Influence of Resource Abundance on Echimyid Rodent Interactions
Casey A. Krause1, Gregory H. Adler2, Laxman M. Hedge3, Erica H. Kennedy4, and Thomas D. Lambert1*
1Department of Biology, Frostburg State University, 101 Braddock Road, Frostburg, MD, 21532. 2Department of Biology, University of Wisconsin-Oshkosh, 800 Algoma Blvd, Oshkosh, WI, 54901. 3Department of Mathematics, Frostburg State University, 101 Braddock Road, Frostburg, MD, 21532. 4Department of Psychology, Frostburg State University, 101 Braddock Road, Frostburg, MD, 21532. *Corresponding author.
Abstract
We studied inter- and intraspecific interactions between two morphologically-similar echimyid rodents Proechimys semispinosus (Tome’s Spiny Rat) and Hoplomys gymnurus (Armored Rat) in central Panama. Due to their cryptic nature and nocturnal activity patterns, many aspects of the natural history of echimyids remain poorly documented, despite their being some of the most abundant mammal species in Neotropical forests. We performed paired behavioral trials with one individual having prior access to a resource during periods of resource abundance and scarcity. Species pairings demonstrated differences in aggression, but refuted seasonality and resource presence as main factors influencing aggression, suggesting other influencing factors.
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Volume 3, 2022 Neotropical Naturalist No. 6
Influence of Resource
Abundance on Echimyid
Rodent Interactions
Casey A. Krause, Gregory H. Adler,
Laxman M. Hedge, Erica H. Kennedy,
and Thomas D. Lambert
NEOTROPICAL NATURALIST
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Cover Photograph: Hoplomys gymnurus from Central Panama. Photograph © Casey Krause.
Neotropical Naturalist
C.A. Krause, G.H. Adler, L.M. Hedge, E.H. Kennedy, and T.D. Lambert
Vol. 3, 2022 No. 6
1
Vol. 3, 2022 NEOTROPICAL NATURALIST 6:1–7
Influence of Resource Abundance on
Echimyid Rodent Interactions
Casey A. Krause1, Gregory H. Adler2, Laxman M. Hedge3,
Erica H. Kennedy4, and Thomas D. Lambert1*
Abstract - We studied inter- and intraspecific interactions between two morphologically-similar
echimyid rodents Proechimys semispinosus (Tome’s Spiny Rat) and Hoplomys gymnurus (Armored
Rat) in central Panama. Due to their cryptic nature and nocturnal activity patterns, many aspects of
the natural history of echimyids remain poorly documented, despite their being some of the most
abundant mammal species in Neotropical forests. We performed paired behavioral trials with one
individual having prior access to a resource during periods of resource abundance and scarcity. Species
pairings demonstrated differences in aggression, but refuted seasonality and resource presence as
main factors influencing aggression, suggesting other influencing factors.
Introduction
Aggressive interactions are energetically costly and risky; thus, individuals increase their
fitness by avoiding direct hostilities. Dominance hierarchies, both within and between species,
help to reduce the frequency and intensity of agonistic interactions, thereby allowing
individuals to assess the likelihood of winning the interaction before the interaction occurs.
In seasonally-fluctuating environments, changes in the availability of a shared resource may
alter species interactions and resource use (Barger and Kitaysky 2011, Correa and Winemiller
2014, Venner et al. 2011). When periods of resource scarcity occur, resource competition increases.
During these times of scarcity, the strength of the dominance hierarchy is predicted
to be greatest (Isbell and Young 2002, Michel et al. 2016, Sterck et al. 1997) and species may
alter their use of the resource to reduce competition and overlap (Correa and Winemiller 2014,
Pianka 1974). However, during periods of scarcity, the relative importance of resources to the
individual is increased, and for non-social species, inter- and intraspecific encounters will become
more frequent. The increase in the value of the resource might increase the likelihood of
an aggressive interaction. Despite the theoretical predictions, studies have had mixed results
when searching for a relationship between resource abundance and the strength of dominance
hierarchies (Michel et al. 2016, Wikberg et al. 2013, Wright et al. 2014).
To test this relationship, we studied two species of echimyid rodents that are known
to have similar resource use and overlapping habitats with seasonal variations in resource
availability. These two species are Proechimys semispinosus (Tomes) (Tome’s Spiny Rat)
and Hoplomys gymnurus (Thomas) (Armored Rat). Both species are nocturnal and morphologically
similar and often occur sympatrically in central Panama (Adler et al. 1998,
Buchanan and Howell 1965, Endries and Adler 2005, Fleming 1971, Tomblin and Adler
1998). Due to the seasonality of their environment and main dietary resources, we predict
that behavioral changes should occur between seasons. Previous studies have focused on
1Department of Biology, Frostburg State University, 101 Braddock Road, Frostburg, MD, 21532.
2Department of Biology, University of Wisconsin-Oshkosh, 800 Algoma Blvd, Oshkosh, WI, 54901.
3Department of Mathematics, Frostburg State University, 101 Braddock Road, Frostburg, MD, 21532.
4Department of Psychology, Frostburg State University, 101 Braddock Road, Frostburg, MD, 21532.
*Corresponding author: tdlambert@frostburg.edu.
Associate Editor: Paulo Bobrowiec, Instituto Nacional de Pesqui sas da Amazônia.
Neotropical Naturalist
C.A. Krause, G.H. Adler, L.M. Hedge, E.H. Kennedy, and T.D. Lambert
Vol. 3, 2022 No. 6
2
encounters between the 2 species, demonstrating that the Tome’s Spiny Rat is generally
more aggressive than and behaviorally dominant to the Armored Rat during encounters
(Alberico and Gonzales 1993, Dupre 2012, Dupre et al. 2015). While some seasonal comparisons
have been made, such comparisons did not have a resource present (Dupre 2012,
Dupre et al. 2015). However, anecdotal observations collected during a previous study
showed increased aggression by the Armored Rat and potentially a reversal of the behavioral
dominance when it had prior access to a resource. In this study, we examine whether
there are seasonal differences in their encounters when a resource is present.
Methods
Data collection occurred during 2 consecutive wet (June and July, 2011 and 2012)
and dry (January, 2012 and 2013) seasons in both Soberania National Park and Barro
Colorado Nature Monument in central Panama. Central Panama experiences seasonality
in rainfall, with an 8-month wet season from May to December, punctuated by an intense
4-month dry season from December through April, during which less than 10% of annual
rainfall occurs (Windsor 1990). This seasonality results in predictable patterns in fruit and
seed availability. From the end of the dry season into the beginning of the wet season,
fruit production increases to where there is an abundance. This production decreases by
the end of the wet season and beginning of the dry season, resulting in a period of severe
resource scarcity (Adler 1998, Adler and Lambert 2008, Foster 1982, Poulin et al. 1999).
Individuals of both species were captured for behavioral trials using Tomahawk Live traps
(41 X 13 X 13 cm), handling and trapping techniques following the American Society
of Mammalogists’ guidelines (Sikes et al. 2011). Traps were checked every morning and
captured rodents were identified, sexed, weighed, and fitted with numbered ear tags for
individual identification. Individuals that were to used in the behavioral trials were then
taken to a protected holding facility where they were kept separately in covered cages
and provided with fresh plantain. The rodents were brought back to the holding facility
at around mid-day; thus, individuals were not held without access to food for no more
than a few hours (time of capture until approximately noon). Individuals were kept no
longer than one night to reduce the harmful effects of stress. Individuals were used in the
behavioral trial only once, recaptured individuals were immediately released, ensuring
that subjects involved in the study were experiencing a novel s ituation.
Behavioral trials were conducted by selecting 2 individual rodents, placing them in an
enclosure separated by an opaque barrier for a 5’ acclimation period, during which time one
individual had access to a food resource. After the acclimation period the barrier was removed,
and interactions were recorded for a 20’ trial period. These dyadic behavioral trials
were performed starting 1 hour after sunset in a 93 X 84 X 77 cm Plexiglass® enclosure.
This enclosure consisted of 4 sides with an open top and bottom and an opaque partition to
separate the rodents at the start and end of each trial. The open bottom was placed on natural
substrate, such as mowed grass or bare earth. The open top allowed for trials to be video recorded
with a camera (DCR-TRV22; Sony Co.) and infrared light (SL-20IR; Sima Products
Co.) stabilized on a tripod above the enclosure. Pairings were determined based on what
species were caught the night before and how many of each type of pairing had previously
been performed. Which individual had initial access to the resource was randomly selected.
Due to their year-round availability and consumption by both species, ripe plantains (Musa
sp.) were used as the resource in the trials.
After video recording the trials, the videos were analyzed for three specific interactive
behaviors using the coding program, Noldus: Observer XT program (8.0, Noldus InformaNeotropical
Naturalist
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tion Technology). Due to the different lengths of time for each trial, standardization of
the behavior counts was performed. The raw count for each behavior was standardized by
dividing the count by the trial duration in seconds and multiplying that by 10,000 (e.g.,
Dupre 2012, Dupre et al. 2015). Each behavior was defined by specific movements and the
directionality of those movements. For each behavior, one bout was counted by the initiation
of that behavior after a period of no movement, immediately after ceasing the actions
of a different behavior, or if that behavior was being performed and stopped for 5” before
restarting. If the behavior lasted longer than 10”, it was counted as a bout and as a new bout
for every 10” increment that it continued.
The three main behaviors analyzed were aggression, retreat, and approach, with aggression
being the focal behavior (e.g., Dupre et al. 2015) (Table 1). Behavioral rates were
compared using 8 explanatory variables, by both bout and individual. For individuals, the
8 explanatory variables were categorized as whether the individual had the resource (yes or
no), the species of the individual (Tome’s Spiny Rat (Ps) or Armored Rat (Hg)), the season
in which that trial took place (wet or dry), the type of pairing in which that individual participated
(Ps/Ps, Ps/Hg, or Hg/Hg), the individual’s age (young or adult), the individual’s sex
(female or male), their weight difference class based on the weight difference between the
two rodents in that trial (WDC) (1 = 1–75 g, 2 = 76–150 g, and 3 = 151 g +), and whether the
individual was reproductive (reproductive or non-reproductive). The explanatory variables
for the bouts of the behaviors were based on age of the pairings (young/young, young/adult,
adult/adult), the sex of the pairings (male/male, male/female, female/female), and the reproductive
state of the pairings (reproductive/reproductive, reproductive/non-reproductive,
non-reproductive/non-reproductive).
Comparisons for each of the behaviors to the explanatory variables were made to determine
if any of those variables influenced the frequency of th e 3 behaviors during a trial.
For instance, we wanted to determine if the pairing type caused different frequencies of
aggression for each species. All behaviors were tested for normality using the Cramer-von
Mises test. The data were non-normal due to the inflations in zero, so the non-parametric
Kruskal-Wallis rank sums test was used to compare the behaviors, with each of the explanatory
variables using an alpha of 0.05 to assess statistical significance (Dupre 2012, Dupre
et al. 2015, Johnson 1999).
Table 1. Descriptions for each of the interactive behaviors performed by individuals during trials.
Behavioral Category Description
Aggression
Lunge Rapid movement towards the other individual
Mount To climb on the back of the other from behind
Stance Standing on hind legs with paws in a boxing position. While
in this position one can bite, “punch”, or place its paws on
the arms or shoulders of the other individual (Alberico and
Gonzalez 1993)
Approach To move within 2” of the other individual
Retreat A responsive movement away from an immediate action
performed by the other individual
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Results
Of the 51 dyadic trials performed, 16 were interspecific, seven were paired Armored
Rats, and 28 were paired Tome’s Spiny Rats. Species pairing was the only explanatory variable
with rate of aggression showing significant variation for individuals (χ2 = 16.3070, df
= 2, p-value = 0.0003), and in bouts (χ2 = 10.6960, df = 2, p-value = 0.0048). Interspecific
pairings had the highest mean aggression of 29.66 ± 72.78, while Tome’s Spiny Rat pairings
had a mean of 2.27 ± 6.86, and Armored Rat pairs had zero acts of aggression (Fig.
1). Levels of aggression were not different during interspecific bouts (χ2 = 2.9376 df = 1,
p-value = 0.0865). However, post hoc testing demonstrated that the Tome’s Spiny Rat was
the more aggressive species based on a higher mean and a greate r variance (Fig. 2).
Rates of aggression did not differ between seasons for either individuals (χ2 = 0.9156
df = 1, p-value = 0.3386) or in bouts (χ2 = 0.7675, df = 1, p-value = 0.3810). Retreat also
did not differ between seasons for individuals (χ2 = 0.1715, df = 1, p-value = 0.6788) or
in bouts (χ2 = 0.1587, df = 1, p-value = 0.6904). Approach was different between seasons,
with higher rates during the dry season for both individuals (χ2 =6.0869, df = 1, p-value =
0.01362) and bouts (χ2 = 5.2250, df= 1, p-value = 0.02223) (Table 2).
Discussion
Our findings supported previous studies in demonstrating that species pairings produced
varying yet predictable levels of aggression (Alberico and Gonzalez 1993, Dupre 2012, Dupre
et al. 2015). As predicted, the two species exhibited higher levels of aggression towards
each other than they did intraspecifically. These findings suggest that interspecific encounters
are more likely to result in aggression and consequently cost more to the individuals.
Therefore, partitioning habitat between species would be more likely than within species.
The higher level of aggression shown by the Tome’s Spiny Rat also suggests that it is the
Figure 1. Number of aggressive
acts per 10,000
second period for bouts
within the three pairing
types. Pairing types included
Tome’s Spiny Rat /
Tome’s Spiny Rat (PsPs),
Tome’s Spiny Rat / Armored
Rat (PsHg), and Armored
Rat / Armored Rat
(HgHg).
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Vol. 3, 2022 No. 6
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Figure 2. Box-plot showing
the number of aggressive
acts per 10,000
second period for individuals
of the two species
Tome’s Spiny Rat (Ps) and
Armored Rat (Hg). Individuals
scored were from
interspecific pairings.
Table 2. Summary of the Kruskal-Wallis rank sum test for each of the three behaviors along with
the eight explanatory variables and grouped by individual and bout. The explanatory variables were
categorized as resource (yes or no), species (Tome’s Spiny Rat or Armored Rat), season (wet or
dry), pairing (Ps/Ps, Ps/Hg, or Hg/Hg), age (young or adult), sex (female or male), weight difference
class (WDC) (1 = 1-75 g, 2 = 76-150 g, and 3 = 151 g +), and reproductive (reproductive or nonreproductive).
Bouts also included pairings of age (young/young, young/adult, adult/adult), sex (male/
male, male/female, female/female), and reproductive state (reproductive/reproductive, reproductive/
non-reproductive, non-reproductive/non-reproductive).
Aggression Approach Retreat
Test Indiv. Bout Indiv. Bout Indiv. Bout
Resource p > 0.05 p > 0.05 p > 0.05
Species p > 0.05 p > 0.05 p > 0.05
Season p > 0.05 p > 0.05 p < 0.05* p < 0.05* p > 0.05 p > 0.05
Pairing p < 0.05* p < 0.05* p > 0.05 p > 0.05 p > 0.05 p > 0.05
Age/Pairing p > 0.05 p > 0.05 p > 0.05 p > 0.05 p < 0.05* p > 0.05
Sex/Pairing p > 0.05 p > 0.05 p > 0.05 p < 0.05* p > 0.05 p > 0.05
WDC p > 0.05 p > 0.05 p > 0.05 p > 0.05 p > 0.05 p > 0.05
Repro/Pairing p > 0.05 p > 0.05 p > 0.05 p > 0.05 p > 0.05 p > 0.05
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more aggressive of the two species. If their current habitat were to drastically change, we
could expect the more aggressive Tome’s Spiny Rat to competitively exclude the Armored
Rat through interference competition, due not only to their higher levels of aggression but
also to their more generalized use of habitat (Adler 1996, Flem ing 1971).
Seasonality did not alter the interactions of the two species in terms of aggression or
retreat. This result suggests that these species have different strategies to reduce overlap
during resource-limited periods and therefore do not need to be aggressive. Individuals
did, however, show an increase in approach during the dry season. This form of behavior is
considered investigative because they are shortening the distance between each other while
not demonstrating aggression. This behavior could be performed either to gain information
about the other individual or to demonstrate affiliation (Moy et al. 2004). The lack of
changes in aggression due to seasonal fluctuations in resources suggests that stabilizing
strategies have already been developed to maintain coexistence. Subordinate species use
subtle changes in behavior and use of space to avoid direct conflict over resources (Monterroso
et al. 2020, Vanak et al. 2013). It is possible that similar mechanisms are at work here,
and further investigation into which strategies are used and how these strategies change with
season should be conducted.
Continued research on resource partitioning strategies for both species would further
assist in understanding interspecific relationships. Studies using more controlled environments
should also be performed to determine if fluctuations in certain resources alter how
individuals interact with one another. Research should aim to understand each species’
intraspecific regulatory behaviors. Not only does more research need to be conducted to
understand this specific relationship, but further understanding their ability to coexist could
improve our understanding of species coexistence in general.
Acknowledgements
This study was conducted through the Smithsonian Tropical Research Institute (STRI). Permits were
granted by Autoridad Nacional del Ambiente and STRI. Support was provided by Frostburg State
University under the Foundation Opportunity Grant. Dr. Frank Ammer, Bebe Elrick and Dr. Scott
Mangan are thanked for their support and assistance.
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