E cological and evolutionary constraints determine the solitary or social character of animal species, but certain solitary mammals such as bears or domestic cats show some social organization in areas with clumped food resources or in captivity (Fagen and Fagen 1990; Sunquist and Sunquist 2002). The European hedgehog (Erinaceus europaeus) is one of these cases (Figure 1); this solitary species is only found together during courtships, captivity conditions, or whenever plentiful food resources occur, such as human-provided food in gardens (Reeve 1994). Hedgehogs do not compete for space and mostly avoid the simultaneous use of a common area (Morris 1969). When an encounter with a conspecific occurs, fights or aggression have rarely been reported (Boitani and Reggiani 1984; Reeve 1994). This tolerance to conspecifics suggests that except for the mating period where males actively search for females, hedgehog movements and space use are not influenced by conspecifics but are mostly driven by the search of food and dry nests (Jones and Sanders 2005). Hedgehogs of both sexes use nests as daytime refugia during the summer and for hibernation during harsh weather; breeding females also rely on nests in which to raise litters of hoglets (Reeve 1994). Hedgehogs can nest in crevices, burrows, or other hollows, but nests are mostly built aboveground using leaves, grasses, tussocks, and other materials. In general, depending on their use, nests can be simple or elaborate, and the number of nests constructed, their spacing, and the frequency of use vary between individuals and sexes (Reeve 1994).
Nests are normally occupied by single hedgehogs, with the exception of breeding females with litters, although nests can be inhabited non-simultaneously by several hedgehogs to optimize the use of available resources (Reeve 1994). To my knowledge, only two old reports of simultaneous nest sharing by hedgehogs exist, and both corresponded to winter nest sharing. One report referred to multiple nest sharing in Olonets, Russia (Ognev 1928), and a second report detailed a dual occupation in a double-chambered nest in London, UK (Morris 1973). The sharing of a summer nest has been reported only for the case of a male and female pair of southern white-breasted hedgehogs (Erinaceus concolor), but lying 20 cm apart (Schoenfeld and Yom-Tov 1985). However, here I report the observation of simultaneous nest sharing during summer by European hedgehogs in New Zealand.
Hedgehogs were introduced to New Zealand during the early European colonization to control slugs and bugs in backyards, and as an endearing reminder of the distant gardens of the UK (Jones and Sanders 2005). Although this prickly insectivore is cherished in its original distribution range and sometimes even considered as a pet, the European hedgehog in New Zealand is the wrong species in the wrong environment. In New Zealand, the hedgehog preys on threatened and endangered native invertebrates and on eggs of ground-nesting birds such as the critically endangered black stilt (Himantopus novaezelandiae); it also competes for resources with the emblematic kiwi (Apteryx spp), and is considered to be a spillover host of bovine tuberculosis (Sanders and Maloney 2002; Jones and Sanders 2005).
Although ecological research of invasive species is often focused on pest management, these necessary efforts sometimes yield interesting and valuable findings regarding unexpected and unique behaviors. I carried out research in a farmland in the eastern margin of the Godley Valley, Mackenzie Basin of the central South Island, New Zealand, during the Austral summer and autumn of 2009. I tagged with GPS-backpacks (Recio et al. 2011, 2013) a total of 27 hedgehogs, 15 in summer (13 males, 2 females) and 15 in autumn (7 males, 8 females); although one female and two males were tracked in both seasons. After the tracking period, I manually recaptured tagged individuals during the day to recover the GPS-backpack. I noted any eye-catching and valuable information during the recapture of the tracked individual, such as the type of nest found, or whether the animal was found alone or with litters. I was not expecting to find any sign of simultaneous nest sharing, considering the individual nature of the species and the scarce reports of this social behavior.
I found simultaneous nest sharing of adult hedgehogs on two occasions during summer (7.7%). The first observation was a tracked hedgehog with two male adults sleeping in a nest under a dense patch of shrubs; all the hedgehogs were in contact with each other. A second observation was two adult males sharing a nest made with a female and three hoglets (Figure 2), all in contact in a dense nest of tussocks. I found no nest sharing among males in autumn, although all the females tracked in autumn were found with 3–4 hoglets in tussock nests sited underneath individual shrubs or in tussock patches alone. The rest of the hedgehogs were recaptured sleeping alone in full nests under shrubs, in dense ball nests made of tussocks, or on one occasion in a rabbit burrow.
It appears that adult male and female hedgehogs in New Zealand are able to exhibit mutual tolerance by simultaneously sharing summer nests with other individuals and even females with litters. Reeve (1994) suggested that hedgehogs may share nests in winter whenever the benefits of this behavior overcome the disadvantages (eg disturbance, mutual injuries, selection of optimum nest room, disease and parasite transmission, or even predation). However, these benefits could only materialize in regions characterized by extreme cold winters and with low availability of nests and materials to build them. This does not explain the presence of three adults in close contact in summer nests detected in my study area.
Hedgehogs have no major predators in New Zealand (Jones and Sanders 2005); consequently, the non-territorial and generalist character of the species, together with a good food supply, could influence intraspecific relationships by improving spatial tolerance. This might have consequences for the spacing patterns, demography, population size, and density of hedgehogs as a pest species in New Zealand (Adams 2001). From a theoretical viewpoint, research would be required to identify family links between individuals simultaneously sharing nests as a case of kin selection. This could apply to the female found with a litter and two adult males in the same nests – these males could be members of a previous litter produced by this female. From an applied and pest management perspective, if this behavior is more frequent in New Zealand than reported in the literature, simultaneous nest sharing could facilitate the transmission of host-specific diseases or parasites as part of a suite of potential biological control measures. Thus, consideration of natural history information may be important for the management of pest species, particularly regarding adaptations to novel or changing environmental conditions.