1
Title: A behavioral logic underlying aggression in an African cichlid fish
1
2
Short title: Aggression in a cichlid
3
4
Authors: Beau A. Alward
1,2,3*
, Phillip H. Cathers
3
, Danielle M. Blakkan
3
, Russell D. Fernald
3
5
6
Affiliations:
1
University of Houston, Department of Psychology;
2
University of Houston,
7
Department of Biology and Biochemistry;
3
Stanford University, Department of Biology
8
9
*Corresponding author: balward@uh.edu
10
11
Competing interest: We have no competing interests.
12
13
Author’s contributions: Conceptualization, B.A.A., P.H.C., D.M.B., R.D.F.; Methodology,
14
B.A.A., P.H.C., D.M.B.; Investigation, B.A.A., P.H.C.; Writing – Original Draft, B.A.A.; Writing –
15
Review & Editing, B.A.A., P.H.C., D.M.B., R.D.F.; Supervision, B.A.A., R.D.F.
16
17
18
Abstract
19
20
Social rank in a hierarchy determines which individuals have access to important resources
21
such as food, shelter, and mates. In the African cichlid fish Astatotilapia burtoni, rank is under
22
social control, such that larger males are more likely than smaller males to be dominant in rank.
23
Although it is well known that the relative size of A. burtoni males is critical in controlling social
24
rank, the specific behavioral strategies underlying responses to males of different sizes are not
25
well understood. In this research, our goal was to characterize these responses by performing
26
resident-intruder assays, in which aggressive behaviors were measured in territorial males in
27
response to the introduction of unfamiliar males that differed in relative standard length (SL). We
28
found that the relative SL of intruders played an important role in determining behavioral
29
performance. Resident males exposed to larger (>5% larger in SL) or matched (between 0 and
30
5% larger or smaller in SL) intruder males performed more lateral displays, a type of non-
31
physical aggression, compared to resident males exposed to smaller (>5% smaller in SL)
32
intruder males. However, physical aggression, such as chases and bites, did not differ as a
33
function of relative SL. Our results suggest that A. burtoni males amplify non-physical
34
aggression to settle territorial disputes in response to differences in relative SL that were not
35
previously considered to be behaviorally relevant.
36
.CC-BY-NC-ND 4.0 International licenseavailable under a
was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made
The copyright holder for this preprint (whichthis version posted July 22, 2020. ; https://doi.org/10.1101/2020.07.22.216473doi: bioRxiv preprint
2
Keywords: Aggression, territorial, resident-intruder, social behavior, cichlid
37
38
Highlights
39
40
• Relative size determines social rank in the African cichlid Astatotilapia burtoni
41
• Resident male A. burtoni respond differently to small size differences in intruder males
42
• Residents perform more non-physical aggression against larger intruders
43
• Residents do not alter physical aggression as a function of differently sized intruders
44
• Distinct behavioral strategies are used against different intruders
45
46
Introduction
47
48
Intraspecific aggression is widespread among social animals (van Staaden, Searcy, &
49
Hanlon, 2011). Aggressive behavior, either through physical attacks or non-physical signaling, is
50
used to resolve conflicts related to access to resources such as food, shelter, territory, and
51
mates. Extraordinary diversity exists in how different species express aggression and the rules
52
that govern aggressive interactions. However, one rule seems to apply across species: physical
53
or injurious behaviors are considered to be escalatory, occurring primarily in response to
54
conflicts that are difficult to resolve (Holekamp & Strauss, 2016; Maynard Smith & Harper, 1988;
55
van Staaden et al., 2011). The degree of conflict has been formally defined in terms of
56
differences in resource holding potential (RHP). RHP can take the form of different levels of
57
fighting ability as measured by body or weapon size. When large asymmetries in RHP exist
58
among animals, aggressive interactions do not escalate from non-physical to physical; however,
59
when asymmetries in RHP among animals are smaller, aggressive interactions are more likely
60
to escalate, involving more physical and injurious forms of aggression.
61
62
.CC-BY-NC-ND 4.0 International licenseavailable under a
was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made
The copyright holder for this preprint (whichthis version posted July 22, 2020. ; https://doi.org/10.1101/2020.07.22.216473doi: bioRxiv preprint
3
Evolution has shaped social dynamics across species to resolve aggressive interactions
63
with as little physical fighting as possible, as this ensures individual and species survival
64
(Holekamp & Strauss, 2016; Maynard Smith & Harper, 1988; van Staaden et al., 2011). This is
65
abundantly clear in social animals that exist in a hierarchy, where rank determines which
66
individuals possess a territory and the behaviors they perform. This is the case for the African
67
cichlid fish Astatotilapia burtoni, where males stratify along a dominance hierarchy and exist as
68
either non-dominant or dominant (Fernald, 2012). Dominant males possess a territory which
69
they defend through aggressive interactions and in which they mate with females, while non-
70
dominant males do not perform these behaviors. Dominant males also possess larger testes
71
and brighter body coloration compared to non-dominant males. Social hierarchies in A. burtoni
72
remain in flux, however, as non-dominant males constantly survey the environment, searching
73
for a social opportunity to ascend in social rank to dominance. Social opportunity for a non-
74
dominant A. burtoni male typically occurs when a larger male is absent from the environment,
75
which a given smaller non-dominant male perceives as an opportunity to ascend to dominant
76
rank. Within minutes of the opportunity, the non-dominant male increases aggressive and
77
reproductive behavior in an attempt to establish a territory. Dominant males who encounter a
78
larger dominant male in their environment will begin to descend in social rank by reducing
79
aggressive and reproductive behavior (Maruska, Becker, Neboori, & Fernald, 2013).
80
81
Size-induced social control of social status in A. burtoni has been shown in several
82
studies. The reliable occurrence of this phenomenon makes size an excellent tool for
83
controlling social environments in the laboratory, with the goal of generating fish with a given
84
social status and studying the associated physiological underpinnings (for examples, see
85
Alward, Hilliard, York, & Fernald, 2019; Maruska, Becker, Neboori, & Fernald, 2013; Maruska &
86
Fernald, 2010). Although it has been shown repeatedly that size influences social status in male
87
A. burtoni, a precise understanding of the relationship between size and behavior has not been
88
.CC-BY-NC-ND 4.0 International licenseavailable under a
was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made
The copyright holder for this preprint (whichthis version posted July 22, 2020. ; https://doi.org/10.1101/2020.07.22.216473doi: bioRxiv preprint
4
established. For instance, while size is something that modifies social decisions in male A.
89
burtoni, it is unclear what size difference males actually perceive as different and how they
90
modify their behavior accordingly. Previous work has defined male A. burtoni as “matched” in
91
size within a large range of standard length (e.g., 0-10% larger or smaller in standard length
92
(SL=measured from the most anterior portion of the mouth to the most anterior portion of the
93
caudal fin); see Alcazar, Hilliard, Becker, Bernaba, & Fernald, 2014; Desjardins & Fernald,
94
2010)). However, recent work suggests that very small size differences between male A. burtoni
95
can affect social interactions. For example, Alcazar et al ( 2014) found that males that were 2.1-
96
4.9% larger in SL than their competitor consistently won during a contest, suggesting that size
97
differences previously regarded as “matched” may actually be behaviorally relevant. However,
98
this study was focused on which fish won each contest and not on the specific behavioral
99
strategies underlying responses to differently sized males.
100
101
Characterizing the specific behavioral patterns in A. burtoni that occur in response to
102
differently sized males may yield insight into the capacity of A. burtoni to discern different levels
103
of social opportunities, which would allow for a deeper understanding of the cognitive abilities
104
required to successfully navigate a social hierarchy. In the present study we characterized
105
behavioral responses in male A. burtoni as a function of differently sized male competitors
106
during resident-intruder assays, in which a dominant male with a territory (i.e., the resident) was
107
exposed to an unfamiliar, non-dominant male intruder that differed in relative standard length
108
(SL). (illustrated in Fig. 1). The results of this study could shed light on the rules of engagement
109
during social interactions in male A. burtoni.
110
.CC-BY-NC-ND 4.0 International licenseavailable under a
was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made
The copyright holder for this preprint (whichthis version posted July 22, 2020. ; https://doi.org/10.1101/2020.07.22.216473doi: bioRxiv preprint
5
111
Figure 1. The behavioral patterns in male Astatotliapia burtoni underlying responses to
112
differently sized males have not been characterized. Male A. burtoni change their social
113
status depending on the social environment. Large males socially suppress smaller males and
114
large males are more likely to be dominant. The specific behaviors males perform in response
115
to differences in relative size, however, have not been determined. We asked during a resident-
116
intruder assay what behavior patterns resident males use when presented with male intruders
117
that were smaller, larger, or matched in size.
118
119
Methods
120
121
Ethical Note
122
123
The protocols and procedures used here were approved by the Stanford University
124
Administrative Panel on Laboratory Animal Care (protocol number: APLAC_9882) and followed
125
the ASAB/ABS Guidelines for the use of animals in research. We were able to monitor the
126
behaviours of all fish throughout each day of the study (see below). Throughout the whole
127
assay, each tank was monitored in real time through a Wi-Fi-enabled camcorder remotely
128
connected to a tablet (iPad). Fish in all other tanks were monitored three times daily by visual
129
inspection, to ensure they experienced no physical harm. No fish were physically harmed at any
130
point during the assay.
131
.CC-BY-NC-ND 4.0 International licenseavailable under a
was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made
The copyright holder for this preprint (whichthis version posted July 22, 2020. ; https://doi.org/10.1101/2020.07.22.216473doi: bioRxiv preprint