When the sun never sets: diverse activity rhythms under continuous daylight in free-living arctic-breeding birds.

Circadian clocks are centrally involved in the regulation of daily behavioural and physiological processes. These clocks are synchronized to the 24 h day by external cues (Zeitgeber), the most important of which is the light-dark cycle. In polar environments, however, the strength of the Zeitgeber is greatly reduced around the summer and winter solstices (continuous daylight or continuous darkness). How animals time their behaviour under such conditions has rarely been studied in the wild. Using a radio-telemetry-based system, we investigated daily activity rhythms under continuous daylight in Barrow, Alaska, throughout the breeding season in four bird species that differ in mating system and parental behaviour. We found substantial diversity in daily activity rhythms depending on species, sex and breeding stage. Individuals exhibited either robust, entrained 24 h activity cycles, were continuously active (arrhythmic) or showed 'free-running' activity cycles. In semipalmated sandpipers, a shorebird with biparental incubation, we show that the free-running rhythm is synchronized between pair mates. The diversity of diel time-keeping under continuous daylight emphasizes the plasticity of the circadian system, and the importance of the social and life-history context. Our results support the idea that circadian behaviour can be adaptively modified to enable species-specific time-keeping under polar conditions.

Evidence for adaptive evolution of olfactory receptor genes in 9 bird species.

It has been suggested that positive selection, in particular selection favoring a change in the protein sequence, plays a role in the evolution of olfactory receptor (OR) gene repertoires in fish and mammals. ORs are 7-transmembrane domain (TM) proteins, members of the G-protein-coupled receptor superfamily in vertebrate genomes, and responsible for odorant binding and discrimination. OR gene repertoires in birds are surprisingly large and diverse, suggesting that birds have a keen olfactory sense. The aim of this study is to investigate signatures of positive selection in an expanded OR clade (group-gamma-c) that seems to be a characteristic of avian genomes. Using maximum-likelihood methods that estimate the d(N)/d(S) ratios and account for the effects of recombination, we show here that there is evidence for positive selection in group-gamma-c partial OR coding sequences of 9 bird species that are likely to have different olfactory abilities: the blue tit (Cyanistes caeruleus), the black coucal (Centropus grillii), the brown kiwi (Apteryx australis), the canary (Serinus canaria), the galah (Eolophus roseicapillus), the kakapo (Strigops habroptilus), the mallard (Anas platyrhynchos), the red jungle fowl (Gallus gallus), and the snow petrel (Pagodroma nivea). Positively selected codons were predominantly located in TMs, which in other vertebrates are involved in odorant binding. Our data suggest that 1) at least some avian OR genes have been subjected to adaptive evolution, 2) the extent of such adaptive evolution differs between bird species, and 3) positive selective pressures may have been stronger on the group-gamma-c OR genes of species that have well-developed olfactory abilities.

Evidence for increased olfactory receptor gene repertoire size in two nocturnal bird species with well-developed olfactory ability.

BACKGROUND: In vertebrates, the molecular basis of the sense of smell is encoded by members of a large gene family, namely olfactory receptor (OR) genes. Both the total number of OR genes and the proportion of intact OR genes in a genome may indicate the importance of the sense of smell for an animal. There is behavioral, physiological, and anatomical evidence that some bird species, in particular nocturnal birds, have a well developed sense of smell. Therefore, we hypothesized that nocturnal birds with good olfactory abilities have evolved (i) more OR genes and (ii) more intact OR genes than closely related and presumably less 'olfaction-dependent' day-active avian taxa. RESULTS: We used both non-radioactive Southern hybridization and PCR with degenerate primers to investigate whether two nocturnal bird species that are known to rely on olfactory cues, the brown kiwi (Apteryx australis) and the kakapo (Strigops habroptilus), have evolved a larger OR gene repertoire than their day-active, closest living relatives (for kiwi the emu Dromaius novaehollandiae, rhea Rhea americana, and ostrich Struthio camelus and for kakapo the kaka Nestor meridionalis and kea Nestor notabilis). We show that the nocturnal birds did not have a significantly higher proportion of intact OR genes. However, the estimated total number of OR genes was larger in the two nocturnal birds than in their relatives. CONCLUSION: Our results suggest that ecological niche adaptations such as daily activity patterns may have shaped avian OR gene repertoires.

A comparison of reptilian and avian olfactory receptor gene repertoires: species-specific expansion of group gamma genes in birds.

BACKGROUND: The detection of odorants is mediated by olfactory receptors (ORs). ORs are G-protein coupled receptors that form a remarkably large protein superfamily in vertebrate genomes. We used data that became available through recent sequencing efforts of reptilian and avian genomes to identify the complete OR gene repertoires in a lizard, the green anole (Anolis carolinensis), and in two birds, the chicken (Gallus gallus) and the zebra finch (Taeniopygia guttata). RESULTS: We identified 156 green anole OR genes, including 42 pseudogenes. The OR gene repertoire of the two bird species was substantially larger with 479 and 553 OR gene homologs in the chicken and zebra finch, respectively (including 111 and 221 pseudogenes, respectively). We show that the green anole has a higher fraction of intact OR genes (approximately 72%) compared with the chicken (approximately 66%) and the zebra finch (approximately 38%). We identified a larger number and a substantially higher proportion of intact OR gene homologs in the chicken genome than previously reported (214 versus 82 genes and 66% versus 15%, respectively). Phylogenetic analysis showed that lizard and bird OR gene repertoires consist of group alpha, theta and gamma genes. Interestingly, the vast majority of the avian OR genes are confined to a large expansion of a single branch (the so called gamma-c clade). An analysis of the selective pressure on the paralogous genes of each gamma-c clade revealed that they have been subjected to adaptive evolution. This expansion appears to be bird-specific and not sauropsid-specific, as it is lacking from the lizard genome. The gamma-c expansions of the two birds do not intermix, i.e., they are lineage-specific. Almost all (group gamma-c) OR genes mapped to the unknown chromosome. The remaining OR genes mapped to six homologous chromosomes plus three to four additional chromosomes in the zebra finch and chicken. CONCLUSION: We identified a surprisingly large number of potentially functional avian OR genes. Our data supports recent evidence that avian olfactory ability may be better developed than previously thought. We hypothesize that the radiation of the group gamma-c OR genes in each bird lineage parallels the evolution of specific olfactory sensory functions.

Avian olfactory receptor gene repertoires: evidence for a well-developed sense of smell in birds?

Among vertebrates, the sense of smell is mediated by olfactory receptors (ORs) expressed in sensory neurons within the olfactory epithelium. Comparative genomic studies suggest that the olfactory acuity of mammalian species correlates positively with both the total number and the proportion of functional OR genes encoded in their genomes. In contrast to mammals, avian olfaction is poorly understood, with birds widely regarded as relying primarily on visual and auditory inputs. Here, we show that in nine bird species from seven orders (blue tit, Cyanistes caeruleus; black coucal, Centropus grillii; brown kiwi, Apteryx australis; canary, Serinus canaria; galah, Eolophus roseicapillus; red jungle fowl, Gallus gallus; kakapo, Strigops habroptilus; mallard, Anas platyrhynchos; snow petrel, Pagodroma nivea), the majority of amplified OR sequences are predicted to be from potentially functional genes. This finding is somewhat surprising as one previous report suggested that the majority of OR genes in an avian (red jungle fowl) genomic sequence are non-functional pseudogenes. We also show that it is not the estimated proportion of potentially functional OR genes, but rather the estimated total number of OR genes that correlates positively with relative olfactory bulb size, an anatomical correlate of olfactory capability. We further demonstrate that all the nine bird genomes examined encode OR genes belonging to a large gene clade, termed gamma-c, the expansion of which appears to be a shared characteristic of class Aves. In summary, our findings suggest that olfaction in birds may be a more important sense than generally believed.

Detection of olfactory receptor transcripts in bird testes.

The sense of smell is mediated through olfactory receptors (ORs) expressed in olfactory sensory neurons of the olfactory epithelium. Interestingly, some OR genes also function in another context: they are expressed in nonolfactory testicular tissue and in sperm of mammals and fish where they mediate sperm flagellar motility. The presence of OR transcripts in testicular tissue of both mammals and fish suggests that this is a conserved trait among vertebrates. In birds, sperm competition is widespread and its outcome depends, in part, on sperm motility. Thus, avian testicular OR gene expression might be particularly interesting to study, especially in the context of current ideas on postcopulatory sexual selection. Using reverse transcription-polymerase chain reaction with degenerate primers specific for OR genes and subsequent cloning, we here demonstrate that multiple OR gene transcripts are present in chicken (Gallus gallus domesticus) testes. Moreover, we show that they belong to the class-gamma OR gene clade. We discuss the potential significance and evolutionary implications of avian testicular OR gene expression.

Low metabolism and inactive lifestyle of a tropical rain forest bird investigated via heart-rate telemetry.

Birds in the lowland tropical rain forest are expected to have low energy turnover. Here, we used heart rate telemetry to estimate nighttime resting metabolic rate (RMR), daily energy expenditure (DEE), and locomotor activity of a small, long-lived tropical rain forest-understory bird, the spotted antbird (Hylophylax naevioides). Heart rate was linearly related to oxygen consumption in respirometry measurements that encompassed 96% of heart rates measured in wild birds. Heart rates in the wild ranged from 260 beats/min at night to 824 beats/min during the day, with a mean of 492 beats/min. Compared with temperate-forest birds of similar body mass, wild spotted antbirds had a low DEE, only 51% of the expected value. Such low metabolism was achieved mainly by being locomotively inactive for 35% of the daytime (i.e., 0 hops or flights/min). On average, spotted antbirds exhibited 1.6 hops or short flights/min during the daytime. In addition, they decreased nighttime RMR in the wild (at ambient temperatures below their thermoneutral zone [TNZ]) to levels equivalent to nighttime RMR in the laboratory at temperatures within their TNZ. This suggests that wild birds reduce their body temperature every night. Our data confirm and extend previous studies showing that tropical passerines have low metabolic rates.

Plasma steroid hormones in two Arctic-breeding shorebirds: monogamy versus polygyny.

Steroid hormones are predicted to vary with mating systems, degree of paternal care and aggression, as proposed in the "challenge hypothesis." We measured plasma concentrations of testosterone (T) and 5alpha-dihydrotestosterone (DHT) in two high Arctic breeding and closely related shorebird species, the polygynous pectoral sandpiper (Calidris melanotos) and the monogamous semipalmated sandpiper (Calidris pusilla) to examine whether the hormonal findings corroborate the predictions of the challenge hypothesis. In both species, males showed significantly higher levels of DHT and T than females, but in pectoral sandpipers median T levels were 34 times greater in males than in females, whereas in semipalmated sandpiper there was only a 4.9-fold difference. T and DHT concentrations correlated in semipalmated sandpipers and in male, but not in female, pectoral sandpipers. In semipalmated sandpipers, androgen levels were highest in the beginning of the breeding season and then declined, whereas male pectoral sandpipers showed extremely high androgen levels which were sustained throughout the breeding season. In both species, androgen levels were independent of body condition. Several incubating male semipalmated sandpipers had high circulating T levels, suggesting that low T is not required to exhibit paternal care. Our results are consistent with the challenge hypothesis in that androgen concentrations were higher throughout the breeding season in the polygynous compared to the monogamous species. Our study also supports previous studies suggesting that the short breeding season in the high Arctic and the importance of male care to nest success may be factors leading to behavioral T insensitivity.