1. Introduction
Species distribution is determined by a set of factors that are vital for the species’ survival. Among them, nest sites (understood as resting places not only for breeding), food availability and the presence of predators seem to be the main variables limiting the presence of European hedgehogs (
Erinaceus europaeus) in an area [
1,
2,
3,
4,
5]. Despite their vital importance, nests and refuges are not fully appreciated, and, as in other mammals, studies of hedgehogs have usually concentrated on other aspects of their biology and the behaviour of active animals [
3].
The European hedgehog (hereafter ‘hedgehog’) is a medium-sized nocturnal insectivorous mammal found naturally from the Iberia Peninsula northwards into Scandinavia. Although timings may differ in relation to climate, sex, body size and condition, hedgehogs typically hibernate between November and April, making winter nests for hibernation particularly important and therefore becoming the focus of many studies [
6,
7,
8]. However, hibernation is a flexible strategy. In warmer climates, such as in Italy, hibernation is reduced by up to two months between January and February [
1], and it completely disappears in South Spain [
9]. Still, as a secretive nocturnal animal, hedgehogs spend the day in well-hidden day nests [
10], which provide security and protection from the elements. Summer nests, in comparison with winter nests, tend to be less robust [
3]. Sometimes, during the summer days, animals simply rest under the cover of vegetation. When asleep by day, hedgehogs do not roll up tightly but sleep on their side, with their muscles and spines relaxed [
3]. In warm weather, they may even stretch out. Given the vulnerability that such a posture offers, a safe choice of day-nest location becomes crucial, not only during wintertime but all year around.
The European hedgehog is a species of conservation concern in Europe due to their reported population declines [
11,
12]. One of the main causes for that decline includes habitat loss due to land use changes like agricultural intensification and urbanisation. Reducing and removing a key element, such as hedgerows to increase field sizes, has become an important threat to hedgehogs [
13]. Therefore, conservation strategies focusing efforts on hedgerow management have proven to be beneficial for biodiversity in agricultural-dominated landscapes [
14]. In line with this idea, we should not forget the constant growth of cities. There has also been rapid urban expansion accompanied by an increasing tendency for mammals to colonise urban settlements [
5]. Urban environments and metropolitan areas will then play an increasingly important role in conserving biodiversity at the local and regional levels. Hedgehogs are classified as urban adaptors [
15,
16], and recent studies have highlighted that hedgehogs are more likely to be found in towns and cities than in rural areas [
2,
17,
18,
19] and have higher densities in urban areas [
5,
20]. Given this scenario, it is important to find what drives nest habitat use by hedgehogs in urban environments. The availability of safe nest locations may be important for their survival in urban areas and for maintaining stable populations.
The aim of this study was to characterise for the first time the key elements of habitat use for nesting in urban populations of European hedgehogs at the southern range of this species distribution. In order to do so, we studied hedgehogs in two areas representative of two types of urban habitats used by this species in the region: The campus of the University of Alicante was used as an example of an intensively gardened park, and Mount Benacantil was used as an urban forest with more natural vegetation. In the present study, we analysed: (1) the number of nests and the frequency of use depending on sex, season and location; (2) the spatial distribution of the nests and their distance to artificial structures; (3) the structures in which the nests were located and the habitat around them; and (4) the nests’ characteristics and nesting materials used.
4. Discussion
Our results have revealed differences between male and female hedgehogs regarding their various aspects of nest use, spatial spread and location in relation to habitat’s characteristics. These differences are likely related to the promiscuous mating system of the species [
3,
32,
33] and the fact that males’ home ranges are between two to five times larger than females [
1,
7,
9,
10,
24]. We also found some differences between the urban habitats studied, which are likely related to differences in the topography and types of habitats available at each locality.
The distances between consecutive nests did not differ between sexes; however, in the urban forest, we estimated that the males presented a larger nest spread, as measured by the minimum convex polygon (MCP) of the nests, compared to the females. We could not test the sex effect at the university campus due to limited data; however, we observed a trend for the male nest MCP being larger in spring–summer compared to winter. Both of these patterns align with expectations due to the larger home range of males and their active search for mating partners [
24]. The combination of both results together shows that the exploratory behaviour of males may be staggered. Males do not change areas drastically from one night to the next (the distance between consecutive nests is similar to that of females); however, it is the cumulative effect over several nights that contributes to larger home range areas for males. Furthermore, we discovered that females exhibited a higher frequency of nest reuse compared to males, with this difference being more pronounced during the summer period. In general, females’ activity is restricted to a smaller area, making it feasible to return to the same nest daily and perhaps this also being more cost-effective in energy terms [
3,
34]. This limited home range is accentuated, especially during the breeding season, when a reduction in home ranges has been observed in female hedgehogs [
24,
35], as in other small mammals [
36,
37].
Hedgehogs spend the entire day sleeping in their resting nests [
3]. Good protection against predators and threats during this vulnerable time is necessary, especially in urban environments, where the daily movement of people and dogs is frequent. Therefore, it is likely that females’ selectivity of nesting locations extends to their choice of cover or structure for nest placement, as well as the surrounding environment. None of the females tagged during the spring–summer period were pregnant or had hoglets, but it is likely that nest selectivity extends beyond the breeding season, possibly even throughout most of the year, as hedgehogs in the Mediterranean region may also breed in autumn and winter (pers. obs.).
The diversity of plants or structures used by females for nest placement was found to be lower compared to males in both study areas. However, this difference reached statistical significance only at the university campus, where it was more pronounced. At the university campus, we identified a total of 23 distinct types of nest locations, while in the urban forest, we found only 12 types. This suggests that the presence of gardens and artificial structures on the campus provides a wider array of potential nesting places [
35]. Hedgehogs utilised six different types of artificial structures at the university campus, whereas they only utilised one type (a hole in a wall) in the urban forest. We lack estimates regarding the abundance of potential nesting sites; however, considering the university campus has a larger surface of built areas, it is expected that the availability of artificial structures would be greater there. Consequently, we cannot determine whether the higher frequency of nesting in artificial structures at the university campus indicates a preference for them or simply results from their greater availability.
The FAMD ordination of nesting sites and their environment clearly revealed a distinction between the two urban study areas, which was statistically significant in the first four dimensions, likely indicating variations in nesting sites’ availability and habitats. The first dimension primarily separates nesting in artificial structures lacking shelter at their entrance from nesting under plants, which typically provide shelter. Nesting in artificial structures is primarily found at the university, although nesting under plants remains predominant there. Interestingly, this analysis revealed differences between the sexes in Dimensions 2 and 3, providing evidence of variations in the average nest selection environments between males and females. Dimension 2 depicts a gradient ranging from nesting in sturdy plants with a complementary structure and ample grass-like plant cover in the surrounding area (positive values of the dimension) to nesting beneath shrub-like plants in an area dominated by large shrubs (negative values of the dimension). Interestingly, and consistently across both locations, females nested more often in the former and males in the latter. Dimension 3 is exclusively correlated with the surrounding habitats of the nests and represents a gradient ranging from areas characterised by paved soil and small shrubs (positive values) to areas dominated by large shrubs (negative values). In this gradient, there is also a consistent difference between the sexes across the two locations, with the majority of female nests situated in areas with a higher abundance of small shrubs and, in the case of the university, more paved soil.
Within the group of sturdy plants where the females established their nests at the university, ivy dominated, followed by pampas grass. Both plant species, unlike many shrubs, are difficult-to-access structures, meaning that they provide highly concealed protection for hedgehogs, with large surfaces around them (e.g., ivy) and/or thick walls of plant tissue (e.g., pampas grass). These types of plants were absent in the urban forest, where all female nests were found within shrub-like plants, primarily Salsola oppositifolia, which was the dominant shrub species in that area. This raises the question of why female nests were more frequently located in sturdy plants at the university campus, while in the urban forest, they were able to utilise mostly shrubs. The answer is likely related to the variability in shape and density of the shrubs, as not all of them would be equally suitable for female nests. All plants used by females at the university and included in the category of shrub-like plants, such as Pittosporum, Lavandula or Cupressus, are quite compact. In contrast, as well as these same species, the males made nests in other shrub-like plants (e.g., Acacia, agapanthus, Nerium oleander) that were never used by the females. Those bushes, unlike others, have less dense branching structures and seem to provide a lower level of protection that may not be suitable for females.
The presence or absence of an immediate shelter, which allows hedgehogs to quickly hide when entering the structure that includes the nest, seems to be important, given that the great majority of used sites (89.2%) offer this type of shelter. Among the shrub-like plants, immediate shelter was present and directly provided by the principal structure in 96% of the cases. Of the remaining 4%, in 26.3% of the cases, shelter was provided by the presence of the secondary structure, which could contribute to increasing its suitability. The nature of the secondary structures varied greatly between localities. In the urban forest, for 93.3% of nests, the secondary structure was composed of live plants, mainly
Brachypodium retusum, while at the university campus, it was predominantly composed of leaf litter (66.7% of the cases). Therefore, at the university campus, leaf litter seems to contribute to the increased suitability of some shrub-like plants besides being a component of the structure and inner lining of the nest. In addition, leaf litter is known to be an important source of food, as it provides a habitat for their natural prey, such as snails, slugs and insects [
3]. Therefore, removing leaf litter from gardens and managed urban parks is likely to contribute to a decreased habitat quality for hedgehogs and is not recommended [
14].
The artificial structures were the only ones that lacked immediate shelter in most cases, with only 18.2% of the used sites having shelter. These structures also did not have a complementary structure that could contribute to hiding the hedgehog. As a consequence, the hedgehog was not immediately protected by the structure when entering and, thus, had to go deeper. We recorded only two cases of artificial structures used by females: a nest box specifically designed for hedgehogs, and therefore likely to be more suitable than the average artificial structures, and an office trailer, whose size and shape could have contributed to being selected by females. But overall, artificial structures were used mainly by males, which supports the idea that males are less selective of nesting sites.
The type of sites used for the nest and the characteristics of its surrounding environment are expected to be related, as some types of structures are more likely to be found in particular habitats. An example of this effect may be the tendency of females to place nests in sites with greater cover of paved soil than males, especially at the university. This could be a consequence of the females selecting (less often than males) shrub-like plants for their nests, as there is a negative relation between the cover of large shrubs and paved soil. On the other hand, females showed a tendency to choose sites with greater grass-like plant cover around. This result may originate from the selection of nesting sites with greater availability of food items nearby since the amount of time spent by females searching for natural prey in lawns has been shown to be greater than that spent by males in urban areas where cat feeders are available [
9]; likewise, female nests were located closer to the streetlights compared to male nests. Although most studies report negative behavioural consequences of artificial light at night (ALAN) in mammals [
38], some species, like hedgehogs, are able to exploit the foraging opportunities created by lighting, such as the accumulation of insects around streetlights [
39].
The contrasting topography of the university campus (flat) and the steep urban forest located on a mountain may affect the cost of movement [
40] and help to explain some differences in nesting ecology between the study areas. At the university, hedgehogs used fewer nests and used each nest more frequently than in the urban forest. Due to the university campus’s flat landscape, hedgehogs can move between nests at a lower cost and need fewer nests to cover their home range compared to the urban forest. Similarly, proximity to stable food sources, like cat feeders [
41], might be more important in areas where the costs of movement are larger. Nests of both sexes were closer to the five available cat feeders in the urban forest but were more distant at the university campus with fourteen feeders.
Differences in the management of the urban parks studied may have also contributed to increasing the dependency on cat feeders in the urban forest. The artificial irrigation employed at the university could enhance the availability of invertebrates, whereas, in the dry and arid urban forest, hedgehogs may rely more on supplementary food from cat feeders. The different distributions of artificial structures between the study areas may also help to explain some of the contrasting patterns found. Roads at the university campus are close to the periphery as it is designed as a pedestrian area, while in the urban forest, there is a road running roughly through the centre. The males placed nests closer to roads at the university campus but farther away in the urban forest. This sex and locality interaction could be attributed to the distinct road distribution. The males’ tendency for longer exploratory movements and larger home ranges [
1,
7,
9,
10,
24] would make it more likely that some male nests will be located near the periphery, where roads are present at the university but farther from the central road in the urban forest. Hence, this pattern is more likely a result of the sex-related differences in behaviour rather than a deliberate choice for habitats near roads.
Finally, our results showed that the nests described here are looser and smaller than those described at northern latitudes [
3,
8,
35]. Many authors have stated that there is a certain plasticity in the construction of the nests [
34]. Hibernacula in the northern latitudes must be well-sited, robust and carefully constructed to protect torpid animals over extended periods [
7,
35]. Captive animals did not build a nest when the ambient temperature exceeded 16 °C [
42], resulting in relatively less substantial summer nests compared to winter nests. The mild winter temperatures in the Mediterranean region may have led to a less robust nest construction compared to the hibernacula found in the northern regions [
35]. Nests from all seasons in our study areas were relatively flimsy, constructed merely of grass and dry leaves, and poorly compacted, as occurs in other mild climates, such as in New Zealand [
43,
44], where the species has been introduced. Adequate construction materials for nest-building limit hedgehog distribution in higher altitudes, often linked to deciduous forest distribution [
3]. In urban areas, planted deciduous trees provide suitable materials, as seen in our results, with 40% of the nests composed of dry leaves and 66.7% with dry grass. The hedgehogs also used other artificial resources as bedding materials, such as plastics and chocolate wrappers [
35], found in 26.7% of the nests. Similar results to those found by Schoenfeld and Yom-Tov with
Erinaceus concolor in Israel [
45] reaffirms the generalist and opportunistic nature of these species.
5. Conclusions
Our study has shown differences in the nest distribution and habitat use between male and female hedgehogs in urban habitats. Females showed a preference for well-concealed structures with the presence of immediate shelter, while males were able to use a greater variety of nesting sites and tended to locate their nests further apart than females. Females tend to reuse the same nest more often than males. These results suggest that females are more selective than males when choosing the structures hosting their nests. There are also differences between the sexes in the distance of nests to some artificial elements present in the urban habitat that may act as barriers, sources of perturbations or, contrarily, attraction points. Females nested closer to paths and streetlights at the university, while males were closer to roads. Opposite results were found in the urban forest, where a road ran through the centre and where female nests were closer to roads than those of males, which is evidence that the distribution of elements in urban areas is important. Different topographies and garden management also affected the dependence of hedgehogs on supplementary food, with the nests closer to the cat feeders in the urban forest.
Nests in both study areas were found to be relatively flimsy, consisting merely of grass and dry leaves, with poor compaction, and significant changes in nest characteristics were not detected across the seasons. This contrasts with the results from northern latitudes, where winter nests are thicker and far better constructed than spring–summer nests, pointing out that in southern latitudes, the selection of good hosting structures seems more important than the nest construction itself.
Differences in nesting ecology between the two urban parks observed are likely to be related to their different topography and habitat characteristics and availability, highlighting the importance of studying hedgehogs’ nest site selections at the microhabitat level. However, our study has some limitations because, despite having analysed more than a hundred nests, they were built by just 31 hedgehogs, which were monitored for a limited period of time. Consequently, the sample sizes of some combinations of sex, season and study site were limited, and we were unable to fully explore the effects of these factors. Future studies with larger sample sizes and in other habitats are needed to confirm or not the generality and strength of the patterns we have found. This type of study would contribute to a better understanding of hedgehogs’ nesting requirements, fostering the design of more effective conservation strategies for hedgehogs and the appropriate management of green urban areas for this species and other wildlife.
Currently, based on our results, we can highlight certain practices that would enhance the suitability of garden habitats for hedgehogs’ nests. These include: (1) using plants with a dense structure, where branches begin close to the ground to provide good shelter for the hedgehogs; (2) maintaining lawn areas as naturally as possible [
46]; and (3) preserving leaf litter in gardens. Furthermore, it is important to consider the possibility of hedgehog nests when conducting maintenance work in gardens, as hedgehogs may utilise unexpected temporary or movable structures for nesting. Finally, our research revealed that hedgehogs often selected nesting sites near roads, which are a significant cause of mortality for the species [
47]. Therefore, where suitable nesting sites are particularly close to roads, corrective measures are encouraged to prevent road casualties.