Territory size and age explain movement patterns in the Eurasian beaver

Abstract

Territoriality is only profitable when the benefits gained from territory exploitation exceed the costs of defence, and territory sizes are usually optimized by time constraints related to resource defence (e.g. patrolling) and exploitation. In this study, we equipped 25 dominant Eurasian beavers (Castor fiber) with GPS units to study spatial movement patterns both on land and in water in relation to territory size, resource availability, the number of neighbours, season, and the beavers’ age. We show a territory size-dependent trade-off between territorial behaviours and foraging distances: Beavers in larger territories moved greater distances each night, thereby spending more time patrolling, and stayed closer to the shoreline when being on land (i.e. when foraging). Inversely, in smaller territories beavers patrolled less and foraged further away from the shoreline. These results suggest that individuals trade-off the costs of patrolling larger territories against the benefits of foraging closer towards the shoreline. Smaller territories might be more prone to resource depletion, thus, making foraging further from the shoreline a strategy to ensure sustainable resource use. Further, older beavers spent more time on land and close to territory borders compared to younger ones, suggesting a behavioural change with age possibly due to increased experience and boldness.

Full text

Mammalian
Biology
81
(2016)
587–594
Contents lists available at ScienceDirect
Mammalian
Biology
journal homepage: www.elsevier.com/locate/mambio
Original
investigation
Territory
size
and
age
explain
movement
patterns
in
the
Eurasian
beaver
Patricia
M.
Grafa,b,1,
Martin
Mayera,∗,1,
Andreas
Zedrossera,b,
Klaus
Hackländerb,
Frank
Rosella
aDepartment
of
Environmental
and
Health
Studies,
Faculty
of
Arts
and
Sciences,
University
College
of
Southeast
Norway,
Bø
i
Telemark,
Norway
bDepartment
of
Integrative
Biology
and
Biodiversity
Research,
Institute
of
Wildlife
Biology
and
Game
Management,
University
of
Natural
Resources
and
Life
Sciences,
Vienna,
Austria
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
28
January
2016
Accepted
28
July
2016
Handled
by
Adriano
Martinoli
Available
online
30
July
2016
Keywords:
Behaviour
Castor
fiber
GPS
Movement
ecology
Territoriality
a
b
s
t
r
a
c
t
Territoriality
is
only
profitable
when
the
benefits
gained
from
territory
exploitation
exceed
the
costs
of
defence,
and
territory
sizes
are
usually
optimized
by
time
constraints
related
to
resource
defence
(e.g.
patrolling)
and
exploitation.
In
this
study,
we
equipped
25
dominant
Eurasian
beavers
(Castor
fiber)
with
GPS
units
to
study
spatial
movement
patterns
both
on
land
and
in
water
in
relation
to
territory
size,
resource
availability,
the
number
of
neighbours,
season,
and
the
beavers’
age.
We
show
a
territory
size-
dependent
trade-off
between
territorial
behaviours
and
foraging
distances:
Beavers
in
larger
territories
moved
greater
distances
each
night,
thereby
spending
more
time
patrolling,
and
stayed
closer
to
the
shoreline
when
being
on
land
(i.e.
when
foraging).
Inversely,
in
smaller
territories
beavers
patrolled
less
and
foraged
further
away
from
the
shoreline.
These
results
suggest
that
individuals
trade-off
the
costs
of
patrolling
larger
territories
against
the
benefits
of
foraging
closer
towards
the
shoreline.
Smaller
territories
might
be
more
prone
to
resource
depletion,
thus,
making
foraging
further
from
the
shoreline
a
strategy
to
ensure
sustainable
resource
use.
Further,
older
beavers
spent
more
time
on
land
and
close
to
territory
borders
compared
to
younger
ones,
suggesting
a
behavioural
change
with
age
possibly
due
to
increased
experience
and
boldness.
©
2016
Deutsche
Gesellschaft
f¨
ur
S¨
augetierkunde.
Published
by
Elsevier
GmbH.
All
rights
reserved.
Introduction
Territoriality
is
linked
to
the
defence
of
a
fixed
area
by
an
indi-
vidual
or
a
group
of
mutually
tolerant
individuals
(Maher
and
Lott,
1995).
Animals
typically
occupy
territories
when
resources,
such
as
food,
cover,
shelter,
and
mating
partners,
are
scarce
(Brown,
1969;
Davies
and
Houston,
1984;
Maher
and
Lott,
1995).
Territorial
behaviour
is
expected
to
evolve
when
the
benefits
gained
from
the
exclusive
use
of
essential
and
restricted
resources
exceed
the
costs
of
defence
(Brown,
1964;
Stamps,
1994).
Defence
mechanisms
are
diverse
and
may
include
aggressive,
physical
disputes
with
intrud-
ers,
which
generally
impose
significant
metabolic
costs
(Parker,
1974;
Viera
et
al.,
2011),
and
advertisement
of
territory
owner-
ship
by
chemical
(e.g.
scent-marking)
(Gosling
and
Roberts,
2001;
∗Corresponding
author
at:
Faculty
of
Arts
and
Sciences,
Department
of
Envi-
ronmental
and
Health
Studies,
University
College
of
Southeast
Norway,
3800
Bø
i
Telemark,
Norway.
E-mail
addresses:
martin.mayer@hit.no,
m.mayer89@web.de
(M.
Mayer).
1These
authors
contributed
equally
to
this
work.
Roberts
and
Dunbar,
2000),
acoustic
(Bee
et
al.,
2000;
McGregor,
1993),
or
visual
signals
(Burst
and
Pelton,
1983;
Penteriani
and
del
Mar
Delgado,
2008).
Patrolling
territory
borders
is
essential
for
effectively
advertising
territory
occupation
(Sillero-Zubiri
and
Macdonald,
1998)
and
is,
besides
foraging,
an
important
driver
of
spatial
movement
behaviour
in
territorial
species
(Fagan
et
al.,
2013;
Ims,
1995).
Animals
are
continually
subject
to
multiple
decisions
regard-
ing
energy
investment
and
thus
need
to
trade-off
which
activity
to
adopt
at
any
time
(Mangel
and
Clark,
1986).
Such
trade-offs
may
impact
an
animal’s
fitness
and
survival
(Ohgushi,
1996;
Stearns,
1989)
and
have
been
a
research
area
of
interest
for
decades.
The
literature
in
this
field
is
extensive,
however,
most
studies
focus
on
the
trade-offs
between
offspring
size
and
offspring
number
(e.g.
Charnov
and
Ernest,
2006;
Fleming
and
Gross,
1990),
foraging
and
predation
risk
(e.g.
Lima
et
al.,
1985;
Sih,
1980;
Verdolin,
2006),
or
growth
and
reproduction
(e.g.
Kozłowski,
1992;
Roff,
1983).
Studies
investigating
the
trade-off
between
foraging
and
territorial
behaviours
are
comparatively
rare.
For
example,
great
tits
(Parus
major)
traded
off
food
intake
for
territory
defence
in
the
pres-
ence
of
an
intruder
(Kacelnik
et
al.,
1981;
Ydenberg
and
Krebs,
http://dx.doi.org/10.1016/j.mambio.2016.07.046
1616-5047/©
2016
Deutsche
Gesellschaft
f¨
ur
S¨
augetierkunde.
Published
by
Elsevier
GmbH.
All
rights
reserved.

588
P.M.
Graf
et
al.
/
Mammalian
Biology
81
(2016)
587–594
1987).
Jaeger
et
al.
(1983)
found
that
red-backed
salamanders
(Plethodon
cinereus)
decreased
foraging
time
and
devoted
more
time
to
territory
defence
when
potential
competitors
intruded.
Wild
chimpanzees
(Pan
troglodytes)
reduced
their
feeding
time
by
at
least
50%
when
on
patrolling
trips
(Amsler,
2010).
Generally,
the
costs
of
territoriality
are
positively
correlated
with
territory
size,
because
larger
areas
are
more
costly
to
defend
(Righton
et
al.,
1998;
Schoener,
1983).
Determining
the
costs
and
benefits
of
different
territory
sizes
in
a
species
is
difficult
(Jaeger
et
al.,
1983)
and
has
been
subject
to
a
range
of
modelling
approaches
(e.g.
Adams,
2001;
Dill,
1978;
Schoener,
1983).
The
optimization
criterion
for
territory
size
is
usually
related
to
time
constraints
between
resource
defence
and
exploitation
(Adams,
2001;
Kacelnik
et
al.,
1981),
and
has
been
described
as
the
minimum
economi-
cally
defensible
area
(Gill
and
Wolf,
1975;
Pyke
et
al.,
1977).
Adams
(2001)
suggested
two
additional
factors
that
may
influence
ter-
ritory
sizes,
i.e.
interactions
among
neighbours
and
interactions
between
established
residents
and
potential
settlers.
Such
inter-
actions
may
be
especially
important
in
high-density
populations
with
contiguous
territory
borders
(Adams,
2001),
and
may
result
in
territory
sizes
smaller
or
larger
than
the
minimum
economically
defensible
area.
Further,
individual
differences
in
movement
pat-
terns
might
be
related
to
age:
e.g.
Cederlund
and
Sand
(1994)
found
that
older
male
moose
(Alces
alces)
had
larger
home
ranges
than
younger
ones,
which
may
be
caused
by
differences
in
nutritional
demands
and
social
activities
like
rutting
behaviour.
Similarly,
in
tailed
frogs
(Ascaphus
truei)
(Daugherty
and
Sheldon,
1982)
and
pinnipeds
(Baker
et
al.,
1995;
Cameron
et
al.,
2007),
older
individ-
uals
exhibited
greater
site
fidelity
than
younger
ones,
which
was
suggested
to
be
related
to
sexual
maturation,
age-specific
varia-
tion
in
ecological
requirements
and
accumulated
knowledge
on
breeding
site
characteristics.
We
used
the
Eurasian
beaver
(Castor
fiber)
to
investigate
fac-
tors
affecting
spatial
movement
patterns
in
a
long-lived,
territorial
animal.
Beavers
(both
the
Eurasian
and
the
North
American
beaver
(Castor
canadensis))
are
semi-aquatic,
nocturnal
rodents
that
are
socially
monogamous
and
live
in
family
groups
consisting
of
the
dominant
pair,
the
young
of
the
year,
yearlings,
and
subadults,
i.e.,
non-dominant
individuals
of
two
years
or
older
(Campbell
et
al.,
2005;
Wilsson,
1971).
The
two
beaver
species
are
in
the
small
percentage
of
mammals
(3–5%)
that
form
monogamous
pair
bonds
(Kleiman,
1977)
with
complex
social
behaviours
including
male
parental
care
and
shared
territorial
defence
(Busher,
2007).
Beavers
build
lodges
or
bank
dens
and
are
central-place
foragers
with
a
preference
for
poplars
(Populus
sp.)
and
willows
(Salix
sp.)
(Haarberg
and
Rosell,
2006;
Vorel
et
al.,
2015).
They
move
rela-
tively
close
to
the
shoreline
and
feed
within
approx.
40
m
from
the
water’s
edge
(Barnes
and
Dibble,
1988;
Parker
et
al.,
2001).
The
beavers’
fusiform
body
with
short
limbs
and
webbed
hind-
feet
make
them
good,
enduring
swimmers,
but
constrain
their
agility
in
terrestrial
environments
(Allers
and
Culik,
1997).
Beavers
hold
larger
territories
during
initial
settlement,
whereas
in
popula-
tions
at
carrying-capacity
territories
of
various
sizes
are
occupied
(Campbell
et
al.,
2005).
To
advertise
territory
occupation,
both
sexes
deposit
scent-marks
within
their
territories,
especially
along
up-
and
downstream
borders
(Hodgdon,
1978;
Rosell
et
al.,
1998;
Sun
and
Müller-Schwarze,
1999).
Scent-marking
activity
increases
dur-
ing
spring
when
subadults
disperse
from
their
natal
colony
(Rosell
et
al.,
1998).
Territorial
behaviour
by
both
sexes
is
suggested
to
have
evolved
from
a
mate-guarding
strategy
and/or
a
resource
defence
strategy
(both
food
and
the
physical
family
area,
Busher,
2007).
In
autumn,
beavers
prepare
food
caches
in
front
of
their
lodges
to
sus-
tain
the
family
during
the
cold
months
(Busher,
1996;
Hartman
and
Axelsson,
2004).
The
dominant
pair
exhibits
similar
space
use
and
movement
behaviour,
and
does
not
reduce
their
patrolling
activity
in
the
presence
of
an
increasing
number
of
subordinate
helpers
in
the
colony
(Herr
and
Rosell,
2004).
We
deployed
GPS
units
on
dominant,
territory-holding
beavers
to
analyse
terrestrial
and
aquatic
movement
patterns
in
relation
to
environmental
and
demographic
factors.
We
hypothesized
that
terrestrial
and
aquatic
movement
patterns
would
depend
on
1)
ter-
ritory
size,
2)
resource
availability,
3)
season,
4)
intruder
pressure
(number
of
neighbours),
and
5)
age.
We
predicted
that
1)
owners
of
larger
territories
would
move
greater
distances
in
water
(i.e.,
have
a
higher
relative
patrolling
effort),
but
2)
also
have
more
oppor-
tunities
to
forage
closer
to
the
shoreline
due
to
higher
resource
availability
than
owners
of
smaller
territories.
Third,
we
predicted
that
beavers
would
patrol
more
in
spring
when
subadults
are
dis-
persing,
and
spent
more
time
on
land
in
autumn
to
prepare
for
winter,
i.e.,
to
build
food
caches
and
repair
lodges.
Fourth,
we
pre-
dicted
that
beavers
would
generally
increase
patrolling
activities
when
facing
higher
intruder
pressure
as
determined
by
the
number
of
individuals
in
neighbouring
colonies.
And
5),
we
hypothesized
that
movement
patterns
would
change
with
increasing
age
due
to
a
shift
in
behavioural
traits
such
as
dominance
and
experience.
Material
and
methods
Study
area,
animals
and
capture
Our
study
was
conducted
between
2009
and
2014
in
Telemark
county,
southeast
Norway
(Fig.
1).
Data
were
collected
in
three
con-
nected
rivers,
the
Straumen,
Gvarv,
and
Saua,
which
flow
through
a
semi-cultural
and
mixed
forest
landscape,
and
empty
into
Lake
Norsjø.
The
rivers
are
mostly
slow
flowing
and
between
10
and
100
m
wide
with
stable
water
levels,
making
it
unnecessary
for
beavers
to
build
dams.
Woody
vegetation
along
the
rivers
mostly
consists
of
grey
alder
(Alnus
incana),
willow
(Salix
spp.),
bird
cherry
(Prunus
padus),
common
ash
(Fraxinus
excelsior),
rowan
(Sorbus
aucuparia),
birch
(Betula
spp.),
and
Norway
spruce
(Picea
abies)
(Haarberg
and
Rosell,
2006).
The
proportion
of
deciduous
habitat
was
similar
between
the
rivers
in
our
study
area
(ANOVA:
F
=
0.544,
p
=
0.586)
(Campbell
et
al.,
2005).
Hunting
pressure
in
the
area
was
presumably
low
(Rosell
et
al.,
2000)
with
eight
known
cases
of
hunted
beavers
(4.6%
of
the
known
population)
between
2009
and
2014
(unpublished
results).
Natural
predators,
predominantly
Eurasian
lynx
(Lynx
lynx),
were
present
in
low
densities
in
our
area
(Herfindal
et
al.,
2005).
Red
fox
(Vulpes
vulpes),
which
is
known
to
occasionally
predate
on
beaver
kits
(Kile
et
al.,
1996),
was
also
present.
Dominant
Eurasian
beavers
were
captured
at
night
from
a
motorboat
using
landing
nets
from
March
to
June
(spring),
and
August
to
October
(autumn)
each
year
as
part
of
a
long
term
study
(Steyaert
et
al.,
2015).
Dominance
status
(i.e.,
being
the
reproduc-
tive
individual)
had
previously
been
assigned
by
multiple
capture
and
sighting
events
in
the
same
territory,
body
weight,
lactation
in
females,
and
evidence
indicating
the
disappearance
of
the
pre-
vious
dominant
same-sex
individual
in
that
territory
(Campbell
et
al.,
2012).
All
individuals
had
been
previously
marked
and
were
sex-determined
based
on
the
colour
of
the
anal
gland
secretion
(Rosell
and
Hovde,
2001;
Rosell
and
Sun,
1999).
The
exact
age
was
known
for
13
individuals
as
they
were
captured
as
kits
or
yearlings;
for
the
other
12
individuals
age
was
determined
as
minimum
age
based
on
body
weight
(Rosell
et
al.,
2010).
There
was
no
differ-
ence
between
individuals
of
known
age
and
ones
of
uncertain
age
suggesting
that
our
age
determination
worked
reliably
(8.72
±
3.44
vs.
7.0
±
3.19
years,
p
=
0.820).
At
capture,
beavers
were
transferred
into
a
cloth
sac
where
they
were
immobilized
and
easier
to
han-
dle
(no
anaesthesia
was
administered).
We
measured
body
mass
and
length,
and
attached
a
unit
consisting
of
a
VHF
transmitter

P.M.
Graf
et
al.
/
Mammalian
Biology
81
(2016)
587–594
589
Fig.
1.
A
Eurasian
beaver
(Castor
fiber)
with
a
GPS
unit
glued
onto
its
lower
back
(a)
in
our
study
area
in
southeast
Norway
(b).
The
main
map
(c)
shows
data
examples
of
GPS
positions
and
calculated
territory
size
(measured
as
bank
length)
for
three
beavers
in
Gvarv
River.
(10
g,
Reptile
glue-on,
series
R1910;
Advanced
Telemetry
Systems,
Isanti
MN,
USA)
and
a
␮GPS
transmitter
(24
g,
model
G1G
134A;
Sir-
track,
Havelock
North,
NZ).
The
unit
was
glued
on
the
lower
back
(ca.
15
cm
from
the
base
of
the
tail,
Fig.
1)
using
a
two-component
epoxy
resin
(System
Three
Resins,
Auburn
WA,
USA).
This
position
was
chosen
to
minimize
drag
and
potential
effects
on
the
animal,
respectively,
but
also
allowed
for
obtaining
GPS
positions
while
the
animal
was
swimming
as
the
tag
was
above
water
level.
GPS
trans-
mitters
were
programmed
to
take
a
position
every
15
min
between
1900
and
0700
h
and
were
set
to
sleep
during
the
day
as
beavers
are
not
active
then
(Sharpe
and
Rosell,
2003).
In
contrast
to
fully
aquatic
endotherms,
beavers
rarely
dive
for
long
periods
(typically
<3
min,
(Graf
et
al.,
2012)),
thus,
diving
events
were
unlikely
to
influence
the
number
of
successful
GPS
fixes
in
water
as
transmitters
were
programmed
to
acquire
a
position
for
3
min.
Handling
time
of
cap-
tured
animals
ranged
between
20
and
50
min.
The
total
weight
of
the
glued-on
unit
did
not
exceed
1%
of
the
beavers’
body
weight.
For
retrieval,
animals
were
re-trapped
after
two
to
six
weeks
and
the
unit
was
cut
off
the
fur
with
a
scalpel.
All
animal
handling
pro-
cedures
were
approved
by
the
Norwegian
Directorate
for
Nature
Management
and
the
Norwegian
Animal
Research
Authority.
Data
preparation
Due
to
our
long-term
individual-based
monitoring
program,
we
had
information
on
the
number
of
individuals
per
colony,
allowing
us
to
estimate
the
number
of
adjacent
neighbours.
For
adjacent
colonies
in
which
we
did
not
obtain
the
number
of
individuals
(i.e.,
in
territories
located
at
the
edge
of
our
study
area),
we
used
the
average
annual
number
of
individuals
per
colony
as
an
estimate.
Kits
were
not
included
in
this
estimate,
because
they
do
not
present
an
intruder
threat.
The
capture
night
and
the
following
night
were
removed
from
the
analysis
to
correct
for
possible
effects
of
capture
(Graf
et
al.,
2016,
in
prep.).
GPS
positions
with
horizontal
dilution
of
precision
(HDOP)
values
of
≥5
and
≤4
available
satellites
were
removed
from
the
analysis
(8.5%
of
the
raw
data),
to
correct
for
imprecise
locations
(Lewis
et
al.,
2007).
As
a
measure
for
distance
from
the
shoreline
(separately
for
land
and
water
positions),
we
calculated
the
average
perpendicular
distance
of
GPS
positions
to
the
shoreline
using
the
join
tool
in
ArcMap
10.1
(Esri,
Redlands,
CA,
USA).
We
used
bank
length
as
a
measure
of
territory
size,
because
the
beavers
in
our
study
area
generally
stayed
close
to
the
shoreline
(on
average
<20
m),
both
when
being
on
land
and
in
water
(see
Results,
Fig.
1).
Other
measures
of
territory
size,
such
as
minimum
con-
vex
polygon
(MCP)
or
kernel
methods,
would
have
resulted
in
an
overestimation
of
territory
size
due
to
the
inclusion
of
unused
habi-
tat,
for
example
in
meandering
rivers.
To
obtain
accurate
estimates
of
bank
length,
we
calculated
the
95%
MCP
based
on
each
indi-
viduals’
GPS
relocations
in
ArcMap
10.1,
and
then
extracted
bank
length
(from
now
on
referred
to
as
territory
size)
for
each
individual
from
the
MCPs.
Explorative
trips
(defined
as
a
one-time
movement
outside
the
territory
lasting
less
than
4
h)
were
removed
when
cal-
culating
territory
sizes,
as
they
were
visibly
outside
the
territorial
borders
(and
within
the
neighbours’
territory).
Land
cover
data
was
derived
from
a
digital
topographic
map
(Felles
KartDatabase,
FKB
data
Geovekst,
http://www.kartverket.no/).
The
amount
of
mixed
and
deciduous
forested
area
within
a
buffer
of
50
m
from
the
shore-
line
was
calculated
to
obtain
a
measure
for
resource
availability
(measured
in
ha).
A
buffer
of
50
m
was
chosen
because
95%
of
all
land
positions
were
located
within
this
buffer
(see
Results).
Time
spent
on
land
was
determined
by
the
proportion
of
land
positions.
The
distance
between
GPS
positions
was
calculated
as
direct
line
distance
between
two
consecutive
GPS
positions
sep-
arately
for
land
and
water,
and
was
averaged
per
hour.
Beavers
typically
travel
in
water,
thus,
this
method
may
have
resulted
in
an
overestimation
of
the
average
distance
moved
per
hour
on
land,
as
it
is
possible
that
beavers
swam
in
between
two
consecutive
land
positions.
Consequently,
instead
of
using
this
estimate
as
measure
for
actual
movement
on
land,
it
should
be
interpreted
as
an
estimate
for
different
foraging
tactics,
i.e.
foraging
more
selectively
between
multiple
patches
versus
less
selective
foraging
within
the
same
or

590
P.M.
Graf
et
al.
/
Mammalian
Biology
81
(2016)
587–594
Table
1
Overview
of
25
Eurasian
beavers
(Castor
fiber)
equipped
with
a
GPS
unit
between
2009
and
2014
in
southeast
Norway.
A
=
autumn,
S
=
spring.
Territory
Beaver
Sex
Year
and
Season
#
GPS
Days
#
GPS
Positions
Group
Size
#
of
Kits
Banklength
(km)
Time
on
land
(%)
Absolute
patrolling
(%)
Bråfjorden
a Andreas
M
2010S
7
252
2
0
4.70
36.4
15.8
Bråfjorden
a
Leslie
F
2010S
17
747
2
0
4.80
41.0
11.2
Bråfjorden
a
Leslie
F
2014A
10
400
8
3
5.21
43.6
6.6
Bråfjorden
b
Moritz
M
2010A
6
159
2
0
2.87
43.0
4.6
Gvarv
Lower
Hazel
F
2010S
22
722
8
0
2.41
46.3
15.0
Gvarv
Lower
Paddy
M
2012S
11
425
6
3
2.35
37.8
10.4
Gvarv Middle Klumpen
M
2014S
11
427
9
1
2.15
56.7
15.0
Lille
Patmos Ida
F
2010A
13
421
5
2
4.70
63.6
10.5
Lille
Patmos
Kjartan
M
2010A
8
242
5
2
4.96
54.6
20.3
Lunde
2
Lasse
M
2011S
9
327
5
1
5.37
31.1
25.5
Lunde
4a
Loran
M
2009A
10
400
3
0
2.31
45.5
56.5
Lunde
4a
Malena
F
2014S
8
254
3
0
7.42
42.6
12.2
Lunde
6a Bram
M
2011S
6
181
3
0
3.67
45.8
18.0
Lunde
6a
Maud
F
2009A
10
414
2
0
3.30
44.6
6.1
Norsjø
1
Jodie
F
2012S
11
407
5
1
5.02
52.6
22.1
Patmos
0
Hanne
F
2010A
10
271
4
1
4.33
34.0
8.2
Patmos
0 Jan
Marc M
2010A
14
461
4
1
4.70
32.7
14.6
Patmos
1
Live
F
2013A
18
247
5
2
2.32
34.8
9.3
Patmos
2a
Apple
F
2013A
16
709
5
0
1.80
44.7
9.1
Patmos
2b
Moses
M
2010A
14
340
5
2
1.84
48.9
48.8
Patmos
3a
Christina
F
2010A
18
539
3
0
1.68
43.4
51.4
Patmos
3b
Erlend
M
2010S
12
470
3
1
1.49
45.6
18.3
Patmos
3b
Erlend
M
2013A
15
448
3
1
1.47
68.4
36.9
Patmos
4 Horst
M
2010A
5
152
3
1
1.49
62.9
7.4
Patmos
5
Tanja
F
2014S
5
102
5
0
3.69
68.5
29.0
Patmos
6
Ase
F
2014S
10
370
4
0
4.67
37.0
14.5
Patmos
6
Edwin
M
2014S
5
182
4
0
5.15
37.7
15.6
fewer
patches.
Similarly,
this
uncertainty
may
have
resulted
in
an
underestimate
of
the
average
distance
moved
per
hour
in
water
since
beavers
could
have
been
on
land
in
between
two
consecutive
water
positions.
By
choosing
a
GPS
sampling
interval
of
15
min,
we
attempted
to
minimize
such
effects.
Moreover,
we
assume
that
uncertainties
were
consistent
among
individuals,
thereby
not
or
only
marginally
influencing
our
analysis.
Relative
patrolling
effort
was
estimated
as
the
time
a
beaver
spent
inside
the
border
zones
of
its
territory,
defined
as
the
pro-
portion
of
GPS
positions
inside
the
upper
(upstream)
and
lower
(downstream)
5%
zones
(ranging
from
74
to
371
m)
of
the
overall
territory
size.
To
obtain
a
measure
of
absolute
patrolling
effort,
i.e.,
how
much
time
a
beaver
spent
at
the
actual
territory
borders,
we
assumed
borders
as
independent
of
territory
size
and
defined
them
as
the
last
75
m
on
each
side
of
the
river
on
the
upper
and
lower
side
of
each
individual
territory.
This
75
m
buffer
was
chosen
because
Rosell
et
al.
(1998)
found
that
the
majority
of
scent
mounds
were
clumped
within
150
m
between
bordering
territories
(i.e.,
75
m
bor-
der
zone
per
territory).
For
both
relative
and
absolute
patrolling
effort,
we
only
used
GPS
positions
inside
water
and
within
two
meters
from
the
shoreline
on
land,
because
scent
marking
activity
is
limited
to
close
proximity
to
water
and
because
positions
further
inland
most
likely
were
foraging
sites
(Rosell
and
Nolet,
1997).
Statistical
analysis
We
used
generalized
linear
models
(GLM)
to
investigate
move-
ment
patterns
separately
for
water
and
land
positions,
as
the
mode
and
purpose
of
movement
differs
on
land
in
comparison
to
water
(swimming
vs.
walking,
and
patrolling
vs.
foraging).
Initially,
we
also
tested
generalized
linear
mixed
models
(GLMM)
as
two
beavers
were
equipped
with
a
GPS
twice;
however,
the
results
were
not
different
and
thus,
we
chose
the
simpler
GLMs.
The
dependent
variables
for
movement
patterns
in
water
were
average
distance
moved/h,
relative
patrolling
effort
(measured
as
the
proportion
of
all
positions
in
water
and
within
2
m
on
land
within
5%
border
zones),
and
absolute
patrolling
effort
(proportion
of
all
positions
in
water
and
within
2
m
on
land
within
75
m
from
the
borders;
three
separate
analyses).
The
dependent
variables
for
movement
pat-
terns
on
land
were
average
distance
from
the
shoreline,
time
spent
on
land
and
average
distance
between
GPS
positions/h
(three
sepa-
rate
analyses).
Average
distance
from
shoreline
was
ln-transformed
to
normalize
residuals
of
the
statistical
models
and
one
outlier
was
excluded
based
on
Cook’s
distance
(Cook,
1977).
The
independent
variables
used
in
all
six
analyses
were
terri-
tory
size,
resource
availability
(i.e.,
area
of
mixed-deciduous
forest
in
ha),
number
of
neighbours,
season
(spring
vs.
autumn),
and
the
beaver’s
age.
No
correlations
between
the
independent
variables
were
detected
(r
<
0.6
in
all
cases),
and
variance
inflation
factors
(VIF)
were
<3
(see
Zuur
et
al.,
2010).
To
avoid
overfitting
the
models
we
initially
tested
for
an
effect
of
sex
and
group
size
in
all
analyses,
but
removed
these
variables
as
there
was
no
effect.
For
the
analysis
of
each
dependent
variable,
we
selected
12
explanatory
models
a
priori
based
on
biological
knowledge.
These
models
included
the
full
model
(all
independent
variables,
no
interactions
due
to
small
sample
size
and
to
avoid
overfitting
the
model);
the
five
indepen-
dent
variables
in
separate
models;
and
six
models
with
a
two-way
interaction:
1)
number
of
neighbours
and
season;
2)
resource
avail-
ability
and
season;
3)
territory
size
and
resource
availability;
4)
territory
size
and
season,
5)
number
of
neighbours
and
age,
and
6)
season
and
age.
Model
selection
was
based
on
Akaike
weights
(Table
S1)
(Wagenmakers
and
Farrell,
2004),
i.e.,
the
model
with
the
highest
conditional
probability
was
chosen,
and
parameters
that
included
zero
within
their
95%
confidence
interval
(CI)
were
considered
as
uninformative
(Arnold,
2010)
as
their
estimated
coef-
ficients
could
not
be
reliably
interpreted.
All
statistical
analyses
were
performed
using
the
software
R
3.1.1
(R
Core
Team,
2015).
Results
Twenty-five
dominant
beavers
(13
males
and
12
females)
of
17
different
territories,
and
ranging
between
3
and
14
years
of
age
(mean
±
SD:
7.3
±
3.2
years)
were
equipped
with
a
GPS
(two
indi-
viduals
were
tagged
twice,
Table
1).
Thirteen
beavers
were
captured
in
spring
and
14
in
autumn.
On
average,
the
GPS
units
delivered
11
nights
of
data
(range:
5–22)
and
356
GPS
positions
(range:

P.M.
Graf
et
al.
/
Mammalian
Biology
81
(2016)
587–594
591
Fig.
2.
Predicted
relationship
between
territory
size
(given
as
bank
length
in
km)
and
(a)
average
distance
moved/h
(in
m)
in
water,
and
(b)
relative
patrolling
effort
defined
as
the
proportion
of
GPS
positions
close
to
the
territory
borders
(within
the
lower
and
upper
5%
of
the
territory)
for
25
Eurasian
beavers
(Castor
fiber)
in
southeast
Norway.
102–747)
per
individual.
The
number
of
neighbouring
colonies
var-
ied
between
two
(n
=
13
territories)
and
three
(n
=
4
territories)
with
the
number
of
neighbours
varying
between
4
and
16
individuals
(7.1
±
2.8).
Territory
sizes
varied
between
1472
and
7425
m
bank
length
(3550
±
1591
m,
Table
1).
Four
beavers
(one
female
and
three
males)
made
explorative
trips
into
neighbouring
territories;
two
individuals
did
three
and
two
individuals
did
one
explorative
trip.
These
trips
lasted
on
average
1.9
±
1.1
h
(range:
0.5–3.5
h).
Movement
patterns
in
water
When
in
water,
beavers
stayed
on
average
14
±
5
m
(range:
0–255
m)
from
the
shoreline,
and
moved
on
average
682
±
204
m/h
(individual
range:
335–1106
m/h).
The
average
distance
moved/h
was
best
explained
by
territory
size
(Tables
2
and
S1),
with
beavers
in
larger
territories
moving
greater
distances
(Fig.
2a).
Relative
patrolling
effort
varied
between
1.4
and
50.5%
(21.8
±
11.5%)
and
was
best
explained
by
territory
size
(Tables
2
and
S1,
Fig.
2b),
with
beavers
in
larger
territories
spending
more
time
patrolling.
On
aver-
age,
beavers
visited
at
least
one
territory
border
in
81.5
±
20.7%
of
the
recorded
days.
The
absolute
patrolling
effort
varied
between
4.6
and
56.6%
(19.0
±
14.1%)
and
was
best
explained
by
the
age
of
an
individual
(Tables
2
and
S1);
older
beavers
were
spending
more
time
at
the
border.
Fig.
3.
Back-transformed
prediction
(solid
line)
between
territory
size
(measured
as
bank
length)
and
average
distance
from
the
shoreline
for
all
land
positions
(a),
and
predicted
relationship
between
the
age
of
an
individual
and
the
time
spent
on
land
(b)
for
25
Eurasian
beavers
(Castor
fiber)
in
southeast
Norway.
Movement
patterns
on
land
When
being
on
land,
beavers
stayed
on
average
16
±
8
m
(range:
0–201
m)
from
the
shoreline.
The
average
distance
from
the
shore-
line
was
best
explained
by
territory
size
(Table
2,
Fig.
3a),
i.e.,
beavers
in
larger
territories
stayed
closer
to
the
water.
Beavers
spent
between
31.1
and
68.5%
of
their
active
time
on
land
(46.1
±
10.5%).
Time
spent
on
land
was
best
explained
by
the
age
of
an
individual
(Tables
2,
S1),
with
older
beavers
spending
more
time
on
land
(Fig.
3b).
The
distance
between
GPS
positions
on
land
was
on
average
355
±
121
m/h
(individual
range:
165–641
m/h),
and
was
best
explained
by
season
(Table
2),
i.e.,
beavers
moved
greater
distances
on
land
in
spring
compared
to
autumn.
Discussion
We
found
that
beavers
adjusted
their
movement
patterns
in
water
and
on
land
in
relation
to
territory
size
and
age.
Beavers
in
larger
territories
moved
greater
distances
in
water
and
spent
more
time
within
relative
territory
borders,
thereby
indicating
that
they
patrolled
more.
Further,
individuals
in
larger
territories
stayed
closer
to
the
shoreline
when
on
land,
i.e.,
when
foraging.
In
contrast,
beavers
in
smaller
territories
had
a
lower
relative
patrolling
effort
and
foraged
further
away
from
the
shoreline.
Age
also
affected
movement
patterns
with
older
beavers
spending
more
time
on
land
and
at
territory
borders.
In
addition,
we
also
found
that
beavers
generally
moved
greater
distances
between
land
positions
during
spring.

592
P.M.
Graf
et
al.
/
Mammalian
Biology
81
(2016)
587–594
Table
2
Results
of
the
model
selection
showing
the
best
model
based
on
Akaike
weight
for
the
six
dependent
variables
for
25
Eurasian
beavers
(Castor
fiber)
that
were
equipped
with
a
GPS
between
2009
and
2014
in
southeast
Norway.
Number
of
observations
=
27,
␤
=
estimated
coefficient,
SE
=
standard
error,
LCI
=
lower
limit
of
the
95%
confidence
interval,
UCI
=
upper
limit
of
the
95%
confidence
interval.
Dependent
variable
AICc
Akaike
weight
Predictor
␤
SE
LCI
UCI
R2
Movement
patterns
in
water
Average
distance
moved/h
348.3
0.77
Territory
size
94.820
17.200
61.111
128.535
0.55
Relative
patrolling
effort
206.3
0.55
Territory
size
3.739
1.242
1.304
6.173
0.27
Absolute
patrolling
effort
221.4
0.34
Age
1.655
0.832
0.025
3.285
0.14
Movement
patterns
on
land
Average
distance
from
shoreline 27.2
0.46
Territory
size −0.132 0.052
−0.233
−0.030
0.21
Time
spent
on
land
201.5
0.55
Age
1.723
0.575
0.596
2.849
0.26
Average
distance
moved/h
327.5
0.43
Season
Spring
151.390
36.590
79.664
223.113
0.41
Territory
sizes
found
in
this
study
are
comparable
with
radio-
tracking
derived
territory
sizes
for
beavers
in
the
same
study
area
(Campbell
et
al.,
2005;
Herr
and
Rosell,
2004),
as
well
as
territory
sizes
of
Eurasian
beavers
in
general
(Heidecke,
1986;
Nolet
and
Rosell,
1994).
Our
results
suggest
that
individuals
may
trade-off
the
costs
of
patrolling
larger
territories
against
the
benefits
of
foraging
closer
towards
the
shoreline.
Changing
movement
patterns
with
age
Interestingly,
we
found
that
older
beavers
spent
more
time
within
the
75
m
border
zones
than
younger
ones
and,
thus,
had
a
greater
absolute
patrolling
effort.
However,
the
distance
moved
in
water
was
not
explained
by
age,
suggesting
that
older
beavers
spent
more
time
per
visit
at
a
border.
Further,
we
found
that
older
beavers
spent
more
time
on
land.
Spending
more
time
at
territory
borders
(patrolling
via
presence)
may
in
effect
allow
beavers
to
spend
more
time
on
land
instead
of
swimming
between
up-
and
downstream
borders.
Beavers
can
reach
20
years
of
age
(Gorbunova
et
al.,
2008)
and
only
five
beavers
in
this
study
were
older
than
10
years,
suggesting
that
the
observed
pattern
was
rather
related
to
a
change
in
personality
than
senescence.
Other
studies
reported
changing
movement
patterns
in
relation
to
sex
or
social
status,
e.g.
Sollmann
et
al.
(2011)
found
that
female
jaguars
(Panthera
onca)
both
had
smaller
home
ranges
and
moved
less
than
males,
and
Messier
(1985)
found
different
amounts
of
extraterritorial
move-
ments
between
adult
and
yearling
wolves
(Canis
lupus).
However,
to
our
knowledge
we
are
the
first
to
report
changing
movement
patterns
in
relation
to
age
within
individuals
of
the
same
social
status
(dominant
territory
holders).
Beavers
possibly
gain
experience
over
the
years
as
a
territory
holder,
leading
to
enhanced
boldness
and
dominance.
Moreover,
experiencing
a
low
abundance
of
natural
predators
coupled
with
a
relatively
low
hunting
pressure
in
the
area
may
lead
to
increased
boldness
of
older
individuals
explaining
the
higher
proportion
of
time
spent
on
land.
Plasticity
in
behavioural
traits
related
to
(social)
learning
allows
for
adjusting
behaviour
based
on
environmental
conditions,
which
is
important
for
individual
fitness
(Dingemanse
et
al.,
2010;
Frost
et
al.,
2007).
An
increase
in
boldness
with
age
was
shown
in
perch
(Perca
fluviatilis)
(Magnhagen
and
Borcherding,
2008),
and
a
shift
in
behavioural
traits
with
age
has
also
been
shown
in
humans
(Martin
et
al.,
2002;
Wilson
et
al.,
1994).
Body
mass
was
shown
to
influence
the
boldness
of
fish
and
reptiles
(Brown
and
Braithwaite,
2004;
Mayer
et
al.,
2016);
this
could
also
partly
explain
an
increasing
boldness
with
age
in
beavers
as
they
reach
their
maximum
body
mass
around
age
seven
(Mayer
et
al.,
unpubl.
results).
Movement
in
water
and
patrolling
effort
Beavers
typically
disperse
along
watersheds;
therefore
intru-
sion
by
dispersers
is
most
likely
to
occur
at
the
up-
or
downstream
borders
of
a
territory
(Herr
and
Rosell,
2004;
Rosell
et
al.,
1998).
Thus,
scent-marking
activity
is
highest
within
border
zones
(Rosell
and
Thomsen,
2006),
and
border
visits
are
crucial
for
beavers,
particularly
in
saturated
populations.
Holders
of
larger
territories
showed
a
higher
relative
patrolling
effort
(as
determined
by
the
presence
within
5%
border
zones)
and
thus
swam
greater
dis-
tances.
This
suggests
that
beavers
in
larger
territories
generally
spent
more
time
patrolling
territory
borders.
As
beavers
in
larger
territories
have
to
cover
greater
distances
to
reach
the
borders,
they
face
higher
patrolling
costs
for
two
reasons:
swimming
has
been
shown
to
decrease
the
body
temperature
compared
to
being
on
land,
especially
during
winter
and
early
spring
(Nolet
and
Rosell,
1994).
In
addition,
an
increased
patrolling
effort
constrains
the
time
that
beavers
can
spend
foraging.
Similarly,
wild
chimpanzees
(Pan
troglodytes)
reduced
their
feeding
time
from
33%
to
10%
during
patrolling
trips
(Amsler,
2010),
which
demonstrates
the
trade-off
between
foraging
and
patrolling.
Foraging
distance
from
the
shoreline
and
distance
between
land
positions
As
central
place
foragers,
beavers
should
deplete
foraging
patches
close
to
the
water
before
exploiting
patches
further
away
(Orians
and
Pearson,
1979).
However,
beavers
need
to
forage
fur-
ther
inland
once
the
majority
of
food
plants
close
to
the
shoreline
are
depleted.
Beavers
in
smaller
territories
were
found
to
move
farther
away
from
the
shoreline
when
on
land,
which
suggests
resource
depletion
along
the
shoreline.
However,
travelling
on
land
to
forage
is
also
considered
to
be
costly
both
energetically
and
in
time
(Belovsky,
1984;
Haarberg
and
Rosell,
2006),
as
terrestrial
for-
ays
enhance
predation
risk
(Basey
and
Jenkins,
1995).
In
addition,
transporting
food
items
on
land
is
an
arduous
task
compared
to
the
efficient,
buoyancy-supported
transport
in
water
(Novak,
1987).
Several
studies
found
beavers
to
be
more
selective
(both
in
food
item
size
and
species)
when
foraging
at
greater
distances
from
the
shore
(Fryxell
and
Doucet,
1991;
Haarberg
and
Rosell,
2006;
Jenkins,
1980),
however,
this
selectivity
diminished
in
low
quality
habitats
(Gallant
et
al.,
2004).
Unfortunately,
the
resource
availabil-
ity
in
our
study
area
was
not
measured
on
the
ground,
but
based
on
land
cover
data.
This
relatively
poor
temporal
and
spatial
reso-
lution
did
not
allow
us
to
measure
changes
in
resource
availability
over
the
years,
and
we
cannot
exclude
the
possibility
that
it
did
influence
the
observed
movement
patterns.
Contrary
to
our
prediction,
we
found
that
beavers
moved
greater
distances
between
land
positions
in
spring,
independent
of
terri-
tory
size.
This
could
be
a
strategy
to
compensate
for
winter
weight
loss
via
more
selective
foraging
(in
patches
further
apart
from
each
other)
during
the
spring
green-up
when
food
quality
is
higher.
For
example,
North
American
beavers
were
shown
to
utilize
differ-
ent
resources
in
different
seasons
(Milligan
and
Humphries,
2010;
Svendsen,
1980),
which
may
result
in
different
movement
patterns
when
foraging.
Similarly,
a
study
on
food-caching
behaviour
of

P.M.
Graf
et
al.
/
Mammalian
Biology
81
(2016)
587–594
593
North
American
beavers
describes
higher
selectivity
of
tree
species
early
in
autumn
compared
to
later
in
the
caching
season,
suggest-
ing
that
beavers
appear
to
balance
energy
content
and
nutritional
diversity
of
the
food
cache
(Busher,
1996).
Another
semi-aquatic
rodent,
the
capybara
(Hydrochaeris
hydrochaeris),
also
displayed
different
seasonal
foraging
patterns,
spending
more
time
for
for-
aging
during
the
dry
season,
but
being
more
selective
during
the
rainy
season
when
food
quality
was
higher
(Barreto
and
Herrera,
1998).
The
trade-off
between
patrolling
and
foraging
distance
In
general,
food
abundance
has
been
shown
to
affect
the
inten-
sity
of
territorial
defence
in
animals,
resulting
in
smaller
territories
(Carpenter,
1987;
Simon,
1975)
or
even
non-territorial
behaviour
(Davies
and
Houston,
1984)
during
high
food
availability.
Beavers,
however,
show
a
strong,
year-round
territorial
defence
(Nolet
and
Rosell,
1994),
which
may
reduce
the
rate
of
resource
depletion
and
increases
food
availability
during
the
cold
months
when
vegetation
is
scarce.
Because
the
study
population
is
at
carrying-capacity
for
the
last
ten
years
(Campbell
et
al.,
2005;
Steyaert
et
al.,
2015),
likely
all
territories
in
our
study
area
are
affected
by
resource
depletion
along
the
shoreline.
Resource
depletion
forces
beavers
to
forage
further
inland
(Goryainova
et
al.,
2014)
and,
in
combination
with
male
feeding
territory
defence,
has
been
suggested
to
have
trig-
gered
the
evolution
of
social
monogamy
in
beavers
(Busher,
2007;
Sun,
2003).
These
findings
could
explain
the
trade-off
we
observed
in
this
study:
In
larger
territories
beavers
have
to
invest
more
time
in
patrolling
activities,
but
can
forage
closer
to
the
shoreline.
In
smaller
territories
beavers
moved
greater
distances
on
land,
pos-
sibly
as
a
consequence
of
resource
depletion
along
the
shoreline.
However,
the
short
distance
between
the
up-
and
downstream
border
reduces
patrolling
costs
and
results
in
greater
efficiency
in
territorial
defence,
thus,
compensating
for
increased
foraging
costs
in
smaller
territories.
Territory
size
seems
to
act
as
a
counterbalancing
factor
for
patrolling
and
foraging,
making
both
owning
larger
and
smaller
territories
a
viable
strategy
in
beaver
populations
at
high
densi-
ties.
This
is
supported
by
the
findings
of
Campbell
et
al.
(2005),
who
found
that
beaver
territories
are
not
configured
to
a
mini-
mum
economically
defensible
area:
they
rather
seem
to
occupy
larger
territories
to
reduce
the
rate
of
resource
depletion
dur-
ing
initial
settlements
in
an
area,
whereas
in
populations
at
or
near
carrying-capacity,
territories
that
become
vacant
are
con-
quered
independent
of
size.
Our
findings
stress
the
need
for
further
investigations
on
whether
constraints
in
foraging
or
constraints
in
territorial
defence
have
greater
impact
on
the
length
of
territory
occupation
and
long-term
life
history
parameters
such
as
life-time
reproductive
success.
Acknowledgements
The
authors
thank
Christian
A.
Robstad,
Manuel
E.
Echeverria,
and
Anders
Mydland
for
capture
and
technical
support,
and
Shane
Frank
for
advice
on
analyses
and
statistics.
Also,
we
thank
the
three
anonymous
reviewers
for
constructive
comments
improving
our
manuscript.
This
study
was
funded
by
the
University
College
of
Southeast
Norway.
Appendix
A.
Supplementary
data
Supplementary
data
associated
with
this
article
can
be
found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.mambio.2016.07.
046.
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