Reproductive Traits of an Invasive Alien Population of Grey Squirrel (Sciurus carolinensis) in Central Italy

SIMPLE SUMMARY: The presence of an invasive alien species is one of the main causes of biodiversity loss, which occurs through the reduction or even the extinction of native populations. In this context, the EU 2020 biodiversity strategy calls for research on invasive alien species in order to prevent their introduction and spread more effectively. The grey squirrel (Sciurus carolinensis), which is native to North America, provides one of the best-known examples of competitive exclusion of a native one, the European red squirrel (Sciurus vulgaris). A grey squirrel population, living in central Italy (Umbria), was studied to provide parameters on its population dynamics, growth rates, and viability, in order to understand its adaptation to its non-native range. Our data revealed that the Umbrian grey squirrel population is well-adapted to its new range, undergoing two annual mating periods and experiencing high reproductive success. Therefore, these invasive grey squirrels pose a substantial threat to the local European red squirrel population. ABSTRACT: The reproductive cycle of an invasive alien Italian grey squirrel population was studied to understand its adaptation and limit its spread, in order to conserve the autochthonous red squirrel. Female and male genital traits were evaluated throughout the reproductive cycle, including the ovary, uterus, testicle, epididymis, seminiferous tubule morphometry, and germinative epithelium histology. Moreover, individual female fecundity was determined by counting uterine scars. Ovary width and uterus weight, length, and width reached their highest values in the luteal and pregnancy phases. On conducting a histological evaluation of the testicular germinal epithelium, four morphotypes related to the different reproductive phases of the male squirrels were identified: immature, pubertal, spermatogenesis, and regressive. Testicle and epididymis weights and seminiferous tubule diameters reached their largest values during spermatogenesis. Uterine scar analysis showed that 69% of the females had given birth to one or two litters, while 31% had no uterine scars. Litters were larger in the first breeding period than in the second; annual fecundity was 4.52 ± 1.88 uterine scars/female. Umbrian grey squirrels have adapted to their non-native range, showing two annual mating periods at times similar to those in their native range, and high reproductive success.