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Wildpro Reference Code: J147.7.w1 The
Survival and Welfare of Hedgehogs N J
Reeve School of Life Sciences, Roehampton Institute London, West Hill, London SW 1 5 3SN, UK Abstract The fate of rescued hedgehogs (Erinaceus europaeus) released back to the wild has now been the subject of several follow-up studies. Although subject to various hazards, released hedgehogs have clearly shown essential 'life-skills'. However, all previous studies have involved releases into hedgehog-rich areas and the observed long-range (≥500m) dispersal movements shown by some subjects, of up to 5km, may be a consequence of local intraspecific competition. This study has reduced a number of potentially confounding subject variables and provides follow-up data on 12 age-matched female hedgehogs with similar histories. A main group (n = 10) was released into a rural woodland area (Surrey, UK) of low natural hedgehog density, and radio-tracked for up to 108 days. A pilot release of two animals in an urban area with an established hedgehog population also took place (tracked for 109 and 131 days respectively). Most of the main group dispersed (up to 3km) from the release site; the two animals in the urban site did not. These data, taken with those from previous studies, suggest that dispersal is not specifically the result of intraspecific competition. Overall survival at week 8 was 42 per cent (5 hedgehogs) plus two lost animals. This is comparable with previous studies. However, survival fell to 25 per cent (3 animals) plus two lost animals by week 15. Of seven recorded deaths, only one was the result of a failure to thrive and all other mortalities were accidental: four road deaths, one drowned in a pond and one predation. The study concludes that the hazards of the human environment were the principal threat to the welfare and survival of released hedgehogs in the area. Keywords: animal welfare, Erinaceus europaeus, hedgehog, rehabilitation, survival Hedgehogs
are one of commonest mammals brought to British wildlife hospitals. Many of
these hedgehogs are ill or injured. However, in the autumn a large number of
juveniles which are either presumed to be orphans, or are judged to be below
the minimum weight widely believed to be necessary for winter survival
(about 450g in the UK [Morris 1984]) are encountered. These ‘autumn
orphans’ commonly present with lungworm infestations (Capillaria
and Crenosoma) and a variety of other health problems (Stocker 1987;
Reeve 1994; Sykes & Durrant 1995). Nevertheless, many are in good
overall health and require little more than food and shelter over the winter
months before being released back into the wild. The fate of such animals
after release has recently become an import investigation, as discussed
below, but wildlife aid organizations require further empirical data if
release and rehabilitation programmes are to be properly evaluated.
Many
constraints
have limited these follow-up studies (and the present study), including:
Such restrictions have limited the development of more rigorous field experiments. Additionally, many of the above studies have involved rather miscellaneous sample groups unevenly matched for factors such as age, sex, state of health on release, previous history and style of release. Despite
such problems, the previous studies have shown some consistent features. The
released hedgehogs were typically found to be able to forage and feed on
natural prey, to orientate, to make suitable day nests and to re-find former
nest-sites which were used in much the same way as previously described for
wild hedgehogs (Reeve 1982; Reeve & Morris 1985). The first day nest was
often not used again, as was found for six of the 12 animals released by
Morris and Warwick (1994), maybe because it
was a poor site, poorly constructed or the hedgehog failed to find
it. Observed interactions with resident hedgehogs included courtships (both
sexes) but no aggressive encounters. Despite these generally positive
indications, the released hedgehogs were subject to a variety of hazards and
their longer-term survival rate has been low. Although 75 per cent of the
eight animals released by Morris et al
(1993) were alive at 5 weeks, by 8 weeks they could confirm only one
survivor (12.5%), with four others unaccounted for. For the 12 animals
released by Morris and Warwick (1994) there was an 8-week survival rate of
33 per cent, with two others unaccounted for. Confounding variables included
two different release protocols, and an uneven sex ratio within the release
groups. Furthermore, there was apparently no attempt to control for
health-related factors such as levels of endoparasite infestation (Sainsbury
et al 1996). Morris and Sharafi (1996) reported the highest survival rates
yet from the predator-free and low traffic density environment of Jersey.
They released 13 animals, all of which survived at least 4 weeks, with a
minimum of 10 animals (77 per cent) surviving 6 weeks. None of the missing
animals were known to have died. There
are no published data regarding summer survival rates in the wild for young
adult hedgehogs in the UK, but with one exception (Morris and Sharafi 1996)
all release studies have shown mortality rates over a period of a few weeks
to be higher than reported annual mortality rates in the wild (an estimated
adult mortality of 30% per year in Morris [1991]). Better data are available
from an 8-year study in southern Sweden (Kristiansson 1990) that found a
mean mortality rate of 47 per cent per year for adults and young
non-breeding adults - although much of this could be accounted for by the
mean winter mortality rate of 33 per cent for this age group. Such figures
are comparable to estimates of 20-40 per cent adult winter mortality in
southern Germany in studies cited by Hoeck (1987). Study Aims The
principal aim was to obtain data on the fates of hedgehogs released into an
area of low natural hedgehog density (low intraspecific competition) to
provide a comparison with other studies. In order to reduce the number of
potentially confounding variables affecting behaviour and survival, the
subjects were matched as closely as possible for gender, age and condition
(see, Methods). All
anaesthetic, diagnostic and drug administration procedures were conducted by
a veterinary surgeon, or by trained staff under the supervision of a
veterinary surgeon, at Wildlife Aid, Surrey. Pre-release matching of subject variables The
present study could not escape the constraints affecting sample size
discussed above and it was
felt that the author’s ability to properly monitor released animals should
not be over-stretched. However, some potentially confounding variation in
the release group was reduced by ensuring that all animals were of the same
approximate age and sex (all autumn 1994 youngsters and all previously
uninjured females). Females were chosen because they have been shown to be
less wide ranging than males under natural conditions (Reeve 1982) and were
considered more likely to remain within the study area. Six
of the 12 subjects (numbers 1-6) were supplied by Wildlife Aid (Leatherhead,
Surrey), while numbers 7-12 came from Hedgehog Care (nr Louth,
Lincolnshire). The difference in origin was considered unlikely to affect
their post-release behaviour because all the hedgehogs were taken into care
when juvenile and therefore had little previous experience of life in the
wild. The
release weights of hedgehogs from the two wildlife hospitals were not
significantly different (t = 0.33,
two-tailed P = 0.75). The Lincolnshire group (mean weight 1001.66 ±
258.97 g) showed a rather greater coefficient of variation (25.8%) than the
Surrey group (mean weight 1044.83 ± 17.98 g, coefficient of variation =
17.9%). There was no significant difference in variance between the two
groups (F5.5 = 1.9, P = 0.50). To reduce the variance in the weight of the main release
group, the heaviest animal (number 11, weighing 1440g) was excluded - as was
number 12 (weighing 850g) which was observed to have a slight limp. The 10
animals released in Nower Wood had a mean weight of 998.9 ± 184.89 g,
coefficient of variation = 18.51%. Oocysts
of coccidia (Isospora rastegaivae) were
detected in moderate numbers in faecal sample extracts (obtained by
flotation) from the Surrey group, and occasional single oocysts in the
Lincolnshire group. Infestation with coccidia is common and usually
asymptomatic in hedgehogs (Saupe & Poduschka 1985) but, as a precaution
against any possible effects on post-release welfare, each animal was given
a single subcutaneous injection of 0.2ml trimethoprim sulphadiazine (Tribrissen®
24%, Schering Plough Animal Health Ltd, Middlesex, UK). Only isolated
oocysts were found in faecal samples obtained the day before release. Pre-release physical examination and transmitter attachment On
6 June 1995 all subjects were given a general physical examination under
gaseous anaesthesia using Halothane RM™ (Merial Animal Health Ltd, Essex,
UK) delivered in an oxygen and nitrous oxide mixture via a face mask.
Anaesthesia was induced using 4 per cent Halothane and maintained according
to effect using 1-2 per cent Halothane RM™. The animals appeared to be in
good physical condition and no fleas or ticks were found. A radio
transmitter was glued to the dorsal spines over the shoulders using epoxy
resin (setting time 10min) as described in Reeve (1982). No
obvious cause for number 12’s limp was found and, as both this animal and
number 11 appeared to be in generally good health, they were released in the
author’s garden in an urban area (Byfleet, Surrey) with a resident
hedgehog population and provided an ad
hoc pilot for future urban releases. Data from these animals are not
included in the main analyses but as both survived to the end of the study
they have provided useful additional information. Release protocol and study sites Releases
took place in Nower Wood (51°16’48’’N, 0°17’6’’W) on 7 June
1995 and in Byfleet (51°20’39’’N, 0°28’12’’W) on 8 June 1995.
On the basis of Morris and Warwick's (1994) reports that supplementary food
and shelter remained unused after release, there was no attempt at a
carefully staged 'soft release' and food was not provided. Byfleet
(Surrey) is an urban area of busy roads, industrial and retail developments,
fairly dense suburban housing and gardens with nearby or adjoining areas of
waste ground and copses. Hedgehogs were commonly seen in the gardens where
the releases took place. Foxes (Vulpes
vulpes), occasional predators of hedgehogs, were also a common sight but
no badgers have been recorded in the area (personal communication Dave
Williams 1997) or were seen by the author during the present study. Radio-tracking equipment The
transmitter package (Biotrack Ltd, Wareham, Dorset, UK) weighed between 7
and 8 g, with a 20cm flexible whip aerial and a lithium battery with a
predicted life in excess of 6 months. Transmitter frequencies ranged from
173.214 to 173.339 MHz. This gave adequate separation of tuning positions
using a Mariner 57 receiver with a 3-element hand-held Yagi (both rigid and
collapsible types) and a vehicle-mounted whip (Mariner Radar Ltd, Lowestoft,
Suffolk, UK). Movements A
major feature of the results was a very high level of dispersal from the
Nower Wood study site. The first dispersal from the wood occurred on day 5 (number
10) and another three animals (numbers 2, 3 and
4) dispersed on day
6; number 3 was never relocated and number 4 was not relocated until day 33,
when she was discovered 2.7km from her former position. Other examples of
movement included number 1 which moved over 1km between day 9 and day 17,
and number 7 which (having remained close to the release point) suddenly
vanished on day 12 to be re-found 2.8km away on day 33. Two
animals emigrated in two phases. Number 5 moved out of Nower Wood to gardens
in Tyrrell’s Wood on day 17 and remained there (about 760m from the
release point) until day 31 when she suddenly moved to the outskirts of
Leatherhead, over 2km from the release site. Number 10 first moved about
600m east to a cottage garden in Headley on day 5 but then disappeared on
day 17, only to be found the next day on the outskirts of Leatherhead almost
2.6km to the west.
Body
weight changes
Survival and welfare The released animals suffered a variety of mishaps. Only three hedgehogs (one from the Nower wood group and both the Byfleet animals) definitely survived in the wild until the end of the main study at 108 days, although the fates of two animals remained unknown. Table 2 shows a summary of the fates of the released hedgehogs and the survival rate at various times during the study. Only one animal (number 6) was unable to cope with life in the wild; her weight loss was not unusual but she never moved further than 76m from the release point and became lethargic, remaining in her nest even by night. She was dug out of her nest on the evening of day 9 and found to be weak and dehydrated. Despite treatment she died the next day. A post-mortem (Vetlab Services, Horsham, West Sussex, UK) revealed clear signs of bacterial pneumonia and pericarditis and a culture of purulent pleural fluid showed heavy growths of Pasteurella multocida and Bacteroides levii. Road traffic accidents were the certain cause of death of two animals, and the probable cause of death of number 8 which was found dead within Nower Wood with a smashed snout (although other accidental causes of death are possible). The jaw of number 4 was broken in a road accident but she was rescued and subsequently recovered at Wildlife Aid. Nevertheless, this incident was counted among the ‘deaths’ of the study as she could not have survived in the wild without intervention. Overall, four of the deaths could be attributed to road traffic - a high proportion of the total mortality. One animal (number 2) was predated, apparently by a badger, shortly after moving to Headley Heath (just under 1.5km away). Number 1 drowned in a steep-sided garden pond; this animal had previously been rescued from a 1.5m deep concrete shaft.
Despite an initial period of weight loss, the animals
seemed to be able to maintain themselves adequately in the wild, build nests
and forage for wild prey. If number 4’s broken jaw is counted as a death,
then six of the seven recorded deaths were accidental in nature rather than
failures to thrive. Ectoparasites Both
fleas (Archaeopsylla crinacei) and
ticks (Ixodes hexagonus) had been
eliminated from the hedgehogs before release, but were found in abundance on
both hedgehogs released in Byfleet within 1 week of release. Among the Nower
Wood release group, numbers 7, 8 and 9 were all found to have numerous
larval I.
hexagonus attached (Paul Hillyard personal communication 1995). Number 7
was particularly badly infested with well over 100 ticks around her eyes, on
the underside and around the forelimbs. No fleas or adult I.
hexagonus were recorded from hedgehogs while in the wood, but numbers 1,
4, 5, 7, 10 were all observed to have both fleas and ticks after moving into
built-up areas. The
present study supports the conclusion that released hedgehogs quickly become
accustomed to life in the wild and learn to feed themselves, make nests and
orientate (Morris et al 1992,
1993; Morris & Warwick 1994; Morris and Sharafi 1996). However, it is
clear that the transition is not easy and some individuals, as in the case
of number 6, may fail to adapt
and do not thrive. In this case, the stress of coping with life in the wild
may have exacerbated the severity of illness. Pasteurella
multocida has been reported as a pathogen of the respiratory tract in
European hedgehogs, but one that usually presents as a secondary infection
(studies reviewed by Reeve 1994). The
rapid infestation of both Byfleet animals with fleas and ticks, and the
eventual infestation of the animals released in Nower Wood after moving into
areas with other hedgehogs, confirms that fleas can spread rapidly through
the adult population. The separate use of the same, established wild
hedgehog’s nest by both Byfleet animals, is one route by which both fleas
and ticks could be rapidly transmitted. Such non-simultaneous nest sharing
was first described in Reeve and Morris (1985) and probably also occurred in
the study by Morris et al (1992).
The rapid colonization of released hedgehogs by ectoparasites suggests that
any putative welfare benefits of specifically treating flea or tick
infestations will be very short lived once the animal is released. None
of the recorded deaths were attributable to illness resulting from
endoparasitic infestation. This contrasts with the results from hedgehogs in
the study by Morris and Warwick (1994) which had not received pre-release
anthelmintic treatment (see Sainsbury et
al 1996). It also suggests that the pre-release ivermectin treatments
used in the present study may have been worthwhile. This
study shows conclusively that long-range dispersal (in this case up to 3km)
following release is a feature common to females and not just a male
characteristic. This confirms suspicions raised by Morris and Warwick's
(1994) study in which three of the furthest dispersing animals were females
(although only one moved much over 500m). A key question for the present
study is, why did the hedgehogs disperse from the apparently suitable site
of Nower Wood and its surroundings? Six
possible explanations are discussed below. The release site was in some way poor habitat There
would appear to have been plenty of suitable nest sites and abundant
invertebrate prey both in the wood and in the surrounding rural pasture,
golf course and low-density housing with large gardens. Although 1995 was a
very dry summer, ponds in the wood did not dry out and were accessible. Only
number 6 (which died of pneumonia) seemed unable to maintain herself while
in the wood. The possibility that the release site was lacking in some key,
but unknown, ecological requirement cannot be ruled out. However, the
apparent absence of wild hedgehogs in the release area does not necessarily
prove this. The area may have been quite suitable habitat but with a low
hedgehog density created by chance population fluctuations. Recolonization
may have been inhibited because the area is bounded to the north and east by
the M25 motorway and to the west by the A24 and other busy roads of the
Leatherhead conurbation. To the south and east, recolonization may have been
inhibited both by the high ground and dense badger population of the North
Downs. The
violent death of number 8, could have resulted from a predator's attack.
However, with no other injuries evident, the most likely cause of death
seems to be a road accident on Mill Way (unrelated to conditions in the
wood) from which she returned to the wood before dying. Predator avoidance Although
there is some good evidence for hedgehogs avoiding badgers and their odour (Doncaster
1992; Jane Ward personal communication 1996), it seems to be an unlikely
explanation for the dispersals in this study. First, the area was not
heavily used by badgers. Furthermore, at least three of the dispersals were
into or straight through areas of high badger density. Youngsters are 'instinctively'
driven to disperse, or the act of release triggers dispersal There
is some evidence that juveniles disperse to their eventual home area in the
first few months of life (Reeve 1994). The animals in this study may not
have been through this phase and might somehow have been driven to disperse.
Similarly, it could be argued that newly released animals may be
disorientated and that this triggers dispersal (or homing) movements.
Evidence against both of these suggestions includes observations that many
released hedgehogs do not disperse (eg Morris & Warwick 1994) and those
that do, seem to disperse after a latent period of a few days or a couple of
weeks. A drop in body weight triggers dispersal It
may be an adaptive response to disperse when an animal experiences a rate of
weight loss exceeding a certain threshold. Exploring new areas when prey
availability in the vicinity
becomes unsustainably low could prevent starvation. All released animals in
the present study (and previous studies) showed significant post-release
weight losses (Figure 2). Such weight losses were probably due to a
combination of factors such as the shedding of excess weight accompanying a
shift from a sedentary lifestyle to full activity, and a lack of practice in
foraging for natural foods. The hedgehogs were seeking other hedgehogs The
two animals released in Byfleet, where there was a resident hedgehog
population, did not disperse. The Nower Wood hedgehogs were all released in
the same area, so dispersal could not simply be a matter of the presence or
absence of other hedgehogs. However, it is possible that the released
females dispersed in search of males as potential mates. All the surviving,
released animals ended up in areas with resident hedgehogs - and the females
were sexually active. This suggestion challenges the image built up by home
range studies (see review by Reeve 1994) of comparatively sedentary females
sought out by highly mobile males with large home ranges encompassing the
ranges of several females. Nevertheless, it would not be necessarily
inconsistent to suggest that females, in the absence of encounters with
potential mates, may seek them out. The apparent absence of other hedgehogs
(none observed by researchers or householders) could explain why numbers 1,
5, and 10 subsequently left areas of apparently suitable gardens reached
after their first dispersal. The adult hedgehog ticks (I.
hexagonus) found on numbers 1 and 10 may have been an indication that
other hedgehogs were in fact present in these areas, but the fairly wide
host range of this tick species mitigates against this assumption. Hedgehogs seek out areas of human habitation It
is interesting that with the exception of three animals (numbers 2,
6 and 8; all of which died) all the released hedgehogs ended up around human
habitation. Urban/suburban areas may be more attractive and favourable
habitats for hedgehogs than much of the countryside. Possible positive
factors include food availability (perhaps enhanced by the irrigation of
gardens), a reduction in the number of badgers, and/or the number of secure
nest sites (eg under garden sheds). One negative aspect of the apparent
preference for areas of human habitation, however, is the risk of death and
injury on the roads. By week 8, the 42 per cent overall survival rate in the present study was comparable to that reported by previous release studies (see, Introduction and Table 2), but the survival rate at 15½ weeks was only 25 per cent - although it must be remembered that a further two lost animals could possibly have survived. The low survival figure may simply be an artefact of this being the longest follow-up study so far conducted. Animal welfare implications One
death (by drowning in a steep-sided pond) claimed an animal which had been
previously rescued, from a deep concrete shaft. Such hazards are a product
of the human environment, accidental in nature and presumably representative
of the risks affecting all hedgehogs. The risk of predation is also faced by
most wild hedgehogs, as they have no effective defence against a determined
badger. Such accidental deaths (six of seven recorded) have little direct
relationship with a lack of adaptive experience on the part of released
animals. The 25 per cent overall survival rate in this study, though lower
than one might hope for, still represents a positive step in compensating
for the various sources of mortality affecting wild hedgehog populations. There
remains a need for further comparative studies using matched groups to
determine the key factors influencing the survival and welfare of released
hedgehogs. These studies should aim to develop a release protocol that
maximizes survival and minimizes health and welfare problems for released
animals. Despite preliminary observations to the contrary (Morris &
Warwick 1994), a carefully staged (‘soft’) release protocol may reduce
potentially hazardous long-distance dispersal movements. This possibility
should be properly investigated. Doncaster P 1992 Testing the role of intraguild predation in regulating hedgehog
populations. Proceedings of the Royal
Society of London Series B 249: 113-117 Hoeck H N 1987 Hedgehog mortality during hibernation- Journal of the Zoological Society of London 213: 755-757 Kristiansson H 1990 Population variables and causes of mortality in a hedgehog population (Erinaceus europaeus) in southern Sweden. Journal of the Zoological Society of London 220: 391-404 Majeed S
K, Morris P
A and Cooper J E 1989
Occurrence of the lungworrns Capillaria and Crenosoma SPP- in British
hedgehogs (Erinaceus europaeus).
Journal of Comparative Pathology 100: 27-36 Morris P A 1984 An estimate of the minimum body weight necessary for hedgehogs (Erinaceus
europaeus) to survive hibernation.
Journal of Zoology 203: 291-294 Morris P A 1991 Family Erinaceidae. In:
Corbet G B & Harris S (eds) The
Handbook of British Mammals, 3rd edition pp 37-43.
Blackwell Scientific Publications: Oxford, UK Morris P A,
Meakin K and Sharafi S 1993
The behaviour and survival of rehabilitated hedgehogs (Erinaceus europaeus). Animal
Welfare 2: 53-66 Morris P A,
Munn S and Craig-Wood S
1992 The effects of releasing captive hedgehogs (Erinaceus curopacus) into the wild. Field Studies 8: 89-99 Morris P A and Sharafi S 1996
Released, rehabilitated hedgehogs: a
follow-up study in Jersey, April~May 1995- Unpublished report, Royal
Holloway College, University of London Morris P A and Warwick H 1994
A study of rehabilitated hedgehogs after release into the wild.
Animal Welfare 3: 163-177 Reeve N J 1982 The home range of the hedgehog as revealed by a radio tracking
study. In: Cheeseman C L and Mitson R B (eds) Telemetric Studies of Vertebrates (Symposium of The Zoological
Society of London 49) pp 207-230, Academic Press: London, UK Reeve N J 1994
Hedgehogs. T
& A D Poyser: London, UK Reeve N J and Morris P A 1985 Construction and use of summer nests by the
hedgehog (Erinaceus europaeus).
Mammalia 49 (2): 187-194 Sainsbury A W,
Cunningham A A, Morris P
A, Kirkwood J K, Macgregor S K
1996 Health and welfare of rehabilitated juvenile hedgehogs (Erinaceus europaeus) before and after release into the
wild. Veterinary Record 138: 61-65 Saupe
E and Poduschka W 1985
Igel. In: Gabrisch K and Zwart
P (eds) Krankheiten der Heimtiere pp 75-96.
Schlütersche: Hanover, Germany Stocker L 1987 The Complete Hedgehog.
Chatto and Windus: London, UK |
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