A cross-taxonomic perspective on the integration of temperature cues in vertebrate seasonal neuroendocrine pathways

The regulation of seasonality has been an area of interest for decades, yet global climate change has created extra urgency in the quest to understand how sensory circuits and neuroendocrine control systems interact to generate flexibility in biological timekeeping. The capacity of temperature to alter endogenous or photoperiod-regulated neuroendocrine mechanisms driving seasonality, either as a direct cue or through temperature-dependent effects on energy and metabolism, is at the heart of this phenological flexibility. However, until relatively recently, little research had been done on the integration of temperature information in canonical seasonal neuroendocrine pathways, particularly in vertebrates. We review recent advances from research in vertebrates that deepens our understanding of how temperature cues are perceived and integrated into seasonal hypothalamic thyroid hormone (TH) signaling, which is a critical regulator of downstream seasonal phenotypic changes such as those regulated by the BPG (brain-pituitary-gonadal) axis. Temperature perception occurs through cutaneous transient receptor potential (TRP) neurons, though sensitivity of these neurons varies markedly across taxa. Although photoperiod is the dominant cue used to trigger seasonal physiology or entrain circannual clocks, across birds, mammals, fish, reptiles and amphibians, seasonality appears to be temperature sensitive and in at least some cases this appears to be related to phylogenetically conserved TH signaling in the hypothalamus. Nevertheless, the exact mechanisms through which temperature modulates seasonal neuroendocrine pathways remains poorly understood.

Olfactory detection of trace amounts of plant volatiles is correlated with testosterone in a passerine bird

In response to damage by insects, plants release herbivore-induced plant volatiles (HIPVs) into the air. Insectivorous birds exploit these cues and, consequently, reduce the damages inflicted to the plants. However, little is known about whether they solely use HIPVs as foraging cues, or if they also use them to modulate traits linked to reproduction. As caterpillars are the primary food source required for insectivorous birds to raise offspring, their ability to locate and predict future peaks in caterpillar biomass using olfaction is likely to be advantageous. Therefore, we tested whether an insectivorous songbird that naturally inhabits oak dominated forests can be trained to detect early spring infestation by hatchling caterpillars, at a time when oaks begin bursting, and birds prepare to breed. Tree buds were either infested with caterpillars or left as a control and visually obscured in a Y-Maze choice test. Additionally, we measured testosterone and 17β-estradiol as they influence olfactory perception in mammals and are linked to reproduction in vertebrates. After being trained to associate the presence of HIPVs with that of food, blue tits spent more time with, were more active around, and more frequently chose to first visit the infested trees, showing that blue tits can smell caterpillar activity. Males with higher testosterone spent more time around infested trees, suggesting that foraging behavior during the pre-breeding season is linked with a major reproductive signal. There was no relationship between foraging and estradiol in females. These results are an important foundation for further investigation of the role of hormones in avian olfaction and how smell may be useful for making breeding decisions that could improve reproductive success.

The ecological and physiological bases of variation in the phenology of gonad growth in an urban and desert songbird

Birds often adjust to urban areas by advancing the timing (phenology) of vernal gonad growth. However, the ecological and physiological bases of this adjustment are unclear. We tested whether the habitat-related disparity in gonad growth phenology of male Abert's towhees, Melozone aberti, is due to greater food availability in urban areas of Phoenix, Arizona USA or, alternatively, a habitat-related difference in the phenology of key food types. To better understand the physiological mechanism underlying variation in gonad growth phenology, we compared the activity of the reproductive system at all levels of hypothalamo-pituitary-gonadal (HPG) axis. We found no habitat-associated difference in food availability (ground arthropod biomass), but, in contrast to the seasonal growth of leaves on desert trees, the leaf foliage of urban trees was already developed at the beginning of our study. Multiple estimates of energetic status did not significantly differ between the non-urban and urban towhees during three years that differed in the habitat-related disparity in gonad growth and winter precipitation levels. Thus, our results provide no support for the hypothesis that greater food abundance in urban areas of Phoenix drives the habitat-related disparity in gonad growth phenology in Abert's towhees. By contrast, they suggest that differences in the predictability and magnitude of change in food availability between urban and desert areas of Phoenix contribute to the observed habitat-related disparity in gonad growth. Endocrine responsiveness of the gonads may contribute to this phenomenon as desert - but not urban - towhees had a marked plasma testosterone response to GnRH challenge.

Scale-dependent phenological synchrony between songbirds and their caterpillar food source

In seasonal environments, the timing of reproduction has important fitness consequences. Our current understanding of the determinants of reproductive phenology in natural systems is limited because studies often ignore the spatial scale on which animals interact with their environment. When animals use a restricted amount of space and the phenology of resources is spatially variable, selection may favor sensitivity to small-scale environmental variation. Population-level studies of how songbirds track the changing phenology of their food source have been influential in explaining how populations adjust to changing climates but have largely ignored the spatial scale at which phenology varies. We explored whether individual great tits (Parus major) synchronize their breeding with phenological events in their local environment and investigated the spatial scale at which this occurs. We demonstrate marked variation in the timing of food availability, at a spatial scale relevant to individual birds, and that such local variation predicts the breeding phenology of individuals. Using a 45-year data set, we show that measures of vegetation phenology at very local scales are the most important predictors of timing of breeding within years, suggesting that birds can fine-tune their phenology to that of other trophic levels. Knowledge of the determinants of variation in reproductive behavior at different spatial scales is likely to be critical in understanding how selection operates on breeding phenology in natural populations.

Spring phenology does not affect timing of reproduction in the great tit (Parus major)

Many seasonal breeders adjust the timing of reproduction in response to year-to-year variations in supplementary environmental cues, amongst which ambient temperature is thought to be most influential. However, it is possible that for species such as the great tit (Parus major L.), phenological cues from sprouting vegetation and the consequent abundance of invertebrate prey, although dependent on temperature, may provide supplementary environmental cues per se. This hypothesis was investigated in breeding pairs of great tits kept in outdoor aviaries. In spring, experimental pairs were provided with access to leafing birch branches and caterpillars as a visual food cue, while control pairs were provided with non-leafing branches. Observations were made on the onset of laying and on concentrations of plasma luteinizing hormone (LH) at regular intervals to monitor changes in reproductive function. The onset of egg laying was not advanced by the presence of leafing branches and caterpillars. LH concentrations increased during the course of the study, but phenological cues did not affect plasma LH levels in females and males. Early spring vegetation, such as the leafing of birch branches, and the appearance of caterpillar prey do not appear to play a significant role in fine-tuning the onset of egg laying in great tits.

Multisensory non-photoperiodic cue advances the onset of seasonal breeding in Island canaries (Serinus canaria)

In most temperate zone vertebrates, photoperiodic change plays the major role in the timing of seasonal breeding. However, direct environmental stimuli such as temperature, rainfall, or availability of food are thought to be important for fine-tuning breeding activities. Building on evidence from wild Island canaries (Serinus canaria), the authors had shown advancing effects of green vegetation on breeding under captive, short-day conditions. So far, the precise, sensory modalities of this stimulatory cue are unknown. Here the authors present new data that confirm advanced breeding activities in the presence of green vegetation and narrow its stimulatory components. They found that direct exposure of the birds to fresh green vegetation represented the strongest stimulus and advanced breeding by up to 2 months compared to controls. In contrast, access to artificial green vegetation, extracts from green vegetation, or olfactory components alone had no such effects. This is, to the best of the authors' knowledge, the first experiment that examines sensory components of an effective, supplementary, non-photoperiodic cue in a temperate zone species. The data suggest that in order to use non-photoperiodic information to time breeding, birds must be able to integrate and process multisensory stimuli. Single non-photoperiodic sensory cues are insufficient to affect the timing of seasonal breeding.

Phenology, seasonal timing and circannual rhythms: towards a unified framework

Phenology refers to the periodic appearance of life-cycle events and currently receives abundant attention as the effects of global change on phenology are so apparent. Phenology as a discipline observes these events and relates their annual variation to variation in climate. But phenology is also studied in other disciplines, each with their own perspective. Evolutionary ecologists study variation in seasonal timing and its fitness consequences, whereas chronobiologists emphasize the periodic nature of life-cycle stages and their underlying timing programmes (e.g. circannual rhythms). The (neuro-) endocrine processes underlying these life-cycle events are studied by physiologists and need to be linked to genes that are explored by molecular geneticists. In order to fully understand variation in phenology, we need to integrate these different perspectives, in particular by combining evolutionary and mechanistic approaches. We use avian research to characterize different perspectives and to highlight integration that has already been achieved. Building on this work, we outline a route towards uniting the different disciplines in a single framework, which may be used to better understand and, more importantly, to forecast climate change impacts on phenology.

Geographically distinct reproductive schedules in a changing world: Costly implications in captive Stonechats

With progressively faster global change, shifts in phenology, and distributional ranges are reported for an increasing number of species. The success of organisms at coping with novel seasonal conditions depends on the mechanisms that determine their schedules. Species that rely on fixed schedules and those that time their activities by predictive cues may be particularly constrained in their ability to accommodate changes. The present study examines rigid scheduling and its implications for breeding in captivity in an avian model taxon, the Stonechat (Saxicola torquata). Within their extensive breeding range, Stonechats differ geographically in migratory behavior and reproduce and molt under a wide range of daylengths (10-17 h). Stonechats time their activities by programs that involve circannual rhythms and photoperiodism. The study reports reproductive timing of four taxa (central European, Irish, Siberian, and Kenyan), relates it to laydates in the field, and investigates modifying influences of housing conditions and of social context. Reproductive consequences of timing programs were then tested by crossbreeding of taxa with different schedules. The study revealed persistent, population-specific breeding windows in captivity. Resident Stonechats from equatorial Kenya synchronized their reproductive cycles with the European summer, presumably in response to local photoperiod, and bred at similar times as northern migrants. In all other taxa schedules matched those in the field, but were timed slightly earlier in captivity and advanced by indoor keeping conditions. Influences of social context were negligible. In pairs with clutches, testes regressed slightly later than in pairs without clutches, but presence of a mate per se had no influence on breeding cycles. Accordingly, crossbreeding Stonechats were predicted to have limited capacity to adjust schedules to those of their mates. This prediction was tested by crossbreeding of single-clutched Siberian long-distance migrants with multiple-clutched European short-distance migrants. Males and females of both taxa retained their characteristic breeding schedules, regardless of their mate's activities. This led to dramatic loss of reproductive success in the population with the longer breeding season, European Stonechats. Siberian Stonechats were unable to profit from the presence of a sexually active mate, but they suffered no disadvantage from crossbreeding. In a changing world, inherited timing programs may severely constrain responses to novel conditions, impose schedule-dependent, asymmetric costs of hybridization, and contribute to directional gene flow or to reproductive isolation.

Circannual basis of geographically distinct bird schedules

To anticipate seasonal change, organisms schedule their annual activities by using calendrical cues like photoperiod. The use of cues must be fitted to local conditions because schedules differ between species and habitats. In complete absence of temporal information, many species show persistent circannual cycles that are synchronised, but not driven, by photoperiod. The contribution of circannual rhythms to timing under natural photoperiodic conditions is still unclear. In a suite of experiments, we examined timing in two closely related songbirds (Siberian and European stonechats) that inhabit similar latitudes but differ in seasonal behaviour. Under a more continental climate, Siberian stonechats breed later, moult faster and migrate further than European stonechats. We tested hypotheses for seasonal timing mechanisms by comparing the birds under constant and naturally changing daylengths. The taxa retained characteristic reproductive and moult schedules and hybrids behaved roughly intermediately. Based on their distinct circannual cycles, we expected European and Siberian stonechats to differ in photoperiodic responses at a given time of year. We found that the taxa responded, as predicted, in opposite ways to photoperiodic simulations as experienced on different migration routes. The findings indicate that circannual rhythms reflect geographically distinct periodic changes in seasonal disposition and cue-response mechanisms. Under natural daylengths, the phase relationship of the underlying circannual rhythm to the external year determines the action of photoperiod. Circannual rhythms are widespread among long-lived species. Accordingly, responses to environmental change, range expansion and novel migration patterns may depend on the particulars of a species' underlying circannual programming.

Seasonality in song behaviour revisited: seasonal and annual variants and invariants in the song of the domesticated canary (Serinus canaria)

The song of the domesticated canary (Serinus canaria) is one of the most widely used models to study the neural correlates of behavioural plasticity and the mechanisms of female mate choice. However, only few studies have described the song behaviour in detail and monitored their changes throughout the year, and these data are restricted to the "Waterslager" strain. Here, we studied the song characteristics of the male common domesticated canary at different times of the year, the spring breeding and autumnal non-breeding season, and monitored the birds' songs up to the following breeding season. During breeding, males have increased plasma levels of testosterone, and songs are on average longer and consist of fewer non-repeated syllable types compared to the non-breeding season. When subsequent seasons are compared, song duration and the proportion of non-repeated syllable types change seasonally but not across years. Repertoire size remains constant throughout seasons although syllable types are exchanged. Syllable carry-over is significantly higher from one breeding season to the next than between the breeding and non-breeding season. Further, the repertoire of the breeding season contains more potentially sexually attractive syllable types than that of the non-breeding season. These data show that overall song structure is retained throughout the year while seasonality occurs in the temporal pattern and in repertoire composition.

Photoperiodic induced changes in reproductive state of border canaries (Serinus canaria) are associated with marked variation in hypothalamic gonadotropin-releasing hormone immunoreactivity and the volume of song control regions

In temperate zone songbirds, such as canaries (Serinus canaria), seasonal variation in gonadal activity and behavior are associated with marked brain changes. These include gonadotropin-releasing hormone (GnRH) expression and the volume of brain areas controlling song production. Questions have been raised about the consistency of seasonal brain changes in canaries. Laboratory studies of the American singer strain raised doubts as to whether this strain exhibits a robust photoperiodic response along with changes in brain GnRH content, and studies of free-living canaries have failed to identify seasonal changes in volume of song control nuclei. We assessed differences in brain GnRH and the song control system associated with photoperiod-induced variation in reproductive state in Border canaries. We found that males and females maintained for 10 weeks on long days (14L:10D) regress their gonads, exhibit a decline in testosterone and initiate molt; a response consistent with the onset of absolute photorefractoriness (i.e., failed to respond to previously stimulating daylengths). All birds regained photosensitivity (i.e., exhibited gonadal response to stimulating daylengths) after experiencing short days (8L:16D) for 6 weeks. Furthermore, comparisons of birds in either a photosensitive, photostimulated, or photorefractory state revealed a marked increase in GnRH protein expression in the photosensitive and photostimulated birds over photorefractory birds. A similar variation was observed in the volume of key forebrain song nuclei. Thus, Border canaries demonstrate measurable neuroplasticity in response to photoperiodic manipulations. These data, along with previous work on other strains of canaries, indicate the presence of intra-specific variation in photoperiodically regulated neuroplasticity.