil and high PAR conditions. Whole-shoot light compensation points were higher for plants raised under higher PAR conditions, but were not significantly influenced by the water treatments. Shoot dark respiration rates decreased under drier conditions. These results do not support the hypothesis of a trade-off between shade and drought tolerance. Instead, they indicate mechanisms that can lead to positive effects of shading by neighbouring plants under dry conditions. Indeed, such facilitation is thought to be important in many plant communities.