Ecological consequences of body size: a model for patch choice in desert rodents

Summary Recent models exploring the ecological consequences of body size have assumed that its primary effect is to determine how easily individual prey of different sizes can be pursued or handled. However, for predators that eat small, particulate food, size-related costs associated with finding and harvesting prey should be at least as important as those associated with consuming individual prey once thay have been harvested. Such predators should have generalized diets, and body size differences would not be expected to influence substantially the sizes of prey eaten. The effect of body size on spatial patterns of foraging could, however, be substantial for these predators if prey have a patchy distribution.I develop a simple model for a particle feeder foraging in patchy environments and use it to examine the special case of patch choice by seed-eating desert rodents. The model implies that for most parameter values large and small animals should specialize to different extents on the most profitable patches. Size differences among coexisting desert rodents therefore can be expected to promote partitioning of food by differential patch choice. Preliminary observations of desert rodent seed dispersion and microhabitat preferences indicate that interspecific differences in patch choice do exist.The model predicts that the nature of the relationship between size and patch choice depends on the values taken by certain model parameters. Thus, although the model predicts that patch choice generally should vary with body size, the spatial scale of patchiness and the way in which within-patch harvest rates and between-patch travel velocities scale with size determine whether, and in what way, body size should affect patch choice. As yet estimates of these parameters for heteromyid rodents are not precise enough for us to have much confidence in specific model predictions about this system. However, it will only be a matter of time before we can derive better estimates; in principle the model is testable, and when suitably modified should be applicable to many systems.     

Foraging mode affects the evolution of egg size in generalist predators embedded in complex food webs

Ecological networks incorporate myriad biotic interactions that determine the selection pressures experienced by the embedded populations. We argue that within food webs, the negative scaling of abundance with body mass and foraging theory predict that the selective advantages of larger egg size should be smaller for sit‐and‐wait than active‐hunting generalist predators, leading to the evolution of a difference in egg size between them. Because body mass usually scales negatively with predator abundance and constrains predation rate, slightly increasing egg mass should simultaneously allow offspring to feed on more prey and escape from more predators. However, the benefits of larger offspring would be relatively smaller for sit‐and‐wait predators because (i) due to their lower mobility, encounters with other predators are less common, and (ii) they usually employ a set of alternative hunting strategies that help to subdue relatively larger prey. On the other hand, for active predators, which need to confront prey as they find them, body‐size differences may be more important in subduing prey. This difference in benefits should lead to the evolution of larger egg sizes in active‐hunting relative to sit‐and‐wait predators. This prediction was confirmed by a phylogenetically controlled analysis of 268 spider species, supporting the view that the structure of ecological networks may serve to predict relevant selective pressures acting on key life history traits.     

Optimal foraging constrains macroecological patterns: body size and dietary niche breadth in lizards

Aim  To explore and identify probable mechanisms contributing to the relationships among body size, dietary niche breadth and mean, minimum, maximum and range of prey size in predaceous lizards.
Location  Our data set includes species from tropical rainforests, semi-arid regions of Brazil, and from deserts of the south-western United States, Australia and the Kalahari of Africa.
Methods  We calculated phylogenetic and non-phylogenetic regressions among predator body size, dietary breath and various prey size measures.
Results  We found a negative association between body size and dietary niche breadth in 159 lizard species sampled across most evolutionary lineages of squamate reptiles and across major continents and habitats. We also show that mean, minimum, maximum and range of prey size were positively associated with body size.
Main conclusions  Our results suggest not only that larger lizards tend to eat larger prey, but in doing so offset their use of smaller prey. Reduction of dietary niche breadth with increased body size in these lizards suggests that large predators target large and more profitable prey. Consequently, the negative association between body size and niche breadth in predators most likely results from optimal foraging. Though this result may appear paradoxical and runs counter to previous studies, resources for predators may be predictably more limited than resources for herbivores, thus driving selection for more profitable prey.     

Optimal foraging on perilous prey: risk of bill damage reduces optimal prey size in oystercatchers

Intake rate maximization alone is not always sufficient in explainingprey size selection in predators. For example, bivalve-feedingoystercatchers regularly select smaller prey than expected ifthey aimed to maximize their intake rate. It has been proposedthat to these birds large prey are "risky," in the sense thatbirds may damage their bills when feeding on large bivalves.Large bivalves yield more energy, but according to this hypothesisthis is achieved at the expense of energy yield in the longterm when (1) the risk of bill damage increases with prey sizeand (2) foraging with a damaged bill is less effective. In accordancewith this hypothesis, we show that captive oystercatchers feedingon large cockles experienced a high probability of bill tipdamage, while bill damage was absent when cockles were small.Moreover, among free-living oystercatchers the prevalence ofbill damage was correlated with mean cockle size near the capturesite, and the data on captive birds fit in this pattern. Foodintake of captive oystercatchers feeding exclusively on cockleswas reduced by 23% after bill damage, and free-living birdswith damaged bills had 14 g lower mass. Because lower body masswas associated with higher mortality probability, these resultsindicate long-term costs associated with feeding on large cockles.We conclude that the risk of bill damage can potentially explainwhy oystercatchers avoid large bivalves and that oystercatchersmay maximize long-term intake rate by selecting prey sizes thatare "suboptimal" from a short-term rate-maximizing point ofview.     

Variability in seed size selection by granivorous passerines: effects of bird size,bird size variability,and ecological plasticity

The niche variation hypothesis predicts a direct relationship between intraspecific variability in feeding ecology and the variability of the morphological traits related to feeding behaviour. The following study tests this prediction by measuring in captivity the seed size preferences and the morphology of 9–11 individuals of seven specialized granivorous bird species. The average seed size preferences of these birds have previously been shown to be related to components of bill size. The ranges of seed sizes selected were related to the mean bill sizes of birds in a way that paralleled the patterns found when analysing average values. Bill and body size variability were not related, however, to the range of seed preferences after controlling for the significant mean-variance relationship showed by morphological traits. Thus, results do not support the niche variation hypothesis. the significant effect of average bill size on diet variability was consistent with the direct relationship between bird size and ecological plasticity expected on the basis of the shape of the family of functions relating seed size and seed profitability for different-sized birds. These findings suggest morphological mechanisms for ecological plasticity whose generality and evolutionary significance merit further research.     

Evolution of reversed sexual size dimorphism and role partitioning among predatory birds, with a size scaling of flight performance

To explain the adaptive significance of sex role partitioning and reversed sexual size dimorphism among raptors, owls and skuas, where females are usually larger than males, we combine several previous hypotheses with some new ideas. Owing to their structural and behavioural adaptations for prey capture, predatory birds have better prospects than other birds of defending their offspring against nest predators. This makes sex role partitioning advantageous; one parent guards the offspring while the other forages for the family. Further, among predators hunting alert prey such as vertebrates, two mates because of interference may not procur much more food than would one mate hunting alone. By contrast, two mates feeding on less alert prey may together obtain almost twice as much food as one mate hunting alone. For these reasons, partitioning of breeding labours might be adaptive only in predatory birds. An initial imbalance favours female nest guarding and male foraging: the developing eggs might be damaged if the female attacks prey; their mass might reduce her flight performance; she must visit the nest to lay; and the male feeds her before she lays (‘courtship feeding’). Increased female body size should enhance egg production, incubation, ability to tear apart prey for the young, and, in particular, offspring protection in predatory birds. Efficient foraging during the breeding period then becomes most important for the male. This imposes great demands on aerial agility in males, particularly among predators of agile prey. Flight performance decreases with increasing size in five of six aspects explored. The male must therefore not be too large in relation to the most important prey. For these reasons, he should be smaller than the female. Among predatory birds, size dimorphism increases with the proportion of birds in the diet, which may be explained as follows. Adult birds have mainly one type of predators: other predatory birds. Because almost only these specialists exploit adult birds, they carry out most of the cropping of this prey. A predator of easier prey competes with many other kinds of predators, which considerably reduce prey abundance in its territory. This is not so for predators of adult birds. Further, because birds are extremely agile, the specialized predator can hunt efficiently only within a limited size range of birds, whose flight skill it can match. Increased size dimorphism among these predators therefore should be particularly important for enlarging the combined food base of the pair. A bird specialist may consume much of the available prey in the suitable size range during the breeding period. When the predator's young are large enough to defend themselves, the female aids better by hunting than by guarding the chicks. It is advantageous among bird specialists if she hunts prey of other sizes than does the male, who has by then reduced prey abundance in his prey size class. But among predatory birds hunting easier prey the female gains little by hunting outside the male's prey spectrum, because other kinds of predators will have reduced the prey abundance outside as well as inside the male's preferred size range. Intra-pair food separation through large sexual size dimorphism therefore should be particularly advantageous among predators of birds. This may be the main reason why the degree of size dimorphism increases with the dietary proportion of birds.     

An asymmetric producer-scrounger game: body size and the social foraging behavior of coho salmon

A tension between cooperation and conflict characterizes the behavioral dynamics of many social species. The foraging benefits of group living include increased efficiency and reduced need for vigilance, but social foraging can also encourage theft of captured prey from conspecifics. The payoffs of stealing prey from others (scrounging) versus capturing prey (producing) may depend not only on the frequency of each foraging strategy in the group but also on an individual’s ability to steal. By observing the foraging behavior of juvenile coho salmon (Oncorhynchus kisutch), we found that, within a group, relatively smaller coho acted primarily as producers and took longer to handle prey, and were therefore more likely to be targeted by scroungers than relatively larger coho. Further, our observations suggest that the frequency of scrounging may be higher when groups contained individuals of different sizes. Based on these observations, we developed a model of phenotype-limited producer-scrounger dynamics, in which rates of stealing were structured by the relative size of producers and scroungers within the foraging group. Model simulations show that when the success of stealing is positively related to body size, relatively large predators should tend to be scroungers while smaller predators should be producers. Contrary to previous models, we also found that, under certain conditions, producer and scrounger strategies could coexist for both large and small phenotypes. Large scroungers tended to receive the highest payoff, suggesting that producer-scrounger dynamics may result in an uneven distribution of benefits among group members that—under the right conditions—could entrench social positions of dominance.     

Filter-feeding dabbling ducks (Anas spp.) can actively select particles by size

The relationship between bill morphology, function and prey use among filter-feeding dabbling ducks (Anas spp.) is poorly understood. In particular, it is not clear if interspecific differences in morphology affect the retention of prey to allow prey partitioning. The size of particles retained by ducks may be determined entirely by the distance between adjacent mandibular lamellae (interlamellar distance), possibly allowing interspecific partitioning of prey by size. Alternatively, articulation of the maxilla and mandible allows ducks to increase the distance between the maxillary and mandibular lamellae (lamellar separation) so that it exceeds their interlamellar distance, possibly allowing them to selectively expel unwanted particles larger than their interlamellar distance. In contrast, if interlamellar distance alone determines the size of particles ingested, particles larger than the interlamellar distance will always be retained because lamellar spacing is fixed. When large, preferred millet and wheat seeds were mixed with small, less preferred poppy seeds, all three ducks in this investigation ingested a greater proportion of the millet and wheat seeds than the poppy seeds, even though all three seed types were larger than the ducks' interlamellar distance. The proportion of millet and poppy seeds ingested when seeds were mixed differed from the proportion expected from foraging rates on unmixed prey, indicating the ducks actively avoided poppy seeds. These results are consistent with the lamellar separation hypothesis and clearly reject the singular role of interlamellar distance in prey retention. Because lamellar separation and water filtration rate are both determined by movement of the maxilla and mandible, there may be a trade-off between particle size selection and prey ingestion rate that allows interspecific partitioning of prey by size.     

Sex-specific niche partitioning and sexual size dimorphism in Australian pythons (Morelia spilota imbricata)

Sexual dimorphism is usually interpreted in terms of reproductive adaptations, but the degree of sex divergence also may be affected by sex-based niche partitioning. In gape-limited animals like snakes, the degree of sexual dimorphism in body size (SSD) or relative head size can determine the size spectrum of ingestible prey for each sex. Our studies of one mainland and four insular Western Australian populations of carpet pythons ( Morelia spilota ) reveal remarkable geographical variation in SSD, associated with differences in prey resources available to the snakes. In all five populations, females grew larger than males and had larger heads relative to body length. However, the populations differed in mean body sizes and relative head sizes, as well as in the degree of sexual dimorphism in these traits. Adult males and females also diverged strongly in dietary composition: males consumed small prey (lizards, mice and small birds), while females took larger mammals such as possums and wallabies. Geographic differences in the availability of large mammalian prey were linked to differences in mean adult body sizes of females (the larger sex) and thus contributed to sex-based resource partitioning. For example, in one population adult male snakes ate mice and adult females ate wallabies; in another, birds and lizards were important prey types for both sexes. Thus, the high degree of geographical variation among python populations in sexually dimorphic aspects of body size and shape plausibly results from geographical variation in prey availability.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 113–125.     

Evolution of body size in Galapagos marine iguanas

Body size is one of the most important traits of organisms and allows predictions of an individual's morphology, physiology, behaviour and life history. However, explaining the evolution of complex traits such as body size is difficult because a plethora of other traits influence body size. Here I review what we know about the evolution of body size in a group of island reptiles and try to generalize about the mechanisms that shape body size. Galapagos marine iguanas occupy all 13 larger islands in this Pacific archipelago and have maximum island body weights between 900 and 12 000g. The distribution of body sizes does not match mitochondrial clades, indicating that body size evolves independently of genetic relatedness. Marine iguanas lack intra- and inter-specific food competition and predators are not size-specific, discounting these factors as selective agents influencing body size. Instead I hypothesize that body size reflects the trade-offs between sexual and natural selection. We found that sexual selection continuously favours larger body sizes. Large males establish display territories and some gain over-proportional reproductive success in the iguanas' mating aggregations. Females select males based on size and activity and are thus responsible for the observed mating skew. However, large individuals are strongly selected against during El Ni?o-related famines when dietary algae disappear from the intertidal foraging areas. We showed that differences in algae sward ('pasture') heights and thermal constraints on large size are causally responsible for differences in maximum body size among populations. I hypothesize that body size in many animal species reflects a trade-off between foraging constraints and sexual selection and suggest that future research could focus on physiological and genetic mechanisms determining body size in wild animals. Furthermore, evolutionary stable body size distributions within populations should be analysed to better understand selection pressures on individual body size.     

Assortative mating,sexual size dimorphism and sex determination in a seabird with plumage polymorphism

Intraspecific plumage polymorphism in seabirds is often attributed to advantages in foraging activities and escape from predators, but its role in sexual selection is not well understood. The Trindade petrel (Pterodroma arminjoniana) presents morphs varying from pale to whole dark, with no apparent sexual size dimorphism (SSD). We tested assortative mating in Trindade petrels based on plumage colours and body size. In addition, genders of Trindade petrels were identified molecularly aiming to test SSD based on morphometrics, which was also used to generate a discriminant function for sex assignment. Within-pair consistency in plumage colour (i.e. birds paired with mates of the same morph) was detected in 9 out of 10 pairs, but not in morphometric traits. Minimum bill depth and bill depth at unguis were traits significantly larger in males. The best model was adjusted with Bill depth at unguis, wing chord and body mass, with global discriminatory power of 78.4%. Our results suggest that plumage colours may be sexually selected in Trindade petrels, which brings evolutionary implications on the persistence of plumage polymorphism. Discriminatory power of the best discriminant function was similar to those found in other Procellariiformes and also among datasets obtained by distinct researchers, demonstrating its robustness.     

Assessing frequency-dependent seed size selection: a field experiment

Seed size is a life history attribute that affects the probability of seed predation, and therefore affects plant fitness. Compared with smaller seeds, those with large size should be more attractive to predators, as they constitute a more profitable food item because of higher energetic and/or nutrient content. However, predator preferences may be frequency-dependent in the sense that they may be modulated by the relative abundance of alternative seeds of different sizes. We set up a field experiment to evaluate frequency-dependent seed predation using seeds of Cryptocarya alba (Lauraceae), at La Campana National Park in central Chile. Predators (rodents and birds) preferentially consumed large seeds in an antiapostatic manner. These selective responses were maintained throughout the experiment and seed selection by predators was not affected by previous seed consumption. Our results suggest that (a) large seeds are very profitable food items actively sought by seed predators even at low relative abundance, (b) seed selection is expressed in a short time scale and (c) seed predators, by consuming large seeds consistently, have the potential to modify significantly the quality of plant progeny.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 307–312.     

Canine size, shape, and bending strength in primates and carnivores

Anthropoid primates are well known for their highly sexually dimorphic canine teeth, with males possessing canines that are up to 400% taller than those of females. Primate canine dimorphism has been extensively documented, with a consensus that large male primate canines serve as weapons for intrasexual competition, and some evidence that large female canines in some species may likewise function as weapons. However, apart from speculation that very tall male canines may be relatively weak and that seed predators have strong canines, the functional significance of primate canine shape has not been explored. Because carnivore canine shape and size are associated with killing style, this group provides a useful comparative baseline for primates. We evaluate primate maxillary canine tooth size, shape and relative bending strength against body size, skull size, and behavioral and demographic measures of male competition and sexual selection, and compare them to those of carnivores. We demonstrate that, relative to skull length and body mass, primate male canines are on average as large as or larger than those of similar sized carnivores. The range of primate female canine sizes embraces that of carnivores. Male and female primate canines are generally as strong as or stronger than those of carnivores. Although we find that seed-eating primates have relatively strong canines, we find no clear relationship between male primate canine strength and demographic or behavioral estimates of male competition or sexual selection, in spite of a strong relationship between these measures and canine crown height. This suggests either that most primate canines are selected to be very strong regardless of variation in behavior, or that primate canine shape is inherently strong enough to accommodate changes in crown height without compromising canine function.     

Phylogenetic relationships of 34 passerines based on mitochondrial Cytochrome b sequences

Phylogenetic relationships of 34 passerines were studied based on mitochondrial Cytochrome b (Cyt b) sequences. Phylogenetic trees were constructed using Neighbor-Joining, Maximum-Parsimony and Minimum evolution methods. The results show that the divergence between Fringillids and Emberizids reaches a family level and they should be grouped into family Fringillidae and Emberizidae, respectively; Accentors has a relatively close relationship with Fringillids and Emberizids; the divergence between robins and flycatchers does not reach a family level and they should be member to family Muscicapidae; long-tailed tits and sylviids should all be listed into families; barn swallow, crowtits and long-tailed tits have close relationships with Sylviidae; in the Fringillidae, brambling should be member to one subfamily Fringillinae, several other birds under the subfamily Carduelinae; in the Sylviidae, although lanceolated warblers and scaly-headed stubtails have a relatively far relationship, they should be member to one subfamily Acrocephalinae and warblers to Phylloscopinae. Muscicapidae, Fringillidae and Emberidae are all monophylies, but Sylviidae is not. The substitution rates of major clades are thought to be the same according to relative rate tests. Divergence time of major clades is estimated at the rate of 1.6% per million years, thus the estimated divergence time between Fringillidae and Emberizidae is 10.5 million years, robins and flycatchers 9.0 Myr, Acrocephalinae and Phylloscopinae 9.0 Myr, Carduelis flammea and Carpodacus erythrinus 7.5 Myr, Luscinia cyane and Tarsiger cyanurus 7.8 Myr, two outgroups 13.5 Myr.     

Allometry and selection in a novel predator–prey system: Australian snakes and the invading cane toad

Because many organismal traits vary with body size, interactions between species can be affected by the respective body sizes of the participants. We focus on a novel predator–prey system involving an introduced, highly toxic anuran (the cane toad, Bufo marinus ) and native Australian snakes. The chance of a snake dying after ingesting a toad depends on the size of the snake and the size of the toad, and ultimately reflects the effect of four allometries: (1) physiological tolerance (the rate that physiological tolerance to toad toxin changes with snake size); (2) swallowing ability (the rate that maximal ingestible toad size (i.e. snake head size) increases with snake body size); (3) prey size (the rate that prey size taken by snakes increases with snake head size) and (4) toad toxicity (the rate that toxicity increases with toad size). We measured these allometries, and combined them to estimate the rate at which a snake's resistance changes with toad toxicity. The parotoid glands (and thus, toxicity) of toads increased disproportionately with toad size (i.e. relative to body size, larger toads were more toxic) but simultaneously, head size relative to body size (and thus, maximal ingestible prey size relative to predator size) declined with increasing body size in snakes. Thus, these two allometries tended to cancel each other out. Physiological tolerance to toxins did not vary with snake body size. The end result was that across snake species, mean adult body size did not affect vulnerability. Within species, however, smaller predators were more vulnerable, because the intraspecific rate of decrease in relative head size of snakes was steeper than the rate of increase in toxicity of toads. Thus, toad invasion may cause disproportionate mortality of juvenile snakes, and adults of the sex with smaller mean adult body sizes.     

Influence of intra‐ and interspecific variation in predator–prey body size ratios on trophic interaction strengths

1. Predation is a pervasive force that structures food webs and directly influences ecosystem functioning. The relative body sizes of predators and prey may be an important determinant of interaction strengths. However, studies quantifying the combined influence of intra‐ and interspecific variation in predator–prey body size ratios are lacking.
2. We use a comparative functional response approach to examine interaction strengths between three size classes of invasive bluegill and largemouth bass toward three scaled size classes of their tilapia prey. We then quantify the influence of intra‐ and interspecific predator–prey body mass ratios on the scaling of attack rates and handling times.
3. Type II functional responses were displayed by both predators across all predator and prey size classes. Largemouth bass consumed more than bluegill at small and intermediate predator size classes, while large predators of both species were more similar. Small prey were most vulnerable overall; however, differential attack rates among prey were emergent across predator sizes. For both bluegill and largemouth bass, small predators exhibited higher attack rates toward small and intermediate prey sizes, while larger predators exhibited greater attack rates toward large prey. Conversely, handling times increased with prey size, with small bluegill exhibiting particularly low feeding rates toward medium–large prey types. Attack rates for both predators peaked unimodally at intermediate predator–prey body mass ratios, while handling times generally shortened across increasing body mass ratios.
4. We thus demonstrate effects of body size ratios on predator–prey interaction strengths between key fish species, with attack rates and handling times dependent on the relative sizes of predator–prey participants.
5. Considerations for intra‐ and interspecific body size ratio effects are critical for predicting the strengths of interactions within ecosystems and may drive differential ecological impacts among invasive species as size ratios shift.

     

Foraging and community consequences of seed size for coexisting Negev Desert granivores

We examined the effects of seed size on patch use and diet selection for three co-existing Negev Desert granivores: Allenby's gerbil ( Gerbillus allenbyi ), greater Egyptian sand gerbil ( Gerbillus pyramidum ), and crested lark ( Galerida cristata ). We manipulated size and spatial distribution of seeds in experimental food patches and quantified foraging behavior by measuring giving-up densities (GUDs: the amount of food remaining in a resource patch following exploitation by a forager). In one experiment, we presented small (<1.4 mm in diameter cracked wheat), medium (2.0–3.3 mm), and large (>3.4 mm) seeds in separate trays; in a second, we presented small and medium seeds separately and mixed together. Gerbils had a higher handling time efficiency on smaller seeds, but a much higher encounter probability on larger seeds (20 times higher on large than medium seeds, and 2–5 times higher on medium than small seeds). This led gerbils to have significantly lower GUDs on larger seeds than smaller seeds and to harvest a higher proportion of the larger seeds. When presented with rich and poor patches, G. allenbyi tended to equalize GUDs in both patches, indicating a quitting harvest rate rule for patch exploitation. In contrast, larks appeared to use a fixed time rule for patch exploitation. For larks, seed size did not influence encounter probabilities, and they showed no seed-size selectivity. Still, larks had higher handling efficiencies on smaller than larger seeds, and consequently had a significantly lower GUD on small than medium seeds. Despite large differences between the gerbils and larks in their foraging, our results do not support species coexistence via seed-size partitioning: the larks had much higher GUDs than the gerbils on all seed sizes. Nonetheless, seed size, seed abundance, seed distribution and the animal's patch use behavior all played major roles in determining gerbils' and larks' diet selectivities and GUDs.     

Mate size and breeding success in a monogamous cichlid fish

Synopsis The patterns of mate size and parental care of a monogamous cichlid fish,Cichlasoma maculicauda, were studied in Gatun Lake, Panama. Males defend territories which serve as courtship and nest sites. Within a population most mates in pairs are of equal size rank. In each pair the male is larger than the female, probably because most mature males are larger than most mature females. Clutch size increases with female body size. Male size affects breeding success in two ways. First, larger males provide nest sites less susceptible to destructive wave action. Second, young of larger males grow faster than young of smaller males. Large males defeat small males in contests for position in feeding areas, and this may provide their young with better feeding conditions. In the laboratory young growth rates increase with food abundance, and at high levels of food surpass those observed in nature. Fast growth of young reduces their vulnerability to predators and should allow parents to breed more often. Young survival rates improve with the size of the parents, so that larger fish raise more offspring at each breeding attempt. These observations suggest why preference for large mates should occur.     

Effects of body size and sociality on the anti‐predator behaviour of foraging bees

Pollinators, like most other animals, often face a tradeoff between increasing food uptake and minimising predation. An earlier model suggests that social bees should be more likely than solitary bees to adopt riskier foraging strategies in order to increase food uptake. In this paper, we extend this model by studying the effect of body size, in addition to sociality, on the predation–intake rate tradeoff. When, following standard practice, we express the foraging strategies in terms of mortality probability and net food uptake, we find that body size should have no effect on the foraging strategies of solitary bees. Social bees, on the other hand, should change their foraging preferences according to their size. Small social bees should tend to maximise food uptake, and large social bees to minimise mortality rate. Mortality, however, is the product of two terms: the probability of suffering an attack and the probability of succumbing to it. Noting that larger bees are less susceptible to succumb to a predation attempt than smaller bees, model predictions change when foraging strategies are expressed in terms of exposure to predators. Following this second approach, exposure to predators should increase monotonically with body size in solitary bees. In social bees it should reach a minimum for medium‐sized bees. We conclude that both bee body size and sociality should be considered when studying the effect of predators on resource use.     

Predator–prey interactions,flight initiation distance and brain size

Prey avoid being eaten by assessing the risk posed by approaching predators and responding accordingly. Such an assessment may result in prey–predator communication and signalling, which entail further monitoring of the predator by prey. An early antipredator response may provide potential prey with a selective advantage, although this benefit comes at the cost of disturbance in terms of lost foraging opportunities and increased energy expenditure. Therefore, it may pay prey to assess approaching predators and determine the likelihood of attack before fleeing. Given that many approaching potential predators are detected visually, we hypothesized that species with relatively large eyes would be able to detect an approaching predator from afar. Furthermore, we hypothesized that monitoring of predators by potential prey relies on evaluation through information processing by the brain. Therefore, species with relatively larger brains for their body size should be better able to monitor the intentions of a predator, delay flight for longer and hence have shorter flight initiation distances than species with smaller brains. Indeed, flight initiation distances increased with relative eye size and decreased with relative brain size in a comparative study of 107 species of birds. In addition, flight initiation distance increased independently with size of the cerebellum, which plays a key role in motor control. These results are consistent with cognitive monitoring as an antipredator behaviour that does not result in the fastest possible, but rather the least expensive escape flights. Therefore, antipredator behaviour may have coevolved with the size of sense organs, brains and compartments of the brain involved in responses to risk of predation.