Urban habitats can shape interactions between hosts and parasites by altering not only exposure rates but also within-host processes. Artificial light at night (ALAN) is common in urban environments, and chronic exposure can impair host immunity in ways that may increase infection. However, studies of causal links between this stressor, immunity, and infection dynamics are rare, particularly in migratory animals. Here, we experimentally tested how ALAN affects cellular immunity and haemosporidian parasite intensity across the annual cycle of migrant and resident subspecies of the dark-eyed junco ( Junco hyemalis ). We monitored an experimental group exposed to light at night and a control group under natural light/dark cycles as they passed through short days simulating early spring to longer days simulating the breeding season, followed by autumn migration. Using generalized additive mixed models, we show that ALAN increased inflammation, and leucocyte counts were greatest in early spring and autumn. At the start of the experiment, few birds had active infections based on microscopy, but PCR revealed many birds had chronic infections. ALAN increased parasitaemia across the annual cycle, with strong peaks in spring and autumn that were largely absent in control birds. As birds were kept in indoor aviaries to prevent vector exposure, this increased parasitaemia indicates relapse of chronic infection during costly life-history stages (i.e. reproduction). Although the immunological and parasitological time series were in phase for control birds, cross-correlation analyses also revealed ALAN desynchronized leucocyte profiles and parasitaemia, which could suggest a general exaggerated inflammatory response. Our study shows how a common anthropogenic influence can shape within-host processes to affect infection dynamics.
The initiation of reproduction in many seasonally breeding animals is controlled by photoperiod and tends to be clinal: populations at higher latitudes breed later than those at lower latitudes, often reflecting a higher photoperiodic threshold. Migratory animals presumably time reproduction to match conditions at their breeding grounds, at least in part, by cues perceived on their wintering grounds. We asked how closely related dark-eyed junco ( Junco hyemalis ) populations that overwinter in sympatry but breed in allopatry respond to their shared winter environment by comparing early spring indices of readiness to migrate (fat and muscle condition) and breed (baseline and elevated testosterone). We measured stable hydrogen isotopes from feathers grown the preceding year and claws grown during winter to estimate breeding and wintering latitudes, respectively. We predicted that if reproductive initiation is adapted to the emergence of resources at their respective breeding destinations, then birds migrating to higher latitudes (slate-coloured junco; J. h. hyemalis ) should delay breeding as compared with those migrating to lower latitudes (pink-sided junco; J. h. mearnsi ) despite a common overwinter environment. We found higher testosterone in pink-sided juncos, consistent with earlier reproductive initiation, suggesting local adaptation in reproductive phenology is achieved through differential responses to predictive environmental cues.
14Urban habitats can shape interactions between hosts and parasites by altering within-host 15 processes such as resistance. Artificial light at night is common in urban environments, and 16 chronic exposure can impair host immunity in ways that may increase infection. However, 17 studies of causal links between this stressor, immunity, and infection dynamics are rare, 18 particularly in migratory animals. Here, we experimentally tested how artificial light at night 19 affects cellular immunity and haemosporidian parasite intensity across the annual cycle of 20 migrant and resident subspecies of the dark-eyed junco (Junco hyemalis). We monitored an 21 experimental group exposed to light at night and a control group under nighttime darkness as 22 they passed through short days simulating early spring to longer days simulating the breeding 23 season, followed by fall migration. Using generalized additive models, we show that artificial 24 light at night increased inflammation, and leukocyte counts were greatest in early spring and fall. 25At the start of the experiment, few birds had active infections based on microscopy, but PCR 26 revealed many birds had chronic infections. Artificial light at night increased parasitemia across 27 the annual cycle, with strong peaks in spring and fall that were largely absent in control birds. As 28 birds were kept in indoor aviaries to prevent vector exposure, this increased parasitemia indicates 29 relapse of chronic infection during costly life history stages (i.e., reproduction). Although the 30 immunological and parasitological time series were in phase for control birds, cross-correlation 31 analyses also revealed artificial light at night desynchronized leukocyte profiles and parasitemia, 32 which could suggest a general exaggerated inflammatory response. Our study shows how a 33 common anthropogenic influence can shape within-host processes to affect infection dynamics. 34 35 As suburban and urban habitats expand globally, certain species are attracted to these 36 environments owing to a combination of altered microclimate, predation risk, and resource 37 availability [1,2]. Such changes in biotic and abiotic conditions can have cascading and complex 38 effects on wildlife [3,4]. When considering infectious disease, urban habitats can shape the 39 interactions between hosts and parasites by altering within-host processes such as resistance or 40 tolerance [5]. Understanding how urbanization affects these processes is especially important, as 41 it can facilitate novel cross-species transmission opportunities between various host species [6]. 42 One aspect of urban habitats that could have strong effects on these within-host processes 43 is artificial light at night (ALAN). By altering animal circadian rhythms, ALAN can cause a suite 44 of physiological changes relevant to infection [7]. In particular, chronic exposure to ALAN can 45 suppress various aspects of host immune function, including cellular response and microbial 46killing ability [8,9]. Prolonged ambient lig...
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