Anne Peters

Declining body size: a third universal response to warming?

A recently documented correlate of anthropogenic climate change involves reductions in body size, the nature and scale of the pattern leading to suggestions of a third universal response to climate warming. Because body size affects thermoregulation and energetics, changing body size has implications for resilience in the face of climate change. A review of recent studies shows heterogeneity in the magnitude and direction of size responses, exposing a need for large-scale phylogenetically controlled comparative analyses of temporal size change. Integrative analyses of museum data combined with new theoretical models of size-dependent thermoregulatory and metabolic responses will increase both understanding of the underlying mechanisms and physiological consequences of size shifts and, therefore, the ability to predict the sensitivities of species to climate change.

Life history trade-offs are influenced by the diversity, availability and interactions of dietary antioxidants

The expression of most life history traits, such as immunity, growth and the development of sexual signals, is negatively affected by high levels of oxidative stress. Dietary antioxidants can reduce oxidative stress and have therefore been the focus of numerous studies in behavioural and evolutionary ecology in the last few decades. Most of this research has focused on carotenoids, neglecting a number of more common, more potent, and thereby potentially more important, antioxidants, such as polyphenolic antioxidants. However, the effects of several classes of antioxidants on different life history traits have been thoroughly investigated in medical and animal-breeding studies. We suggest that behavioural and evolutionary studies will benefit from incorporating these advances. By reviewing the literature on the effects of antioxidants on life history traits in fish, birds and mammals, we develop a broad framework for dietary antioxidants. Fundamental properties of antioxidants, in particular their biochemistry, their potency and the interactions between them affect their relative relevance for life history traits. Based on tissue affinity, we distinguish between two categories of dietary antioxidants: focal antioxidants that are intrinsically important for a given trait and nonfocal antioxidants that influence traits only indirectly. Furthermore, we show how temporal and spatial environmental variability in antioxidant availability, as well as individual variation in food selection, may generate interindividual differences in the expression of life history traits. Finally, we suggest future research lines and experimental designs that may provide basic information needed to advance our knowledge of the ecological and evolutionary relevance of dietary antioxidants.

Sources of individual variation in plasma testosterone levels

The steroid hormone testosterone (T) plays a central role in the regulation of breeding in males, because many physiological, morphological and behavioural traits related to reproduction are T dependent. Moreover, in many seasonally breeding vertebrates, male plasma T levels typically show a pronounced peak during the breeding season. While such population-level patterns are fairly well worked out, the sources and the implications of the large variability in individual T levels within the seasonal cycle remain surprisingly little understood. Understanding the potential sources of individual variation in T levels is important for behavioural and evolutionary ecologists, for at least two reasons. First, in ‘honest signalling’ theory, T is hypothesized to play a critical role as the assumed factor that enforces honesty of the expression of sexually selected quality indicators. Second, T is often considered a key mediator of central life-history trade-offs, such as investment in survival versus reproduction or in mating versus parental care. Here, we discuss the patterns of within- and between-individual variation in male plasma T levels in free-living populations of birds. We argue that it is unclear whether this variability mainly reflects differences in underlying individual quality (intrinsic factors such as genetic or maternal effects) or in the environment (extrinsic factors including time of day, individual territorial status and past experience). Research in avian behavioural endocrinology has mainly focused on the effects of extrinsic factors, while other sources of variance are often ignored. We suggest that studies that use an integrative approach and investigate the relative importance of all potential sources of variation are essential for the interpretation of data on individual plasma T levels.

Carotenoid-based bill colour as an indicator of immunocompetence and sperm performance in male mallards.

Female mate choice is often based on exaggerated sexual traits, signals of male qualities that females cannot assess directly. Two such key qualities are male immune and/or sexual competence, whereby honesty in signalling could be maintained by physiological trade‐offs. Carotenoid‐based ornaments likely constitute such honest signals, as there is direct competition for (limited) carotenoids between ornament deposition and anti‐oxidant support of immune or sperm functioning. Using spectrometry, we assessed the potential signalling function of the yellow, carotenoid‐based colour of the bill of male mallards, a target of female mate choice. Here we demonstrate that bill reflectance varied with plasma carotenoid level, indicating antioxidant reserves. Moreover, lower relative UV reflectance during autumn pairing predicted immune responsiveness and correlated positively with sperm velocity during breeding, a trait that affects fertility. Our data provide support for current theories that females could use carotenoid‐based sexual signals to detect immune vigour and fertilizing ability of prospective mates.

Paternity analysis reveals opposing selection pressures on crown coloration in the blue tit (Parus caeruleus)

In socially monogamous species, extra–pair paternity can increase the variance in reproductive success and thereby the potential for sexual selection on male ornaments. We studied whether male secondary sexual ornaments are selected through within– and/or extra–pair reproductive success in the blue tit (Parus caeruleus). Male blue tits display a bright blue crown plumage, which reflects substantially in the ultraviolet (UV) and previously has been indicated to be an important sexual signal. We show that males with a more UV–shifted crown hue were less cuckolded, which probably resulted from female preference for more ornamented mates. By contrast, however, older males and males with a less UV–shifted hue sired more extra–pair young. This probably did not reflect direct female preference, since cuckolders were not less UV–ornamented than the males they cuckolded. Alternatively, a trade–off between UV ornamentation and other traits that enhance extra–pair success could explain this pattern. Our results might reflect two alternative male mating tactics, where more UV–ornamented males maximize within–pair success and less UV–ornamented males maximize extra–pair success. Since crown colour was selected in opposite directions by within–pair and extra–pair paternity, directional selection through extra–pair matings seemed weak, at least in this population and breeding season. Reduced intensity of sexual selection due to alternative mating tactics constitutes a potential mechanism maintaining additive genetic variance of male ornaments.

Male sexual attractiveness and parental effort in blue tits: a test of the differential allocation hypothesis

When the reproductive value of a breeding attempt is related to attributes of the breeding partner, an individual is expected to allocate more resources to parental care when mated to a high-quality partner. We tested predictions of the differential allocation hypothesis, by experimentally increasing and decreasing male blue tit, Parus caeruleus, sexual attractiveness and recording subsequent measures of male and female parental effort during the chick-feeding period. We used marker pens, to create two distinct male phenotypes: one more attractive phenotype with a shift in peak reflectance towards the ultraviolet (UV) part of the spectrum (UV+) and one less attractive phenotype with a shift towards the human-visible part of the spectrum (UV−). There was no significant difference in absolute or relative female feeding rate with respect to treatment. However, there were significant interaction effects between treatment and female age on female feeding rate, indicating that 1-year-old females provisioned more when mated to a UV+ male than a UV− male. UV− males fed their chicks at a higher rate than UV+ males, but there was no significant difference between the groups in total feeding rate. Females contributed less to nest defence relative to their mates when they were mated to UV− males, whereas the opposite was true for females mated to UV+ males. The behavioural responses did not translate into differences in measures of reproductive output. Our study suggests that male phenotypic appearance at the chick-feeding stage influences female decisions about level of parental effort.

Trade-offs between immune investment and sexual signaling in male mallards.

Allocation trade‐offs between the immune system and sexual traits are central to current sexual selection hypotheses but remain contentious. Such trade‐offs could be brought about by the dual action of testosterone that stimulates sexual signals but also suppresses immune functions and/or by competition for carotenoids that can be deposited in ornaments or used as antioxidants in support of immune functions. We investigated the trade‐off between investment in immunity and maintenance of testosterone, carotenoids, and sexually selected, carotenoid‐based bill color in male mallards. Following a nonpathogenic immune challenge, facultative immune investment resulted in a syndrome of changes in allocation. Plasma carotenoids disappeared from circulation proportional to antibody production. In addition, the reflectance spectrum of the bill was affected; greater antibody production was associated with an increase in relative UV reflectance. Although changes in bill reflectance and plasma carotenoids were related, the relationship appeared more complex than direct competition with immunity. Finally, maintenance of testosterone was affected by immune investment: testosterone levels declined substantially when males produced more antibodies. Because males with high testosterone are preferred by females, the decline in testosterone, in addition to carotenoid depletion and effects on bill reflectance, could constitute a significant cost of immune investment.

Compensatory Constitutionalism: The Function and Potential of Fundamental International Norms and Structures

The article conceives international (or global) constitutionalism as a legal argument which recommends and strengthens efforts (legal and political ones) to compensate for ongoing de-constitutionalization on the domestic level. Although the notions ‘international constitution’ and ‘international constitutionalism’ have in the recent years served as buzz-words in various discourses, the many meanings of those concepts have not yet been fully explored and disentangled. The paper suggests a specific understanding of those concepts. It highlights various aspects and elements of micro- and macro-constitutionalization in international law, and identifies anti-constitutionalist trends. On this basis, the paper finds that, although no international constitution in a formal sense exists, fundamental norms in the international legal order do fulfill constitutional functions. Because those norms can reasonably be qualified as having a constitutional quality, they may not be summarily discarded in the event of a conflict with domestic constitutional law. Because the relevant norms form a transnational constitutional network, and cannot be aligned in an abstract hierarchy, conflict-solution requires a balancing of interests in the concrete case. Finally, because constitutionalism historically and prescriptively means to ask for a legitimate constitution, a constitutionalist reading of the international legal order provokes the question of its legitimacy. This question is pressing, because state sovereignty and consent are – on good grounds – no longer accepted as the sole source of legitimacy of international law. International constitutionalism – as understood in this paper – does not ask for state-like forms of legitimacy of a world government, but stimulates the search for new mechanisms to strengthen the legitimacy of global governance.

Visual mimicry of host nestlings by cuckoos

Coevolution between antagonistic species has produced instances of exquisite mimicry. Among brood-parasitic cuckoos, host defences have driven the evolution of mimetic eggs, but the evolutionary arms race was believed to be constrained from progressing to the chick stage, with cuckoo nestlings generally looking unlike host young. However, recent studies on bronze-cuckoos have confounded theoretical expectations by demonstrating cuckoo nestling rejection by hosts. Coevolutionary theory predicts reciprocal selection for visual mimicry of host young by cuckoos, although this has not been demonstrated previously. Here we show that, in the eyes of hosts, nestlings of three bronze-cuckoo species are striking visual mimics of the young of their morphologically diverse hosts, providing the first evidence that coevolution can select for visual mimicry of hosts in cuckoo chicks. Bronze-cuckoos resemble their own hosts more closely than other host species, but the accuracy of mimicry varies according to the diversity of hosts they exploit.

The Condition‐Dependent Development of Carotenoid‐Based and Structural Plumage in Nestling Blue Tits: Males and Females Differ

Despite the importance of proximate mechanisms for understanding costs and functions of signals, relatively little is known about physiological processes underlying colorful plumage production. We investigated yellow carotenoid‐based breast plumage and ultraviolet (UV)/blue structural color of tail feathers in nestling blue tits. At peak plumage production and at fledging, we examined plasma concentration of protein, the substance of feathers, and levels of carotenoids, which are deposited in yellow plumage and also required for support of growth and immune development. Males showed more UV‐shifted hue and higher chroma and UV chroma of the tail feathers. In males only, tail chroma and UV chroma were strongly but negatively related to circulating plasma protein at the period of peak feather production. Breast plumage was more chromatic in males, and yellow coloration was strongly condition dependent in males only. Production of more chromatic plumage appeared to deplete carotenoids from plasma, since male fledglings had less circulating carotenoids than females. This had no obvious consequences for these males, since coloration was unrelated to cell‐mediated immune responses, humoral immune status, stress, and parasitism. Nonetheless, the pronounced sex specificity of condition dependence of structural and carotenoid‐based coloration is suggestive of a signaling function of the fledgling plumage.