Thousands of hedgehogs are taken into care each year, mostly injured animals or late-born young unlikely to survive hibernation. Many are returned to the wild, but until recently there was little information about their welfare, behaviour or survival after release. A review of three studies undertaken in different regions of Britain, showed that the majority of 33 released adults and juveniles adapted quickly to life in the wild despite the juveniles having been raised in captivity. They found food readily, made nests and rapidly learned their way about. They all lost weight initially but, after two to three weeks, stabilised at a similar weight to wild animals. The hedgehogs which were heaviest at release lost the greatest percentage of their bodyweight, suggesting they had become overweight in captivity. The hedgehogs which were known not to have survived at least six weeks after release included three road casualties and three eaten by badgers. Although wild hedgehogs may die in the same ways, it is possible that the released hedgehogs' susceptibility to such dangers may be increased by their becoming tame during captivity. However, the high survival rate suggests that the release of rehabilitated hedgehogs back into the wild is worthwhile.
Increasing numbers of British hedgehogs (Erinaceus europaeus) are taken into care each year. One wildlife hospital claims to handle 3000 annually, and a single private carer took in 171 during 1996 alone (M. Rhigini, personal communication). The RSPCA treats over 1000 hedgehogs each year and these represent an increasing proportion of all wildlife casualties treated by the RSPCA (T. Thomas, personal communication), rising from 15 per cent in 1993 to 20 per cent in 1996. These animals consist of sick and injured individuals, together with many late-born young which are too small to survive hibernation (Morris 1984). Some of these hedgehogs die in captivity, but survival rates are generally good and have improved steadily with the increasing experience of wildlife carers.
Rehabilitated hedgehogs are released into the wild, but until recently there has been little information about their survival rates or how they cope with being moved to unfamiliar sites. Information is also lacking about how animals raised or treated in captivity adapt to hunting for food, building and maintaining nests or establishing their own home range. Do they attempt to ‘home’ to their original area of capture (or place of hospitalisation)? Are they integrated into the wild hedgehog population?
Field studies and methods
Thirty-three hedgehogs were studied. They had been rescued from circumstances that would have resulted in early death, cared for in captivity for many weeks, and were destined for release irrespective of the follow-up study for which they were later selected. They were fitted with a small radio transmitter (Biotrack) and tracked by means of a direction-finding receiver. Their survival and movements were monitored for a period of up to eight weeks, after which battery life was exhausted and the transmitters had to be removed.
Some of the animals were adults with experience of living free in the wild, but 25 (76 per cent) were juveniles, selected particularly because they had little or no experience of independent life. These juveniles had been taken into care for various reasons, but were probably too young to have ever built their own nest. Most had never lived alone, many had never found or eaten natural prey and had not travelled beyond the confines of a cage. It was presumed that if they could survive release, so would adults.
Eight hedgehogs were released in Suffolk (Morris and others 1993a), 12 in Devon (Morris and Warwick 1994; the sample described by Sainsbury and others 1996) and 13 in Jersey (Morris 1997). After release, each animal was weighed nightly if possible, its daytime nest was located and its interactions with wild hedgehogs were recorded. In Devon, some of the hedgehogs were kept in pre-release cages on site for up to 10 days before they were released and food was made available in the cages afterwards, an attempt at 'soft release'.
Results
Bodyweight
Most of the hedgehogs lost weight progressively for at least two weeks after their release, but most had stabilised by about the third week. Data from 24 hedgehogs (two released in Suffolk, the rest in Devon or Jersey) which survived more than 19 days after release and whose weights were monitored regularly for up to seven weeks were combined. When they were released the bodyweights of these individuals ranged from 560 to 1220 g (coefficient of variation [CV] 18.2 per cent). Subsequently, all the animals lost weight and the range of minimum weights recorded was 500 to 840 g (CV=12.3 per cent) as they approached a 'standard' mass. The maximum weight loss recorded was 38 per cent and the majority lost at least 20 per cent in the weeks after their release. The animals which lost most weight were those which were heaviest at release. Several were more than 50 per cent heavier than would be expected for wild hedgehogs of similar age, owing to generous feeding in captivity and lack of exercise. The relationship between maximum weight loss and weight at release was statistically significant (linear regression, P<0.001, Fig 1).
Nesting and orientation
All three studies showed that the released hedgehogs quickly adjusted to their new surroundings and did not attempt to 'home' to a distant location by heading off in a consistent direction. Typically, the hedgehogs travelled several hundred metres in a night, comparable with distances travelled by wild hedgehogs (Morris 1988) despite their previous confinement. They built nests successfully and repeatedly found their way back to them again after a night's foraging. Most re-used a previous nest (sometimes over 200 m away) after an absence of more than two days, indicating an ability to orientate in a new habitat.
In all three studies, after a week or more of consistent behaviour, some hedgehogs undertook sudden, long distance movements away from the release area. In Suffolk, half the animals travelled at least 2 km from the release area and never returned. Two swam across the River Stour. In Devon, one travelled 2 km then established a new regular pattern of movements, two others dispersed 0.5 km before settling down. In the Jersey study, of the eight animals located six weeks after release, two were still within 20 m of the release garden, one was more than 250 m away and a ninth hedgehog was later found alive 5.2 km from the release point.
Survival and predation
The combined results showed that 30 of the 33 released hedgehogs were still alive after three weeks (Table 1). At least 23 animals were alive four to five weeks after release and 17 were known to be still alive after seven weeks; it is likely that more were still alive because the 'lost' animals were not necessarily dead. Two could not be located by radio tracking because they had shed their transmitters, and at least three of these 'lost' animals had travelled long distances. Known causes of death included two hedgehogs which had not fully recovered from their original disease (nematode infection in one case) and subsequent treatment. One hedgehog in Suffolk drowned and there were three deaths due to road traffic (two in Devon and one in Suffolk). Three were killed by badgers (Meles meles), all in Devon, but there are no badgers on Jersey and none was killed by traffic on Jersey, although it was busier than at the Devon or Suffolk sites.
Pre-release cages and acclimatisation
None of the hedgehogs released in the Devon study returned to eat the food left for them in the pre-release cages.
Social behaviour
No aggressive interactions were observed between the released animals and resident wild hedgehogs. On the contrary, in all three studies, there was frequent courtship behaviour between wild and released hedgehogs, often protracted, at the expense of foraging behaviour. In one case (in Devon) it is believed that a successful pregnancy resulted (Sainsbury and others 1996).
Discussion
A consistent feature of these studies was weight loss, and the large percentage of bodyweight lost by some of the animals may, suggest that they were unable to find sufficient food and were starving. However, the significant (P<0.001) relationship between bodyweight at release and maximum weight loss, indicates that these animals were shedding excess weight put on in captivity.
For this species, weight loss should be interpreted carefully. Substantial changes are quite normal in hibernators like the hedgehog (Kristoffersson and Suomalainen 1964) and even from day to day the weights of wild hedgehogs, studied simultaneously with the released animals in Suffolk, varied noticeably. The coefficient of variation of the bodyweight of three wild hedgehogs weighed there each night for at least a week, was 6.6 per cent, slightly more than for six released captives (4.5 per cent). Wide daily fluctuations are natural and it is unwise to read much into individual weight measurements, it is the trend over time which is significant. Being overweight at release is not necessarily bad for the animals. Extra fat reserves might help support released hedgehogs while they adjust to finding natural food or if they have to cope with unexpectedly cold or dry conditions when such food is scarce. They will also benefit from substantial fat reserves in advance of hibernation.
During the Devon study, the nights became very wet and cold, with temperatures around 0°C, posing an additional challenge to survival, because invertebrates are less active in the cold. Heavy rain also threatens the ability of homoeotherms to maintain body temperature, especially if dry nesting sites cannot be located. It was for this reason that the animals had not been released even earlier in the year. Some had sustained small bite wounds in captivity (Sainsbury and others 1996), but in every case these healed quickly after release and several of the affected animals survived until the end of the study. There is no evidence that these minor wounds represented a significant hazard, although it would have been better had the animals not received them. For this reason, it seems wise to cage the animals individually, particularly males in the breeding season, before they are released.
A regular change of nest site is normal in wild hedgehogs, especially among males (Reeve and Morris 1985). The released hedgehogs repeatedly found their way back to the nests they had built and most re-used a previous nest at least once, indicating an ability to orientate in a new habitat. Similar results were obtained in a study of three hedgehogs released in Yorkshire (Morris and others 1993b).
Although none of the release studies showed a consistent pattern suggestive of 'homing', several of the hedgehogs disappeared from the study areas, having travelled considerable distances. In Jersey, at least one moved over 5 km, and four of the Suffolk animals were last seen over 2 km from the release point. Although these animals were 'lost' from the study, their dispersal suggests that they had sufficient stamina to travel significant distances and were thus probably healthy.
In Cheshire, 22 rehabilitated hedgehogs were released into a wood and radio tracked, but all of them departed to nearby villages within two weeks (Key 1997). A similar result was obtained when hedgehogs were released into a woodland in Surrey (N. J. Reeve, personal communication). Dispersal may be a response to being moved to an unfamiliar type of habitat, suggesting that hedgehogs should be released in the habitat to which they were accustomed (Morris and others 1993a). However, in Jersey there was no statistically significant difference between the behaviour of animals released close to where they had first been captured and of those unfamiliar with the release site (Morris 1997). All the animals in that study were juveniles, and were perhaps not old enough to have become fixed in their behaviour towards their home site before being taken into captivity.
Thirty of the 33 hedgehogs released were still alive after three weeks, long enough for them to have starved if they had been unable to obtain natural food. A high survival rate is particularly significant because 25 of them were juveniles with little or no previous experience of life in the wild. If they could be fully reinstated, then adults should also be capable of survival.
Although several hedgehogs died in the two months after they were released, this is to be expected in a species whose average life expectancy is about two years. Moreover, but for the care the animals received in captivity, they would all have died much earlier. Over half not only survived, but some may also have bred. Premature deaths were due largely to accidents (drowning, or being killed by traffic or badgers) unrelated to their health. After an experimental release of 30 wild hedgehogs with no history of ill health or veterinary care in to Wytham Wood in Oxfordshire, 11 were killed by badgers or traffic within 10 weeks (Doncaster 1992). Rehabilitated animals will inevitably be exposed to such dangers.
It has been argued that hedgehogs should not be released into areas frequented by badgers because of the likelihood of predation. However, the most recent published distribution map (Arnold 1993) shows that badgers are present throughout Wales and most of southern England. They occur in all but two of the 10 km grid squares between Land's End and central Wiltshire. Avoiding them, at least in the south west, is not a realistic option. Where badgers are scarce, it is often because the habitat is unsuitable, and it is frequently inadequate for hedgehogs too.
Similarly, avoiding traffic is unrealistic. Hedgehogs can travel up to 3 km in a night (Morris 1988), and few suitable areas in the lowlands are more than 3 km from a road. The road deaths in these studies occurred on tiny lanes, used by very few vehicles per night. In all cases, wild hedgehogs were already present at the release sites, irrespective of badgers or road traffic.
Drowning is a common cause of death in hedgehogs, not because they cannot swim, but usually because they fall into water and cannot climb steep smooth surfaces to escape. Such mishaps are likely to occur irrespective of the animal's health or previous life in captivity. Nevertheless, despite all these difficulties, survival among rehabilitated hedgehogs was very high, compared, for example, with the minimal success of oiled seabirds after they have been cleaned and released (Schmidt 1997).
A probable risk factor is that the released hedgehogs were accustomed to being handled. Many had been captive almost since birth and were quite tame. They were insufficiently wary of ‘attack’ and several barely bothered to roll up when caught for weighing each night. Tameness may increase the danger posed by badgers and road traffic. It would therefore seem wise to discourage tameness in animals intended for release.
The assertion that rehabilitated animals should be released in urban areas because they will be safer there remains unsubstantiated. It may be true in respect of predation by badgers (although badgers are common in some suburban areas), but avoiding one hazard may simply increase exposure to another. In the studies reviewed here, as many hedgehogs were lost to motor traffic as to predators, despite the relative quietness of the country lanes in the three study areas. The lanes were also narrow, slowing the few vehicles present after dark. In urban areas, traffic will be heavier, and persist late into the night, almost certainly increasing the risk of accidental death.
Studies of dormice (Muscardinus avellanarius) highlight the benefit of 'soft release' methods involving use of pre-release cages in which the animals can become accustomed to their new environment before release (Bright and Morris 1994). Some rehabilitators also take considerable trouble to accustom hedgehogs to the release site before they are freed, and provide supplementary food afterwards. There was no evidence that the provision of food or acclimatisation to the release site through the use of pre-release cages had any beneficial effect. A similar result was obtained in Yorkshire (Morris and others 1993b) where none of the three released hedgehogs returned to feed at a bowl of dog food put out for them for several nights, despite their continuing weight losses. Nevertheless, soft release techniques which reduce the abruptness of the transition from captivity to the wild are unlikely to be harmful.
One of the Devon hedgehogs had defective teeth, which led to assertions that it should not have been released without 'effective veterinary treatment'. However, dental anomalies (including missing teeth) were found in 39 of 77 wild E europaeus (Brockie 1964) and the hedgehog released in Devon showed no evidence of progressive weight loss that might have been expected if it had been hampered by dental deficiencies. It lost a maximum of 21 per cent of its weight at release, then regained much of it, and maintained its weight stable for three weeks at 81 to 90 per cent of the initial weight, until it was killed by a badger.
The popularity of hedgehogs ensures that relevant press stories are widely read, and misleading reports and comments may have significant negative effects. Articles in the press following the publication of the paper by Sainsbury and others (1996) caused confusion among animal carers and dismay concerning the validity of hedgehog rehabilitation. Animal careers became concerned that dog food may be a dangerously inappropriate diet for hedgehogs because press reporting emphasised a comment by Sainsbury and others (1996) that eating soft dog food may lead to an accumulation of tartar on the teeth. Claims that hedgehogs had contracted lungworm while in care as a result of being fed on a diet of dog food instead of earthworms were particularly misleading. In fact, earthworms are a natural source of lungworm, Capillaria erinacei, common in wild hedgehogs (Romashov 1980).
These studies have clearly demonstrated that released hedgehogs, even inexperienced juveniles, coped well with life in the wild, found food, nested normally and integrated well with resident wild hedgehogs, often in spite of bad weather and other adverse factors. The rehabilitation of hedgehogs is therefore a justifiable use of resources to the extent that full integration into the wild is an achievable aim. There is no evidence that welfare is significantly at risk, except from accidents to which all hedgehogs are exposed. However, captive hedgehogs should not be encouraged to become tame and unwary of potential dangers. Substantial weight loss is to be expected after they are released, but this appears to be a natural readjustment to weight gain in captivity.
Thu Oct 11, 2012 8:43 am
