3-Methylhistidine excretion by molting and non-molting sparrows

No evidence of metabolic costs following adaptive immune activation or reactivation in house sparrows

The energy cost of adaptive immune activation in endotherms is typically quantified from changes in resting metabolic rate following exposure to a novel antigen. An implicit assumption of this technique is that all variation in energy costs following antigenic challenge is due solely to adaptive immunity, while ignoring potential changes in the energy demands of ongoing bodily functions. We critically assess this assumption by measuring both basal metabolic rate (BMR) and exercise-induced maximal metabolic rate (MMR) in house sparrows before and after the primary and two subsequent vaccinations with either saline (sham) or two novel antigens (keyhole limpet haemocyanin and sheep red blood cells; KLH and SRBC, respectively). We also examined the effect of inducing male breeding levels of testosterone (T) on immune responses and their metabolic costs in both males and females. Although there was a moderate decrease in KLH antibody formation in T-treated birds, there was no effect of T on BMR, MMR or immunity to SRBC. There was no effect of vaccination on BMR but, surprisingly, all vaccinated birds maintained MMR better than sham-treated birds as the experiment progressed. Our findings caution against emphasizing energy costs or nutrient diversion as being responsible for reported fitness reductions following activation of adaptive immunity.

The energy cost of feather replacement is not intrinsically inefficient

Feathers serve a diversity of functions in birds and their continuous use and exposure to the environment requires a scheduled moult to maintain their full functionality. As feathers represent about 25% of a bird’s protein content, moult is expected to impose substantial energy and nutrient demands, but perhaps not to the extent reported. Energy conversion efficiencies for feather formation are among the lowest for any biological structure examined, but this assumes that increases in maintenance energy requirements (minimum resting metabolic rate (RMRmin)) during moult are predominately due to feather synthetic costs. We tested this assumption by comparing the RMRminand protein turnover rates of House Sparrows (Passer domesticus (Linnaeus, 1758)) during peak moult and in a non-moulting cohort before and 12 days after having a similar amount of feathers plucked. Replacement of plucked feathers had no effect on metabolic rate, whereas RMRminwas 28% higher in moulting than in non-moulting House Sparrows. Protein turnover rates were lowest in non-moulting birds, but rate differences between non-moulting and moulting birds were threefold higher than those between non-moulting and plucked House Sparrows. Thus, the energy inefficiencies reported for feather replacement are mainly due to costs associated with coincident processes rather than being a direct cost of feather synthesis per se.

Lack of seasonal and moult-related stress modulation in an opportunistically breeding bird: The white-plumed honeyeater (Lichenostomus penicillatus)

This article is part of a Special Issue "SBN 2014". In most vertebrate species, glucocorticoid levels and stress sensitivity vary in relation to season and life-history stage. In birds, baseline corticosterone (CORT) and stress sensitivity are typically highest while breeding and decrease substantially during moult. Because elevated CORT adversely affects protein synthesis, moult-related CORT suppression is thought to be necessary for forming high-quality feathers. Surprisingly, some passerine species lack moult-related CORT suppression, but these are distinguished by having slow rates of moult and being opportunistic breeders. We examined baseline and stress-induced CORT levels in an opportunistically breeding Australian passerine, the white-plumed honeyeater (Lichenostomus penicillatus). Although this species has a slower moult rate than high-latitude breeders, it differs little from north-temperate passerines. Neither baseline nor stress-induced CORT levels varied with season (winter, spring or summer), sex or moult status in adult birds. While breeding tended to be highest in early spring through late summer, laparotomies revealed only limited reduction in testicular size in males the year round. In all but one sampling period, at least some females displayed follicular hierarchy. Breeding usually coincides with outbreaks of phytophagous insects, which can happen at any time of the year. This results in moult/breeding overlap when infestations occur in late spring or summer. The ability of this species to moult and breed at the same time while having breeding-levels of CORT demonstrates that CORT suppression is not a prerequisite for synthesis of high-quality feathers. An experimental design incorporating moulting and non-moulting phenotypes is suggested to test the functional significance of CORT suppression in other species.

No evidence for general condition-dependence of structural plumage colour in blue tits: an experiment

Condition-dependence is a central but contentious tenet of evolutionary theories on the maintenance of ornamental traits, and this is particularly true for structural plumage colour. By providing diets of different nutritional quality to moulting male and female blue tits, we experimentally manipulated general condition within the natural range, avoiding deprivation or stressful treatments. We measured reflectance of the structural-coloured UV/blue crown, a sexually selected trait in males, and the white cheek, a nonpigmented structural colour, directly after moult and again during the following spring mating season. We employed a variety of colour indices, based on spectral shape and avian visual models but, despite significant variation in condition and coloration, found no evidence for condition-dependence of UV/blue or white plumage colour during either season. These and previously published results suggest that structural colour might be sensitive to stress, rather than reduced body condition, during moult.

Hypercarnivory and the brain: protein requirements of cats reconsidered

The domestic hypercarnivores cat and mink have a higher protein requirement than other domestic mammals. This has been attributed to adaptation to a hypercarnivorous diet and subsequent loss of the ability to downregulate amino acid catabolism. A quantitative analysis of brain glucose requirements reveals that in cats on their natural diet, a significant proportion of protein must be diverted into gluconeogenesis to supply the brain. According to the model presented here, the high protein requirement of the domestic cat is the result of routing of amino acids into gluconeogenesis to supply the needs of the brain and other glucose-requiring tissues, resulting in oxidation of amino acid in excess of the rate predicted for a non-hypercarnivorous mammal of the same size. Thus, cats and other small hypercarnivores do not have a high protein requirement per se, but a high endogenous glucose demand that is met by obligatory amino acid-based gluconeogenesis. It is predicted that for hypercarnivorous mammals with the same degree of encephalisation, endogenous nitrogen losses increase with decreasing metabolic mass as a result of the allometric relationships of brain mass and brain metabolic rate with body mass, possibly imposing a lower limit for body mass in hypercarnivorous mammals.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

Neurobiology of molt in avian species

Postnuptial molt is the major molt that occurs in most wild and domestic avian species each year. The process is much more than the replacement of feathers. Studies have shown that a significant increase in metabolic rate, increase in whole body protein synthesis, osteoporosis, loss of body fat, and a suppression of the immune system occur during this event of a bird's annual cycle. Several procedures have been developed to initiate feather replacement, and this review addresses hormonal and neuropeptide treatments that effected molt. The administration of thyroxine, progesterone, gonadotropin-releasing hormone agonist, and prolactin were examined. Of the four, the two most effective were thyroxine and prolactin administration. The neural modulators known to release thyroxine and prolactin, respectively, are thyroid hormone releasing hormone (TRH) and vasoactive intestinal polypeptide (VIP). To gain insight into the possible functions of VIP and TRH, the distribution of these neuromodulators and their terminal fields were reviewed in the avian brain. It was found that VIP-containing neurons and fibers identified a neural system in birds comparable to the visceral forebrain system (VFS) described in mammals. The VFS functions to regulate the balance of the autonomic nervous system. The autonomic nervous system and, therefore, the VFS have been proposed to regulate the many behavioral and physiological events in the annual cycle of a bird, including postnuptial molt. In contrast to VIP that has an extensive brain distribution throughout the forebrain and brainstem, TRH is relatively restrictive and has a main concentration in nerve cells in and about the paraventricular nucleus, a key neural component of the VFS. Due to the roll of TRH in regulating the thyroid axis and operating within the framework of the VFS, it is proposed that the peptide functions to shift the balance of the autonomic nervous system in the direction of the sympathetic nervous system. A shift toward the dominance of the sympathetic nervous system appears to be required during this phase of a bird's yearly cycle.

Feather mites, pectoral muscle condition, wing length and plumage coloration of passerines

I compared the feather mite (Acari, Proctophyllodidae) loads of moulting birds with features of the new plumage that they were growing. I examined 21 samples, each sample containing individuals of the same species, sex and age class (juvenile, yearling or adult). I used nine species: wren, Troglodytes troglodytes; dunnock, Prunella modularis; robin, Erithacus rubecula; blue tit, Parus caeruleus; great tit, P. major; chaffinch, Fringilla coelebs; greenfinch, Carduelis chloris; linnet, C. cannabina; and yellowhammer, Emberiza citrinella. As previously reported for the house finch, Carpodacus mexicanus, birds with more feather mites grew duller plumage and relatively shorter wings than less infested individuals of the same sex and age class. They also had lower protein reserves judged by the shape of their pectoral muscle. Mite load was usually a better predictor of coloration than pectoral muscle score, but the opposite was true for changes in wing length. In contrast to house finches, birds whose plumage was dull before moult had higher mite loads while moulting. Even if feather mites do not cause dull plumage or short wings, they appear to be a good indicator of birds in poor physiological condition. Copyright 1999 The Association for the Study of Animal Behaviour.