Conspecific aggression by beavers (Castor canadensis) in the sangamon river basin in central Illinois: Correlates with habitat, age, sex and season

TLDR: The results suggest both sexes participate in territorial defense through physical confrontations and such encounters can be costly to both dispersing juveniles and resident adults.

Abstract: Conspecific aggression may play an important role in partitioning resources and maintaining territories among beavers (Castor canadensis), yet few studies have examined physical evidence of agonistic encounters. We trapped and examined pelts from 147 beavers harvested between 2006 and 2012 from the Sangamon River (n  =  96) and tributary streams (n  =  51) in central Illinois. We modeled the influence of sex, age class, season (predispersal or dispersal), and habitat (river or tributary stream) on the number of recent injuries caused by conspecifics. One-third (51/147) of beavers had ≥1 injury; of those, the median number of injuries was 2.0. Kits had fewer injuries than adults (βKit  =  −2.24 ± 0.63), but yearlings and subadults did not (βyearling  =  0.02 ± 0.38, βsubadult  =  −0.22 ± 0.48). Beavers on small streams had only one-quarter of the injuries recorded for beavers on the river (βStream  =  −1.34 ± 0.82). We failed to detect differences in injuries between the sexes. Our results suggest...

Figures

Table 1.— Model selection results of negative binomial models used to model the number of 400 injuries detected upon necropsy for beavers trapped from the Sangamon River and (n = 96) and 401 tributary streams (n = 51) in central Illinois during 2006. For brevity, only models within 3 AICc 402 units of the top model are shown; k is the number of parameters in the model and wi/wj is the 403 relative weight of the model out of the candidate set (n = 36). All candidate models were used to 404 estimate model averaged parameters and standard errors. 405

FIG. 4. —Frequency of injuries due to conspecific aggression detected in beavers trapped along 423 the main channel of the Sangamon River (n = 96) and tributary streams (n = 51) from tributary 424 streams in central Illinois 425 426

Full text

Southern Illinois University Carbondale
OpenSIUC
8&0-'%7-326 )4%571)273*#3303+<

!( !!'$ !!"
!!"$ !
" ! "!%"""&
!
 % 
Southern Illinois University Carbondale'5%:*35(.3%22)+1%-0'31
 "#)
Illinois Department of Natural Resources&3&&08)@-00-23-6+39
 #
Southern Illinois University Carbondale6',%8&)56-8)(8
3003:7,-6%2(%((-7-32%0:35/6%7 ,@434)26-8'0-&6-8)(8=330$48&6
>-657-'0)-6&538+,773<38*35*5))%2(34)2%'')66&<7,))4%571)273*#3303+<%74)2!"7,%6&))2%'')47)(*35-2'086-32-28&0-'%7-326
&<%2%87,35-=)(%(1-2-675%7353*4)2!"35135)-2*351%7-3240)%6)'327%'7 34)26-8'0-&6-8)(8
)'311)2()(-7%7-32
5%:*35(3%22)08)@ 3&)57%2(!',%8&)55-'3264)'-?'++5)66-32&<)%9)56%6735'%2%()26-6-27,)
!%2+%132 -9)5%6-2-2)275%000-23-6355)0%7)6:-7,%&-7%7+)!);%2(!)%632 American Midland Naturalist 3 %2
(3-

4
Corresponding author present address: Department of Fisheries and Wildlife, Michigan State
University, East Lansing, 48824. Email: crawford.joanne@gmail.com
Joanne C. Crawford, Department of Fisheries and Wildlife, Michigan State University, 480 1
Wilson Road, Room 13 Natural Resources Bldg. East Lansing, Michigan 48824, Tel: 517. 2
.355.4478, Fax: 517.432.1699, email: crawford.joanne@gmail.com 3
Running Head: CRAWFORD ET AL.—AGGRESSION IN BEAVERS 4
Conspecific Aggression by Beavers (
Conspecific Aggression by Beavers (Conspecific Aggression by Beavers (
Conspecific Aggression by Beavers (
Castor canadensis
Castor canadensisCastor canadensis
Castor canadensis
) in the Sangamon River Basin
) in the Sangamon River Basin ) in the Sangamon River Basin
) in the Sangamon River Basin 5
in Central Illinois: Correlates with Habitat, Age, Sex and Season
in Central Illinois: Correlates with Habitat, Age, Sex and Seasonin Central Illinois: Correlates with Habitat, Age, Sex and Season
in Central Illinois: Correlates with Habitat, Age, Sex and Season
6
JOANNE C. CRAWFORD
1, 4
7
1
Cooperative Wildlife Research Laboratory, Department of Forestry, and Center for Ecology,
8
Southern Illinois University, Carbondale, 62901
9
ROBERT D. BLUETT
2
10
2
Illinois Department of Natural Resources, One Natural Resources Way, Springfield, 62702,
11
AND 12
ERIC M. SCHAUBER
3
13
3
Cooperative Wildlife Research Laboratory, Department of Zoology, and Center for Ecology,
14
Southern Illinois University, Carbondale, 62901
15
16
ABSTRACT.— Conspecific aggression may play an important role in partitioning resources and 17
maintaining territories among beavers (Castor canadensis), yet few studies have examined 18
physical evidence of agonistic encounters. We trapped and examined pelts from 147 beavers 19
harvested between 2006 and 2012 from the Sangamon River (n = 96) and tributary streams (n = 20
51) in central Illinois. We modeled the influence of sex, age class, season (predispersal or 21
dispersal), and habitat (river or tributary stream) on the number of recent injuries caused by 22
conspecifics. One-third (51/147) of beavers had ≥ 1 injury; of those, the median number of 23
injuries was 2.0. Kits had fewer injuries than adults (β
Kit
= -2.24 ± 0.63), but yearlings and 24
subadults did not (β
yearling
= 0.02 ± 0.38, β
subadult
= -0.22 ± 0.48). Beavers on small streams had 25

4
Corresponding author present address: Department of Fisheries and Wildlife, Michigan State
University, East Lansing, 48824. Email: crawford.joanne@gmail.com
only one-quarter of the injuries recorded for beavers on the river (β
Stream
= -1.34 ± 0.82). We 26
failed to detect differences in injuries between the sexes. Our results suggest that both sexes 27
participate in territorial defense through physical confrontations, and that such encounters can be 28
costly to both dispersing juveniles and resident adults. 29
INTRODUCTION 30
Conspecific aggression plays an important role in partitioning resources and maintaining 31
social order among beavers (Castor canadensis). These benefits entail risks to individuals 32
because agonistic encounters can lead to injuries that reduce fitness or cause mortality 33
(Svendsen, 1980; DeStefano et al., 2006). Svendsen (1980) noted bite wounds on two dispersing 34
subadults and one kit that had died from bite wounds inflicted by an unrelated adult. Sun (2003) 35
speculated that intercolony agonistic encounters were rare, but noted a lack of empirical evidence 36
to support these views. Bradt (1938) suggested that subadults were actively driven from the 37
colony by adults, but videos of beavers in dens have revealed few agonistic behaviors among 38
colony members (0.1% of time-activity budget; Mott et al., 2011). Behaviors of beavers 39
engaged in agonistic encounters (e.g., pushing, lunging, sham-biting and biting; Bradt, 1938; 40
Wilsson, 1971; Hodgdon, 1978; Hodgdon and Lancia, 1983) resemble those of rats (Rattus 41
norvegicus; Takhashi and Blanchard, 1982), mice (Mus musculus; Litvin et al., 2007) and other 42
rodents observed in captivity (Blanchard et al., 1979). Thus, it is reasonable to assume defensive 43
strategies reduce the likelihood of injuries, most bites are inflicted on the back or other non-vital 44
parts of an opponent, and few bites pierce the skin (Blanchard et al., 1979; Takhashi and 45
Blanchard, 1982; Blanchard et al., 2003; Litvin et al., 2007). 46

4
Corresponding author present address: Department of Fisheries and Wildlife, Michigan State
University, East Lansing, 48824. Email: crawford.joanne@gmail.com
Behavioral adaptations also should reduce the incidence and severity of injuries caused by 47
conspecific aggression. For example, scent marking reduces the incidence of agonistic 48
encounters by advertising territorial boundaries, as does the ability to distinguish scents of kin 49
and neighbors from those of strangers (Welsh and Müeller-Schwarze, 1989; Davis et al., 1994; 50
Sun and Müeller-Schwarze, 1997, 1998; Rosell and Bjørkøyli, 2002; Herr et al., 2006). 51
Territory-holding adults advertise their status and defend colony boundaries through scent 52
marking, but they may be less likely to initiate aggressive attacks given that they risk losing their 53
colony during such encounters. In contrast dispersing subadults searching for a colony may have 54
little choice but to challenge territory-holders by overmarking and aggressive behavior 55
(Tinnesand et al., 2013). Scent marking increases with increasing colony density, suggesting 56
that beavers must spend more time defending territorial boundaries (Davis et al., 1994; Muller-57
Schwarze and Heckman, 1980). Accordingly we would expect aggressive encounters to increase 58
with increasing colony density or along primary dispersal corridors. Therefore aggressive 59
encounters may be more frequent in saturated or nearly saturated populations, in which juveniles 60
make exploratory movements, but often fail to disperse (Havens, 2006; DeStefano et al., 2006; 61
Bloomquist and Nielsen, 2010). 62
Common hypotheses regarding aggression and territoriality in beavers include: (1) males are 63
more likely than females to engage in aggressive encounters; (2) dispersing subadults are more 64
prone to attacks than residents of established colonies; (3) aggressive encounters are more likely 65
to occur during the dispersal season; and (4) colonies with discrete, easily defended territories 66
such as those on small streams are less prone to aggressive encounters than beavers inhabiting 67
open systems such as large wetlands, lakes or rivers (Nordstrom, 1972; Müeller-Schwarze and 68

4
Corresponding author present address: Department of Fisheries and Wildlife, Michigan State
University, East Lansing, 48824. Email: crawford.joanne@gmail.com
Heckman, 1980; Davis et al., 1994; Baker and Hill, 2003, Tinnesand et al., 2013). Most 69
attempts to quantify aggressive behaviors of North American and European beavers (C. fiber) 70
have relied on staged responses of residents to scents of intruders (Welsh and Müeller-Schwarze, 71
1989; Sun and Müeller-Schwarze, 1997, 1998; Rosell and Bjørkøyli, 2002; Herr et al., 2006, 72
Tinnesand et al., 2013). Reports of injuries caused by conspecific aggression are rare (e.g., 73
Müeller-Schwarze and Schulte, 1999). To that end we examined pelts of beavers trapped in the 74
Sangamon River Basin of central Illinois to identify injuries caused by agonistic encounters. 75
Counts of injuries were used to test hypotheses about sex, age, habitat (main stem of Sangamon 76
River or 1
st
-3
rd
order streams), and season (predispersal or dispersal). 77
METHODS 78
STUDY AREA 79
The Sangamon River originates in McLean County, Illinois. Its main stem flows 386 km 80
before emptying into the Illinois River (Illinois Department of Natural Resources, 2000). This 81
7
th
order stream drains 14,985 km
2
(ca. 10% of the state; Illinois Department of Natural 82
Resources, 2001). Sampling occurred on the main stem of the Sangamon River near Petersburg, 83
Illinois as well as on 1
st
to 3
rd
order streams in a wider geographic area of the river basin (Fig. 1; 84
40°1′N, 89°50′W). We lacked data about densities of beaver in our study area, but assumed 85
they were high, as in other parts of the state (0.1–0.6 colonies/km of stream; Woolf et al., 2003; 86
Cox and Nelson, 2009). 87
CAPTURE AND HANDLING 88
We set body-gripping traps (25.4 x 25.4-cm frame) at artificial scent mounds, dam 89
crossovers, channels and den entrances (Baker and Dwyer, 1987) during legal trapping seasons 90