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Facing Danger: Exploring Personality and Reactions of European Hedgehogs (Erinaceus europaeus) towards Robotic Lawn Mowers

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The populations of European hedgehog (Erinaceus europaeus) are in decline, and it is essential that research identifies and mitigates the factors causing this. Hedgehogs are increasingly sharing habitats with humans, being exposed to a range of dangers in our backyards. Previous research has documented that some models of robotic lawn mowers can cause harm to hedgehogs. This study explored the personality and behaviour of 50 live hedgehogs when facing an approaching, disarmed robotic lawn mower. By combining a novel arena and novel object test, we found that 27 hedgehogs could be categorised as “shy” and 23 as “bold”, independently of sex and age. The encounter tests with a robotic lawn mower showed that the hedgehogs positioned themselves in seven different ways. Personality did not affect their reactions. Adult hedgehogs tended to react in a shyer manner, and the hedgehogs, generally, acted less boldly during their second encounter with the robotic lawn mower. Additionally, our results show that bold individuals reacted in a more unpredictable way, being more behaviourally unstable compared to the shy individuals. This knowledge will be applied in the design of a standardised hedgehog safety test, eventually serving to produce and approve hedgehog-friendly robotic lawn mowers.
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Citation: Rasmussen, S.L.; Schrøder,
B.T.; Berger, A.; Macdonald, D.W.;
Pertoldi, C.; Briefer, E.F.; Alstrup,
A.K.O. Facing Danger: Exploring
Personality and Reactions of
European Hedgehogs
(Erinaceus europaeus) towards Robotic
Lawn Mowers. Animals 2024,14, 2.
https://doi.org/10.3390/
ani14010002
Academic Editor: Brian L. Cypher
Received: 23 November 2023
Revised: 11 December 2023
Accepted: 14 December 2023
Published: 19 December 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
animals
Article
Facing Danger: Exploring Personality and Reactions of European
Hedgehogs (Erinaceus europaeus) towards Robotic Lawn Mowers
Sophie Lund Rasmussen 1,2,3,* , Bettina Thuland Schrøder 4, Anne Berger 5, David W. Macdonald 1,
Cino Pertoldi 2,6 , Elodie Floriane Briefer 4and Aage Kristian Olsen Alstrup 7,8
1Wildlife Conservation Research Unit, The Recanati-Kaplan Centre, Department of Biology,
University of Oxford, Tubney House, Abingdon Road, Tubney, Abingdon OX13 5QL, UK;
david.macdonald@biology.ox.ac.uk
2
Department of Chemistry and Bioscience, Aalborg University, Fredrik Bajers Vej 7H, 9220 Aalborg, Denmark;
cp@bio.aau.dk
3Linacre College, University of Oxford, St. Cross Road, Oxford OX1 3JA, UK
4Behavioral Ecology Group, Section for Ecology and Evolution, Department of Biology,
University of Copenhagen, Universitetsparken 15, 2100 Copenhagen, Denmark;
catsandconservation@gmail.com (B.T.S.); elodie.briefer@bio.ku.dk (E.F.B.)
5Leibniz Institute for Zoo and Wildlife Research, Alfred-Kowalke-Straße 17, 10315 Berlin, Germany;
berger@izw-berlin.de
6Aalborg Zoo, Mølleparkvej 63, 9000 Aalborg, Denmark
7Department of Nuclear Medicine and PET, Aarhus University Hospital, Palle Juul-Jensens Boulevard 165,
8200 Aarhus, Denmark; aagealst@rm.dk
8
Department of Clinical Medicine, Aarhus University, Palle Juul-Jensens Boulevard 165, 8200 Aarhus, Denmark
*Correspondence: sophie.rasmussen@biology.ox.ac.uk or sophielundrasmussen@gmail.com
Simple Summary: The European hedgehog is a generally welcomed but nowadays less common
guest in residential gardens, as the species is in decline. Sharing habitats with humans comes at
a cost: a residential garden holds many potential dangers for hedgehogs. Previous research has
shown that certain models of robotic lawn mowers may harm hedgehogs. This study sought to
investigate the personality and reactions of live hedgehogs towards a disarmed, approaching robotic
lawn mower. Personality tests revealed that the hedgehogs could be divided into categories of “shy”
and “bold” individuals, independently of age and sex. The encounter tests with a disarmed robotic
lawn mower showed that they behaved and positioned themselves in seven different ways, and the
individuals with a bold personality reacted in a more unpredictable way. Adult hedgehogs tended
to react in a shyer manner, and the tested hedgehogs, generally, acted less boldly the second time
they encountered a robotic lawn mower. This knowledge will be used in the process of designing
a standardised hedgehog safety test to eventually produce and approve hedgehog-friendly robotic
lawn mowers that pose no hazards to hedgehogs, ultimately, serving to eliminate their influence on
hedgehog survival and, thereby, improve hedgehog conservation.
Abstract: The populations of European hedgehog (Erinaceus europaeus) are in decline, and it is
essential that research identifies and mitigates the factors causing this. Hedgehogs are increasingly
sharing habitats with humans, being exposed to a range of dangers in our backyards. Previous
research has documented that some models of robotic lawn mowers can cause harm to hedgehogs.
This study explored the personality and behaviour of 50 live hedgehogs when facing an approaching,
disarmed robotic lawn mower. By combining a novel arena and novel object test, we found that
27 hedgehogs could be categorised as “shy” and 23 as “bold”, independently of sex and age. The
encounter tests with a robotic lawn mower showed that the hedgehogs positioned themselves in
seven different ways. Personality did not affect their reactions. Adult hedgehogs tended to react
in a shyer manner, and the hedgehogs, generally, acted less boldly during their second encounter
with the robotic lawn mower. Additionally, our results show that bold individuals reacted in a
more unpredictable way, being more behaviourally unstable compared to the shy individuals. This
knowledge will be applied in the design of a standardised hedgehog safety test, eventually serving to
produce and approve hedgehog-friendly robotic lawn mowers.
Animals 2024,14, 2. https://doi.org/10.3390/ani14010002 https://www.mdpi.com/journal/animals
Animals 2024,14, 2 2 of 19
Keywords: Erinaceus europaeus; animal personality; applied animal behaviour research;
shyness–boldness
; wildlife conservation; anthropogenic disturbance; robotic lawn mowers; garden
technology; lawn care; behavioural instability
1. Introduction
The European hedgehog (Erinaceus europaeus), hereafter referred to as “hedgehog”, is
a widely distributed species that can survive across a range of diverse habitat types [
1
,
2
].
Despite the species’ ability to adapt to many different settings [
3
], recent research has
either documented or suggested declines in the populations of European hedgehogs in
several Western European countries [
4
14
]. For the time being, the investigated factors con-
tributing to this decline include habitat loss; habitat fragmentation; inbreeding; intensified
agricultural practices; road traffic accidents; a reduction in biodiversity and, hence, food
items; lack of suitable nest sites in residential gardens; accidents caused by garden tools;
netting and other anthropogenic sources in residential gardens; infections with pathogens
and endoparasites; molluscicide and rodenticide poisoning; and, in some areas, badger
predation [
3
,
5
,
15
31
]. With the potential to reach 16 years of age [
32
], it is of concern that
the majority of research into the lifespan of European hedgehogs has found a mean age of
only two years (see Rasmussen, Berg, Martens and Jones [
32
], Table 1 for an overview). It is,
therefore, essential to investigate the possible causes for this early mortality and the overall
population decline in order to optimise the conservation initiatives directed at this species.
1.1. Hedgehogs and Robotic Lawn Mowers
Robotic lawn mowers are becoming increasingly popular in Europe and the US.
According to market insight reports, it is expected that the global robotic lawn mower
market will expand from US 0.8–1.5 billion in 2020–2022 to US 2.7–4 billion in 2032, and the
market is anticipated to develop at a compound annual growth rate (CAGR) of 11.5–15.5%
during the forecasted period [
33
,
34
]. With the increasing number of robotic lawn mowers
operating in residential gardens throughout the distribution range of hedgehogs in Europe,
combined with the fact that research indicates that hedgehogs are, nowadays, increasingly
associated with human-occupied areas [
10
,
20
,
31
,
35
], it seems likely that many individual
hedgehogs will encounter several robotic lawn mowers during their lifetime.
Human activities negatively impact the welfare of countless wild vertebrates [
36
], and
hedgehogs are no exception. Injured hedgehogs are frequently found by members of the
public and are admitted to hedgehog rehabilitation centres with different types of cuts
and injuries, most often caused by garden tools and netting, or predators such as dogs,
foxes, or badgers [
37
40
]. In some cases, the injuries are fatal, whilst others necessitate
euthanasia. However, because of a growing concern reported by hedgehog carers and
members of the public that an increasing number of these incidents could have been caused
by robotic lawn mowers, Rasmussen et al. (2021) [
29
] investigated the effects of robotic
lawn mowers on hedgehogs. Tests were carried out on deceased hedgehogs and showed
that some models of robotic lawn mowers did, indeed, injure hedgehog carcasses, whereas
other models only had to touch a hedgehog carcass lightly to detect it before changing
direction accordingly to avoid it. Consequently, it was observed that all models of robotic
lawn mowers included in the tests had to physically interact with the hedgehog carcasses
to detect them [
29
]. This led to the suggestion that research should be initiated to develop
more hedgehog-friendly robotic lawn mowers and that a standardised safety test should
be designed for the evaluation and approval of new models of robotic lawn mowers for
the market in terms of hedgehog safety as an addition to the current mandatory general
safety guidelines [
29
,
41
]. This test should, ideally, be performed on a specially designed
hedgehog crash test dummy placed in realistic fixed positions.
Having previously tested the effects of robotic lawn mowers on dead hedgehogs,
positioning the carcasses in the tests based on the knowledge of hedgehog behaviour, we
Animals 2024,14, 2 3 of 19
decided to test the reactions of live hedgehogs to an approaching robotic lawn mower
to optimise the design of a future standardised safety test by ensuring that it is realistic.
Furthermore, testing and quantifying the behaviour of live animals should also consider
the effects of personality on the outcome.
1.2. Measuring Personality in Animals
When facing danger, hedgehogs tend to stand completely still, as if they are in a
frozen state, often in an upright position with the snout pointing inwards (i.e., head bent
inwards in a partially curled-up position, later referred to as behavioural category 3),
whilst deciding whether the next step should be to curl up or run away [
42
]. The strategy
of curling up in front of an approaching car or robotic lawn mower appears to be less
successful than running away. Could personality determine the reaction of hedgehogs
towards an approaching robotic lawn mower?
Personality, defined as individual differences that are stable over time and across
situations, affects how individuals react to challenging situations [
43
] and may influence
their survival [
44
]. Several studies have shown that it is possible to estimate the shy-
ness/boldness of individuals, including hedgehogs [45], by analysing how they explore a
novel environment or arena, or by measuring their latency to approach a novel object in
a familiar environment [
44
,
46
48
]. Previous research has explored and documented the
occurrence of a shyness–boldness gradient in natural populations [
44
,
49
51
]. In addition, it
has been demonstrated that personality potentially influences fitness through reproductive
success and survival [
44
,
52
,
53
]. Natural selection affects factors such as boldness at a
population level [
54
], which is why individuals with inappropriate levels of boldness may
suffer reduced fitness in the wild due to extensive risk-taking behaviour [
44
]. Accordingly,
Bremner-Harrison, Prodohl, and Elwood (2004) [
44
] found that bolder juvenile captive-bred
swift foxes had a lower postrelease survival compared to their shyer conspecifics.
Another important aspect that characterises the personality of an individual is the
degree of behavioural instability, which quantifies the degree of unpredictability of a
behavioural response. The notion of behavioural instability, as suggested by Pertoldi et al.
(2016) and others [
55
57
], goes beyond being solely described by the variance and/or
interquartile range (IQR). It also encompasses the kurtosis and skewness (i.e., asymmetry)
of the distributions. These parameters collectively impact the median absolute deviation, a
measure of the variability in a dataset, which is estimated by the median distance of the
data values from the median (MAD). The higher levels of behavioural instability exhibited
by certain individuals could have relevant implications, as an increased variability in the
behavioural repertoire could enhance the probability of survival in a dangerous situation,
such as an encounter between a hedgehog and a robotic lawn mower [55].
1.3. Aim of the Research
In this study, we tested the reactions of live hedgehogs towards a disarmed, approach-
ing robotic lawn mower to optimise the test design of a future standardised realistic safety
test. Additionally, we investigated the effects of personality, measured as shyness/boldness,
on behavioural responses and associated predictability and, thereby, the risk-taking be-
haviour of these animals when facing the approaching robotic lawn mower to ensure
that this test would account for differences in the hedgehog reactions linked to their per-
sonality. We predicted that shy individuals would have a higher tendency to run away
and that bolder individuals would be more inquisitive towards the approaching robotic
lawn mower.
Measuring the responses of live hedgehogs towards an approaching robotic lawn
mower will facilitate the optimal design of a standardised safety test for robotic lawn
mowers, which will become an important tool for enhancing the safety of hedgehogs
entering gardens. This will, ultimately, result in the improved conservation of the declining
populations of European hedgehogs.
Animals 2024,14, 2 4 of 19
2. Materials and Methods
The research included both personality tests and encounter tests with a robotic lawn
mower (Figure 1), performed on 50 live rehabilitated Danish hedgehogs that had been
assessed as ready for release back into the wild. All individuals were released at suitable
sites within a few days after the tests. Some individuals were adults originally admitted into
care because of disease or injury (N = 15), and some were orphaned juveniles (
N = 35
) that
were hand-raised by a hedgehog rehabilitator. They had reached the age and capabilities
similar to wild juveniles at the age of independence, having been raised under natural
conditions, and had been deemed ready for release back into the wild.
Animals 2023, 13, x FOR PEER REVIEW 4 of 19
away and that bolder individuals would be more inquisitive towards the approaching
robotic lawn mower.
Measuring the responses of live hedgehogs towards an approaching robotic lawn
mower will facilitate the optimal design of a standardised safety test for robotic lawn
mowers, which will become an important tool for enhancing the safety of hedgehogs en-
tering gardens. This will, ultimately, result in the improved conservation of the declining
populations of European hedgehogs.
2. Materials and Methods
The research included both personality tests and encounter tests with a robotic lawn
mower (Figure 1), performed on 50 live rehabilitated Danish hedgehogs that had been
assessed as ready for release back into the wild. All individuals were released at suitable
sites within a few days after the tests. Some individuals were adults originally admied
into care because of disease or injury (N = 15), and some were orphaned juveniles (N = 35)
that were hand-raised by a hedgehog rehabilitator. They had reached the age and capa-
bilities similar to wild juveniles at the age of independence, having been raised under
natural conditions, and had been deemed ready for release back into the wild.
The experimental design was created with the welfare of the hedgehogs in mind,
taking precautions to reduce the levels of stress caused by transportation and handling.
Therefore, all tests took place in the garden surrounding the wildlife rehabilitation centre,
where the hedgehogs had been in care, and the hedgehogs were exclusively handled by
the wildlife rehabilitator, who had been nursing them back to health.
The tests were divided into a pilot study on ten individuals, which took place on two
consecutive nights from the 3rd5th of June 2022, and further tests on 40 individuals tak-
ing place from the 6th–8th of September 2022 (N = 20) and 15th17th of September 2022
(N = 20). As hedgehogs are nocturnal, the experiments were performed during their nat-
ural activity period, from sunset to sunrise, varying based on the time of year and the
number of individuals tested on the specic nights of testing.
Figure 1. Experimental design and timeline. Design adapted from the “Mouse Experimental Time-
line”, template by BioRender.com, agreement number: RS262OWRC1. Photographs and illustra-
tions by Cloud b; Encyclopaedia Britannica, and Husqvarna.
2.1. Personality Tests
The personality tests were performed as a combination of a novel object test [48,58,59]
and a novel arena test [59,60] for the purpose of assessing each individuals tendency to
respond with either curiosity or fearfulness and, hence, exploration or avoidance of the
novel object in a novel environment. The novel object was a 45 × 25 × 20 cm blue plush toy
(Cloud b “Charley the Chameleon”). The toy stimulated both visual and auditory senses
Figure 1. Experimental design and timeline. Design adapted from the “Mouse Experimental Time-
line”, template by BioRender.com, agreement number: RS262OWRC1. Photographs and illustrations
by Cloud b; Encyclopaedia Britannica, and Husqvarna.
The experimental design was created with the welfare of the hedgehogs in mind,
taking precautions to reduce the levels of stress caused by transportation and handling.
Therefore, all tests took place in the garden surrounding the wildlife rehabilitation centre,
where the hedgehogs had been in care, and the hedgehogs were exclusively handled by the
wildlife rehabilitator, who had been nursing them back to health.
The tests were divided into a pilot study on ten individuals, which took place on
two consecutive nights from the 3rd–5th of June 2022, and further tests on 40 individuals
taking place from the 6th–8th of September 2022 (N = 20) and 15th–17th of September
2022 (
N = 20
). As hedgehogs are nocturnal, the experiments were performed during their
natural activity period, from sunset to sunrise, varying based on the time of year and the
number of individuals tested on the specific nights of testing.
2.1. Personality Tests
The personality tests were performed as a combination of a novel object test [
48
,
58
,
59
]
and a novel arena test [
59
,
60
] for the purpose of assessing each individual’s tendency to
respond with either curiosity or fearfulness and, hence, exploration or avoidance of the
novel object in a novel environment. The novel object was a 45
×
25
×
20 cm blue plush toy
(Cloud b “Charley the Chameleon”). The toy stimulated both visual and auditory senses
through a sound emission, a soothing melody at a maximum of 5 dB, and blinking RGB
LED lights in the colours red, blue, and green.
The novel arena consisted of a wooden pallet frame with a solid, wooden pallet base
(dimensions inside the arena: length 104 cm, width 76 cm, and height 38 cm). The base was
covered by a large piece of cardstock thick paper in a light grey colour, covering the whole
base of the arena. The novel object was placed in the end of the test arena, 60 cm from the
nest box containing the hedgehog subjected to the personality test, which was placed in the
opposite end of the novel arena. The nest box was added to the novel arena just before the
Animals 2024,14, 2 5 of 19
test started. The novel arena was placed under a cover by a dark-blue garden gazebo for
the purpose of standardising the weather conditions and shielding the arena against rain.
The procedure for each personality test lasted for 30 min (including rest) in total
(Figure 1). Before the test started, the hedgehog was transported to the novel arena in its
individual, familiar nest box, from its own enclosure situated in the housing ward of the
hedgehog rehabilitation centre. The closed nest box was placed in the novel arena, allowing
the hedgehog to rest and acclimatise for 15 min after transportation. During this resting
period, the novel object was not activated and was not visible to the hedgehog. After
15 min of rest, the nest box entrance was opened, and the novel object’s light and sound
effects were switched on (Figure 2). Over the following 15 min, two cameras recorded any
activity in the novel arena. A Ring Stick Up Cam video camera (Ring
TM
, Santa Monica, CA,
USA), which was suspended from the ceiling of the garden gazebo at around the centre
of the novel arena, provided a complete overview of the novel arena. The Ring camera
was controlled through the Ring app and was set to record with a live view continuously
for 12 min at a time, as this was the maximum possible duration per recording. The Ring
camera was manually re-activated after 12 min using the Ring app. A Nedis HD Wildlife
CameraWCAM130GN (Nedis, ’s-Hertogenbosch, The Netherlands) was also set to record
for 30 s after the detection of any locomotion and to record back to back with no delay
between recordings. The Nedis camera was placed on a tripod halfway along the long side
of the novel arena allowing for all activity during a test to be detected and recorded.
Animals 2023, 13, x FOR PEER REVIEW 5 of 19
through a sound emission, a soothing melody at a maximum of 5 dB, and blinking RGB
LED lights in the colours red, blue, and green.
The novel arena consisted of a wooden pallet frame with a solid, wooden pallet base
(dimensions inside the arena: length 104 cm, width 76 cm, and height 38 cm). The base
was covered by a large piece of cardstock thick paper in a light grey colour, covering the
whole base of the arena. The novel object was placed in the end of the test arena, 60 cm
from the nest box containing the hedgehog subjected to the personality test, which was
placed in the opposite end of the novel arena. The nest box was added to the novel arena
just before the test started. The novel arena was placed under a cover by a dark-blue gar-
den gazebo for the purpose of standardising the weather conditions and shielding the
arena against rain.
The procedure for each personality test lasted for 30 min (including rest) in total (Fig-
ure 1). Before the test started, the hedgehog was transported to the novel arena in its indi-
vidual, familiar nest box, from its own enclosure situated in the housing ward of the
hedgehog rehabilitation centre. The closed nest box was placed in the novel arena, allow-
ing the hedgehog to rest and acclimatise for 15 min after transportation. During this rest-
ing period, the novel object was not activated and was not visible to the hedgehog. After
15 min of rest, the nest box entrance was opened, and the novel objects light and sound
eects were switched on (Figure 2). Over the following 15 min, two cameras recorded any
activity in the novel arena. A Ring Stick Up Cam video camera (Ring
TM
, Santa Monica,
California, USA), which was suspended from the ceiling of the garden gazebo at around
the centre of the novel arena, provided a complete overview of the novel arena. The Ring
camera was controlled through the Ring app and was set to record with a live view con-
tinuously for 12 min at a time, as this was the maximum possible duration per recording.
The Ring camera was manually re-activated after 12 min using the Ring app. A Nedis HD
Wildlife CameraWCAM130GN (Nedis, USA) was also set to record for 30 s after the de-
tection of any locomotion and to record back to back with no delay between recordings.
The Nedis camera was placed on a tripod halfway along the long side of the novel arena
allowing for all activity during a test to be detected and recorded.
An experimenter (the same person for all personality tests) silently monitored all
tests in complete darkness at a distance of 4 m from the test arena.
Figure 2. A photo from the Ring surveillance camera, which was used during the personality tests,
illustrating the experimental design.
An experimenter (the same person for all personality tests) silently monitored all tests
in complete darkness at a distance of 4 m from the test arena.
2.2. Encounter Tests with a Robotic Lawn Mower
The robotic lawn mower encounter tests were conducted on the lawn of the residential
garden surrounding the wildlife rehabilitation centre, where the hedgehogs were housed.
The experimental area was fenced off with a green, 45 cm tall wire mesh fence to ensure the
hedgehogs tested could not run away (Figure 3). The area was 1.5 m in width and 6.75 m in
length. Green tentor poles were placed alongside the fence inside the test area and marked
Animals 2024,14, 2 6 of 19
each 1 m distance from the hedgehog. A black folding fence, which was 1.2 m in height,
consisting of three panels that were 0.8 m in width, was placed outside the fenced area to
surround the zone in which the hedgehog was placed. This allowed for the experimenters
recording the behaviour and stepping into the test zone to manually stop the robotic lawn
mower and to remain visually hidden from the hedgehog during the tests.
Animals 2023, 13, x FOR PEER REVIEW 6 of 19
Figure 2. A photo from the Ring surveillance camera, which was used during the personality tests,
illustrating the experimental design.
2.2. Encounter Tests with a Robotic Lawn Mower
The robotic lawn mower encounter tests were conducted on the lawn of the residen-
tial garden surrounding the wildlife rehabilitation centre, where the hedgehogs were
housed. The experimental area was fenced o with a green, 45 cm tall wire mesh fence to
ensure the hedgehogs tested could not run away (Figure 3). The area was 1.5 m in width
and 6.75 m in length. Green tentor poles were placed alongside the fence inside the test
area and marked each 1 m distance from the hedgehog. A black folding fence, which was
1.2 m in height, consisting of three panels that were 0.8 m in width, was placed outside
the fenced area to surround the zone in which the hedgehog was placed. This allowed for
the experimenters recording the behaviour and stepping into the test zone to manually
stop the robotic lawn mower and to remain visually hidden from the hedgehog during
the tests.
Figure 3. A photo from the surveillance camera used during the encounter tests, illustrating the
experimental design. A range of safety precautions for the tests can be seen: (1) a person standing
on the right side of the test area stepping in to manually turn o the robotic lawn mower by pushing
the (visual) buon on the shield of the mower; (2) the rope aached to the back of the mower (with
a person pulling it in the other end); and (3) the hedgehog rehabilitator standing inside the test area
next to the hedgehog ready to intervene, if necessary. The two persons standing in the vicinity of
the hedgehog tested (the hedgehog rehabilitator and the person recording with the handheld FLIR
E54 Thermal Imaging camera) remained the same throughout all tests and were cautious to behave
in the same manner during all tests.
The robotic lawn mower (Husqvarna Automower
®
415X, Husqvarna, Huskvarna,
Sweden) was positioned on a spot marked with duct tape at a distance of 5.75 m from the
hedgehog, allowing it to move in a straight line through the test area in the direction of
the hedgehog. At the beginning of each test, the hedgehog was placed inside the fenced
area, on a spot on the lawn marked with duct tape, at a distance of 1 m from the surround-
ing fence. The hedgehog was positioned in an upright standing position with the front
Figure 3. A photo from the surveillance camera used during the encounter tests, illustrating the
experimental design. A range of safety precautions for the tests can be seen: (1) a person standing on
the right side of the test area stepping in to manually turn off the robotic lawn mower by pushing the
(visual) button on the shield of the mower; (2) the rope attached to the back of the mower (with a
person pulling it in the other end); and (3) the hedgehog rehabilitator standing inside the test area
next to the hedgehog ready to intervene, if necessary. The two persons standing in the vicinity of the
hedgehog tested (the hedgehog rehabilitator and the person recording with the handheld FLIR E54
Thermal Imaging camera) remained the same throughout all tests and were cautious to behave in the
same manner during all tests.
The robotic lawn mower (Husqvarna Automower
®
415X, Husqvarna, Huskvarna,
Sweden) was positioned on a spot marked with duct tape at a distance of 5.75 m from the
hedgehog, allowing it to move in a straight line through the test area in the direction of the
hedgehog. At the beginning of each test, the hedgehog was placed inside the fenced area,
on a spot on the lawn marked with duct tape, at a distance of 1 m from the surrounding
fence. The hedgehog was positioned in an upright standing position with the front facing
the mower and the head folding inwards (later referred to as behavioural category 3). When
the hedgehog was in place, the mower was turned on, causing it to move in the direction of
the hedgehog and gradually approaching the individual at 380 mm/s, until a distance of
0.5 m, at which the mower was stopped. During the pilot test on the first ten individuals,
the robotic lawn mower was stopped at a 1 m distance from the hedgehogs, but this was
changed to 0.5 m for the tests with the last 40 individuals due to a concern that the 1 m
distance was too far away to properly trigger a reaction from the hedgehogs.
Each hedgehog was tested with the robotic lawn mower twice: one test in which the
headlights of the mower were switched off and another with the headlights switched on.
Animals 2024,14, 2 7 of 19
The second test was performed immediately after the first (Figure 1). The order of the
tests was counterbalanced among the individuals (Figure 1). In the case that the test was
interrupted because of a technical failure, or if the hedgehog ran away before the robotic
lawn mower was turned on, we restarted the test once, resulting in a maximum of three
tests and a duration of a maximum of 15 min in total, during which the hedgehog was
situated in the test area. In the majority of cases, we only tested the hedgehogs twice.
The tests were recorded with a Ring Stick Up Cam video camera placed on a tripod
overlooking the experimental area, as well as a handheld FLIR E54 Thermal Imaging
camera (Teledyne FLIR, Wilsonville, OR, USA) recording from behind the folding fence
surrounding the hedgehog being tested. With the use of these cameras, the reactions (i.e.,
behavioural categories) of each individual towards the approaching robotic lawn mower
during the two tests, were recorded.
After the tests, the hedgehog was returned to its enclosure in the housing ward at the
hedgehog rehabilitation centre by the hedgehog rehabilitator.
Safety Precautions during the Encounter Tests
Multiple initiatives were implemented to ensure that the safety measures were met
during the tests. The pivoting blades (i.e., knives) were removed from the robotic lawn
mower, and the mower was controlled via remote control (Husqvarna Automower
®
Con-
nect App, Husqvarna, Huskvarna, Sweden). Additionally, a rope was fastened to the
robotic lawn mower, and a staff member held this rope during the tests, generating manual
control to ensure the mower never got closer to the hedgehog than the 0.5 m (1 m in the
pilot study) distance allowed in the test. Furthermore, a person standing alongside the test
area, hidden behind the folding fence, stepped in to stop the robotic lawn mower manually
when it reached the 0.5 m (1 m in pilot study) limit by pressing the red stop button on the
dorsal shield of the robotic lawn mower. The hedgehog carer remained standing next to the
hedgehog during the tests and was ready to intervene and pick up the hedgehog in case
it tried to run away or the other safety precautions failed (which never occurred during
the tests).
2.3. Data Analysis
The data analyses and graphs for publication were prepared in R v 4.2.3. [
61
] using
the packages lme4, DHARMa, pbkrtest, multcomp, ordinal, car, and tidyverse.
2.3.1. Personality Tests
On the basis of their behaviour during the combined novel arena and novel object test,
the hedgehogs were categorised as either shy or bold as follows: individuals that remained
in their nest box during the full 15 min duration of the test were categorised as “shy”,
whilst those that left the nest box and entered the arena to explore the novel arena and
novel object during the test were categorised as “bold”. This categorisation was established
during the pilot study after observing a large number (6/10) of hedgehogs remaining in
their nest box for the entire test.
We used a generalised linear model (GLM) to analyse the results of the personality
tests due to a binary outcome (shy versus bold, analysed as 0 vs. 1) and having no repeated
measures of the same individuals. The GLM with binomial family included personality
(0 versus 1) as a response variable and age (adult versus juvenile), sex (female versus male),
and weight (in kg, z-score transformed) as fixed effects.
On the basis of the variance inflation factor (VIF), using the VIF function in R, testing
for multicollinearity, we found that the fixed effects of age and weight were correlated,
as VIF > 5. Therefore, we removed the response variable “weight” from the initial model
(
VIF = 5.42
). Accordingly, the final model was: glm (personality_category ~ sex + age,
family = binomial, data = personality).
Animals 2024,14, 2 8 of 19
2.3.2. Encounter Tests with a Robotic Lawn Mower
During the encounter tests with a robotic lawn mower, the behaviour of each individ-
ual was scored in five ordinal behavioural categories, with 1 being the more exploratory
behavioural (bold) reaction and 5 being the more cautious (shy) reaction. Behaviour was
recorded at the point in time at which the robotic lawn mower was situated just in front of
the hedgehog and before the mower was manually stopped.
The ordinal categories were:
(1)
Upright position with snout facing 9–10 o’clock, 2–3 o’clock, or 6 o’clock (rump
towards the mower).
(2)
Upright position with snout facing the mower.
(3) Upright position with snout pointing inwards (head bent inwards in a partially curled-
up position). This was the position the hedgehog was originally placed in when the
test was initiated.
(4)
Hedgehog running away from the robotic lawn mower.
(5)
Hedgehog curling up.
To analyse the results of the encounter tests, a cumulative link mixed model (CLMM)
was chosen, as the response is ordinal (ordered categorical data), and no assumption was
made concerning the spacing between behavioural categories 1 and 5.
The CLMM included the ordinal behavioural category (1–5) as a response variable;
lights (on vs. off), personality (shy vs. bold, coded as 0 vs. 1), age (adult versus juvenile),
sex (female vs. male), and test number (1 vs. 2 as a control) as fixed effects; and individual
ID crossed with the date of the test as random effects (to control for repeated measures,
as each hedgehog was tested twice, and the differences between the days). The variance
inflation factor was below 5 for all fixed effects, indicating the absence of multicollinearity,
and allowing us to include all factors in the same model. The final model was: clmm
(responseF ~ lights + personality_cat + weight + sex + test_number + (1|individual) +
(1|date), data = behaviour).
Furthermore, by comparing a model with to a model without the random effect
“individual” using a likelihood ratio test, we tested whether individuals differed in their
responses in general. Finally, the degree of behavioural instability, estimated as the median
absolute deviation (MAD) of the ordinal behavioural category was tested with the Mann–
Whitney U test and Levene’s test from the medians for differences among the individuals
belonging to the two categories of personality (shy vs. bold). In the two categories of
personality, individuals of different sex, weight, and age were pooled, as these factors did
not seem to affect personality (see Section 3, results below).
2.4. Ethical Approval
Ethical approval for this study was provided by the Animal Experiments Inspectorate
under the Danish National Committee for the Protection of Animals used for Scientific
Purposes (license number: 2021-15-0201-00865) in accordance with 2010/63/EU [
62
]. We
followed the 3R concept for use of animals in research: we used a paired design to reduce
the number of experimental animals needed (i.e., reduction), and we ensured that no animal
suffered harm during the study through specified safety measures (i.e., refinements). It was
not possible to replace the studies with alternative methods (i.e., replacement). Permission
was also obtained from the Danish Nature Agency to work with this protected species. All
animals completed the tests without injury and were released back into the wild within a
few days.
3. Results
The 50 individuals tested comprised 15 adults and 35 independent juveniles, 30 females
and 20 males.
Animals 2024,14, 2 9 of 19
3.1. Personality Tests
On the basis of their behaviour during the combined novel arena and novel object test,
27 (54%) individuals were categorised as shy and 23 (46%) as bold.
Our data analysis showed that neither sex (GLM: Z =
0.93, p= 0.35) nor age (and
thereby weight) (GLM: Z = 1.31, p= 0.19) had a significant effect on the personality of the
hedgehogs (see Appendix Afor the raw data).
3.2. Encounter Tests with a Robotic Lawn Mower
The distribution of the behaviours shown by the 50 individuals tested during the total
100 encounter tests with a robotic lawn mower is illustrated in Figure 4(see Appendix A
for a full overview of the results). The most frequent behavioural response (43%) was
the upright position with the snout pointing inwards, which was also the position the
hedgehogs were placed in when the tests started. In 15% of the cases, the strategy of the
hedgehogs was to run away. Curling up was only observed once in the 100 tests that
were conducted.
Animals 2023, 13, x FOR PEER REVIEW 10 of 19
Figure 4. An overview of the results from the encounter tests with a robotic lawn mower. “Behav-
iour” describes the behaviour exhibited by the hedgehogs.Category scores” indicate the degree of
shyness or boldness of the behaviour, with 1 being the boldest and 5 being the shyest. “Distribution”
denotes the frequency of the specic behaviours, in percentage, out of 100 tests conducted with a
total of 50 dierent hedgehogs. The line with arrows indicates the gradient of shyness or boldness
of the behaviour exhibited by the hedgehogs tested. At the beginning of each test, the hedgehog was
placed in an “upright position with snout pointing inwards”.
Figure 5. Behavioural responses as a function of the age category of the individuals during the en-
counter test with a robotic lawn mower (CLMM: p = 0.0002). “Response” shows behavioural
Behaviour
Upright position with sno u t
fa c in g 6 o clock (rum p fa cin g
th e m o w e r)
Upright position
w it h sn o u t f a cin g
2-3 o’clo ck
Upright position with
sn ou t fa cin g 9 -1 0 o’clock
Upright position
with sn o u t fac in g
th e mower
Upright position with
sn o u t p o in tin g in w a rd s Running aw ay C u rlin g up
Category
scores
1 1 1 2 3 4 5
Distribution
(out of 100)
6% 11% 4% 20% 43% 15% 1%
Bold
S h y
Figure 4. An overview of the results from the encounter tests with a robotic lawn mower. “Behaviour”
describes the behaviour exhibited by the hedgehogs. “Category scores” indicate the degree of shyness
or boldness of the behaviour, with 1 being the boldest and 5 being the shyest. “Distribution” denotes
the frequency of the specific behaviours, in percentage, out of 100 tests conducted with a total of
50 different hedgehogs. The line with arrows indicates the gradient of shyness or boldness of the
behaviour exhibited by the hedgehogs tested. At the beginning of each test, the hedgehog was placed
in an “upright position with snout pointing inwards”.
We found that age had an effect on the behavioural responses of the hedgehogs
(CLMM: Z =
3.71, p= 0.0002), with younger and, thereby, lighter individuals showing
a bolder behavioural response to the approaching robotic lawn mower compared to the
older and heavier individuals (Figure 5). In addition, the behavioural responses were also
affected by the test number (CLMM: Z = 2.32, p= 0.021), with hedgehogs reacting in a
shyer manner during the second of the two encounter tests with the robotic lawn mower,
regardless of whether the lights were turned on or off during the first test (Figure 6). The
Animals 2024,14, 2 10 of 19
other fixed factors (lights on/off, personality, and sex) did not reach a low p-value (CLMM:
p0.54 for all).
Animals 2023, 13, x FOR PEER REVIEW 10 of 19
Figure 4. An overview of the results from the encounter tests with a robotic lawn mower. “Behav-
iour” describes the behaviour exhibited by the hedgehogs.Category scores” indicate the degree of
shyness or boldness of the behaviour, with 1 being the boldest and 5 being the shyest. “Distribution”
denotes the frequency of the specic behaviours, in percentage, out of 100 tests conducted with a
total of 50 dierent hedgehogs. The line with arrows indicates the gradient of shyness or boldness
of the behaviour exhibited by the hedgehogs tested. At the beginning of each test, the hedgehog was
placed in an “upright position with snout pointing inwards”.
Figure 5. Behavioural responses as a function of the age category of the individuals during the en-
counter test with a robotic lawn mower (CLMM: p = 0.0002). “Response” shows behavioural
Behaviour
Upright position with sn o u t
fa c in g 6 o clock (rum p fa cin g
th e m o w e r)
Upright position
w it h sn o u t f ac in g
2-3 o’clock
Upright position with
sn ou t fa cin g 9 -1 0 o’clock
Upright position
with sn o u t fa cin g
th e mower
Upright position with
sn o u t p o inting in w a rd s Running aw ay C urling up
Category
scores
1 1 1 2 3 4 5
Distribution
(out of 100)
6% 11% 4% 20% 43% 15% 1%
Bold
S h y
Figure 5. Behavioural responses as a function of the age category of the individuals during the
encounter test with a robotic lawn mower (CLMM: p= 0.0002). “Response” shows behavioural
categories 1–5, with 1 being the most inquisitive (i.e., bold) and 5 being the most timid (i.e., shy)
reactions. Violin and box plots: the horizontal line shows the median, with the box extending from
the lower to the upper quartiles and the whisker to the data extremes.
Figure 6. Behavioural responses of the individuals during the first and the second encounter tests
with a robotic lawn mower (CLMM: p= 0.021). “Response” indicates the behavioural categories
1–5, with 1 being the boldest and 5 being the shyest. Violin and box plots: the horizontal line shows
the median, with the box extending from the lower to the upper quartiles and the whiskers to the
data extremes. The dashed lines indicate repeated measures (i.e., connecting responses by the same
individual between tests 1 and 2).
Animals 2024,14, 2 11 of 19
The random effect “individual” did not significantly affect the behavioural responses,
indicating an absence of clear differences among subjects (CLMM:
χ2
= 0.50, df = 1, p= 0.48;
Figure 6).
The behavioural instability of the ordinal behavioural category estimated with the
MAD showed a significantly higher variability in the individuals belonging to the bold
category compared to the individuals belonging to the shy category (MAD bold > MAD shy,
Mann–Whitney U-test; H = 5.26; p= 0.022 Levene’s test: p< 0.0029).
4. Discussion
Our research showed that the 50 hedgehogs tested could be categorised as shy and
bold, regardless of sex and age. Personality was not found to influence their reaction
towards an approaching robotic lawn mower. However, we observed a higher level of
behavioural instability exhibited by the bold individuals. During the encounter tests, seven
different behavioural reactions were observed in the hedgehogs. There was a tendency for
adults and, therefore, more experienced individuals to behave in a more eluding manner
when facing an approaching robotic lawn mower. The hedgehogs, in general, acted less
boldly during their second encounter test. This could be important from a conservation
and welfare perspective if the experience gained from an encounter with a robotic lawn
mower could reduce the risk of this individual coming into contact with such a potential
risk in the future.
4.1. Distribution of Personality and Behavioural Instability
We found that 54% (N = 27/50) of the individuals could be categorised as shy, as
they did not exit their nest box during the whole test, whilst the rest (46%; N = 23/50)
could be categorised as bold. Neither sex nor weight (and, thereby, age) had an effect on
personality. This is in line with the suggestion that individuals in a given population can
be divided into a shy–bold continuum across age classes [
48
]. The findings of a balanced
distribution of shy and bold individuals in the test group are also supported by the current
lack of evidence for the pace-of-life syndrome (POLS) in populations of hedgehogs. The
POLS predicts that behavioural traits such as high boldness, exploration, aggressiveness, or
activity increase the acquisition of resources at the expense of life span, causing individuals
expressing these traits to exhibit a faster life history, i.e., a higher growth rate compared to
other conspecifics [45,6365].
However, the higher levels of behavioural instability in the bold individuals could
have some relevant implications, as a higher variability in the behavioural repertoire
could enhance the probability of survival when an individual encounters a dangerous
situation, as proposed by Pertoldi et al. (2016) [
55
], which suggests that heightened
behavioural instability could possess adaptive value in an unpredictable environment. If a
higher variability in the behavioural repertoire can increase the probability of surviving an
encounter with a robotic lawn mower, then bold individuals could be favoured.
4.2. Insight Gained from the Encounter Tests with a Robotic Lawn Mower
After the 100 encounter tests performed with 50 live hedgehogs exposed to an ap-
proaching robotic lawn mower, it became evident that the reactions of the hedgehogs
could be divided into seven different behavioural categories, resulting in six different test
positions (excluding the behaviour of running away). These positions adopted by the
hedgehogs in reaction to the robotic lawn mower will be applied in a test design with the
purpose of describing the safety and effects of particular models of robotic lawn mowers
on hedgehogs, using both dead hedgehogs and hedgehog crash test dummies to provide
realistic test scenarios for the standardised safety tests.
Interestingly, we found that the tested individuals tended to behave in a more timid
manner during the second trial. This could indicate that the hedgehogs adjusted their
behaviour towards the robotic lawn mower based on their experience, which might imply
that they learned to avoid future encounters with the robotic lawn mower after a single
Animals 2024,14, 2 12 of 19
undramatic episode. If this is, indeed, the case in real life, this could potentially prove very
beneficial to the survival of hedgehogs, provided that they are not critically harmed in an
encounter with a robotic lawn mower. In testing for differences in the behavioural responses
among the individuals, in general, no significant effects were found. This suggests that
there were no marked differences in the responses among the individuals and/or that
each individual might have reacted differently in the two encounter tests. This is in line
with the findings that the hedgehogs reacted in a manner that was more timid during the
second of the two encounter tests with the robotic lawn mower. Similar results were found,
for example, in rainbow trout, whereby boldness was reduced as an adaptive response to
negative experiences in previous tests [47].
Older individuals exhibited shyer behaviour in the encounter tests with the robotic lawn
mower. We suggest that this could be due to experience and that the independent juveniles
we tested were, in general, more naïve because of their young age and lack of experience.
The independent juveniles we tested (N = 35/50 individuals in total) had been hand raised in
captivity. They, hence, likely had never encountered a robotic lawn mower before. Unfortunately,
we have no knowledge of whether the adults (N = 15/50) included in the tests had had any
previous confrontations with robotic lawn mowers in the wild before they were admitted
into care. This finding could suggest that there should be a focus on the size/age category of
independent juveniles in the experimental design of future hedgehog safety tests in case they
are more likely to come into physical contact with robotic lawn mowers.
4.3. Potential Biases in the Test Designs
When working with live animals, especially wildlife, it is often challenging to obtain
a balanced experimental test design, as the distribution of, for example, the sex and age
of the individuals being tested depend on factors such as the season, as well as chance.
To prioritise animal welfare and minimise the stress caused to the hedgehogs included in
the tests, by eliminating factors such as capture, transport, and handling by strangers, all
of the tests took place at a single wildlife rehabilitation centre, where the hedgehogs had
been in care and were considered ready for release back into the wild. We conducted the
experiments on three occasions, limiting the availability and selection of individuals for the
study, ultimately, resulting in a somewhat unbalanced composition of the 50 hedgehogs
tested (15 adults and 35 independent juveniles; 30 females and 20 males). This should be
taken into consideration when interpreting the results of our tests.
As the novel object was covered in fabric and had permanent electronic parts dis-
tributed throughout, the plush toy was nonwashable. It was, therefore, decided that it
could not be cleaned between the tests. This might have caused the odours of hedgehogs
previously exposed to and exploring the novel object to accumulate, potentially rendering
the novel object more stimulating for the last individuals tested, as a few cases of self-
anointing were observed in the vicinity of the novel object [
42
]. The same applied to the
cardstock paper covering the floor of the novel arena, as it remained the same throughout
the tests. However, no hedgehogs defecated on the floor, and any nesting material or other
foreign objects were removed in between the tests. As we decided to exclusively base
our categorisation of shyness–boldness on the distribution of the individuals entering the
arena or staying in the nest box for the duration of the 15 min test, we assumed that these
factors (i.e., the novel object and the floor cover being more olfactory stimulating for the
last individuals tested) likely did not influence the outcome of the tests.
During the encounter tests with the robotic lawn mower, it was also not possible to clean
the designated spot where each individual was placed, which potentially caused the area to
become more olfactorily stimulating for the last individuals tested. This could, in principle,
have influenced the behaviour of the hedgehogs tested. However, we did not notice any
excessive sniffing activity from the hedgehogs directed at the ground where the individuals
were placed. The hedgehogs, when deciding to expose their noses, away from the original
position with their heads folded inwards, investigated their surroundings by smelling the air
with the noses pointing upwards and not pointed in the direction of the ground.
Animals 2024,14, 2 13 of 19
The wildlife rehabilitation centre and its surrounding garden, where the tests took
place, was situated in a residential area near Aarhus in Denmark. During the tests we
observed variability in the sensory stimuli coming from the surroundings. Some nights had
heavy rainfall, whilst other nights had a range of different anthropogenic noise disturbances.
These factors were beyond our control but, nevertheless, constituted a realistic scenario for
hedgehogs inhabiting suburban residential areas. To the best of our abilities, we tried to
adjust the timing of the tests to reduce the influence of these disturbances on our results.
It has been suggested that confidently determining personality in individuals requires
repeated measures to determine whether these personality traits remain consistent dur-
ing different test scenarios [
66
]. As we prioritised an experimental design to reduce the
duration of the tests and handling of the hedgehogs to reduce stress [
45
] and allowing
for the hedgehogs to be released into the wild as fast as possible, it was not possible to
accommodate more repeated measures in our test design. It would be relevant to consider
a method to increase the repeated measures of personality extending beyond the combined
novel object and novel arena test and the encounter tests with a robotic lawn mower.
Previous traumatic experiences may have influenced the reactions of the individual
hedgehogs during the tests. However, the lack of knowledge about the life history of the
hedgehogs prior to admission to the wildlife rehabilitation centre prevents any meaningful
analysis of the influence of different traumas on the outcomes of the personality and
encounter tests.
4.4. Next Steps
Knowing that certain models of robotic lawn mowers may cause injuries or even kill
hedgehogs, understanding the extent of the problem is critical for hedgehog conservation
and welfare. Therefore, we encourage the establishment of an open access international
hedgehog database that can function as a daily record-keeping system for hedgehog carers,
gathering and storing vital information on the hedgehogs coming into care. Additionally,
collecting photographic evidence of hedgehogs injured or killed by electronic garden tools,
such as trimmers and robotic lawn mowers, in order to quantify, document, and describe
the types of damage caused by these machines is important (Berger et al. in prep.).
On the basis of the present behavioural study, through the knowledge gained on how
live hedgehogs position themselves when confronted by an approaching robotic lawn
mower, it is now possible to prepare a realistic framework for a standardised safety test
to measure the impacts of robotic lawn mowers on hedgehogs. Furthermore, complete
standardisation requires the use of hedgehog crash test dummies, which can be 3D printed
and applied by manufacturers of robotic lawn mowers in the process of designing and
testing prototypes of new and more hedgehog-friendly machines. Ultimately, hedgehog
crash test dummies should be used as proxies for dead hedgehogs in the standardised
safety test and be designed to mimic reality and yield the same results compared to tests
on dead hedgehogs.
The final step is to have the standardised hedgehog safety test implemented in the
CENELEC protocol [
41
], testing and approving robotic lawn mowers for sale on the Euro-
pean market. This test would, furthermore, allow for a labelling system for hedgehog-safe
robotic lawn mowers to be established, guiding the consumers to make the hedgehog-
friendly choice when purchasing these tools.
Work is currently well underway to achieve these described goals. But, for now, our
advice remains to restrict the running of robotic lawn mowers to daylight hours and to
check lawns for any wildlife species that may be vulnerable to an encounter with a robotic
lawn mower, before turning on the machine.
5. Conclusions
The robotic lawn mower market is growing rapidly, and, consequently, it is essential
to help inform manufacturers on how to design more hedgehog-friendly machines in the
future if we wish to eliminate this potentially negative influence on hedgehog survival in
Animals 2024,14, 2 14 of 19
our backyards. This study sought to explore the personality and reactions of live hedgehogs
when facing a disarmed, approaching robotic lawn mower to inform future standardised
hedgehog safety tests allowing manufacturers to evaluate the performance of the new
models being designed and developed at their facilities.
In testing the reactions of hedgehogs towards an approaching (disarmed) robotic lawn
mower, we conclude that that personality did not appear to affect the outcome; that adult
and, thereby, more experienced hedgehogs tended to react in a more timid manner; and
that the hedgehogs generally acted less boldly during their second encounter with the
robotic lawn mower.
The important insights gained from this study will be applied in the process of testing
and refining the design of a hedgehog crash test dummy to be used in a future, standardised
hedgehog safety test, which will be informed by the present results. This test will eventually
serve to produce and approve hedgehog-friendly robotic lawn mowers and, ultimately,
improve hedgehog conservation.
Author Contributions: Conceptualisation, S.L.R., C.P., D.W.M. and A.K.O.A.; methodology, S.L.R.,
C.P., E.F.B. and A.K.O.A.; validation, S.L.R., A.B., C.P., D.W.M. and A.K.O.A.; formal analysis, B.T.S.,
S.L.R. and E.F.B.; investigation, S.L.R., B.T.S., A.B., C.P., D.W.M. and A.K.O.A.; resources, S.L.R.; data
curation, B.T.S., S.L.R. and E.F.B.; writing—original draft preparation, S.L.R.; writing—review and
editing, S.L.R., D.W.M., B.T.S., A.B., C.P., E.F.B. and A.K.O.A.; visualisation, B.T.S., S.L.R. and E.F.B.;
supervision, C.P., E.F.B., D.W.M. and A.K.O.A.; project administration, S.L.R.; funding acquisition,
S.L.R. and D.W.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Husqvarna; STIHL; British Hedgehog Preservation Society;
Carlsberg Foundation, grant number: CF20_0443.
Institutional Review Board Statement: The animal study protocol was approved by the Animal
Experiments Inspectorate under the Danish National Committee for the Protection of Animals used
for Scientific Purposes (2021-15-0201-00865, dates of approval: 20th of September 2021 and 29th of
June 2022).
Informed Consent Statement: Not applicable.
Data Availability Statement: All relevant data from this research have been made available in
the publication.
Acknowledgments: The authors are grateful to hedgehog rehabilitator Dorthe Madsen from Animal
Protection Denmark (Dyrenes Beskyttelse) for hosting the tests and caring for the hedgehogs that
participated in the research project.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the analyses, or interpretation of data; in the writing of the manuscript; or in the
decision to publish the results.
Appendix A
Table A1. Results from the encounter tests with a disarmed robotic lawn mower. “Personality” refers
to the categories shy = 0 and bold = 1. Test number refers to the order of the two tests on each
individual. Light describes whether the headlights of the robotic lawn mower were turned on or
off during the specific test. Response denotes the behavioural response category of the individual,
as defined in Section 2, “Materials and Methods”, with category 1 being the boldest and category 5
being the shyest.
Individual
Personality
Age Sex Weight (Gram) Date Time Test Number Lights
Response
Hedgehog 1 1 Adult Female 1154 03.06.2022 23:17 Two Off 3
Hedgehog 1 1 Adult Female 1154 03.06.2022 23:15 One On 4
Animals 2024,14, 2 15 of 19
Table A1. Cont.
Individual
Personality
Age Sex Weight (Gram) Date Time Test Number Lights
Response
Hedgehog 2 1 Adult Female 1071 04.06.2022 00:00 One Off 3
Hedgehog 2 1 Adult Female 1071 04.06.2022 00:02 Two On 3
Hedgehog 3 0 Adult Male 774 04.06.2022 01:07 One On 4
Hedgehog 3 0 Adult Male 774 04.06.2022 01:10 Two Off 4
Hedgehog 4 0 Juvenile Female 497 04.06.2022 01:33 One Off 3
Hedgehog 4 0 Juvenile Female 497 04.06.2022 01:35 Two On 3
Hedgehog 5 0 Adult Female 996 04.06.2022 02:09 One On 4
Hedgehog 5 0 Adult Female 996 04.06.2022 02:13 Two Off 4
Hedgehog 6 0 Adult Female 924 04.06.2022 23:05 One Off 4
Hedgehog 6 0 Adult Female 924 04.06.2022 23:07 Two On 4
Hedgehog 7 1 Adult Female 1137 04.06.2022 23:40 One On 3
Hedgehog 7 1 Adult Female 1137 04.06.2022 23:42 Two Off 4
Hedgehog 10 0 Adult Female 772 05.06.2022 01:32 Two On 1
Hedgehog 10 0 Adult Female 772 05.06.2022 01:30 One Off 3
Hedgehog 8 0 Adult Female 645 05.06.2022 00:17 One Off 3
Hedgehog 8 0 Adult Female 645 05.06.2022 00:19 Two On 3
Hedgehog 9 1 Adult Female 699 05.06.2022 00:55 One On 3
Hedgehog 9 1 Adult Female 699 05.06.2022 00:56 Two Off 3
Hedgehog 11 0 Adult Male 1225 06.09.2022 21:03 Two Off 3
Hedgehog 11 0 Adult Male 1225 06.09.2022 21:01 One On 4
Hedgehog 12 0 Adult Male 1361 06.09.2022 21:37 One Off 3
Hedgehog 12 0 Adult Male 1361 06.09.2022 21:38 Two On 3
Hedgehog 13 1 Juvenile Female 361 06.09.2022 22:02 One On 1
Hedgehog 13 1 Juvenile Female 361 06.09.2022 22:06 Two Off 3
Hedgehog 14 1 Juvenile Male 381 06.09.2022 22:57 Two On 2
Hedgehog 14 1 Juvenile Male 381 06.09.2022 22:56 One Off 3
Hedgehog 15 1 Juvenile Male 328 06.09.2022 23:23 One On 1
Hedgehog 15 1 Juvenile Male 328 06.09.2022 23:27 Two Off 2
Hedgehog 16 1 Juvenile Female 361 06.09.2022 23:57 One Off 1
Hedgehog 16 1 Juvenile Female 361 06.09.2022 23:58 Two On 1
Hedgehog 17 1 Juvenile Female 328 07.09.2022 00:27 One On 3
Hedgehog 17 1 Juvenile Female 328 07.09.2022 00:30 Two Off 3
Hedgehog 18 1 Juvenile Male 216 07.09.2022 00:58 One Off 3
Hedgehog 18 1 Juvenile Male 216 07.09.2022 01:02 Two On 3
Hedgehog 19 1 Juvenile Female 321 07.09.2022 01:35 One On 2
Hedgehog 19 1 Juvenile Female 321 07.09.2022 01:35 Two Off 2
Hedgehog 20 1 Juvenile Female 264 07.09.2022 02:05 One Off 1
Hedgehog 20 1 Juvenile Female 264 07.09.2022 02:06 Two On 4
Hedgehog 21 0 Juvenile Female 318 07.09.2022 02:34 One On 2
Hedgehog 21 0 Juvenile Female 318 07.09.2022 02:36 Two Off 3
Hedgehog 22 1 Juvenile Female 343 07.09.2022 21:07 One On 2
Hedgehog 22 1 Juvenile Female 343 07.09.2022 21:10 Two Off 2
Hedgehog 23 0 Juvenile Male 383 07.09.2022 21:39 One Off 1
Hedgehog 23 0 Juvenile Male 383 07.09.2022 21:41 Two On 3
Animals 2024,14, 2 16 of 19
Table A1. Cont.
Individual
Personality
Age Sex Weight (Gram) Date Time Test Number Lights
Response
Hedgehog 24 0 Juvenile Male 398 07.09.2022 22:10 One On 1
Hedgehog 24 0 Juvenile Male 398 07.09.2022 22:11 Two Off 1
Hedgehog 25 1 Juvenile Female 308 07.09.2022 22:45 One Off 1
Hedgehog 25 1 Juvenile Female 308 07.09.2022 22:45 Two On 4
Hedgehog 26 1 Juvenile Male 387 07.09.2022 23:11 One On 3
Hedgehog 26 1 Juvenile Male 387 07.09.2022 23:13 Two Off 3
Hedgehog 27 1 Juvenile Female 383 07.09.2022 23:43 One Off 1
Hedgehog 27 1 Juvenile Female 383 07.09.2022 23:43 Two On 4
Hedgehog 28 1 Juvenile Female 231 08.09.2022 00:11 One On 1
Hedgehog 28 1 Juvenile Female 231 08.09.2022 00:13 Two Off 2
Hedgehog 29 1 Juvenile Male 221 08.09.2022 00:41 One Off 1
Hedgehog 29 1 Juvenile Male 221 08.09.2022 00:42 Two On 1
Hedgehog 30 1 Juvenile Male 196 08.09.2022 01:11 One On 3
Hedgehog 30 1 Juvenile Male 196 08.09.2022 01:12 Two Off 3
Hedgehog 31 0 Adult Male 1301 15.09.2022 20:34 One On 2
Hedgehog 31 0 Adult Male 1301 15.09.2022 20:35 Two Off 4
Hedgehog 32 0 Juvenile Female 230 15.09.2022 21:06 Two On 1
Hedgehog 32 0 Juvenile Female 230 15.09.2022 21:05 One Off 2
Hedgehog 33 0 Adult Female 718 15.09.2022 21:34 One On 1
Hedgehog 33 0 Adult Female 718 15.09.2022 21:36 Two Off 2
Hedgehog 34 1 Juvenile Female 322 15.09.2022 22:05 One Off 2
Hedgehog 34 1 Juvenile Female 322 15.09.2022 22:07 Two On 2
Hedgehog 35 1 Juvenile Male 300 15.09.2022 22:35 One On 1
Hedgehog 35 1 Juvenile Male 300 15.09.2022 22:36 Two Off 2
Hedgehog 36 1 Juvenile Male 316 15.09.2022 23:05 One Off 1
Hedgehog 36 1 Juvenile Male 316 15.09.2022 23:07 Two On 2
Hedgehog 37 0 Juvenile Female 298 15.09.2022 23:37 Two Off 1
Hedgehog 37 0 Juvenile Female 298 15.09.2022 23:35 One On 2
Hedgehog 38 0 Adult Female 668 16.09.2022 00:05 One Off 3
Hedgehog 38 0 Adult Female 668 16.09.2022 00:07 Two On 3
Hedgehog 39 1 Adult Female 1529 16.09.2022 00:35 One On 4
Hedgehog 39 1 Adult Female 1529 16.09.2022 00:36 Two Off 4
Hedgehog 40 0 Juvenile Female 318 16.09.2022 01:06 One Off 2
Hedgehog 40 0 Juvenile Female 318 16.09.2022 01:07 Two On 3
Hedgehog 41 0 Juvenile Male 236 16.09.2022 01:35 One On 2
Hedgehog 41 0 Juvenile Male 236 16.09.2022 01:36 Two Off 2
Hedgehog 42 0 Juvenile Male 213 16.09.2022 20:36 One Off 3
Hedgehog 42 0 Juvenile Male 213 16.09.2022 20:37 Two On 3
Hedgehog 43 0 Juvenile Male 478 16.09.2022 21:01 One Off 1
Hedgehog 43 0 Juvenile Male 478 16.09.2022 21:02 Two On 2
Hedgehog 44 0 Juvenile Male 226 16.09.2022 21:32 One Off 3
Hedgehog 44 0 Juvenile Male 226 16.09.2022 21:33 Two On 5
Hedgehog 45 0 Juvenile Female 326 16.09.2022 22:02 One On 3
Animals 2024,14, 2 17 of 19
Table A1. Cont.
Individual
Personality
Age Sex Weight (Gram) Date Time Test Number Lights
Response
Hedgehog 45 0 Juvenile Female 326 16.09.2022 22:04 Two Off 3
Hedgehog 46 0 Juvenile Male 413 16.09.2022 22:32 One Off 3
Hedgehog 46 0 Juvenile Male 413 16.09.2022 22:33 Two On 3
Hedgehog 47 0 Juvenile Female 302 16.09.2022 23:02 One On 3
Hedgehog 47 0 Juvenile Female 302 16.09.2022 23:03 Two Off 3
Hedgehog 48 0 Juvenile Female 295 16.09.2022 23:33 One Off 3
Hedgehog 48 0 Juvenile Female 295 16.09.2022 23:35 Two On 3
Hedgehog 49 0 Juvenile Female 300 17.09.2022 00:03 One On 1
Hedgehog 49 0 Juvenile Female 300 17.09.2022 00:04 Two Off 3
Hedgehog 50 0 Juvenile Male 168 17.09.2022 00:33 One Off 3
Hedgehog 50 0 Juvenile Male 168 17.09.2022 00:34 Two On 3
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... Unfortunately, sharing habitats with humans comes at a cost, as residential gardens provide many anthropogenic sources of danger to hedgehog survival. One of these potentially harmful features is certain models of robotic lawn mowers [26,35,36]. With robotic lawn mowers becoming increasingly popular throughout the distribution range of hedgehogs in Europe, there is a high likelihood for a hedgehog to encounter numerous robotic lawn mowers throughout its lifespan. ...
... six different positions ( Figure 1) in an attempt to mimic the behaviour of a live individual. The most commonly recorded position during the tests on live hedgehogs was "upright position with snout pointing inwards" (43%) [36] which could not be properly mimicked with a dead hedgehog as the head would not bend inwards and stay in place, leaving us to combine this with the second most frequently recorded behaviour (20%), test position 3: The tests were recorded with two Ring Stick Up Cam ® (Ring TM , Santa Monica, CA, USA) cameras placed on tripods. ...
... Based on the results reported by Rasmussen et al. (2024) [36] testing the behaviour of live hedgehogs facing a disarmed, robotic lawn mower, each individual was tested in six different positions ( Figure 1) in an attempt to mimic the behaviour of a live individual. The most commonly recorded position during the tests on live hedgehogs was "upright position with snout pointing inwards" (43%) [36] which could not be properly mimicked with a dead hedgehog as the head would not bend inwards and stay in place, leaving us to combine this with the second most frequently recorded behaviour (20%), test position 3: ...
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