Seasonal Timing Of Reproduction Research Articles

Association of reproduction with seasonality in a subtropical viviparous fish, Jenynsia tucumana (Cyprinodontiformes: Anablepidae)

For fishes in temperate environments, the timing of seasonal reproduction has been correlated with changes in photoperiod and temperature. In tropical environments, seasonal hydrological patterns have been found to be drivers of the onset of reproduction. Despite these established relationships, data on reproductive seasonality for viviparous fish are limited. Here, we investigate aspects of reproduction in the viviparous fish Jenynsia tucumana Aguilera & Mirande, 2005 over the course of 12 months in the Salí River basin (Tucumán, Argentina), which is characterized by dry, short winters, and long, tropical summers. Size and reproductive data were collected each month. We found J. tucumana displayed marked sexual dimorphism in size, with females having higher length and weight when compared to males. Larger females have higher fecundities compared to smaller females. The breeding season corresponds with seasonal changes in photoperiod, air temperature, and rainfall, but ends with the arrival of heavy rainfall later in the season. Interestingly, J. tucumana produce and mature oocytes year-round, unlike congeneric species. This may be an adaptive strategy imposed by the irregular flooding of subtropical mountainous river stream habitats, which maintain high productivity well into early autumn.

Variation in responses to photoperiods and temperatures in Japanese medaka from different latitudes

Seasonal changes are more robust and dynamic at higher latitudes than at lower latitudes, and animals sense seasonal changes in the environment and alter their physiology and behavior to better adapt to harsh winter conditions. However, the genetic basis for sensing seasonal changes, including the photoperiod and temperature, remains unclear. Medaka (Oryzias latipes species complex), widely distributed from subtropical to cool-temperate regions throughout the Japanese archipelago, provides an excellent model to tackle this subject. In this study, we examined the critical photoperiods and critical temperatures required for seasonal gonadal development in female medaka from local populations at various latitudes. Intraspecific differences in critical photoperiods and temperatures were detected, demonstrating that these differences were genetically controlled. Most medaka populations could perceive the difference between photoperiods for at least 1 h. Populations in the Northern Japanese group required 14 h of light in a 24 h photoperiod to develop their ovaries, whereas ovaries from the Southern Japanese group developed under 13 h of light. Additionally, Miyazaki and Ginoza populations from lower latitudes were able to spawn under short-day conditions of 11 and 10 h of light, respectively. Investigation of the critical temperature demonstrated that the Higashidori population, the population from the northernmost region of medaka habitats, had a critical temperature of over 18 °C, which was the highest critical temperature among the populations examined. The Miyazaki and the Ginoza populations, in contrast, were found to have critical temperatures under 14 °C. When we conducted a transplant experiment in a high-latitudinal environment using medaka populations with different seasonal responses, the population from higher latitudes, which had a longer critical photoperiod and a higher critical temperature, showed a slower reproductive onset but quickly reached a peak of ovarian size. The current findings show that low latitudinal populations are less responsive to photoperiodic and temperature changes, implying that variations in this responsiveness can alter seasonal timing of reproduction and change fitness to natural environments with varying harshnesses of seasonal changes. Local medaka populations will contribute to elucidating the genetic basis of seasonal time perception and adaptation to environmental changes.

The influence of plant odours on sexual readiness in an insectivorous songbird.

Many organisms rely on environmental cues to predict and anticipate the annual optimal timing of reproduction. In insectivorous birds, preparation for breeding often coincides with the time vegetation starts to develop in spring. Whether there is a direct relationship between the two, and through which mechanisms this link could come about, has rarely been investigated. Plants release Herbivore-Induced Plant Volatiles (HIPVs) when they are attacked by insects, and recent studies have shown that birds can detect and orient to those odours when searching for food. If those volatiles also stimulate sexual reproductive development and timing of reproduction remains to be discovered. We tested this hypothesis by monitoring gonadal growth in pairs of blue tits (Cyanistes caerueus) exposed to air from caterpillar infested oak trees or from a control, in spring. We found that while males and females grew their gonads over time, gonads grew at the same rate in both odor treatments. More exploratory (i.e. a proxy of personality) females did however have larger ovarian follicle sizes when exposed to the HIPVs than to the control air, which is consistent with earlier results showing that fast explorers have larger gonads in spring and are more sensitive to HIPVs. If HIPVs constitute powerful attractants in foraging birds, their influence on gonadal development prior to breeding appears to be relatively subtle and only enhance reproductive readiness in some individuals. These results are nevertheless important as they set olfaction as a new player in seasonal timing of reproduction in birds.

Size‐associated energetic constraints on the seasonal onset of reproduction in a species with indeterminate growth

The seasonal onset of reproduction is constrained in many systems by a need to first accumulate energetic reserves. Consequently, the observation that larger individuals reproduce earlier may be due to a negative relationship between size and mass‐specific basal metabolic rate that is shared across diverse taxa. However, an untested prediction of this hypothesis is that individuals should be metabolically efficient enough to escape energetic constraints above a certain size threshold. Seasonally reproducing species, such as temperate fishes, that must recover winter energy losses before reproduction and exhibit indeterminate growth are ideal models to test this prediction. We harness decade‐long behavioral data on parental male smallmouth bass, Micropterus dolomieu, to investigate contributions of energetic allometry to differences in reproductive timing. At the population level, peak seasonal reproductive timing (i.e. the median date on which eggs were found in nests each year) was negatively related to degree days – a measure of thermal energy experienced – before reproduction. At the individual level, degree days accumulated by males before reproduction was related to male size and condition in every year, but the impact of temperature on reproductive timing by the largest males was relaxed in most years. Additionally, we used our data to replicate the analyses of two previous studies of M. dolomieu populations and found virtually identical negative associations between male body size and degree days accumulated before reproduction. Our results suggest that in smallmouth bass the onset of seasonal reproduction is constrained by basal metabolic rate – as indicated by total length – and that large individuals can escape size‐associated energetic constraints. We reveal a more complicated relationship between size and reproductive timing than earlier studies, which may be relevant for many species. Knowledge of this relationship is critical to understanding how a changing climate will influence population dynamics of economically, ecologically and recreationally important species like M. dolomieu.

Seasonal Adaptation: Geographic Photoperiod-Temperature Patterns Explain Genetic Variation in the Common Vole Tsh Receptor.

The vertebrate photoperiodic neuroendocrine system uses the photoperiod as a proxy to time the annual rhythms in reproduction. The thyrotropin receptor (TSHR) is a key protein in the mammalian seasonal reproduction pathway. Its abundance and function can tune sensitivity to the photoperiod. To investigate seasonal adaptation in mammals, the hinge region and the first part of the transmembrane domain of the Tshr gene were sequenced for 278 common vole (Microtus arvalis) specimens from 15 localities in Western Europe and 28 localities in Eastern Europe. Forty-nine single nucleotide polymorphisms (SNPs; twenty-two intronic and twenty-seven exonic) were found, with a weak or lack of correlation with pairwise geographical distance, latitude, longitude, and altitude. By applying a temperature threshold to the local photoperiod-temperature ellipsoid, we obtained a predicted critical photoperiod (pCPP) as a proxy for the spring onset of local primary food production (grass). The obtained pCPP explains the distribution of the genetic variation in Tshr in Western Europe through highly significant correlations with five intronic and seven exonic SNPs. The relationship between pCPP and SNPs was lacking in Eastern Europe. Thus, Tshr, which plays a pivotal role in the sensitivity of the mammalian photoperiodic neuroendocrine system, was targeted by natural selection in Western European vole populations, resulting in the optimized timing of seasonal reproduction.

Photoperiodic regulation of avian physiology: From external coincidence to seasonal reproduction.

Seasonal cycles of environmental cues generate variation in the timing of life-history transition events across taxa. It is through the entrainment of internal, endogenous rhythms of organisms to these external, exogenous rhythms in environment, such as cycling temperature and daylight, by which organisms can regulate and time life history transitions. Here, we review the current understanding of how photoperiod both stimulates and terminates seasonal reproduction in birds. The review describes the role of external coincidence timing, the process by which photoperiod is proposed to stimulate reproductive development. Then, the molecular basis of light detection and the photoperiodic regulation of neuroendocrine timing of seasonal reproduction in birds is presented. Current data indicates that vertebrate ancient opsin is the predominant photoreceptor for light detection by the hypothalamus, compared to neuropsin and rhodopsin. The review then connects light detection to well-characterized hypothalamic and pituitary gland molecules involved in the photoperiodic regulation of reproduction. In birds, Gonadotropin-releasing hormone synthesis and release are controlled by photoperiodic cues via thyrotropin-stimulating hormone-β (TSHβ) independent and dependent pathways, respectively. The review then highlights the role of D-box and E-box binding motifs in the promoter regions of photoperiodic genes, in particular Eyes-absent 3, as the key link between circadian clock function and photoperiodic time measurement. Based on the available evidence, the review proposes that at least two molecular programs form the basis for external coincidence timing in birds: photoperiodic responsiveness by TSHβ pathways and endogenous internal timing by gonadotropin synthesis.

Does larval food availability ultimately select for seasonal reproduction in marine invertebrates with feeding larvae? A field test of Crisp’s Rule with the temperate sea star Pisaster ochraceus

AbstractMost organisms reproduce seasonally, including most species of marine invertebrates that live across wide ranges in habitat, depth and environmental conditions. We asked: What ultimately selects for seasonal reproduction in benthic marine invertebrates with planktotrophic larvae (characteristic of ≥70% of marine invertebrates)? We hypothesized that seasonal variation in food available for the larvae ultimately selects for timing of adult reproduction. Testing this hypothesis requires a whole life cycle perspective and approach. Using a known proximate cue, daylength, we shifted gametogenesis in the laboratory by six months in a seasonally breeding temperate sea star, the Ochre Star Pisaster ochraceus (Brandt 1835). We were then able to induce spawning, fertilize gametes and culture resulting embryos in vitro to produce feeding larvae six months out of phase with natural photoperiod. We field‐reared these out‐of‐season larvae, similarly produced in‐season conspecifics and similar larvae of an aseasonally breeding asteroid, the Bat Star Patiria miniata (Brandt 1835) in mesh‐covered flow‐through containers that were deployed in seasonally contrasting oceanographic conditions reflecting different productivity regimes and larval food availability in spring and fall. Larval development and survival were similar between seasons: for example, planktonic larval duration was 48 d to first metamorphosis in spring vs. 45 d in fall for P. ochraceus. Hence, temporal variation in available phytoplankton may not be an ultimate selection factor acting on these larvae to regulate timing of seasonal reproduction in the adults. Alternatively, in this and other species of marine invertebrates, selection acting on early benthic juveniles or other life stages may determine the timing of adult reproduction.

Food and temperature change photoperiodic responses in two vole species.

Seasonal timing of reproduction in voles is driven by photoperiod. We hypothesized that a negative energy balance can modify spring-programmed photoperiodic responses in the hypothalamus, controlling reproductive organ development. We manipulated energy balance by the 'work-for-food' protocol, in which voles were exposed to increasing levels of food scarcity at different ambient temperatures under long photoperiod. We found that in common voles (Microtus arvalis) and tundra voles (Microtus oeconomus), photoperiod-induced pars tuberalis thyroid-stimulating hormone β-subunit (Tshβ) expression is reduced to potentially inhibit gonadal development when food is scarce. Reduction in gonadal size is more pronounced in tundra voles, in which anterior hypothalamic Kiss1 is additionally downregulated, especially in males. Low temperature additionally leads to decreased hypothalamic Rfrp expression, which potentially may facilitate further suppression of gonadal growth. Shutting off the photoperiodic axis when food is scarce in spring may be an adaptive response to save energy, leading to delayed reproductive organ development until food resources are sufficient for reproduction, lactation and offspring survival. Defining the mechanisms through which metabolic cues modify photoperiodic responses will be important for a better understanding of how environmental cues impact reproduction.

Mechanisms of temperature modulation in mammalian seasonal timing.

Global warming is predicted to have major effects on the annual time windows during which species may successfully reproduce. At the organismal level, climatic shifts engage with the control mechanism for reproductive seasonality. In mammals, laboratory studies on neuroendocrine mechanism emphasize photoperiod as a predictive cue, but this is based on a restricted group of species. In contrast, field-oriented comparative analyses demonstrate that proximate bioenergetic effects on the reproductive axis are a major determinant of seasonal reproductive timing. The interaction between proximate energetic and predictive photoperiodic cues is neglected. Here, we focused on photoperiodic modulation of postnatal reproductive development in common voles (Microtus arvalis), a herbivorous species in which a plastic timing of breeding is well documented. We demonstrate that temperature-dependent modulation of photoperiodic responses manifest in the thyrotrophin-sensitive tanycytes of the mediobasal hypothalamus. Here, the photoperiod-dependent expression of type 2 deiodinase expression, associated with the summer phenotype was enhanced by 21°C, whereas the photoperiod-dependent expression of type 3 deiodinase expression, associated with the winter phenotype, was enhanced by 10°C in spring voles. Increased levels of testosterone were found at 21°C, whereas somatic and gonadal growth were oppositely affected by temperature. The magnitude of these temperature effects was similar in voles photoperiodical programmed for accelerated maturation (ie, born early in the breeding season) and in voles photoperiodical programmed for delayed maturation (ie, born late in the breeding season). The melatonin-sensitive pars tuberalis was relatively insensitive to temperature. These data define a mechanistic hierarchy for the integration of predictive temporal cues and proximate thermo-energetic effects in mammalian reproduction.

Exposure to artificial light at night accelerates but does not override latitude-dependent seasonal reproductive response in a North American songbird.

In the modern era of industrialization, illuminated nights have become a common defining feature of human-occupied environments, particularly cities. Artificial light at night (ALAN) imposes several known negative impacts on the neuroendocrine system, metabolism, and seasonal reproduction of species living in the wild. However, we know little about the impact of ALAN on populations of birds that either live year-round in the same location or move to different latitudes across seasons. To test whether ALAN has a differing impact on the reproductive timing of bird populations that winter in sympatry but breed at different latitudes, we monitored sedentary and migratory male dark-eyed juncos that were or were not exposed to low intensity (∼2.5±0.5 lux) ALAN. All groups were held in common conditions and day length was gradually increased to mimic natural day length changes (NDL). We assessed seasonal reproductive response from initiation to termination of the breeding cycle. As expected based on earlier research, the sedentary birds exhibited earlier gonadal recrudescence and terminated breeding later than the migratory birds. In addition, resident and migrant birds exposed to ALAN initiated gonadal recrudescence earlier and terminated reproductive events sooner as compared to their conspecifics experiencing NDL. Importantly, the difference in the reproductive timing of sedentary and migratory populations was maintained even when exposed to ALAN. This variation in the seasonal reproductive timing may likely have a genetic basis or be the result of early developmental effects imposed due to different light regimes related to the latitude of origin. This study reveals first that ALAN accelerated reproductive development across both migrants and residents and second that latitude-dependent variation in reproductive timing is maintained despite exposure to ALAN. These results corroborate a relationship between latitude, population, and reproductive timing while also revealing ALAN's impact on seasonal reproductive timing. This study reveals that, ALAN accelerated reproductive development but maintained latitude-dependent variation in reproductive timing across both migrant and resident bird populations.

Interocean patterns in shallow water sponge assemblage structure and function.

Sponges are a major component of benthic ecosystems across the world and fulfil a number of important functional roles. However, despite their importance, there have been few attempts to compare sponge assemblage structure and ecological functions across large spatial scales. In this review, we examine commonalities and differences between shallow water (<100 m) sponges at bioregional (15 bioregions) and macroregional (tropical, Mediterranean, temperate, and polar) scales, to provide a more comprehensive understanding of sponge ecology. Patterns of sponge abundance (based on density and area occupied) were highly variable, with an average benthic cover between ~1 and 30%. Sponges were generally found to occupy more space (percentage cover) in the Mediterranean and polar macroregions, compared to temperate and tropical macroregions, although sponge densities (sponges m-2 ) were highest in temperate bioregions. Mean species richness standardised by sampling area was similar across all bioregions, except for a few locations that supported very high small-scale biodiversity concentrations. Encrusting growth forms were generally the dominant sponge morphology, with the exception of the Tropical West Atlantic, where upright forms dominated. Annelids and Arthropods were the most commonly reported macrofauna associated with sponges across bioregions. With respect to reproduction, there were no patterns in gametic development (hermaphroditism versus gonochorism), although temperate, tropical, and polar macroregions had an increasingly higher percentage of viviparous species, respectively, with viviparity being the sole gamete development mechanism reported for polar sponges to date. Seasonal reproductive timing was the most common in all bioregions, but continuous timing was more common in the Mediterranean and tropical bioregions compared to polar and temperate bioregions. We found little variation across bioregions in larval size, and the dominant larval type across the globe was parenchymella. No pattens among bioregions were found in the limited information available for standardised respiration and pumping rates. Many organisms were found to predate sponges, with the abundance of sponge predators being higher in tropical systems. While there is some evidence to support a higher overall proportion of phototrophic species in the Tropical Austalian bioregion compared to the Western Atlantic, both also have large numbers of heterotrophic species. Sponges are important spatial competitors across all bioregions, most commonly being reported to interact with anthozoans and algae. Even though the available information was limited for many bioregions, our analyses demonstrate some differences in sponge traits and functions among bioregions, and among macroregions. However, we also identified similarities in sponge assemblage structure and function at global scales, likely reflecting a combination of regional- and local-scale biological and physical processes affecting sponge assemblages, along with common ancestry. Finally, we used our analyses to highlight geographic bias in past sponge research, and identify gaps in our understanding of sponge ecology globally. By so doing, we identified key areas for future research on sponge ecology. We hope that our study will help sponge researchers to consider bioregion-specific features of sponge assemblages and key sponge-mediated ecological processes from a global perspective.

It takes two: Heritable male effects on reproductive timing but not clutch size in a wild bird population.

Within-population variation in the traits underpinning reproductive output has long been of central interest to biologists. Since they are strongly linked to lifetime reproductive success, these traits are expected to be subject to strong selection and, if heritable, to evolve. Despite the formation of durable pair bonds in many animal taxa, reproductive traits are often regarded as female-specific, and estimates of quantitative genetic variation seldom consider a potential role for heritable male effects. Yet reliable estimates of such social genetic effects are important since they influence the amount of heritable variation available to selection. Based on a 52-year study of a nestbox-breeding great tit (Parus major) population, we apply "extended" bivariate animal models in which the heritable effects of both sexes are modeled to assess the extent to which males contribute to heritable variation in seasonal reproductive timing (egg laying date) and clutch size, while accommodating the covariance between the two traits. Our analyses show that reproductive timing is a jointly expressed trait in this species, with (positively covarying) heritable variation for laydate being expressed in both members of a breeding pair, such that the total heritable variance is 50% larger than estimated by traditional models. This result was robust to explicit consideration of a potential male-biased environmental confound arising through sexually dimorphic dispersal. In contrast to laydate, males' contribution to heritable variation in clutch size was limited. Our study thus highlights the contrasting extent of social determination for two major components of annual reproductive success, and emphasizes the need to consider the social context of what are often considered individual-level traits.

Migrant and resident female songbirds differ in gonadal response to upstream stimulation during seasonal sympatry

Timing of seasonal reproduction is driven by environmental cues acting on the hypothalamic-pituitary–gonadal (HPG) axis. Groups of individuals, or populations, of the same species can exhibit different phenology despite facing similar environmental cues or living in the same habitat (i.e., seasonal sympatry). The mechanisms giving rise to population-level differences in reproductive timing are not fully understood, particularly for females. We studied the dark-eyed junco, a songbird with migratory and sedentary (i.e., resident) populations that live in overlapping distributions during winter. In early spring, residents initiate breeding and associated behaviors, including territory establishment and formation of pair bonds, while migrants prepare to depart for their breeding grounds. We tested whether migrant and resident hormonal response to upstream hormonal stimulation differed during this time period. We collected blood from free-living females in early spring, and challenged them with repeated gonadotropin-releasing hormone (GnRH) injections to measure testosterone (T) response. We predicted that if migrants are less sensitive to upstream stimulation than residents, then they would exhibit lower response to the repeated GnRH challenges in migrants. We found that migrant and resident females both responded to an initial challenge by elevating T, but residents responded more robustly, indicating that the ovary plays a role in population-level differences in reproductive timing. We also found that migrants and residents attenuated their response to repeated challenges, and did not differ from one another in final T levels. We speculate that the explanation for the generally reduced T response after repeated GnRH injections need not be the same for migrants and residents, but possible explanations include suppression of upstream stimulation owing to negative feedback after the initial injection oraromatization of T to estradiol between sampling time points. We suggest that future studies experimentally explore how the ovarian response to upstream stimulation changes during the transition to reproduction.

Variation in chronotype is associated with migratory timing in a songbird.

Like many organisms, birds exhibit daily (circadian) and seasonal biological rhythms, and within populations both daily and seasonal timing often vary among individuals. Because photoperiod interacts with the circadian rhythms of many organisms to induce seasonal changes in behaviour and physiology, it is hypothesized that differences in daily timing, called chronotypes, underpin differences among individuals in the timing of seasonal events. For seasonal events stimulated by increasing daylength, this hypothesis predicts a positive relationship between the timing of daily and seasonal activities of individuals, with advanced chronotypes expressing events earlier in the year. The few previous tests of this hypothesis have focused on seasonal reproductive timing in birds. However, the hypothesis predicts that this relationship should extend to other photoinduced seasonal events. Therefore, we tested whether variation in chronotype was associated with variation in spring migratory timing in a captive songbird model, the pine siskin (Spinus pinus). We found that pine siskins expressing migratory restlessness exhibited repeatable chronotypes in their timing of nocturnal activity. Further, chronotype was significantly associated with the onset date of migratory behaviour, consistent with the hypothesized relationship between chronotype and seasonal timing.

Temporally replicated DNA methylation patterns in great tit using reduced representation bisulfite sequencing

Seasonal timing of reproduction is an important fitness trait in many plants and animals but the underlying molecular mechanism for this trait is poorly known. DNA methylation is known to affect timing of reproduction in various organisms and is therefore a potential mechanism also in birds. Here we describe genome wide data aiming to detect temporal changes in methylation in relation to timing of breeding using artificial selection lines of great tits (Parus major) exposed to contrasting temperature treatments. Methylation levels of DNA extracted from erythrocytes were examined using reduced representation bisulfite sequencing (RRBS). In total, we obtained sequencing data from 63 libraries over four different time points from 16 birds with on average 20 million quality filtered reads per library. These data describe individual level temporal variation in DNA methylation throughout the breeding season under experimental temperature regimes and provides a resource for future studies investigating the role of temporal changes in DNA methylation in timing of reproduction.

Behavioural thermoregulation hastens spring mating activity in Pyrrhocoris apterus (Heteroptera: Pyrrhocoridae)

Post-diapause, overwintered adults of the true bug Pyrrhocoris apterus L. (Heteroptera, Pyrrhocoridae) form conspicuous aggregations at warm spots in early spring. Using a combination of laboratory experiments and field observations, we assessed the influence of this behaviour on the seasonal timing of reproduction. In the laboratory, post-diapause pairs mated after an accumulation of 80° days (dd), and females started to lay eggs after an additional 80 dd (considering 10.3 °C as a lower temperature threshold for post-diapause development). In the field, however, females mated as early as late March (based on data from four seasons), which corresponded to the accumulation of <15 dd (based on summing the temperatures recorded at a meteorology station). Such a 'discrepancy' between laboratory and field data is explained by thermoregulation. Aggregated adults bask in sunny spots, thereby increasing their body temperatures to ~25 °C, which is 7-16 °C above the temperature of the surrounding ground. This thermal excess speeds up their post-diapause development and enables early mating and oviposition. As a result, behavioural thermoregulation hastens the seasonal start of reproduction in P. apterus by > 1 month.

Genetic and phenotypic responses to genomic selection for timing of breeding in a wild songbird

Abstract The physiological mechanisms underlying avian seasonal timing of reproduction, a life‐history trait with major fitness consequences, are not well understood. Comparing individuals that have been selected to differ in their timing of breeding may prove to be a promising in studying these mechanisms, making selection lines a valuable tool. We created selection lines for early and late timing of breeding in great tits (Parus major) using genomic selection, that is selection based on multi‐marker genotypes rather than on the phenotype. We took in nestlings (F1 generation) from wild broods of which the mother was either an extremely early (“early line”) or extremely late (“late line”) breeder. These chicks were then genotyped and, based on their “genomic breeding values” (GEBVs), we selected individuals for early and late line breeding pairs to produce the F2 generation in captivity. The F2 offspring was hand‐reared, genotyped and selected to produce an F3 generation, which were then again genotyped and selected. This way we obtained laying dates in aviaries for F1, F2 and F3 birds. We studied the genetic response to the artificial selection and found increased genetic differentiation between the early and late reproducing selection lines over generations (F1–F3), indicated by both diverging GEBVs and increased fixation indices (FST). We studied the phenotypic response to selection for birds breeding in outdoor breeding aviaries. We found that early line birds laid earlier than late line birds, and this difference increased over the generations (F1–F3), with non‐significant line effects for the F1 and F2, but highly significant line differences for the F3. We also assessed whether there was correlated selection on two traits that are potentially part of the mechanisms underlying seasonal timing: the endogenous free‐running period of the day/night clock (tau) and basal metabolic rate, but found no correlated selection. We have successfully created selection lines on seasonal timing in a wild bird species and obtained an instrument for future studies to investigate the physiological mechanisms underlying timing of breeding, and the genetic variation in these mechanisms, an essential component for evolutionary change in timing of reproduction. A free Plain Language Summary can be found within the Supporting Information of this article.

Personality and gonadal development as sources of individual variation in response to GnRH challenge in female great tits.

Seasonal timing of reproduction is a key life-history trait, but we know little about the mechanisms underlying individual variation in female endocrine profiles associated with reproduction. In birds, 17β-oestradiol is a key reproductive hormone that links brain neuroendocrine mechanisms, involved in information processing and decision-making, to downstream mechanisms in the liver, where egg-yolk is produced. Here, we test, using a simulated induction of the reproductive system through a Gonadotropin-Releasing Hormone (GnRH) challenge, whether the ovary of pre-breeding female great tits responds to brain stimulation by increasing oestradiol. We also assess how this response is modified by individual-specific traits like age, ovarian follicle size, and personality, using females from lines artificially selected for divergent levels of exploratory behaviour. We show that a GnRH injection leads to a rapid increase in circulating concentrations of oestradiol, but responses varied among individuals. Females with more developed ovarian follicles showed stronger responses and females from lines selected for fast exploratory behaviour showed stronger increases compared to females from the slow line, indicating a heritable component. This study shows that the response of the ovary to reproductive stimulation from the brain greatly varies among individuals and that this variation can be attributed to several commonly measured individual traits, which sheds light on the mechanisms shaping heritable endocrine phenotypes.

Seasonal Variation in Genome-Wide DNA Methylation Patterns and the Onset of Seasonal Timing of Reproduction in Great Tits.

In seasonal environments, timing of reproduction is a trait with important fitness consequences, but we know little about the molecular mechanisms that underlie the variation in this trait. Recently, several studies put forward DNA methylation as a mechanism regulating seasonal timing of reproduction in both plants and animals. To understand the involvement of DNA methylation in seasonal timing of reproduction, it is necessary to examine within-individual temporal changes in DNA methylation, but such studies are very rare. Here, we use a temporal sampling approach to examine changes in DNA methylation throughout the breeding season in female great tits (Parus major) that were artificially selected for early timing of breeding. These females were housed in climate-controlled aviaries and subjected to two contrasting temperature treatments. Reduced representation bisulfite sequencing on red blood cell derived DNA showed genome-wide temporal changes in more than 40,000 out of the 522,643 CpG sites examined. Although most of these changes were relatively small (mean within-individual change of 6%), the sites that showed a temporal and treatment-specific response in DNA methylation are candidate sites of interest for future studies trying to understand the link between DNA methylation patterns and timing of reproduction.

Detailed biological data are informative, but robust trends are needed for informing sustainability of wildlife harvesting: A case study of reptile offtake in Southeast Asia

Conservation biologists often use sophisticated models of population functioning to predict the levels at which harvesting of wild populations will be sustainable. We argue that this approach is not useful for species that, although abundant, are near impossible to survey cost-effectively with enough precision to gather key data to input as model parameters. Our examination of >7000 Reticulated Pythons (Malayopython reticulatus), collected for the commercial skin industry, provides extensive information on life-history traits, population size structure, sex ratio and diets across sites in Kalimantan, Sumatra and Peninsular Malaysia. Our data reveal many interesting divergences between populations from north vs south of the equator (e.g., in body sizes, reproductive frequencies, seasonal timing of reproduction), and a trend for snakes to be smaller from more intensively harvested sites. In combination with earlier analyses, >10,000 individuals of M. reticulatus have been dissected for information on ecological attributes (more than for any other snake species worldwide). Nonetheless, in the absence of demographic data (e.g., on underlying abundances and rates of survival and growth of free-ranging snakes), the most robust estimate of sustainability of the harvest comes from long-term trends in mean body sizes, life history traits, and offtake rates. Because mathematical models depend upon accurate estimation of abundance and demography, parameters unobtainable for these secretive, well-camouflaged reptiles, managers should prioritise gathering long term data on harvest trends instead.