How within-group behavioural variation and task efficiency enhance fitness in a social group

How task specialization, individual task performance and within-group behavioural variation affects fitness is a longstanding and unresolved problem in our understanding of animal societies. In the temperate social spider, Anelosimus studiosus, colony members exhibit a behavioural polymorphism; females either exhibit an aggressive 'asocial' or docile 'social' phenotype. We assessed individual prey-capture success for both phenotypes, and the role of phenotypic composition on group-level prey-capture success for three prey size classes. We then estimated the effect of group phenotypic composition on fitness in a common garden, as inferred from individual egg-case masses. On average, asocial females were more successful than social females at capturing large prey, and colony-level prey-capture success was positively associated with the frequency of the asocial phenotype. Asocial colony members were also more likely to engage in prey-capture behaviour in group-foraging situations. Interestingly, our fitness estimates indicate females of both phenotypes experience increased fitness when occupying colonies containing unlike individuals. These results imply a reciprocal fitness benefit of within-colony behavioural variation, and perhaps division of labour in a spider society.     

EVALUATING A HYPOTHESIS ABOUT HETEROCHRONY: LARVAL LIFE-HISTORY TRAITS AND JUVENILE HIND-LIMB MORPHOLOGY IN HYLA CRUCIFER

This paper reports the results of an investigation into whether selection on genetically based differences in the timing or rate of development (heterochrony) can give rise to nonadaptive morphological differences among individual frogs. We used a quantitative-genetics approach to examine the relationships among the life-history characters time to metamorphosis and larval-growth rate and a functionally significant morphological features, relative hind-limb length, in the spring peeper, Hyla crucifer. Time to metamorphosis and growth rate had low heritabilities in our population. Morphological traits had moderate heritabilities. There were positive genetic correlations between the life-history traits and the components of relative hind-limb length but no significant correlations with the shape variable itself. We used field observations of pond-drying time and experimental results of selection on growth rate to simulate the correlated responses of hind-limb shape to four reasonable selection regimes on the life-history traits. We found little evidence to suggest that relative hind-limb length would display much of a correlated response to such selection. The differences in relative hind-limb length seen among closely related species or among populations of a single species that appear to be unrelated to performance differences are not obviously explicable as neutral correlated responses to selection on larval traits.     

Sexual differences in morphology and niche utilization in an aquatic snake,Acrochordus arafurae

Filesnakes (Acrochordus arafurae) are large (to 2 m), heavy-bodied snakes of tropical Australia. Sexual dimorphism is evident in adult body sizes, weight/length ratios, and body proportions (relative head and tail lengths). Dimorphism is present even in neonates. Two hypotheses for the evolution of such dimorphism are (1) sexual selection or (2) adaptation of the sexes to different ecological niches. The hypothesis of sexual selection is consistent with general trends of sexually dimorphic body sizes in snakes, and accurately predicts, for A. arafurae, that the larger sex (female) is the one in which reproductive success increases most strongly with increasing body size. However, the sexual dimorphism in relative head sizes is not explicable by sexual selection.The hypothesis of adaptation to sex-specific niches predicts differences in habitats and/or prey. I observed major differences between male and female A. arafurae in prey types, prey sizes and habitat utilization (shallow versus deep water). Hence, the sexual dimorphism in relative head sizes is attributed to ecological causes rather than sexual selection. Nonetheless, competition between the sexes need not be invoked as the selective advantage of this character divergence. It is more parsimonious to interpret these differences as independent adaptations of each sex to increase foraging success, given pre-existing sexually-selected differences in size, habitat or behavior. Data for three other aquatic snake species, from phylogenetically distant taxa, suggest that sexual dimorphism in food habits, foraging sites and feeding morphology, is widespread in snakes.     

Hydrodynamic image formation by the peripheral lateral line system of the Lake Michigan mottled sculpin, Cottus bairdi

Lake Michigan mottled sculpin (Cottus bairdi) have a lateral-line-mediated prey-capture behaviour that consists of an initial orientation towards the prey, a sequence of approach movements, and a final strike at the prey. This unconditioned behaviour can be elicited from blinded sculpin in the laboratory by both real and artificial (vibrating sphere) prey. In order to visualize what Lake Michigan mottled sculpin might perceive through their lateral line when approaching prey, we have combined anatomical, neurophysiological, behavioural and computational modelling techniques to produce three-dimensional maps of how excitation patterns along the lateral line sensory surface change as sculpin approach a vibrating sphere. Changes in the excitation patterns and the information they contain about source location are consistent with behavioural performance, including the approach pathways taken by sculpin to the sphere, the maximum distances at which approaches can be elicited, distances from which strikes are launched, and strike success. Information content is generally higher for laterally located sources than for frontally located sources and this may explain exceptional performance (e.g. successful strikes from unusually long distances) in response to lateral sources and poor performance (e.g. unsuccessful strikes) to frontal sources.     

Functional and ecological relevance of intraspecific variation in body size and shape in the lizard Podarcis melisellensis (Lacertidae)

Within populations, individual animals may vary considerably in morphology and ecology. The degree to which variation in morphology is related to ecological variation within a population remains largely unexplored. We investigated whether variation in body size and shape among sexes and age classes of the lizard Podarcis melisellensis translates in differential whole-animal performance (sprint speed, bite force), escape and prey attack behaviour in the field, microhabitat use and diet. Male and female adult lizards differed significantly in body size and head and limb proportions. These morphological differences were reflected in differences in bite strength, but not in sprint speed. Accordingly, field measurements of escape behaviour and prey attack speed did not differ between the sexes, but males ate larger, harder and faster prey than females. In addition to differences in body size, juveniles diverged from adults in relative limb and head dimensions. These shape differences may explain the relatively high sprint and bite capacities of juvenile lizards. Ontogenetic variation in morphology and performance is strongly reflected in the behaviour and ecology in the field, with juveniles differing from adults in aspects of their microhabitat use, escape behaviour and diet.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 251–264.     

Functional and ecological significance of relative growth in Alligator

Allometric coefficients are calculated for 27 cranial and 39 postcranial measurements of a growth series of Alligator mississipiensis that spans a size range of an order of magnitude. Developmental patterns are quite-well canalized, as expressed in coefficients of variation of 8 to 10 for isometric variables. A multivariate expression of allometry is discovered using principal components analysis. A number of allometric coefficients have expression in known aspects of the life history of Alligator. Negative allometry of limb lengths and limb proportions shows an ontogenetic decrease in importance of the limbs throughout life, and observations show large animals to be more dependent on water than small ones. Isometry of skull length with respect to body length represents an adaptation to ever-increasing size of prey items as body size increases. Positive allometry of snout length and size of the upper temporal fenestrae finds parallel in the structure of the highly aquatic gavial.     

Artificial selection sheds light on developmental mechanisms of limb elongation

Species diversity in limb lengths and proportions is thought to have evolved adaptively in the context of locomotor and habitat specialization, but the heritable cellular processes that drove this evolution within species are poorly understood. In this study, we take a novel “micro‐evo‐devo” approach, using artificial selection on relative limb length to amplify phenotypic variation in a population of mice, known as Longshanks, to examine the cellular mechanisms of postnatal limb development that contribute to intraspecific limb length variation. Cross‐sectional growth data indicate that differences in bone length between Longshanks and random‐bred controls are not due to prolonged growth, but to accelerated growth rates. Histomorphometric and cell proliferation assays on proximal tibial growth plates show that Longshanks’ increased limb bone length is associated with an increased number of proliferative chondrocytes. In contrast, we find no differences in other growth plate cellular features known to underlie interspecific differences in limb bone size and shape, such as the rates of chondrocyte proliferation or the size and number of hypertrophic cells in the growth plate. These data suggest that small differences among individuals in the number of proliferating chondrocytes are a potentially important determinant of selectable intraspecific variation in individual limb bone lengths, independent of body size.     

Energetic benefits and adaptations in mammalian limbs: Scale effects and selective pressures

Differences in limb size and shape are fundamental to mammalian morphological diversity; however, their relevance to locomotor costs has long been subject to debate. In particular, it remains unknown if scale effects in whole limb morphology could partially underlie decreasing mass‐specific locomotor costs with increasing limb length. Whole fore‐ and hindlimb inertial properties reflecting limb size and shape—moment of inertia (MOI), mass, mass distribution, and natural frequency—were regressed against limb length for 44 species of quadrupedal mammals. Limb mass, MOI, and center of mass position are negatively allometric, having a strong potential for lowering mass‐specific locomotor costs in large terrestrial mammals. Negative allometry of limb MOI results in a 40% reduction in MOI relative to isometry's prediction for our largest sampled taxa. However, fitting regression residuals to adaptive diversification models reveals that codiversification of limb mass, limb length, and body mass likely results from selection for differing locomotor modes of running, climbing, digging, and swimming. The observed allometric scaling does not result from selection for energetically beneficial whole limb morphology with increasing size. Instead, our data suggest that it is a consequence of differing morphological adaptations and body size distributions among quadrupedal mammals, highlighting the role of differing limb functions in mammalian evolution.     

The evolution of plumage polymorphism in birds of prey and owls: the apostatic selection hypothesis revisited

Co-evolution between phenotypic variation and other traits is of paramount importance for our understanding of the origin and maintenance of polymorphism in natural populations. We tested whether the evolution of plumage polymorphism in birds of prey and owls was supported by the apostatic selection hypothesis using ecological and life-history variables in birds of prey and owls and performing both cross taxa and independent contrast analyses. For both bird groups, we did not find any support for the apostatic selection hypothesis being the maintaining factor for the polymorphism: plumage polymorphism was not more common in taxa hunting avian or mammalian prey, nor in migratory species. In contrast, we found that polymorphism was related to variables such as sexual plumage dimorphism, population size and range size, as well as breeding altitude and breeding latitude. These results imply that the most likely evolutionary correlate of polymorphism in both bird groups is population size, different plumage morphs might simply arise in larger populations most likely because of a higher probability of mutations and then be maintained by sexual selection.     

Prey size selection and cannibalistic behaviour of juvenile barramundi Lates calcarifer

This study assessed the cannibalistic behaviour of juvenile barramundi Lates calcarifer and examined the relationship between prey size selection and energy gain of cannibals. Prey handling time and capture success by cannibals were used to estimate the ratio of energy gain to energy cost in prey selection. Cannibals selected smaller prey despite its capability of ingesting larger prey individuals. In behavioural analysis, prey handling time significantly increased with prey size, but it was not significantly affected by cannibal size. Conversely, capture success significantly decreased with the increase of both prey and cannibal sizes. The profitability indices showed that the smaller prey provides the most energy return for cannibals of all size classes. These results indicate that L. calcarifer cannibals select smaller prey for more profitable return. The behavioural analysis, however, indicates that L. calcarifer cannibals attack prey of all size at a similar rate but ingest smaller prey more often, suggesting that prey size selection is passively orientated rather than at the predator's choice. The increase of prey escape ability and morphological constraint contribute to the reduction of intracohort cannibalism as fish grow larger. This study contributes to the understanding of intracohort cannibalism and development of strategies to reduce fish cannibalistic mortalities.     

Hybridization between two insectivorous bird species and the effect on prey-handling efficiency

Gene flow between closely related species is not always impeded by hybrid inviability or sterility but may be limited by behavioural or ecological factors. European common redstarts, Phoenicurus phoenicurus, and black redstarts, P. ochruros, are insectivorous passerine bird species which regularly interbreed in the wild and produce viable and fertile progeny. Yet, the isolation barriers, which prevent extensive gene flow between these distinct species, are unknown. I studied prey-handling time and efficiency in both species and the F₁-hybrids in captivity. All birds changed prey-handling mode from a single pick to more complex behaviour with increasing prey length and their handling time was a positive function of prey length. Common redstarts tended to handle their prey quicker than black redstarts. Hybrids appeared to be intermediate. However, individual differences were significant, but group membership was not. Handling efficiency decreased with increasing prey length and was determined by individual and family effects. Hybrid females handled their prey as efficient as hybrid males. Small differences in beak morphology could not explain individual differences in prey-handling behaviour. The results of this study suggest that prey-handling in F₁-hybrids is not a postzygotic barrier which prevent gene flow between common and black redstarts in the wild.     

The dicey dinner dilemma: Asymmetry in predator–prey risk‐taking,a broadly applicable alternative to the life‐dinner principle

Forty years ago, the ‘life‐dinner principle’ was proposed as an example of an asymmetry that may lead prey species to experience stronger selection than their predators, thus accounting for the high frequency with which prey escape alive from interaction with a predator. This principle remains an influential concept in the scientific literature, despite several works suggesting that the concept relies on many under‐appreciated assumptions and does not apply as generally as was initially proposed. Here, we present a novel model describing a very different asymmetry to that proposed in the life‐dinner principle, but one that could apply broadly. We argue that asymmetries between the relative costs and benefits to predators and prey of selecting a risky behaviour during an extended predator–prey encounter could lead to an enhanced likelihood of escape for the prey. Any resulting advantage to prey depends upon there being a behaviour or choice that introduces some inherent danger to both predator and prey if they adopt it, but which if the prey adopts the predator must match in order to have a chance of successful predation. We suggest that the circumstances indicated by our model could apply broadly across diverse taxa, including both risky spatial or behavioural choices.     

‘A prey in the hand’, multi-prey capute behaviour in a sit-and-wait predator,Ranatra dispar (Heteroptera: Nepidae), the Water Stick Insect

The Water Stick Insect,Ranatra dispar, is shown to be able to capute and hold a number of prey simultaneously. Capture of prey characteristically occurs in 3 distinct patterns (Type 1, 2 and 3), each characterized by a different number of prey caught. The time since las feeding by the predator has a significant effect on wether the predator will capture more than one prey. Once feeding starts, there is a critical period during which, if an encounter takes place the predator will attempt to capture either a second or third prey. The critical peroid is longer, the higher the motivation level of the predator. It is suggested that this prey-capture behaviour potentially increase the size of a meal as groups of prey move past the stationary prdator.     

Coevolving avian eye size and brain size in relation to prey capture and nocturnality

Behavioural adaptation to ecological conditions can lead to brain size evolution. Structures involved in behavioural visual information processing are expected to coevolve with enlargement of the brain. Because birds are mainly vision-oriented animals, we tested the predictions that adaptation to different foraging constraints can result in eye size evolution, and that species with large eyes have evolved large brains to cope with the increased amount of visual input. Using a comparative approach, we investigated the relationship between eye size and brain size, and the effect of prey capture technique and nocturnality on these traits. After controlling for allometric effects, there was a significant, positive correlation between relative brain size and relative eye size. Variation in relative eye and brain size were significantly and positively related to prey capture technique and nocturnality when a potentially confounding variable, aquatic feeding, was controlled statistically in multiple regression of independent linear contrasts. Applying a less robust, brunching approach, these patterns also emerged, with the exception that relative brain size did not vary with prey capture technique. Our findings suggest that relative eye size and brain size have coevolved in birds in response to nocturnal activity and, at least partly, to capture of mobile prey.     

Use of prey-specific predatory behaviour by North American jumping spiders (Araneae, Salticidae) of the genus Phidippus

Prey-capture behaviour of seven species of Phidippus is studied using two types of prey, house flies (adult Musca domestica ) and caterpillars (cabbage loopers, larval Trichoplusia ni ). Each species is shown to be a versatile predator which uses a different prey-specific prey-capture behaviour depending on whether prey is a fly or a caterpillar: spiders approached the two types of prey differently and leapt on them from different distances. Interspecific differences among Phidippus species are not pronounced, cxcept for variation in the distance from which spiders jumped and the spider's success at capturing a fly on the first try.     

Assessing the patterns and drivers of shape complexity in the amblypygid pedipalp

Amblypygi is an arachnid order possessing a unique pair of spined pedipalps: appendages that perform in prey capture, courtship, and contest. Pedipalp length, hypothesized to be under sexual selection, varies markedly across amblypygid species, and pedipalp spination, thought to reflect selection for function in prey capture, also differs interspecifically. Differences in pedipalp shape between species may indicate that the relative strength of selection for prey capture and sexual selection vary across the group. However, interspecific differences in pedipalp shape have not been quantified, due to difficulties in identifying homologous features. For the first time, we quantify trends in amblypygid pedipalp shape complexity. We use elliptical Fourier analysis to quantify 2D complexity in pedipalp outlines across eleven species and six genera. We find that complexity significantly decreases as pedipalp length increases. This appears to be driven by relative spine length, suggesting that a trade‐off exists between pedipalp length and spination. Furthermore, significant female‐biased sexual dimorphism in shape complexity is present in the tibial segment of the amblypygid pedipalp. Our results provide novel insights into the drivers of amblypygid pedipalp evolution and suggest that a functional trade‐off between performance in prey capture and other functions under sexual selection exist in this enigmatic structure.     

Do juvenile gape-limited predators compensate for their small size when feeding?

When juvenile and adult animals occur syntopically, juveniles are at a distinct performance disadvantage due to their absolutely small size. Yet, optimal foraging theory predicts that juvenile predators should feed efficiently in order to compete with adults for food, and to minimize their exposure to predators. Previous authors have suggested that one way for juvenile animals to accomplish these ecological tasks is by increasing their overall feeding performance relative to adults (compensation hypothesis). Nonetheless, only a handful of studies have tested whether juvenile animals have increased feeding performance (e.g. decreased ingestion and/or handling times relative to body size) compared with adults. We tested this hypothesis by examining the ontogeny of head dimensions and feeding performance (ingestion time and number of mandibular protractions) on fish prey for broad-banded water snakes Nerodia fasciata . Individuals were fed fish scaled in a 1:1 ratio to their head width. All head dimensions scaled with significant negative allometry versus body size, and thus smaller snakes had relatively larger heads for their body size compared with larger snakes. By contrast, most head variables (except head volume) exhibited positive allometry versus head length, demonstrating that larger snakes had larger head dimensions relative to head size compared with smaller snakes. In the performance trials, smaller snakes had worse feeding performances when feeding on similarly sized fish prey (relative to their head width) compared with larger snakes. Therefore, these data show that smaller water snakes do not compensate for their size through increased feeding performance.     

Evaluating the influence of diet-related variables on breeding performance and home range behaviour of a top predator

Diet composition is linked to reproductive performance directly or indirectly by other life-history traits, including home range behaviour. The relationships between prey abundance, diet and individual fitness have often been explored. However, these relationships are complex and difficult to disentangle, especially in vertebrate top predators. Here, we present the results of a long-term study using multi-model inference procedures to elucidate the influence of diet-related variables on breeding parameters and home range behaviour of a top predator, the eagle owl Bubo bubo. Superpredation, diet diversity, rat biomass and rabbit mean weight were the most important variables when analysing reproductive parameters, suggesting that less diverse diets with greater rabbit biomass percentage may benefit reproductive performance, whereas rat biomass percentage is apparently associated with greater variation of breeding success. Earlier laying dates seem to be associated with the consumption, on average, of smaller rabbits. On the other hand, edge density was the most relevant factor determining the variation in home range behaviour, with individual characteristics, such as age and sex, also being important. Although the relative importance of the diet-related variables was generally low, mean weight of alternative prey, diet diversity and rabbit biomass also helped to explain home range parameters. In an optimal foraging context, centred on the abundance of the main prey species, our results suggest that when rabbits are less available eagle owls may increase home range size in order to obtain alternative prey, increasing at the same time their dietary diversity, which may also require higher movement speed.     

Adhesive performance of the stick-capture apparatus of rove beetles of the genus Stenus (Coleoptera, Staphylinidae) toward various surfaces

Rove beetles of the genus Stenus possess a unique adhesive prey-capture apparatus that enables them to catch elusive prey such as springtails over a distance of several millimeters. The prey-capture device combines the hierarchically organized morphology of dry adhesive systems with the properties of wet ones, since an adhesive secretion is released into the contact zone. We hypothesize that this combination enables Stenus species successfully to capture prey possessing a wide range of surface structures and chemistries. We have investigated the influence of both surface energy and roughness of the substrate on the adhesive performance of the prey-capture apparatus in two Stenus species. Force transducers have been used to measure both the compressive and adhesive forces generated during the predatory strike of the beetles on (1) epoxy resin surfaces with defined roughness values (smooth versus rough with asperity diameters ranging from 0.3 to 12 μm) and (2) hydrophobic versus hydrophilic glass surfaces. Our experiments show that neither the surface roughness nor the surface energy significantly influences the attachment ability of the prey-capture apparatus. Thus, in contrast to the performance of locomotory adhesive systems in geckos, beetles, and flies, no critical surface roughness exists that might impede adhesion of the prey-capture apparatus of Stenus beetles. The prey-capture apparatus of Stenus beetles is therefore well adapted to adhere to the various unpredictable surfaces with diverse roughness and surface energy occurring in a wide range of potential prey.     

Sex differences in impaling behaviour of Great Grey Shrike Lanius excubitor: Do males have better impaling skills than females?

Prey impaling in shrikes Laniidae is considered to be a feeding adaptation to dismember and consume large prey and is unique among food-storing animals. However, other exaptations of this behaviour were recorded, including signals in mate choice, where cache size is a sign of male quality. Thus, due to a strong sexual selection, male and female birds might differ in their behavioural patterns of impaling behaviour. We examined sex differences in impaling behaviour of the Great Grey Shrike Lanius excubitor - one of the species where caches are known to be sexual signals. Data were collected in western Poland during breeding seasons in the years 2006-2010. In the studied population, we recorded several sex-specific differences in impaling behaviour. Males impaled prey, invertebrates as well as vertebrates, faster and with fewer attempts per impaling event than females. Sexes differed in the location of impaled prey; males selected more visible places, especially during the mating and courtship phase, whereas females impaled prey in concealed locations. Males also had slightly better impaling success compared to females. We suggest that sex differences in impaling behaviour may be due to different uses of impaled prey, and the better impaling skills of males may be the result of better experience in impaling which is forced by sexual selection in this species. We also discuss other factors which might trigger sex-specific differences in food caching by shrikes.