The energy cost of flight: do small bats fly more cheaply than birds?

Flapping flight is one of the most expensive activities in terms of metabolic cost and this cost has previously been considered equal for the two extant vertebrate groups which evolved flapping flight. Owing to the difficulty of obtaining accurate measurements without disturbing flight performance, current estimates of flight cost within the group of small birds and bats differ by more than a factor of five for given body masses. To minimize the potential problem that flight behaviour may be affected by the measurements, we developed an indirect method of measuring flight energy expenditure based on time budget analysis in which small nectar-feeding bats (Glossophaginae) could continue their natural rhythm of flying and resting entirely undisturbed. Estimates of metabolic flight power based on 172 24-h time and energy budget measurements were obtained for nine individual bats from six species (mass 7–28 g). Metabolic flight power (P_F) of small bats was found to increase with body mass following the relation P_F = 50.2 M^0.771 (r² = 0.96, n = 13, P_F in W, M in kg). This is about 20–25% below the majority of current predictions of metabolic flight cost for small birds. Thus, either the flight cost of small birds is significantly lower than has previously been thought or, contrary to current opinion, small bats require less energy to fly than birds. Accepted: 29 September 1997     

Why do mountains support so many species of birds?

Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two‐fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within‐cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest “effect” on montane bird species richness everywhere. Topographic roughness had weaker “effects” on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow‐ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad‐scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic‐based processes, as has long been argued.     

Constraints on host choice: why do parasitic birds rarely exploit some common potential hosts?

1. Why are some common and apparently suitable resources avoided by potential users? This interesting ecological and evolutionary conundrum is vividly illustrated by obligate brood parasites. Parasitic birds lay their eggs into nests of a wide range of host species, including many rare ones, but do not parasitize some commonly co-occurring potential hosts. 2. Attempts to explain the absence of parasitism in common potential hosts are limited and typically focused on single-factor explanations while ignoring other potential factors. We tested why thrushes Turdus spp. are extremely rarely parasitized by common cuckoos Cuculus canorus despite breeding commonly in sympatry and building the most conspicuous nests among forest-breeding passerines. 3. No single examined factor explained cuckoo avoidance of thrushes. Life-history traits of all six European thrush species and the 10 most frequently used cuckoo hosts in Europe were similar except body/egg size, nest design and nestling diet. 4. Experiments (n = 1211) in several populations across Europe showed that host defences at egg-laying and incubation stages did not account for the lack of cuckoo parasitism in thrushes. However, cross-fostering experiments disclosed that various factors during the nestling period prevent cuckoos from successfully parasitizing thrushes. Specifically, in some thrush species, the nest cup design forced cuckoo chicks to compete with host chicks with fatal consequences for the parasite. Other species were reluctant to care even for lone cuckoo chicks. 5. Importantly, in an apparently phylogenetically homogenous group of hosts, there were interspecific differences in factors responsible for the absence of cuckoo parasitism. 6. This study highlights the importance of considering multiple potential factors and their interactions for understanding absence of parasitism in potential hosts of parasitic birds. In the present study, comparative and experimental procedures are integrated, which represent a novel approach that should prove useful for the understanding of interspecific ecological relationships in general.     

Why do earlier‐arriving migratory birds have better breeding success?

In migratory birds, early arrival on breeding sites is typically associated with greater breeding success, but the mechanisms driving these benefits are rarely known. One mechanism through which greater breeding success among early arrivers can potentially be achieved is the increased time available for replacement clutches following nest loss. However, the contribution of replacement clutches to breeding success will depend on seasonal variation in nest survival rates, and the consequences for juvenile recruitment of hatching at different times in the season. In particular, lower recruitment rates of late‐hatched chicks could offset the benefits to early arrivers of being able to lay replacement clutches, which would reduce the likelihood of replacement clutch opportunities influencing selection on migratory timings. Using a simulation model of time‐constrained capacity for replacement clutches, paramaterized with empirically‐derived estimates from avian migratory systems, we show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers, even when later renesting attempts contribute fewer recruits to the population. However, these relationships vary depending on the seasonal pattern of nest survival. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although replacement clutches can mitigate this effect. The time benefits of early arrival facilitating replacement clutches following nest loss may therefore be an important but overlooked source of selection on migratory timings. Empirical measures of seasonal variation in nest survival, renesting, and juvenile recruitment rates are therefore needed in order to identify the costs and benefits associated with individual migration phenology, the selection pressures influencing migratory timings, and the implications for ongoing shifts in migration and breeding phenology.     

When do altricial birds reach maximum of their brood defence intensity?

It has been suggested that the brood defence by parents of altricial birds should increase during the breeding attempt until the young depart from the nest. The two proximate hypotheses provide alternative predictions about the peak of brood defence intensity: (1) the vulnerability hypothesis predicts a rapid rise in brood defence after hatching of the chicks, with maximum defence intensity just before fledging and strong decline afterwards; (2) the feedback hypothesis predicts that brood defence intensity will, after a rapid rise, reach a plateau at the end of the nestling period and early after fledging and then slowly decline. I compared brood defence behaviour of altricial meadow pipit (Anthus pratensis) breeding in the Czech Republic during the late nestling stage and during the fledging time. A stuffed stoat (Mustela erminea) was placed 5 m from a meadow pipit nest and the defence behaviour of parents was recorded for 10 min from a hide. Brood defence intensity was higher during the fledgling time than during the late nestling stage, and this trend was more evident in males than in females. Regardless of the proportion of already fledged chicks and those still present in the nest, brood defence did not significantly decrease during the fledgling time in males or females. The results do not agree with the predictions of the vulnerability hypothesis and support the predictions of the feedback hypothesis.     

Introduction: why do we need a science of well-being?

Introduction: why do we need a science of well-being?     

Is the productivity of vegetation plots higher or lower when there are more species? Variable predictions from interaction of the ‘sampling effect’ and ‘competitive dominance effect’ on the habitat templet

Using a habitat templet model, we predict that the productivity (total biomass) of plots within a plant community may be positively, negatively or not at all related to variation in the number of species per plot, depending on successional stage (time since major disturbance) and habitat carrying capacity (reflecting the total resource supplying power of the habitat). For plots of a given size, a positive relationship between productivity and species richness is predicted in recently disturbed habitats because local neighbourhoods here will have been assembled largely stochastically, usually from a pool of available species with a right‐skewed size frequency distribution. Hence, in the earliest stages of succession, plots will have relatively high total biomass only if they contain at least some of the relatively uncommon larger species which will, in turn, be more likely in those neighbourhoods that contain more species (the sampling effect). Among these will also be some of the more common smaller species; hence, these high biomass, species‐rich plots should have relatively low species evenness, in contrast to what is predicted under effects involving species complementarity. In late succession, the plots with high total biomass will still be those that contain relatively large species but these plots will now contain relatively few species owing to increased competitive exclusion over time (the competitive dominance effect). In intermediate stages of succession, no relationship between plot productivity and species richness is predicted because the opposing sampling and competitive dominance effects cancel each other out. We predict that the intensity of both the sampling and competitive dominance effects on the productivity/species richness relationship will decrease with decreasing habitat carrying capacity (e.g. decreasing substrate fertility) owing to the inherently lower variance in between‐plot productivity that is predicted for more resource‐impoverished habitats.     

The timing of hibernation in Tasmanian echidnas: why do they do it when they do?

We investigated the patterns of hibernation and arousals in seven free-ranging echidnas Tachyglossus aculeatus setosus (two male, five female) in Tasmania using implanted temperature data loggers. All echidnas showed a ‘classical’ pattern of mammalian hibernation, with bouts of deep torpor interrupted by periodic arousals to euthermia (mean duration 1.04±0.05 (n=146). Torpor bout length increased as body temperature fell during the hibernation season, and became more variable as temperature rose again. Hibernation started in late summer (February 28±5 days, n=6) and males aroused just before the winter solstice (June 15±3 days, n=3), females that subsequently produced young aroused 40 days later (July 25±3, n=4) while females that did not produce young hibernated for a further two months (arousal Sept 27±5, n=7). We suggest that hibernation in Tasmanian echidnas can be divided into two phases, the first phase, marked by declining minimum body temperatures as ambient temperature falls, appears to be obligatory for all animals, while the second phase is ‘optional’ and is utilised to varying amounts by females. We suggest that early arousal and breeding is the favoured option for females in good condition, and that the ability to completely omit breeding in some years, and hibernate through to spring is an adaptation to an uncertain climate.     

Feather mites on birds: costs of parasitism or conditional outcomes?

Feather mites (suborder Astigmata, superfamilies Analgoidea, Pterolichoidea and Freyaniidae) are among the commonest ectosymbionts of birds. Most researchers have assumed they are parasites, having negative effects on hosts. Here we present evidence that suggests that feather mites may not be parasites. We develop a framework for considering conditional outcomes in these interspecific associations, dealing with different kinds of relationships between symbionts. The non-parasitic status of feather mites is supported by a literature review as well as by preliminary data on mites' food. We illustrate symbiotic relationships with a graphical model showing different scenarios in which hosts' cost-benefit relations are determined by the interactions among their symbionts.     

Sexual dimorphism in birds: why are there so many different forms of dimorphism?

Variation in the extent of sexual dimorphism among bird species is traditionally attributed to differences in social mating system. However, there are many different forms of dimorphism among birds, and not all of them show an obvious correlation with social mating system. For example, recent work has shown that many highly polygamous species are, in fact, monomorphic, whereas many putatively monogamous species are dimorphic. In this paper we break up sexual dimorphism into subcomponents and then use comparative analyses to examine the pattern of covariation between these subcomponents and various aspects of sexual, social, and parental behaviour. Our first finding is that size dimorphism and plumage-colour dimorphism do not show the same pattern of covariation. Differences in size dimorphism are associated with variation in social mating system and sex differences in parental care, whereas differences in plumage-colour dimorphism are associated with variation in the frequency of extra-bond paternity. These results suggest that size dimorphism is associated with the sort of intrasexual competition described by traditional classifications of social mating system, whereas plumage-colour dimorphism is associated with cryptic female choice. However, when we break up plumage-colour dimorphism according to whether it is due to melanins, carotenoids or structural colours, we find that each category of plumage-colour dimorphism shows a different pattern of covariation. The correlation between overall plumage-colour dimorphism and the rate of extra-bond paternity is due to structural colours, whereas melanin-based dimorphism is associated with sex differences in parental care. The former result is particularly interesting given that new work suggests structural colours are associated with active sexual displays and the reflection of ultraviolet light.     

How do sucker‐footed bats hold on,and why do they roost head‐up?

Individuals of most bat species hang head‐down by their toenails from rough surfaces, but Madagascar's endemic sucker‐footed bat (Myzopoda aurita) clings head‐up to smooth leaves using specialized pads on its wrists and ankles. We investigated the adhesive performance of 28 individuals and found that attachment performance on brass was not affected by the presence or absence of a seal around the pad–surface interface. Furthermore, on smooth acrylic, the wrist pads were more than nine‐fold weaker when lifted perpendicular to the surface than when pulled parallel to it. The unimportance of a seal and the difference in strength in those directions on a smooth surface are characteristic of wet adhesion, but not of suction. Thus, despite its name, the sucker‐footed bat appears to adhere using wet adhesion. We observed that when wrist pads were pushed anteriorly, they unpeeled easily from the surface because of deformation of the pads. This most likely permits rapid detachment during crawling, but would also cause passive detachment if bats roosted head‐down. This provides an ecomorphological explanation to the head‐up roosting behaviour of these unique bats. The results obtained in the present study thus link morphology, behaviour, and roosting ecology for an enigmatic Malagasy endemic. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 233–240.     

Influence of pregnancy on the thermal biology of the lizard, Sceloporus jarrovi: why do pregnant females exhibit low body temperatures?

1. Selected body temperatures of female lizards, Sceloporus jarrovi , were measured on a photothermal gradient during late pregnancy and again when postpartum, and pregnant females were subjected to one of three fluctuating temperature regimes that simulated body temperatures of (1) pregnant females, (2) postpartum females or (3) allowed normal thermoregulation.
2. Overall, females selected lower body temperatures when pregnant (mean = 32·0°C) than when postpartum (mean = 33·5°C).
3. Females regulated body temperature more precisely when pregnant than when postpartum as judged by their smaller variances in body temperature throughout the day.
4. When pregnant, females selected a lower mean maximum body temperature (mean: pregnant = 32·8°C; postpartum = 34·5°C) than when postpartum, but selected mean minimum body temperatures did not differ.
5. None of the experimental temperature treatments was detrimental to pregnant females. Female body length increased during pregnancy but the rate of increase did not differ among treatments. Moreover, length-adjusted body mass of postpartum females did not differ among treatments.
6. Pregnant females that experienced postpartum body temperatures produced neonates that were smaller in body mass and length than pregnant females that experienced pregnant body temperatures and females that were allowed to thermoregulate.
7. For neonates resulting from the postpartum body temperature treatment, the disparity in the body length, but not mass, was still observed at 9 days of age, although survival and growth of neonates was high and did not differ among treatments.
8. The results demonstrate that pregnant females could maintain higher postpartum body temperatures without compromising their physical condition, but select relatively low body temperatures, presumably to avoid decrements in offspring fitness.     

Assimilation efficiency in birds: A function of taxon or food type?

• 1.1. We review the available measurements of assimilation efficiency in birds and find a strong dependence upon food ingested, regardless of bird taxa.
• 2.2. Assimilation efficiency in birds is about 75% for most food types, except plants (36.9%) and fruits (41%).

     

The paradox of invasion in birds: competitive superiority or ecological opportunism?

Why can alien species succeed in environments to which they have had no opportunity to adapt and even become more abundant than many native species? Ecological theory suggests two main possible answers for this paradox: competitive superiority of exotic species over native species and opportunistic use of ecological opportunities derived from human activities. We tested these hypotheses in birds combining field observations and experiments along gradients of urbanization in New South Wales (Australia). Five exotic species attained densities in the study area comparable to those of the most abundant native species, and hence provided a case for the invasion paradox. The success of these alien birds was not primarily associated with a competitive superiority over native species: the most successful invaders were smaller and less aggressive than their main native competitors, and were generally excluded from artificially created food patches where competition was high. More importantly, exotic birds were primarily restricted to urban environments, where the diversity and abundance of native species were low. This finding agrees with previous studies and indicates that exotic and native species rarely interact in nature. Observations and experiments in the field revealed that the few native species that exploit the most urbanized environments tended to be opportunistic foragers, adaptations that should facilitate survival in places where disturbances by humans are frequent and natural vegetation has been replaced by man-made structures. Successful invaders also shared these features, suggesting that their success is not a paradox but can be explained by their capacity to exploit ecological opportunities that most native species rarely use.     

Shared components of protein complexes—versatile building blocks or biochemical artefacts?

Protein complexes perform many important functions in the cell. Large-scale studies of protein-protein interactions have not only revealed new complexes but have also placed many proteins into multiple complexes. Whilst the advocates of hypothesis-free research touted the discovery of these shared components as new links between diverse cellular processes, critical commentators denounced many of the findings as artefacts, thus questioning the usefulness of large-scale approaches. Here, we survey proteins known to be shared between complexes, as established in the literature, and compare them to shared components found in high-throughput screens. We discuss the various challenges to the identification and functional interpretation of bona fide shared components, namely contaminants, variant and megacomplexes, and transient interactions, and suggest that many of the novel shared components found in high-throughput screens are neither the results of contamination nor central components, but appear to be primarily regulatory links in cellular processes.