Wildlife Conservation at a Garden Level: The Effect of Robotic Lawn Mowers on European Hedgehogs (Erinaceus europaeus)

Abstract

Simple Summary
Injured European hedgehogs are frequently admitted to hedgehog rehabilitation centres with different types of cuts and injuries. Although not rigorously quantified, a growing concern is that an increasing number of cases may have been caused by robotic lawn mowers. Research indicates that European hedgehogs are in decline. It is therefore important to identify and investigate the factors responsible for this decline to improve the conservation initiatives directed at this species. Because hedgehogs are increasingly associated with human habitation, it seems likely that numerous individuals will encounter several robotic lawn mowers during their lifetimes. Consequently, this study aimed to describe and quantify the effects of robotic lawn mowers on hedgehogs, and we tested 18 robotic lawn mowers in collision with dead hedgehogs. Some models caused extensive damage to the dead hedgehogs, but there were noteworthy differences in the degree of harm inflicted, with some consistently causing no damage. None of the robotic lawn mowers tested was able to detect the presence of dead, dependent juvenile hedgehogs, and no models could detect the hedgehog cadavers without physical interaction. We therefore encourage future collaboration with the manufacturers of robotic lawn mowers to improve the safety for hedgehogs and other garden wildlife species.

Abstract
We tested the effects of 18 models of robotic lawn mowers in collision with dead European hedgehogs and quantified the results into six damage categories. All models were tested on four weight classes of hedgehogs, each placed in three different positions. None of the robotic lawn mowers tested was able to detect the presence of dependent juvenile hedgehogs (<200 g) and all models had to touch the hedgehogs to detect them. Some models caused extensive damage to the hedgehog cadavers, but there were noteworthy differences in the degree of harm inflicted, with some consistently causing no damage. Our results showed that the following technical features significantly increased the safety index of the robotic lawn mowers: pivoting blades, skid plates, and front wheel drive. Based on these findings, we encourage future collaboration with the manufacturers of robotic lawn mowers to improve the safety for hedgehogs and other garden wildlife species.

Full text

animals
Article
Wildlife Conservation at a Garden Level: The Effect of Robotic
Lawn Mowers on European Hedgehogs (Erinaceus europaeus)
Sophie Lund Rasmussen 1, 2, * , Ane Elise Schrøder 3,4 , Ronja Mathiesen 5, Jeppe Lund Nielsen 2,
Cino Pertoldi 2and David W. Macdonald 1


Citation: Rasmussen, S.L.; Schrøder,
A.E.; Mathiesen, R.; Nielsen, J.L.;
Pertoldi, C.; Macdonald, D.W.
Wildlife Conservation at a Garden
Level: The Effect of Robotic Lawn
Mowers on European Hedgehogs
(Erinaceus europaeus). Animals 2021,11,
1191. https://doi.org/10.3390/
ani11051191
Academic Editors: Krishna
N. Balasubramaniam and Stefano
S.K. Kaburu
Received: 5 March 2021
Accepted: 16 April 2021
Published: 21 April 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
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iations.
Copyright: © 2021 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/).
1Wildlife Conservation Research Unit, The Recanati-Kaplan Centre, Department of Zoology,
University of Oxford, Tubney House, Abingdon Road, Tubney, Abingdon OX13 5QL, UK;
david.macdonald@zoo.ox.ac.uk
2Department of Chemistry and Bioscience, Aalborg University, Fredrik Bajers Vej 7H,
DK-9220 Aalborg, Denmark; jln@bio.aau.dk (J.L.N.); cp@bio.aau.dk (C.P.)
3Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15,
DK-2100 Copenhagen Ø, Denmark; aneelises@snm.ku.dk
4Fossil and Moclay Museum, Museum Mors, Skarrehagevej 8, DK-7900 Nykøbing Mors, Denmark
5Agilent Technologies Denmark ApS, Produktionsvej 42, DK-2600 Glostrup, Denmark;
ronjamathiesen@gmail.com
*Correspondence: sophie.rasmussen@zoo.ox.ac.uk or sophielundrasmussen@gmail.com;
Tel.: +45-2211-7268 or +44-787-1837-510
Simple Summary:
Injured European hedgehogs are frequently admitted to hedgehog rehabilitation
centres with different types of cuts and injuries. Although not rigorously quantified, a growing
concern is that an increasing number of cases may have been caused by robotic lawn mowers.
Research indicates that European hedgehogs are in decline. It is therefore important to identify and
investigate the factors responsible for this decline to improve the conservation initiatives directed
at this species. Because hedgehogs are increasingly associated with human habitation, it seems
likely that numerous individuals will encounter several robotic lawn mowers during their lifetimes.
Consequently, this study aimed to describe and quantify the effects of robotic lawn mowers on
hedgehogs, and we tested 18 robotic lawn mowers in collision with dead hedgehogs. Some models
caused extensive damage to the dead hedgehogs, but there were noteworthy differences in the degree
of harm inflicted, with some consistently causing no damage. None of the robotic lawn mowers tested
was able to detect the presence of dead, dependent juvenile hedgehogs, and no models could detect
the hedgehog cadavers without physical interaction. We therefore encourage future collaboration
with the manufacturers of robotic lawn mowers to improve the safety for hedgehogs and other
garden wildlife species.
Abstract:
We tested the effects of 18 models of robotic lawn mowers in collision with dead European
hedgehogs and quantified the results into six damage categories. All models were tested on four
weight classes of hedgehogs, each placed in three different positions. None of the robotic lawn
mowers tested was able to detect the presence of dependent juvenile hedgehogs (<200 g) and all
models had to touch the hedgehogs to detect them. Some models caused extensive damage to the
hedgehog cadavers, but there were noteworthy differences in the degree of harm inflicted, with
some consistently causing no damage. Our results showed that the following technical features
significantly increased the safety index of the robotic lawn mowers: pivoting blades, skid plates, and
front wheel drive. Based on these findings, we encourage future collaboration with the manufacturers
of robotic lawn mowers to improve the safety for hedgehogs and other garden wildlife species.
Keywords:
animal behaviour; applied conservation biology; Erinaceus europaeus; human–wildlife
conflicts; robotic lawn mowers; wildlife conservation
Animals 2021,11, 1191. https://doi.org/10.3390/ani11051191 https://www.mdpi.com/journal/animals

Animals 2021,11, 1191 2 of 13
1. Introduction
Research on both national and local scales has either documented, or expressed
concern about the likelihood of, a decline in European hedgehog (Erinaceus europaeus)
populations in several western European countries [
1
–
10
]. It is therefore a priority to
identify and investigate the factors responsible for this decline to provide the evidence
necessary to underpin remedial conservation interventions.
Injured hedgehogs are frequently admitted to hedgehog rehabilitation centres with
different types of cuts and injuries. Some injuries are consistent with known risks to
hedgehogs in the form of garden trimmers and dog bites [
11
–
13
]. However, although not
rigorously quantified, a concern has arisen in several European countries that an increasing
number of cases may have been caused by robotic lawn mowers. Although not previously
investigated, these growing rumours have led to several articles in the media and on social
media claiming that these mowers are lethal to hedgehogs. If the threat is real, then it
would indeed be a cause for concern, as the global market for robotic lawn mowers is
expanding dramatically and was expected to reach USD 1.3 billion in 2020, growing at an
annual rate of more than 12 percent during the period 2019–2025 [14].
As research indicates that European hedgehogs are increasingly associated with
human habitation [
7
,
8
,
15
–
17
] and are often seen foraging on grassy turf in the gardens
and green spaces of urban areas [
18
–
22
], it seems likely that numerous individuals will
encounter several robotic lawn mowers during their lifetimes. To our knowledge, there
has thus far been no systematic scientific research evaluating whether this risk of physical
damage is mere hearsay or a real and present threat to be added to the already vulnerable
species. Therefore, the aims of this study are to describe and quantify the physical effects
of robotic lawn mowers on hedgehogs and provide information on potential technical
features of the machines that could increase the safety index of the robotic lawn mowers.
The main purpose of providing this information is to improve the conservation of European
hedgehogs living in residential areas by reducing the plausible negative anthropogenic
effects potentially caused by robotic lawn mowers.
2. Materials and Methods
In total, 18 designs of robotic lawn mowers were selected for the study. The selection
was based on the advice of a product specialist in robotic lawn mowers and is considered
to represent the spectrum of brands, models, and specifications of the products available
on the European market (Table 1). The cutting height of the machines was adjusted to the
highest setting to keep the grass at the test site intact to ensure equal test conditions for
all trials.

Animals 2021,11, 1191 3 of 13
Table 1.
Overview of the models of robotic lawn mowers tested. In the column “Blades”, Pivoting indicates “low energy
pivoting blades” and Fixed indicates “heavy duty fixed blades”. WMCC detection is short for “wheel motor current collision
detection”.
Test
Number Brand Model Blades Collision
Sensor
WMCC
Detection Wheels Front/Rear
Wheel Drive
Skid
Plate Headlights Ultrasonic
Sensors
Camera
Vision
1 Husqvarna Automower®105 Pivoting Yes 3 Front Yes
2 Husqvarna Automower®305 Pivoting Yes 4 Rear Yes
3 Husqvarna Automower®315X Pivoting Yes 4 Rear Yes Yes
4 Husqvarna Automower®450X Pivoting Yes 4 Rear Yes Yes Yes
5Gardena Sileno City Pivoting Yes 3 Front
6Gardena Sileno Life Pivoting Yes 4 Front
7 Worx Landroid L (WR153E) Pivoting Yes 4 Rear
8 Worx Landroid M (WR143E) Pivoting Yes 4 Rear Yes
9 Kress Mission KR111 Pivoting Yes 4 Rear Yes
10 LandXcape LX8212i Pivoting Yes 3 Rear Yes
11 Honda Miimo HRM 40 Live Pivoting Yes 4 Rear
12 Honda Miimo HRM 3000 Pivoting Yes 4 Rear
13 Robomow RS635 PRO Fixed Yes 3 Rear Yes
14 AL-KO Robolinho®1150 Fixed Yes 4 Rear
15 Ambrogio
Robot 4.0 Elite Fixed Yes 4 Rear
16 Stiga Autoclip 530 SG Fixed Yes 4 Rear
17 Stihl iMow®422PC Fixed Yes 4 Rear
18 DAYE Grouw M900 Pivoting Yes 4 Rear
Of the 18 robotic lawn mowers tested, 5 had fixed blades (Figure 1A) and 13 had
pivoting blades (Figure 1B).
Animals 2021, 11, x 3 of 13
Table 1. Overview of the models of robotic lawn mowers tested. In the column “Blades”, Pivoting indicates “low energy
pivoting blades” and Fixed indicates “heavy duty fixed blades”. WMCC detection is short for “wheel motor current col-
lision detection”.
Test
Number
Brand Model Blades Collision
Sensor
WMCC
Detection Wheels Front/Rear
Wheel Drive
Skid
Plate
Head-
lights
Ultrasonic
Sensors
Camera
Vision
1 Husqvarna Automower® 105
Pivoting Yes 3 Front Yes
2 Husqvarna Automower® 305
Pivoting Yes 4 Rear Yes
3 Husqvarna Automower® 315X
Pivoting Yes 4 Rear Yes Yes
4 Husqvarna Automower® 450X
Pivoting Yes 4 Rear Yes Yes Yes
5 Gardena Sileno City Pivoting Yes 3 Front
6 Gardena Sileno Life Pivoting Yes 4 Front
7 Worx
Landroid L
(WR153E) Pivoting Yes 4 Rear
8 Worx Landroid M
(WR143E) Pivoting Yes 4 Rear Yes
9 Kress Mission KR111 Pivoting Yes 4 Rear Yes
10 LandXcape LX8212i Pivoting Yes 3 Rear Yes
11 Honda Miimo HRM 40
Live Pivoting Yes 4 Rear
12 Honda Miimo HRM 3000
Pivoting Yes 4 Rear
13 Robomow RS635 PRO Fixed Yes 3 Rear Yes
14 AL-KO Robolinho® 1150 Fixed Yes 4 Rear
15 Ambrogio
Robot 4.0 Elite Fixed Yes 4 Rear
16 Stiga Autoclip 530 SG Fixed Yes 4 Rear
17 Stihl iMow® 422PC Fixed Yes 4 Rear
18 DAYE Grouw M900 Pivoting Yes 4 Rear
Of the 18 robotic lawn mowers tested, 5 had fixed blades (Figure 1A) and 13 had
pivoting blades (Figure 1B).
Figure 1. Fixed or pivoting blades. (A) A robotic lawn mower with fixed blades. (B) A robotic lawn
mower with pivoting blades. Pivoting blades will fold into a protective frame when hitting some-
thing harder than grass, as opposed to fixed blades, which are constantly exposed. Photographs by
Petrus Ekbladh and Ronja Mathiesen.
The robotic lawn mower tests were performed on dead hedgehogs, henceforth re-
ferred to as “hedgehogs”. These animals had died in, and were secured from, hedgehog
Figure 1.
Fixed or pivoting blades. (
A
) A robotic lawn mower with fixed blades. (
B
) A robotic
lawn mower with pivoting blades. Pivoting blades will fold into a protective frame when hitting
something harder than grass, as opposed to fixed blades, which are constantly exposed. Photographs
by Petrus Ekbladh and Ronja Mathiesen.
The robotic lawn mower tests were performed on dead hedgehogs, henceforth re-
ferred to as “hedgehogs”. These animals had died in, and were secured from, hedgehog
rehabilitation centres in Denmark from June to August 2020. All hedgehogs chosen for this

Animals 2021,11, 1191 4 of 13
study were intact with no visible injuries. The hedgehogs were stored in freezers at
−
20
◦
C
and were thawed before the tests. The 70 selected hedgehog cadavers were divided into
four different weight classes to represent four stages of life (Table 2).
Each robotic lawn mower model was tested on four hedgehogs representing each
of the four described weight classes. If an individual was injured by the mower during
a test, it would be discarded to avoid confusion or interaction with previous injuries in
subsequent tests (with one uncomplicated exception, where we reused a cadaver with
superficial injuries in weight class 4, due to a shortage of individuals in this category).
Table 2.
Weight classes. Graphical representation of the four weight classes of dead hedgehogs used in the study. The
pictures of live hedgehogs were provided to illustrate the sizes of individuals belonging to the four weight classes. No live
hedgehogs were tested in this study. The ruler on the pictures indicates length of the individuals in cm. Photographs by
Michela Dugar.
Weight
Class Weight (g) No. of
Individuals
Total No. of
Individuals per
Weight Class
Stages of
Life Representation
1 Up to 199 22 22 Dependent
juveniles
Animals 2021, 11, x 4 of 13
rehabilitation centres in Denmark from June to August 2020. All hedgehogs chosen for
this study were intact with no visible injuries. The hedgehogs were stored in freezers at
−20 °C and were thawed before the tests. The 70 selected hedgehog cadavers were di-
vided into four different weight classes to represent four stages of life (Table 2).
Each robotic lawn mower model was tested on four hedgehogs representing each of
the four described weight classes. If an individual was injured by the mower during a
test, it would be discarded to avoid confusion or interaction with previous injuries in
subsequent tests (with one uncomplicated exception, where we reused a cadaver with
superficial injuries in weight class 4, due to a shortage of individuals in this category).
Table 2. Weight classes. Graphical representation of the four weight classes of dead hedgehogs used in the study. The
pictures of live hedgehogs were provided to illustrate the sizes of individuals belonging to the four weight classes. No
live hedgehogs were tested in this study. The ruler on the pictures indicates length of the individuals in cm. Photographs
by Michela Dugar.
Weight
Class Weight (g)
No. of Individuals
Total No. of Individuals
per Weight Class Stages of Life
Representation
1 Up to 199 22 22 Dependent
juveniles
Weight 46 g
Length 7.5 cm
2
200 3
21 Independent
juveniles
Weight 530 g
Length 19.5 cm
300 3
400 9
500 6
3
600 8
20 Adults Weight 860 g
Length 23 cm
700 8
800 4
4
900 4
7 Large adults Weight 1080 g
Length 25 cm
1000 2
1100 1
Each individual was tested in three different positions (Figure 2), as an attempt to
mimic the behaviour of a live individual:
1. Lying on the side with the back pointing towards the approaching robotic lawn
mower, mimicking the curled up position a hedgehog often adopts as a defence
mechanism against approaching danger [19,23].
2. Lying on the side with the stomach pointing towards the approaching robotic lawn
mower (somewhat unnatural, but extremely vulnerable position).
3. Standing upright on its feet with the head pointing towards the approaching robotic
lawn mower (an expression of curiosity but not alarm).
To sum up, each robotic lawn mower was tested 12 times:
Weight 46 g
Length 7.5 cm
2
200 3
21 Independent
juveniles
Animals 2021, 11, x 4 of 13
rehabilitation centres in Denmark from June to August 2020. All hedgehogs chosen for
this study were intact with no visible injuries. The hedgehogs were stored in freezers at
−20 °C and were thawed before the tests. The 70 selected hedgehog cadavers were di-
vided into four different weight classes to represent four stages of life (Table 2).
Each robotic lawn mower model was tested on four hedgehogs representing each of
the four described weight classes. If an individual was injured by the mower during a
test, it would be discarded to avoid confusion or interaction with previous injuries in
subsequent tests (with one uncomplicated exception, where we reused a cadaver with
superficial injuries in weight class 4, due to a shortage of individuals in this category).
Table 2. Weight classes. Graphical representation of the four weight classes of dead hedgehogs used in the study. The
pictures of live hedgehogs were provided to illustrate the sizes of individuals belonging to the four weight classes. No
live hedgehogs were tested in this study. The ruler on the pictures indicates length of the individuals in cm. Photographs
by Michela Dugar.
Weight
Class Weight (g)
No. of Individuals
Total No. of Individuals
per Weight Class Stages of Life
Representation
1 Up to 199 22 22 Dependent
juveniles
Weight 46 g
Length 7.5 cm
2
200 3
21 Independent
juveniles
Weight 530 g
Length 19.5 cm
300 3
400 9
500 6
3
600 8
20 Adults Weight 860 g
Length 23 cm
700 8
800 4
4
900 4
7 Large adults Weight 1080 g
Length 25 cm
1000 2
1100 1
Each individual was tested in three different positions (Figure 2), as an attempt to
mimic the behaviour of a live individual:
1. Lying on the side with the back pointing towards the approaching robotic lawn
mower, mimicking the curled up position a hedgehog often adopts as a defence
mechanism against approaching danger [19,23].
2. Lying on the side with the stomach pointing towards the approaching robotic lawn
mower (somewhat unnatural, but extremely vulnerable position).
3. Standing upright on its feet with the head pointing towards the approaching robotic
lawn mower (an expression of curiosity but not alarm).
To sum up, each robotic lawn mower was tested 12 times:
Weight 530 g
Length 19.5 cm
300 3
400 9
500 6
3
600 8
20 Adults
Animals 2021, 11, x 4 of 13
rehabilitation centres in Denmark from June to August 2020. All hedgehogs chosen for
this study were intact with no visible injuries. The hedgehogs were stored in freezers at
−20 °C and were thawed before the tests. The 70 selected hedgehog cadavers were di-
vided into four different weight classes to represent four stages of life (Table 2).
Each robotic lawn mower model was tested on four hedgehogs representing each of
the four described weight classes. If an individual was injured by the mower during a
test, it would be discarded to avoid confusion or interaction with previous injuries in
subsequent tests (with one uncomplicated exception, where we reused a cadaver with
superficial injuries in weight class 4, due to a shortage of individuals in this category).
Table 2. Weight classes. Graphical representation of the four weight classes of dead hedgehogs used in the study. The
pictures of live hedgehogs were provided to illustrate the sizes of individuals belonging to the four weight classes. No
live hedgehogs were tested in this study. The ruler on the pictures indicates length of the individuals in cm. Photographs
by Michela Dugar.
Weight
Class Weight (g)
No. of Individuals
Total No. of Individuals
per Weight Class Stages of Life
Representation
1 Up to 199 22 22 Dependent
juveniles
Weight 46 g
Length 7.5 cm
2
200 3
21 Independent
juveniles
Weight 530 g
Length 19.5 cm
300 3
400 9
500 6
3
600 8
20 Adults Weight 860 g
Length 23 cm
700 8
800 4
4
900 4
7 Large adults Weight 1080 g
Length 25 cm
1000 2
1100 1
Each individual was tested in three different positions (Figure 2), as an attempt to
mimic the behaviour of a live individual:
1. Lying on the side with the back pointing towards the approaching robotic lawn
mower, mimicking the curled up position a hedgehog often adopts as a defence
mechanism against approaching danger [19,23].
2. Lying on the side with the stomach pointing towards the approaching robotic lawn
mower (somewhat unnatural, but extremely vulnerable position).
3. Standing upright on its feet with the head pointing towards the approaching robotic
lawn mower (an expression of curiosity but not alarm).
To sum up, each robotic lawn mower was tested 12 times:
Weight 860 g
Length 23 cm
700 8
800 4
4
900 4
7Large adults
Animals 2021, 11, x 4 of 13
rehabilitation centres in Denmark from June to August 2020. All hedgehogs chosen for
this study were intact with no visible injuries. The hedgehogs were stored in freezers at
−20 °C and were thawed before the tests. The 70 selected hedgehog cadavers were di-
vided into four different weight classes to represent four stages of life (Table 2).
Each robotic lawn mower model was tested on four hedgehogs representing each of
the four described weight classes. If an individual was injured by the mower during a
test, it would be discarded to avoid confusion or interaction with previous injuries in
subsequent tests (with one uncomplicated exception, where we reused a cadaver with
superficial injuries in weight class 4, due to a shortage of individuals in this category).
Table 2. Weight classes. Graphical representation of the four weight classes of dead hedgehogs used in the study. The
pictures of live hedgehogs were provided to illustrate the sizes of individuals belonging to the four weight classes. No
live hedgehogs were tested in this study. The ruler on the pictures indicates length of the individuals in cm. Photographs
by Michela Dugar.
Weight
Class Weight (g)
No. of Individuals
Total No. of Individuals
per Weight Class Stages of Life
Representation
1 Up to 199 22 22 Dependent
juveniles
Weight 46 g
Length 7.5 cm
2
200 3
21 Independent
juveniles
Weight 530 g
Length 19.5 cm
300 3
400 9
500 6
3
600 8
20 Adults Weight 860 g
Length 23 cm
700 8
800 4
4
900 4
7 Large adults Weight 1080 g
Length 25 cm
1000 2
1100 1
Each individual was tested in three different positions (Figure 2), as an attempt to
mimic the behaviour of a live individual:
1. Lying on the side with the back pointing towards the approaching robotic lawn
mower, mimicking the curled up position a hedgehog often adopts as a defence
mechanism against approaching danger [19,23].
2. Lying on the side with the stomach pointing towards the approaching robotic lawn
mower (somewhat unnatural, but extremely vulnerable position).
3. Standing upright on its feet with the head pointing towards the approaching robotic
lawn mower (an expression of curiosity but not alarm).
To sum up, each robotic lawn mower was tested 12 times:
Weight 1080 g
Length 25 cm
1000 2
1100 1
Each individual was tested in three different positions (Figure 2), as an attempt to
mimic the behaviour of a live individual:
1.
Lying on the side with the back pointing towards the approaching robotic lawn
mower, mimicking the curled up position a hedgehog often adopts as a defence
mechanism against approaching danger [19,23].
2.
Lying on the side with the stomach pointing towards the approaching robotic lawn
mower (somewhat unnatural, but extremely vulnerable position).

Animals 2021,11, 1191 5 of 13
3.
Standing upright on its feet with the head pointing towards the approaching robotic
lawn mower (an expression of curiosity but not alarm).
To sum up, each robotic lawn mower was tested 12 times:
•Three times per individual (once in each of the three positions).
•One individual from each of the four weight classes.
The tests were filmed with a GoPro Hero 8 Black action camera placed on a tripod. If
a hedgehog was injured during the tests, we recorded the injuries and documented them
with the camera.
The tests of 17 out of 18 machines were carried out in a private garden in Hok, Sweden,
with a flat and well-trimmed lawn, on 25 and 26 August 2020. The last machine (model:
Grouw M900) was tested in a private garden with a flat and well-trimmed lawn in Aarhus,
Denmark on 25 September 2020. All 216 tests were performed during daylight.
The setup for most of the tests was as follows (Figure 2): The hedgehog was placed on
the lawn at a 3 m distance from the robotic lawn mower. The camera was placed next to
the hedgehog on the left-hand side at a 1.5 m distance. The mower was then turned on
and manually directed to move towards the hedgehog. The distance of 3 m was sufficient
to ensure the machine was operating at maximum speed, and the blades were in action,
before reaching the hedgehog. If the machine did not move in a straight line towards the
hedgehog, it was then relocated back to the initial position and turned on again. This was
done to standardise the tests and to ensure that the hedgehog was located to the centre of
the front of each approaching machine.
Animals 2021, 11, x 5 of 13
 Three times per individual (once in each of the three positions).
 One individual from each of the four weight classes.
The tests were filmed with a GoPro Hero 8 Black action camera placed on a tripod. If
a hedgehog was injured during the tests, we recorded the injuries and documented them
with the camera.
The tests of 17 out of 18 machines were carried out in a private garden in Hok,
Sweden, with a flat and well-trimmed lawn, on 25 and 26 August 2020. The last machine
(model: Grouw M900) was tested in a private garden with a flat and well-trimmed lawn
in Aarhus, Denmark on 25 September 2020. All 216 tests were performed during day-
light.
The setup for most of the tests was as follows (Figure 2): The hedgehog was placed
on the lawn at a 3 m distance from the robotic lawn mower. The camera was placed next
to the hedgehog on the left-hand side at a 1.5 m distance. The mower was then turned on
and manually directed to move towards the hedgehog. The distance of 3 m was sufficient
to ensure the machine was operating at maximum speed, and the blades were in action,
before reaching the hedgehog. If the machine did not move in a straight line towards the
hedgehog, it was then relocated back to the initial position and turned on again. This was
done to standardise the tests and to ensure that the hedgehog was located to the centre of
the front of each approaching machine.
Figure 2. An overview of the test setup. Each robotic lawn mower was tested 12 times in total, 3 times per dead hedgehog
representing 1 of 4 weight classes. A hedgehog from each of the four weight classes was placed in three different posi-
tions. The three positions were (1) Lying on the side with the back oriented towards the approaching robotic lawn mower;
(2) Lying on the side with the stomach oriented towards the approaching robotic lawn mower; (3) Standing upright on its
feet with the head oriented towards the approaching robotic lawn mower. The damage recorded from each test was
categorised as 0-A according to the damage categories.
In the cases of two models (tests on Stiga Autoclip 530 SG (Stiga, Castelfranco Ve-
netto, Iltaly) and Ambrogio Robot 4.0 Elite (Zucchetti Centro Sistemi Spa, Arezzo, Italy))
the machine was turned on at a greater distance than 3 m from the hedgehog cadaver,
because these particular robotic lawn mowers took longer distances to gain momentum
and for their blades to be functioning fully. Due to its specifications (causing erratic
movements), the tests of the model Honda Miimo HRM 40 Live (Honda France Manu-
Figure 2.
An overview of the test setup. Each robotic lawn mower was tested 12 times in total, 3 times per dead hedgehog
representing 1 of 4 weight classes. A hedgehog from each of the four weight classes was placed in three different positions.
The three positions were (
1
) Lying on the side with the back oriented towards the approaching robotic lawn mower; (
2
)
Lying on the side with the stomach oriented towards the approaching robotic lawn mower; (
3
) Standing upright on its feet
with the head oriented towards the approaching robotic lawn mower. The damage recorded from each test was categorised
as 0-A according to the damage categories.
In the cases of two models (tests on Stiga Autoclip 530 SG (Stiga, Castelfranco Venetto,
Iltaly) and Ambrogio Robot 4.0 Elite (Zucchetti Centro Sistemi Spa, Arezzo, Italy)) the
machine was turned on at a greater distance than 3 m from the hedgehog cadaver, because
these particular robotic lawn mowers took longer distances to gain momentum and for

Animals 2021,11, 1191 6 of 13
their blades to be functioning fully. Due to its specifications (causing erratic movements),
the tests of the model Honda Miimo HRM 40 Live (Honda France Manufacturing, Ormes,
France) were filmed with a mobile camera, with the hedgehog placed in front of the
approaching machine once it had gained full speed.
2.1. Quantifying the Damage
We divided severity of damage caused by the robotic lawn mowers into six damage
categories:
0.
No physical contact between the machine and the hedgehog. The machine senses
the hedgehog from a distance, changes direction, and drives on without touching the
hedgehog. No damage is caused to the hedgehog cadaver.
1.
The robotic lawn mower approaches the hedgehog and the front of the machine
touches the hedgehog lightly (a “nudge”) and thereby detects the corpse. Immediately,
the machine changes direction and drives on without touching the hedgehog further.
No damage is caused to the hedgehog cadaver.
2.
The robotic lawn mower approaches the hedgehog and the front of the machine
touches the hedgehog (a “flip”) to detect the hedgehog. The physical interaction
causes the hedgehog to be moved into a different body position (flipped from lying
on one side of the body to the other side of the body) or being lifted partly from the
ground before settling in the same position again. Afterwards, the machine changes
direction and drives on without touching the hedgehog further. The damage to the
hedgehog is at most minimal and involves no contact with the blades (at worst this
might cause a slight bruise).
3.
The robotic lawn mower fails to detect the presence of the hedgehog and continues to
drive across the hedgehog. The front panel of the machine is lifted as the machine
drives over the cadaver, which causes the blades to stop running [
24
]. In some
cases the machine withdraws and changes direction, so that only part of the dead
hedgehog’s body was situated underneath the machine. The blades of the robotic
lawn mower may have come into contact with the dead hedgehog but have not
punctured the skin. The damages observed ranged from undetectable to the cutting of
a small number of spines, but might have involved minor bruising to a live hedgehog.
4.
The robotic lawn mower fails to detect the presence of the hedgehog and continues
to drive across it. The blades of the machine have come into contact with the dead
hedgehog and have caused injuries to the cadaver. The severity of the injuries range
from small puncture wounds on the skin (1 cm) to clipping of limbs or complete
exposure of the entire abdominal region and decapitation.
A.
The machine does not detect the juvenile hedgehog (<200 g, weight class 1) and
continues to drive across it. As the body of the small hedgehog is situated below
the blades of the robotic lawn mower, the juvenile hedgehog is left with no visible
injuries. It is possible that in life this could have caused injury or bruising, perhaps by
the wheels rather than the blades (and much would depend on the response of the
juvenile hedgehog in life).
2.2. Data Analyses
The proportion (ratio) of “no damage” or “damage” during the tests, the safety index,
was calculated for the following features on the mowers: (1) blade type, (2) front or rear
wheel drive, (3) wheel numbers, (4) skid plate, (5) ultrasonic sensor, (6) camera vison, (7)
collision sensors, and (8) wheel motor current collision detection.
For the data analyses, the damage categories were divided into two definitions of “no
damage” and “damage”:
•“No damage”:
1. Pooled damage categories 0, 1, and 2.
2. Pooled damage categories 0, 1, 2, and 3.

Animals 2021,11, 1191 7 of 13
•“Damage”:
1. Pooled damage categories 3 and 4.
2. Only damage category 4.
The ratios of “no damage”/”damage” were interpreted as an overall index of safety
for the hedgehogs. The higher the number of “no damage” events compared to “damage”,
the higher the safety index, and hence the judgement that the robotic lawn mower was
more "hedgehog friendly".
The statistical significance for each of the “no damage”/”damage” ratios was tested
with 2
×
2 Chi square tests with Yate’s correction (Chi
Yate’s correction
) to investigate if the
presence or absence of a given technical feature on a robotic lawn mower significantly
affected the ratios.
The 2
×
2 Chi square tests with Yate’s correction were firstly conducted for each of
the four weight classes and three positions of the hedgehogs, separately. However, due to
the low statistical power caused by analysing weight and position separately, all weight
classes and positions were combined. Subsequently, 2
×
2 Chi square tests with Yate’s
correction for all weight classes and positions combined were calculated for each of the
two definitions of damage, testing the effect of each of the eight chosen technical features
on the robotic lawn mowers on the safety index.
Lastly, we calculated the percentage distribution of damage to hedgehogs during the
12 tests on each mower based on the total number of cases where damage was recorded
(either damage category 3 + 4 or damage category 4). Damage category A was omitted from
the analyses, because including it resulted in different sample sizes of tests for different
models of robotic lawn mowers. Therefore, the percentage distribution was chosen as a
measure of safety.
3. Results
Regardless of brand, model, and specifications, none of the robotic lawn mowers
detected the dependent juvenile hedgehogs (<200 g, weight class 1). Some machines did,
however, move over the individuals resulting in no apparent damage, as the juveniles were
sufficiently small, i.e., smaller than the minimum mowing height, thereby avoiding the
running blades of the mowers (damage category A).
In all tests of weight category classes > 200 g (weight classes 2–4), the robotic lawn
mowers had to physically interact with the hedgehog cadaver to detect it. None of the
machines, not even models with camera vision and ultrasonic sensors, was able to detect
the hedgehog in advance and change direction before touching the hedgehog. Therefore,
we did not record any damage category 0. In many cases, the mowers would only touch the
hedgehog (damage category 1 or 2), subsequently detect it, and change direction. However,
some machines did not detect the hedgehogs and ran straight over them. In some cases
the mandatory safety measures of the machine [
24
] caused the blades to stop rotating
within seconds of contact, leaving the hedgehog undamaged or with slight cuts to the
spines (damage category 3). In the event that the safety features of the machine failed to
detect the hedgehog, the result was injury to the cadaver (damage category 4) ranging from
lighter skin abrasions and puncture wounds, to the amputation of extremities like legs and
penises, to complete disembowelment, and in one case a partial decapitation. The injuries
appeared on all areas of the body in no particular pattern, as it depended on the position in
which the hedgehog was caught under the robotic lawn mower, as well as the angles of the
blades. Figure 3provides an overview of the damage categories recorded for each of the 12
different tests performed on the 18 robotic lawn mowers.

Animals 2021,11, 1191 8 of 13
Animals 2021, 11, x 8 of 13
Figure 3. The test results for each of the 18 robotic lawn mowers tested. Every result for each of the four weight classes in
each of the three positions have been described based on a categorisation of damage ranging from 0 to 4, with damage
category 4 being the most severe. Damage category A represents the events where the machine does not detect the juve-
nile hedgehog (<200 g, weight class 1) and continues to drive across the juvenile hedgehog, but as the body of the small
hedgehog is situated below the blades of the robotic lawn mower, the juvenile hedgehog is left with no visible injuries or
bruises.
Comparing the effect of fixed and pivoting blades, the results showed that pivoting
blades significantly reduced the number of damages during the tests, regardless of the
definition of the category “damage” (either damage category 3 + 4 or damage category 4)
(ChiYate’s correction = 28.95 and 26.62, p < 0.0001). The same applied to machines with front
wheel drive compared to rear wheel drive (ChiYate’s correction (4) = 7.25, p = 0.007; ChiYate’s correc-
tion (3 + 4) = 8.99, p = 0.003) as well as the presence of skid plates on the machines (ChiYate’s
correction = 11.39 and 10.99, p = 0.001). Robotic lawn mowers with three wheels instead of
four had a significantly higher safety index, meaning that there were fewer cases of
damage to the hedgehogs during the tests for the damage categorisation based on both
damage category 3 and 4 (ChiYate’s correction = 4.37, p = 0.037), but not for the damage catego-
risation based only on damage category 4. Ultrasonic sensors also appeared to increase
the safety index for the damage categorisation based on damage category 3 and 4 (ChiYate’s
correction = 3.84, p = 0.05), but not for the damage categorisation based only on damage cat-
egory 4. The presence of collision sensors, compared to wheel motor current collision
detection, reduced the safety index for damage category 4 (ChiYat e’s correction = 13.23, p =
0.0003). Table 3 provides a summary of the Chi square statistics.
Figure 3.
The test results for each of the 18 robotic lawn mowers tested. Every result for each of the four weight classes
in each of the three positions have been described based on a categorisation of damage ranging from 0 to 4, with damage
category 4 being the most severe. Damage category A represents the events where the machine does not detect the juvenile
hedgehog (<200 g, weight class 1) and continues to drive across the juvenile hedgehog, but as the body of the small
hedgehog is situated below the blades of the robotic lawn mower, the juvenile hedgehog is left with no visible injuries
or bruises.
Comparing the effect of fixed and pivoting blades, the results showed that pivoting
blades significantly reduced the number of damages during the tests, regardless of the
definition of the category “damage” (either damage category 3 + 4 or damage category
4) (Chi
Yate’s correction
= 28.95 and 26.62, p< 0.0001). The same applied to machines with
front wheel drive compared to rear wheel drive (Chi
Yate’s correction
(4) = 7.25, p= 0.007;
Chi
Yate’s correction
(3 + 4) = 8.99, p= 0.003) as well as the presence of skid plates on the
machines (Chi
Yate’s correction
= 11.39 and 10.99, p= 0.001). Robotic lawn mowers with three
wheels instead of four had a significantly higher safety index, meaning that there were
fewer cases of damage to the hedgehogs during the tests for the damage categorisation
based on both damage category 3 and 4 (Chi
Yate’s correction
= 4.37, p= 0.037), but not for the
damage categorisation based only on damage category 4. Ultrasonic sensors also appeared
to increase the safety index for the damage categorisation based on damage category 3 and
4 (Chi
Yate’s correction
= 3.84, p= 0.05), but not for the damage categorisation based only on
damage category 4. The presence of collision sensors, compared to wheel motor current
collision detection, reduced the safety index for damage category 4 (Chi
Yate’s correction
=
13.23, p= 0.0003). Table 3provides a summary of the Chi square statistics.

Animals 2021,11, 1191 9 of 13
Table 3.
Results from the data analyses investigating if the presence or absence of a given technical feature on a robotic
lawn mower significantly influenced the safety index. Damage category A was omitted from the analyses.
Features
Damage
Categories
Included
Type No
Damage Damage Safety Index (No
Damage/Damage) Safety Index
Chi Square
with Yates
Correction
p-Value
Fixed or
pivoting
blades
4Fixed 23 27 23/27 0.85 28.95 <0.0001 ***
Pivoting 112 18 112/18 6.22
3+4 Fixed 14 36 14/36 0.39 26.62 <0.0001 ***
Pivoting 93 37 93/37 2.51
Front or rear
wheel drive
4Front 28 1 28/1 28.00 7.25 0.007 **
Rear 107 44 107/44 2.43
3+4 Front 25 4 25/4 6.25 8.99 0.003 **
Rear 82 69 82/69 1.19
3 or 4 wheels
43 wheels 35 5 35/5 7.00 3.47 0.062
4 wheels 100 40 100/40 2.50
3+4 3 wheels 30 10 30/10 3.00 4.37 0.037 *
4 wheels 77 63 72/63 1.22
Skid plate
4Yes 37 1 37/1 37.00 11.39 0.001 ***
No 98 44 98/44 2.23
3+4 Yes 32 6 32/6 5.33 10.99 0.001 ***
No 75 67 75/67 1.12
Ultrasonic
sensors
4Yes 34 5 34/5 6.80 3.15 0.076
No 101 40 101/40 2.53
3+4 Yes 29 10 29/10 2.90 3.84 0.050 *
No 78 63 78/63 1.24
Camera
vision
4Yes 8 3 8/3 2.67 0.03 0.857
No 127 42 127/47 3.02
3+4 Yes 7 4 7/4 1.75 0.01 0.980
No 100 69 100/69 1.45
Collision
sensors
4Yes 49 31 49/31 1.58 13.23 0.0003 ***
No 86 14 86/14 6.14
3+4 Yes 45 35 45/35 1.29 0.39 0.53
No 62 38 62/38 1.63
Wheel motor
current
collision
detection
4Yes 86 14 86/14 6.14 13.23 0.0003 ***
No 49 31 49/31 1.58
3+4 Yes 62 38 62/38 1.63 0.39 0.53
No 45 35 45/35 1.29
*p-value ≤0.05, ** p-value ≤0.01, *** p-value ≤0.001.
The percentage distribution of damages to the hedgehogs during the tests of each
robotic lawn mower (Table 4) provides an overview of the performance of each machine.
The lower percentage of damages during the tests, the safer the mower is for hedgehogs,
insofar as the injuries are a good approximation to what could be sustained on live hedge-
hogs. The percentage distribution of damages varied accordingly, as some models may
have caused no or few category 4 damages but had a higher occurrence of damage category
3 during the tests.

Animals 2021,11, 1191 10 of 13
Table 4.
The percentage distribution of tests resulting in damage to the hedgehogs, defined either as damage category 4 or
damage category 3 + 4. Damage category A was omitted from the analyses, leaving the total number of tests between 9
and 12 depending on the amount of damage category A results recorded per robotic lawn mower. The lower percentage
of cases of damage during the tests, the safer the robotic lawn mower. The robotic lawn mower models have been listed
in accordance with the percentage distribution of damage defined as damage category 3 + 4. Models showing the lowest
damage percentage are listed first.
Robotic Lawn Mowers Tests with Damage Category 4 Tests with Damage Category 3 + 4
Brand Model No Damage
(0–3)
Damage
(4)
Cases of Damage
in Tests (%)
No Damage
(0–2)
Damage
(3–4)
Cases of Damage
in Tests (%)
Gardena Sileno Life 10 0 010 0 0
Husqvarna Automower®105 9 1 10 9 1 10
Husqvarna Automower®315X 9 0 08 1 11
Honda Miimo HRM 40 Live 12 0 010 2 17
Husqvarna Automower®450X 10 0 08 2 20
Worx Landroid M (WR143E) 10 0 08 2 20
LandXcape LX8212i 9 1 10 8 2 20
Husqvarna Automower®305 9 0 07 2 22
DAYE Grouw M900 9 3 25 9 3 25
Gardena Sileno City 9 0 06 3 33
Robomow RS635 PRO 8 3 27 7 4 36
Kress Mission KR111 5 4 44 5 4 44
Worx Landroid L (WR153E) 10 1 95 6 55
Ambrogio Robot 4.0 Elite 4 7 64 4 7 64
Stihl iMow®422PC 2 8 80 2 8 80
AL-KO Robolinho®1150 2 7 78 1 8 89
Honda Miimo HRM 3000 1 8 89 0 9 100
Stiga Autoclip 530 SG 7 2 22 0 9 100
4. Discussion
As the results showed that none of the robotic lawn mowers tested was able to detect
the hedgehogs without physical interaction and none detected the dependent juveniles,
we cannot be confident that any of the robotic lawn mowers tested were entirely safe
to hedgehogs. Preferably, the machines should not interact physically at all with the
hedgehogs. However, the damages categorised as 1–2 do not appear to harm the hedgehogs,
and perhaps the hedgehogs may even learn to avoid robotic lawn mowers after such an
encounter. Furthermore, there were obvious differences in the outcome on the hedgehogs
depending on the machines tested, with some models consistently causing no damage on
collision (Figure 3and Table 4).
Some of the injuries recorded would have been immediately lethal, and all of the
damages in category 4 would have had the potential to become lethal if left untreated. A
small puncture wound, if untreated, might get infected and progress to balloon syndrome,
a potentially lethal condition caused by subcutaneous emphysema, which makes the skin
of the hedgehog blow up like a balloon [
25
], or a general systemic infection. As hedgehogs
are considered quite elusive even when damaged and in pain, it must be assumed that a
proportion of hedgehogs injured by robotic lawn mowers will not be found and helped in
time and will likely die from their injuries in the wild.
In some cases the robotic lawn mowers failed to detect the hedgehog but met the
safety regulations insofar as the blades stopped when the surface of the machine was lifted
(activating a tilt-, lift- or obstruction sensor) leaving the skin of the hedgehog unbroken
(damage category 3) [
24
]. However, there were situations where the mower continued to
run over and hence injure the hedgehog cadaver. We reduced our recording of injuries

Animals 2021,11, 1191 11 of 13
to one category (damage category 4), as the outcome may be influenced by a range of
different factors, such as the soil softness and type, height of the grass, position of the
hedgehog as the robotic lawn mower runs over the individual, and how the collision with
the hedgehog positions the individual underneath the blades. As these different factors
may have influenced the results of the tests, causing uncertainty of the potential outcome in
all scenarios where the robotic lawn mowers failed to detect the hedgehogs and continued
to run over the individuals (damage categories 3 and 4), we decided to represent both
types of damage categories in our analyses of the results (damage category 4 and damage
category 3 + 4) as a precautionary measure.
During our experiments, there was a greater likelihood that robotic lawn mowers
with fixed blades would fail to detect the dead hedgehogs, causing more extensive damage
to them. These results may be explained by various factors. In contrast to fixed blades,
which are constantly exposed, pivoting blades fold into a protective frame when they hit
something harder than grass. Furthermore, robotic lawn mowers with fixed blades require
more heavy-duty machine power to run the blades, and this greater power appeared to
render the machines less controllable and less sensitive in their detection technology. The
engineering of front- compared to rear-wheel drive, as well as the use of three compared to
four wheels, influenced the safety index positively. This may also be because models with
front-wheel drive and three wheels all had pivoting blades. The same explanation may
apply to the significantly lower incidence of damage for tests on robotic lawn mowers with
ultrasonic sensors, all of which were fitted with pivoting blades. Lastly, the presence of
skid plates significantly reduced the number of tests causing damage to the hedgehogs.
The skid plate is designed to protect the pivoting blades from hard objects and thereby
also protects foreign objects, such as a hedgehog, from the blades. Only one of the models
tested contained a combination of these beneficial features (except ultrasonic sensors).
These should be the focus of future designs of robotic lawn mowers with hedgehog safety
in mind.
We cannot rule out the possibility that the results were also influenced by the lift
detection sensitivity of the robotic mowers. We could not test this, but presume that if
lift detection sensitivity was sufficiently high, the machines would detect the hedgehogs
and change direction or stop the blades rotating as soon as the surface of the machine was
lifted, reducing the risk of injuries.
4.1. Using Dead Hedgehogs as Test Subjects
Working with dead hedgehogs as test subjects may not perfectly mimic the outcomes
of natural collisions. Firstly, live hedgehogs might detect and evade the robotic lawn
mower. Secondly, they might curl up, and their tightened muscles and raised spines could
provide protection. We sought to mimic these behaviours in the positions we chose for the
cadavers, but of course their muscle tone and reactions were different. Alternative insights
would come from simulations using live hedgehogs with safely modified mowers.
4.2. Failed Detection of Dependent Juveniles and the Consequences
None of the tested robotic lawn mowers was able to detect the dependent juvenile
hedgehogs (<200 g, weight class 1). In most cases, these small individuals passed beneath
the rotating blades. We do not know how mother hedgehogs accompanied by their litters
would react to an active robotic lawn mower, but reports from the public indicate that they
generally tend to stay in the nests during ordinary human garden activity, although this
would have to be investigated further in future work. An orphaned juvenile hedgehog
is more likely to be exposed to running robotic lawn mowers. However, such an individ-
ual is already very vulnerable with a low chance of survival, regardless of the presence
of a robotic lawn mower, unless found in good time and taken into care at a wildlife
rehabilitation centre.

Animals 2021,11, 1191 12 of 13
4.3. Results in Relation to Discussions with the Public
The public discourse has raised questions of whether hedgehogs can outrun robotic
lawn mowers and whether hedgehogs are able to detect them properly. As we used
cadavers, we were not able to test this, but we do know that hedgehogs can run at up to
50 m per minute [
26
], whereas the maximum speed of Husvarna’s robotic lawn mowers
ranges between 21 m per minute and 39 m per minute (pers. comm. Husqvarna). In terms
of cues likely to alert the hedgehogs, these machines make characteristic sounds and smells
detectable by human senses. We made no observations of the behavioural responses of live
hedgehogs to the mowers, although this could be done at no risk to the hedgehogs using
disarmed machines.
As hedgehogs are nocturnal, it has been widely recommended that any problem
would be circumvented by running robotic mowers only by day. This might indeed largely
obviate the problem, nonetheless being mindful that hedgehogs may be active during the
daytime for several different reasons [19,23].
In the light of the results from the present study, we encourage manufacturers, dis-
tributors, and sellers of robotic lawn mowers to educate customers on the importance
of refraining from using robotic lawn mowers at night time and to check the lawn for
wildlife species that are potentially vulnerable to the machines, such as hedgehogs, leverets,
fledglings, and amphibians, before mowing.
5. Conclusions
As hedgehogs are increasingly associated with human habitation, they are likely to
encounter robotic lawn mowers, and our results show the encounters, depending on the
model, could be injurious and even fatal. That said, while our study answers critical
questions regarding the likely nature and extent of injuries, we cannot comment on the
likelihood of these encounters or the hedgehogs’ responses to them. However, a major
step towards resolving the risk of robotic lawn mowers on hedgehog survival involves
the design and purchase of hedgehog-friendly mowers, a topic of potentially fruitful
collaboration between hedgehog conservationists, behavioural ecologists, and mower
manufacturers.
Author Contributions:
Conceptualization, S.L.R., A.E.S., R.M., J.L.N., C.P., and D.W.M.; Data cu-
ration, S.L.R., A.E.S., and R.M.; Formal analysis, S.L.R. and C.P.; Funding acquisition, S.L.R.; In-
vestigation, S.L.R., A.E.S., and R.M.; Methodology, S.L.R., A.E.S., and R.M.; Project administration,
S.L.R.; Resources, S.L.R.; Supervision, J.L.N., C.P., and D.W.M.; Visualization, S.L.R., A.E.S., and R.M.;
Writing—original draft, S.L.R.; Writing—review and editing, S.L.R., A.E.S., R.M., J.L.N., C.P., and
D.W.M. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the British Hedgehog Preservation Society. The APC was
funded by the QATO Foundation and Dyrenes Beskyttelse (Animal Protection Denmark).
Institutional Review Board Statement:
Ethical review and approval were waived for this study,
due to the fact that, regardless of the status of European hedgehogs as protected animals in the
Danish legislation, the hedgehogs used in the study had already died in care at Danish hedgehog
rehabilitation centres authorised and monitored by the Danish Nature Agency.
Data Availability Statement:
Further data from the research is available from Zenodo (DOI: 10.5281/
zenodo.4707658, https://doi.org/10.5281/zenodo.4707658).
Acknowledgments:
The authors thank robotic lawn mower product specialist Petrus Ekbladh for
advice and technical assistance. We thank the Danish hedgehog carers from Pindsvine Plejerne and
Dyrenes Beskyttelse (Inge Tornkjær, Dorte Sigga Lund, Janne Montell, Tine Deth Christiansen, Leon
and Maria Holm Kragh, and Dorthe Madsen) for collecting hedgehogs dying in care for the tests. We
also thank Michela Dugar from Progetto Riccio Europeo—The Wild Hedgehog Project for providing
pictures of hedgehogs for Table 2.

Animals 2021,11, 1191 13 of 13
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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