Reproduction results in parallel changes of oxidative stress and immunocompetence in a wild long-living mammal - edible dormouse <em>Glis glis</em>

Oxidative stress (OS) and impaired immune function (IF) have been proposed as key physiological costs of reproduction. The relationship between IF and OS, particularly in long-living iteroparous species, remains unresolved. Here, we studied OS and IF in breeding and non-breeding females of edible dormice- a long-living rodent prioritizing self-maintenance. We tested whether free-ranging reproducing females experience the trade-off between breeding investments, preventing OS, and sustaining IF. Because of senescence, the OS markers (oxidative damage - ROMs, lower antioxidant capacity - OXY) should increase with age and reproductive activity; the IF (indexed as the level of white blood cells – WBC and increased neutrophil to lymphocyte ratio - N/L) should decrease with age and reproductive effort. WBC decreased, N/L increased, while OS markers remained unaffected by age. ROMs (but not OXY) and age-adjusted WBC were the lowest in lactating, higher in post-lactating, and the highest in non-breeding females. The path analysis testing the causal relationship between ROMs and WBC revealed that IF is more likely to affect OS than vice versa. Our study indicates the trade-off between reproductive investments and sustaining IF and demonstrates that females suppress IF to reduce the risk of OS and, therefore, mitigate oxidative costs of reproduction. Methods The population of edible dormice was studied in the central part of Kozienice Forest (51°30′15″N, 21°27′27″E) in central Poland. We monitored this population from 2013 and marked animals individually (RF-IDW-1, CBDZOE, Poland). In August-September 2019-2021, females were captured (21 lactating, 17 post-lactating, and 44 non-lactating) and transported to the laboratory of Forest Research Institute in Sękocin Stary. Blood sampling protocol was performed after ~30 hours spent in the animal room to reduce the stress associated with capture and transport to the laboratory. During the procedure (which lasted less than 3 minutes), animals were anaesthetised under a 2 % isoflurane and atmospheric air. Blood samples (~50 µL) were taken from the retro-orbital sinus by heparinized microhematocrit capillary tubes. To establish the leukocyte status, the blood smear was prepared immediately from 4µL of full blood. 15 min after sampling~~ blood was centrifuged at 13,000 rpm for 20 min at 4°C to separate red blood cells from plasma. Plasma was immediately stored at -80°C until the assays were performed.