Exposure to artificial light at night accelerates but does not override latitude-dependent seasonal reproductive response in dark-eyed juncos

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 differing impact on reproductive timing of the 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 reproduction 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 ground or early developmental effects imposed due to different latitude of origin. This study reveals first that latitude-dependent variation in reproductive timing is maintained despite exposure to ALAN, and second that ALAN accelerated reproductive development across both migrants and residents. The results corroborating relationship between latitude, population, and ALAN impact on seasonal reproductive timing, may provide a potential mechanism to test the fitness of a population and its range expansion to exploit urban environment.

Breeding Latitude and Annual Cycle Timing in a Songbird

In spring, songbirds undergo physiological changes such as migratory fattening and gonadal recrudescence in response to increasing day length. Past research suggests that the day length required to initiate physiological changes, known as the photoperiodic threshold, can vary by breeding latitude. In this study, we explored whether migrants breeding at higher latitudes require longer days in spring before physiological changes occur (i.e., whether breeding latitude of origin predicts photoperiodic threshold). We caught and housed male migrant and resident dark-eyed juncos (Junco hyemalis) in an indoor aviary. Photoperiod was increased incrementally from nine to sixteen hours over fourteen weeks. During each photocycle, morphological measurements of mass, subcutaneous body fat, and cloacal protuberance were measured as indicators of migratory and reproductive condition. Stable isotope signatures of hydrogen were used to estimate breeding latitude as an index of migratory distance. Our results show that migrants and residents differed in physiological changes, as migrants accumulated more subcutaneous fat, increased body mass, and displayed a significant delay in gonadal recrudescence relative to residents. Additionally, individuals breeding at higher latitudes deposited fat at a faster rate than individuals breeding at lower latitudes. These results supported our hypothesis that migratory strategy and breeding latitude may predict differences in photoperiodic threshold for both migratory and reproductive timing. Our findings contribute to the understanding of regulation of timing in annual cycles and improve predictions of how species might respond to changing environments.

Paddlefish: Ecological, Aquacultural, and Regulatory Challenges of Managing a Global Resource

<i>Abstract</i>.—In the past decade, advances in our understanding of Paddlefish <i>Polyodon spathula</i> life history have provided additional insight into the information needed for sustainable harvest management of this long-lived species. Recovery of known-age fish in some stocks has enabled stock assessment biologists and managers to not only validate ages of individual fish, but to begin to validate the life histories. A framework for potentially recruited Paddlefish life history can be broken into five stages: 1) immature, 2) maturing, 3) somatic growth and reproduction, 4) prime reproduction, and 5) senescence to death. These stages involve measurable changes in growth in length and weight, gonadosomatic index (GSI), gonadal fat storage (GFBs), reproductive periodicity, natural mortality rates, and, in some cases, fish migrations. Stages 2–5 are typically initiated at younger ages for males than for females. Metabolic demands on Paddlefish result in them progressing through these life history stages more rapidly in southern stocks, inhabiting warmer waters, than in northern ones, inhabiting colder waters. Lifespans in most northerly stocks tend to be 2–3 times longer than for southern stocks. Natural mortality is also typically lower in northern stocks. These differences necessitate fundamentally different harvest management strategies among stocks. Regardless of the stock, however, in the prime reproduction stage, somatic growth is slow or negative, as energy is routed more strongly into reproduction, GSI is at a maximum, the period of gonadal recrudescence (i.e., spawning interval) is minimized, and GFBs are largely or completely depleted in females. Consistent with recommendations for other long-lived freshwater and marine species, harvest management strategies should be specifically planned to retain some older, prime spawning females in the population. In addition, sporadic or episodic recruitment in many stocks makes steady-state harvest models unrealistic, necessitating that harvest be appropriately matched to recruitment rates or events.

Seasonally sympatric but allochronic: differential expression of hypothalamic genes in a songbird during gonadal development

Allochrony, the mismatch of reproductive schedules, is one mechanism that can mediate sympatric speciation and diversification. In songbirds, the transition into breeding condition and gonadal growth is regulated by the hypothalamic–pituitary–gonadal (HPG) axis at multiple levels. We investigated whether the difference in reproductive timing between two seasonally sympatric subspecies of dark-eyed juncos ( Junco hyemalis ) was related to gene expression along the HPG axis. During the sympatric pre-breeding stage, we measured hypothalamic and testicular mRNA expression of candidate genes via qPCR in captive male juncos. For hypothalamic mRNA, we found our earlier breeding subspecies had increased expression of gonadotropin-releasing hormone ( GnRH ) and decreased expression of androgen receptor, oestrogen receptor alpha and mineralocorticoid receptor ( MR ). Subspecies did not differ in expression of hypothalamic gonadotropin-inhibitory hormone ( GnIH ) and glucocorticoid receptor ( GR ). While our earlier breeding subspecies had higher mRNA expression of testicular GR , subspecies did not differ in testicular luteinizing hormone receptor, follicle-stimulating hormone receptor or MR mRNA expression levels. Our findings indicate increased GnRH production and decreased hypothalamic sensitivity to sex steroid negative feedback as factors promoting differences in the timing of gonadal recrudescence between recently diverged populations. Differential gene expression along the HPG axis may facilitate species diversification under seasonal sympatry.

The effect of chronic and acute stressors, and their interaction, on gonadal function: an experimental test during gonadal recrudescence

Organisms are expected to invest less in reproduction in response to a stressor, but theory predicts that this effect should depend on the frequency of stressors in the environment. Here we investigated how an acute stressor affected gonadal function in a songbird, and how long-term differences in the stress environment influenced these acute stress responses. We exposed male Dark-eyed Juncos (Junco hyemalis) either to chronic or minimal (control) disturbance during gonadal recrudescence, after which we measured baseline testosterone, testosterone after an acute handling stressor, and the ability to elevate testosterone in response to hormonal stimulation. In a 2x2 design, we then euthanized males from the two chronic treatment groups either immediately or after an acute stressor to investigate the effect of these treatments on the gonadal transcriptome. We found that chronically disturbed birds had marginally lower testosterone. The acute stressor suppressed testosterone in control birds, but not in the chronic disturbance group. The ability to elevate testosterone did not differ between the chronic treatments. Surprisingly, chronic disturbance had a weak effect on the testicular transcriptome, and did not affect transcriptomic response to the acute stressor. The acute stressor, on the other hand, upregulated cellular stress response, and affected expression of genes associated with hormonal stress-response. Overall, we show that both chronic and acute stressors affect reproductive function, and that chronic stress changes how acute stressors affect testosterone physiology. Our findings also suggest that acute and chronic stressors affect testes differently, and that gonadal function is relatively robust to long-term stressors.Summary statementAn acute stressor downregulated testosterone production, but this effect was absent in chronically disturbed birds. The acute stressor had a strong effect on the gonadal transcriptome, whereas chronic disturbance had a negligible effect.

Age and size compositions, habitats, growth and reproductive characteristics of a terapontid (Pelates octolineatus) in coastal waters

This study of Pelates octolineatus is the first to use individually aged fish to describe the life cycle of a terapontid, a speciose and abundant Indo-West Pacific family. On the lower west Australian coast, this species uses dense seagrass as a nursery area and, after ~1 year of life when approaching 100mm in total length (TL), moves into deeper waters over sparser seagrass where it matures at the end of its second year at ~140–170mm. The maximum TL and age were 256mm and 10 years. A modified von Bertalanffy curve, allowing for a linear increase in the growth coefficient with age, improved the fit to the lengths at age of older P. octolineatus. Growth was even better described by extending this model to allow for seasonality through incorporating a sine-based curve. This model described well the seasonality exhibited by modal progressions in monthly length–frequency distributions. Instantaneous growth rates, particularly of the youngest age classes, peaked in the warm, summer months and the amplitude of seasonal change in these rates declined with increasing age. Gonadal recrudescence occurred in early spring as temperature and day length increased and spawning peaked in late-spring to mid-summer when temperatures were approaching their maxima.