Ecology

The Milos viper (Macrovipera schweizeri), endemic to the western Cyclades islands of Milos, Kimolos, Polýaigos, and Sifnos, diverged from its mainland relatives around four to five million years ago, most likely through geographic isolation during the Pliocene. This prolonged insular isolation has shaped its ecological traits and behaviour, making it one of the most distinctive vipers in Europe. The species was originally described as a subspecies of the blunt-nosed viper (Macrovipera lebetina), but subsequent morphological and biochemical studies supported its elevation to full species rank. More recent studies, however, have questioned this status, and its exact taxonomic position remains debated. Forthcoming comprehensive genomic studies are expected to resolve this issue definitively. Fossil evidence indicates that the isolation of M. schweizeri in the western Cyclades was most likely the result of vicariance associated with rising sea levels, rather than human-mediated dispersal. 

The species inhabits rocky, semi-arid maquis landscapes characterised by gravelly ground, low to medium shrub vegetation and scattered rocky outcrops. It is most often found in open shrublands dominated by Pistacia lentiscus, Genista acanthoclada, Juniperus phoenicea, Nerium oleander and Myrtus communis, with the presence of large bushes proving to be especially important. Studies on Milos suggest that more than three-quarters of the vipers are located within or immediately adjacent to larger shrubs, which provide both shelter and sites for cryptic basking. South-facing slopes are often selected for hibernacula, while on Sifnos the snakes are frequently associated with dry stone walls bordered by vegetation, which offer both refuge and hunting opportunities. Optimal habitats supporting densities of around 50 individuals per km². The altitudinal range extends from sea level to around 400 meters. It is primarily nocturnal during the hot summer months (mid-summer to mid-September) and crepuscular during cooler seasons. With the arrival of summer and high daytime temperatures, activity shifts to the night and early morning hours, and vipers remain largely inactive during the day. In winter they retreat to underground hibernacula, although on mild days in February individuals have occasionally been seen basking near the entrances of their shelters.

Movement patterns are relatively restricted, though they vary between sexes and seasons. Males typically occupy larger home ranges, estimated at ten to twenty hectares, while females are more localized and restricted to around two hectares. Radio-tracking studies have shown average daily movements of less than thirty metres, though this increases considerably in May and September, when males roam more widely in search of mates and both sexes adjust their foraging strategies. Unlike many viper species that predominantly feed on rodents, M. schweizeri has adapted on a diet focused on passerine birds. The viper's foraging strategy aligns with bird migration cycles, with peak feeding periods occurring in spring and autumn. During spring, vipers are frequently found near water pools, taking advantage of increased bird activity. In autumn, they have been observed climbing into shrubs and small trees to ambush roosting birds. This reliance on avian prey makes them vulnerable to fluctuations in bird populations and climatic changes affecting migration patterns. Juveniles, however, feed mostly on lizards such as Podarcis milensis and Ablepharus kitaibelii, gradually shifting to an avian diet as they grow. On Sifnos and Kimolos rodents are opted for, probably due to abundance.

The reproductive cycle of the Milos viper seems to follow a biennial pattern, with mating occurring in mid-May often in proximity to water pools. Females reproduce only every second year, reflecting the energetic costs of egg production and the dependence of body condition on prey availability, particularly the fluctuations of migratory bird abundance. The reproductive output is therefore relatively low, with an estimated 600 individuals reaching sexual maturity and reproducing each year. The sex ratio in populations appears to be relatively balanced, and an estimated 600 individuals reach adulthood and participate in reproduction annually, according to an older study.

The species faces predation pressures from raptors such as golden eagles (Aquila chrysaetos) and buzzards (Buteo buteo), while feral cats, particularly abundant on some islands, are thought to be a significant threat. The Milos viper faces several challenges, including habitat destruction due to mining, road mortality, illegal collection, and direct persecution. Mining activities, particularly for bentonite and perlite, threaten key habitats by reducing the availability of suitable shelter and hunting grounds. Additionally, road-killed specimens contribute significantly to annual mortality, with estimates suggesting the removal of 500-600 adult vipers each year, roughly 25% of the adult population. This level of turnover poses a severe risk to long-term population stability. It is a unique species with specialized ecological adaptations that make it highly vulnerable to environmental changes and human activity. Understanding its biology, behavior, and habitat requirements is crucial for developing effective conservation strategies. Protecting this species requires direct action to mitigate habitat loss, reduce human-induced mortality, and ensure the long-term viability of its populations.