Behavioral Ecology Vol. 10 No. 5: 578-584
© 1999 International Society for Behavioral Ecology
Pheromonally mediated mate attraction by males of the burying beetle Nicrophorus orbicollis: alternative calling tactics conditional on both intrinsic and extrinsic factors
Department of Entomology, University of Kentucky, Lexington, KY 40546-0091, USA
Address correspondence to C. M. Rauter, S-225 Agricultural Science Center North, Department of Entomology, University of Kentucky, Lexington KY 40546-0091, USA. E-mail: cmraut0{at}pop.uky.edu . A. J. Moore is currently at the School of Biological Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester, M13 9PT, UK.
Received 2 October 1998; accepted 19 February 1999.
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ABSTRACT |
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Male burying beetles attract females using a pheromonal signal and can provide parental care and a food resource, vertebrate carrion, for their developing offspring. But males attempt to attract females even when they have no carrion. We examined the factors that influence male behavior directed toward finding or attracting mates in both field-caught and laboratory-reared Nicrophorus orbicollis, a North American burying beetle. We investigated whether male behavior differed based on both intrinsic (size) and extrinsic (resources held) differences among males. Further, we examined repeatability of individual behaviors and the effect of holding or lacking resources on these repeatabilities. Field-caught and laboratory-reared individuals differed in overall activity but not in their behavioral repertoire, making studies of laboratory-reared males relevant. The behavior of individual males was very consistent within a condition, but plastic between resource conditions. The frequency of calling (adopting a posture that indicates pheromone release to attract females) depended on male size when males did not hold resources, but this relationship disappeared when males held resources. Without carrion, smaller males called more frequently than did larger males. When holding carrion, smaller males reduced their calling, whereas larger males significantly increased the frequency with which they attempted to attract females and reduced the amount of time they spent searching. Thus, calling behavior of males was conditional on not only intrinsic and extrinsic factors, but also an interaction between them. We suggest that the changes in calling represent alternative tactics based on the costs and benefits of attracting both potential mates and competitors, which differ for males of different sizes.
Key words: body size, burying beetles, male-male competition, Nicrophorus orbicollis, pheromone, repeatabilities, resource defense.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Behavioral plasticity is common in connection with mating, and many of the factors that influence plasticity in mating tactics are known (Henson and Warner, 1997
).
Plasticity is generally thought to result from responses to both immediate
environments (extrinsic factors) or developmental or past influence (intrinsic
factors). Many different forms of extrinsic factors (i.e., immediate
environments such as population density or prey resources) have been shown to
lead to the expression of alternative mating behavior
(Andersson, 1994;
Gross, 1996;
Moore, 1989;
Thornhill and Alcock, 1983).
Individuals also display tactics based on intrinsic factors such as fixed
morphological differences (e.g., alternative mating tactics based on
morphology or size; Cook et al.,
1997; Gross, 1996;
Thornhill and Alcock, 1983).
Such intrinsic factors can reflect the effects of genetic alternatives
(Shuster and Sassman, 1997;
Shuster and Wade, 1991),
differential allocation of resources during development
(Nijhout and Emlen, 1998), or
variation in previous experiences or learning
(Caro and Bateson, 1986;
Dugatkin, 1996,
1998). Although recognizing the individual effects of intrinsic and extrinsic factors has facilitated our understanding of alternative mating tactics, focusing exclusively on the separate effects of intrinsic and extrinsic factors may be misleading. All organisms reflect both past and current experiences. Examining the interaction between factors is therefore important and should further our understanding of behavioral plasticity.
Burying beetles (Nicrophorus spp.) offer an opportunity to study
effects of intrinsic and extrinsic influences on behavior, as well as their
interactions. Considerable research has been conducted on burying beetle
behavior and ecology in both North America and Europe (recently summarized in
two excellent reviews: Eggert and
Müller, 1997;
Scott, 1998a). Several aspects
of burying beetle biology suggest they may have plastic mate-attraction
behavior. In burying beetles, one or both of the sexes must locate, prepare,
and bury carrion for their developing larvae. Males and females may mate
either before or after the carrion is located. Individuals looking for carrion
search for small, dead vertebrates. Competition over carrion is often fierce
both within and between burying beetle species. Larger size is associated with
successful takeover or defense of carrion, especially for males. In addition
to searching for carrion, sexually mature male burying beetles can spend a
considerable amount of time attempting to attract mates
("calling"). To attract mates, male burying beetles adopt a
characteristic posture and emit a pheromone signal either in the absence of
defendable resources or after they have located a carrion resource. If no
female is present on carrion when it is located, the male buries the carrion
superficially and calls. If another male is present on the carrion, but no
female, the males may fight over the carrion. If a female is also present on
the carrion, however, the males compete for the carrion and mating
opportunities. Intruders larger than the resident male are most likely to be
successful in takeover attempts. Eggert
(1992) has suggested that,
because successful reproduction in burying beetles is dependent on locating
both mates and carrion, males could adopt alternative mating tactics. Males
can attempt to locate carrion first and then attract females pheromonally and
mate, or they can attempt to attract females even though they have no carrion
to offer.
In this paper we examine the role of, and interaction between, intrinsic
and extrinsic factors that might influence the expression of alternative
mating tactics in the burying beetle, Nicrophorus orbicollis. Eggert
(1992) examined the role of
time-of-day and genetic variation underlying male calling and searching for
carrion in the related N. vespilloides. We also examine timing of
calling, but in addition we examine how size and presence or absence of
carrion may influence mate attraction strategies. We chose to examine body
size because the benefits of searching for carrion before attempting to
attract a mate may depend on more than simply locating a resource; males may
have to takeover or defend the carrion from other males. Size is strongly
associated with potential to secure and defend resources in N.
orbicollis, with larger males being more successful
(Robertson, 1993;
Scott and Gladstein, 1993;
Trumbo, 1990a,
b,
1991;
Wilson and Fudge, 1984). The
benefits of calling without carrion, however, may be unrelated to male size.
Although females cannot reproduce without having carrion to lay their eggs,
there can still be net fitness gains for males because females do mate with
males even when they lack carrion (Eggert
and Müller, 1989). These copulations
may provide females with fresh sperm that allows them to reproduce on carrion
in the absence of a male (Eggert,
1992). Scott
(1998a) found that N.
orbicollis males from a population in New Hampshire do not emit pheromone
when there is no carrion, yet we often observe males calling without carrion
present. This further suggests that male N. orbicollis behavior is
plastic, so we investigated whether the presence or absence of carrion
influenced the frequency of individual male calling. Given that size and
carrion ownership are not independent, we further studied whether the
extrinsic influence of resources and the intrinsic factor of size interact to
affect the likelihood of calling by males. Finally, we documented both within-
and between-individual variation by repeatedly measuring male behavior in the
same and different environments. In this way we partitioned intra- and
interindividual variation in plasticity of male behavior. Although this study
was conducted in a laboratory setting, we validated our observations by
comparing field-caught and laboratory-reared males.
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MATERIALS AND METHODS |
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Study animals and rearing conditions
We used field-caught males and first-generation laboratory offspring of field-caught burying beetles (Nicrophorus orbicollis) in this study. We collected beetles from 2 to 29 May 1997 at Cowbell near Berea, Kentucky, USA, using commercially available Japanese beetle traps. We captured beetles with traps filled with 2 cm of dirt, baited with a piece of rotting beef. We transported beetles to the laboratory and housed them separately in clear plastic containers (17x12x6 cm) filled two-thirds with humid peat under a 15 h:9 h light:dark cycle and a temperature of 20-25°C. We fed adult beetles mealworms twice a week. We reared first-generation beetles (hereafter termed "laboratory-reared males") under the same conditions, except that after eclosion we arbitrarily paired two laboratory-reared males together for 1.5-2.5 months, then separated them 1 month before the experiment.
Influence of extrinsic and intrinsic factors on male calling
We compared the frequency of calling of 63 laboratory-reared, sexually
mature males in the presence and absence of carrion to investigate the
influence of an extrinsic factor on calling in N. orbicollis. We also
studied the influence of an intrinsic factor, size, on male calling. We
examined an interaction between extrinsic and intrinsic factors by comparing
the relationship of male size and male calling in the presence and absence of
carrion. We first observed all males without carrion then, at least 5 days
later, with carrion. We observed the behavior of all males twice under the
same conditions to assess the consistency of male behavior. The first and
second observation occurred 6 days apart when no carrion was present. We
observed males 2 days in a row when carrion was present. There were no
sequence effects within a condition (see repeatabilities below).
We started each behavioral observation session 30 min before the onset of
"night" (lights out) and finished 1 h after the lights came on. We
collected behavioral data (Table
1) using scan sampling as the sampling rule instantaneous as the
recording rule (Altmann, 1974):
during each 15-min observation session, we recorded every behavior performed
by every male (including inactivity). We recorded non-reproductive behavior
(resting, grooming, burrowing, and feeding) in addition to the alternative
reproductive tactics of pheromone calling and searching.
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Five days to 2 weeks after observing males without carrion, we added a mouse (26-28 g) 1 h before onset of night to study male behavior in the presence of carrion. This then became the "with carrion" condition. We recorded all the behaviors as before, but we also added those that occur only when carrion is present (digging and preparation of the mouse; Table 1).
Comparison of calling between wild-caught and laboratory-reared male
beetles
We compared the calling of 43 field-caught beetles to the calling of 63
different laboratory-reared beetles in the absence of carrion to investigate
whether behavioral data collected on laboratory-reared beetles can be
extrapolated to field populations. We recorded the behavior of field-caught
beetles as for laboratory-reared beetles, with the following exceptions: (1)
we sampled the behavior of each male every 30 min instead of every 15 min, and
(2) we subdivided each of the two observation sessions into two sessions. We
collected the first 4.5 h of data during two sessions on 3 June 1997 and 22
June 1997. We collected the remaining 6 h of behavioral data during two
sessions on 11 June 1997 and 21 June 1997. Therefore, we were able to collect
behavioral data for the whole daily activity period of these beetles.
Measurement of size
We used length of pronotum as measure for size. We measured the length of
the pronotum along the suture line in the middle of the pronotum, and each
measurement was done twice on the same image. This measure is highly
repeatable (t = 0.995 ± 7.6 x 10-7, 132
beetles, 2 measurements each). We imaged all of the beetles and measured the
pronotum length using the NIH Image analysis package (publicly available at
http://rsb.info.nih.gov/nih-image/ or at ftp://zippy.nimh.nih.gov/pub). We
used the mean of the two measurements for the size measure of each beetle. We
froze field-caught beetles immediately after death and then thawed them for
imaging. A month before the behavioral observation, we imaged live
laboratory-reared beetles that had been anesthetized with CO2.
Statistical analyses
We analyzed our data using SYSTAT 7.0 for Windows. Unless otherwise stated,
we present means±1 SE. We calculated repeatabilities from variances
derived from the means squares obtained from one-way ANOVA
(Becker, 1992).
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RESULTS |
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Behavior of N. orbicollis males
For the behaviors we measured, there was complete overlap in observed behavior of the two categories of males (with and without carrion to defend). The only exceptions were behaviors directly related to preparing the carrion (Table 2). With the exception of two males in the no-carrion treatment that were inactive and buried under the soil and therefore were never seen, males attempted to attract females, searched for carrion, and engaged in non-reproductive behaviors in various combinations. Field-caught and laboratory-reared males also did not differ in the categories of behavioral acts performed.
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We found significant differences in the likelihood of a male attempting to
attract a female under the two resource conditions. When no carrion was
present 17 (27%) males never called, compared to only 2 (3%) males that never
called when carrion was present (
2 = 14.566, df = 1,
p <.001). Field-caught males were even less likely to call; 23
(53%) never called. This is significantly fewer than laboratory-reared males
that also lacked carrion (2 = 6.996, df = 1, p
=.008).
There was also variation among males in the timing and frequency of
behavioral acts performed (Table
2). These differences were greatest between field-caught and
laboratory-reared individuals. Difference in the time behavioral activity
began or ended cannot be compared statistically between field-caught and
laboratory-reared males because of differences in our sampling schemes.
Frequencies of behavioral acts, however, can be compared. Field-caught beetles
were significantly less active than laboratory-reared beetles
(Table 2; F1,102 = 19.410, p <.001). Laboratory-reared
beetles called more frequently (F1,102 = 5.702, p
=.019), but there was no significant difference in frequency of searching
(F1,102 = 1.498, p =.224). In addition to
difference in activity, field-caught males were significantly larger than
laboratory-reared males (pronotum 
± 1 SD; field-caught
males: 5.368±0.593 mm; laboratory-reared males: 4.895±0.326 mm;
F1,104 = 27.871, p <.001). Neither
distribution differed significantly from a normal distribution (field:
Lilliefors test maximum difference = 0.0845, n = 43, p
=.619; laboratory: Lilliefors test maximum difference = 0.0652, n =
63, p =.704).
Within each treatment, individual male behavior was consistent (Table 3). Repeatabilities ranged from 0.80 (± 0.002) for the frequency of pheromone release by laboratory-reared males without a mouse, which is extraordinarily high for behavior, to 0.01 (± 0.03) for the end of pheromone release by field-caught beetles. The timing of behavior was generally less repeatable, especially for the termination of behavioral acts. This could be statistical rather than biological, as the lower repeatabilities are also consistent with our sampling scheme that introduces a fair amount of error. The repeatability of beginning and ending pheromone calling, however, increased substantially when males held carrion.
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Plasticity in male behavior when carrion is not present
Among field-caught males, there was a significant relationship among male
size and frequency of pheromone calling
(Figure 1a). When there was no
mouse present, smaller males were significantly more likely to call than were
larger males (y = 0.381-0.060x, r2 =.18;
F1,41 = 8.755, p =.005). Smaller males were also
significantly more likely to end pheromone calling later than larger males
(Figure 1b; y = 4.369
- 1.065x; r2 =.13; F1,41 =
6.304, p =.016). None of the other behavioral acts were significantly
related to size (all p >.22).
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Among laboratory-reared males without a mouse, smaller males were again more likely to call than were larger males (Figure 2a; y = 1.086-0.188x; r2 =.13; F1,59 = 8.651, p =.005). None of the other behavioral acts of males without a mouse were significantly related to male size (all p >.14). This significant relationship between size and frequency of pheromone calling disappeared when these same males held a mouse (Figure 2b; y = 0.406-0.039x; r2 =.009; F1,61 = 0.574, p =.452). Again, none of the other behavioral acts were significantly associated with male size (all p >.26).
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Plasticity in male behavior when carrion is present
Individual males were generally consistent in their behavior, regardless of
whether they were held without or with the resource of a mouse. Individuals
did not increase their overall activity (repeated-measures ANOVA;
F1,60 = 0.155, p =.695), or differ in when they
began to be active (F1,60 = 0.626, p =.432), or
differ in when they terminated their activity (F1,60 =
3.906, p =.086). They did, however, significantly decrease the amount
of time spent searching (F1,60 = 13.665, p
<.001). Males with carrion also significantly increased the frequency of
their pheromone calling (F1,60 = 3.906, p =.05),
although there were no significant differences in the time they began calling
(F1,42 = 2.396, p =.129) or ended calling
(F1,42 = 2.535, p =.119) comparing individuals
with and without a mouse.
Although size was continuously distributed, we can get some indication of
those males that changed their behavior by creating post-hoc size categories
and examining how relatively small, medium, or large males behave with and
without vertebrate carrion. To do this we categorized our sample of males
based on deviations from the mean size of males in our sample. "Large
males" were either 
1 SD above the mean size (pronotum length
>5.221 mm; n = 11), "small males" were >1 SD
smaller than the mean (pronotum length 
4.569 mm; n = 12). Medium
males were those between these categories (n = 40).
The change in frequency of pheromone calling was due to relatively large
males increasing their calling when carrion was added (repeated-measures
ANOVA; F1,10 = 4.980, p =.05). Large males more
than doubled their frequency of calling (without mouse =
0.109; with mouse = 0.253). Medium-sized males significantly
increased pheromone calling as well (from = 0.141 to
= 0.213; F1,37 = 6.367, p
=.037). Small males decreased the frequency of calling (from
= 0.302 to = 0.220), though not significantly with these
small sample sizes (F1,11 = 2.208, p =.165).
As expected for a change in tactic, from searching for carrion to calling
for females, large males showed a significant decrease in the frequency of
searching (from = 0.358 to 0.221; repeated-measures ANOVA;
F1,10 = 4.610, p =.057). Medium-sized males also
decreased the frequency of their searching (from = 0.305 to
0.215; F1,37 = 10.628, p =.002). Small males did
not change their frequency of searching ( = 0.26 under both
conditions; F1,11 = 0.008, p =.929).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Like its European relative N. vespilloides (Eggert, 1992
), N.
orbicollis males varied in the timing and frequency of the different
behaviors they performed. Also like N. vespilloides, individual males
are consistent in their behavior. Male N. orbicollis began their
activity shortly after the onset of the night and either searched for carrion
to provide to mates and offspring, or called to attract mates. Given these
alternatives, we found, like Eggert
(1992), that males of the
burying beetle N. orbicollis used alternative mate-finding tactics.
We have gone further in this study and investigated different causes for this
plasticity. We found that differences among males in the tactic used were
conditional based on both intrinsic (size) and extrinsic (resources held)
factors and on the interaction between these factors. Below, we interpret our
results in light of our expectations on how male fitness might be affected by
these factors, and the expected costs and benefits of each strategy for each
male.
Comparisons between field-caught and laboratory-reared males and
consistency of behavior
Field-caught and laboratory-reared males differed in their overall activity
levels but did not differ in the behaviors that they performed.
Laboratory-reared males were generally more active and performed each behavior
more frequently. Differences in our sampling regime preclude comparing the
specific timing of behavior, but our subjective impression was that there were
few differences. Most important, both field-caught and laboratory-reared
individuals performed the same behaviors at the same relative frequencies. It
is likely that field-caught males can be just as active in the field, but that
bringing them into the laboratory causes them to become somewhat inactive. In
addition, we may have seen less activity among our field-caught males because
they were larger. Larger males tend to call less, and our laboratory
population was significantly smaller than our field-caught males. Nonetheless,
the overall consistency of the behavioral repertoire suggests that we can use
data from laboratory-reared individuals as indicative of patterns of behavior
in N. orbicollis. Scott
(1998b) found similar
consistency between field-caught and laboratory-reared N. orbicollis
in duration of parental care.
The behavioral acts we examined, with the exception of timing of pheromone
release (initiation and cessation), were highly repeatable. Intraindividual
consistency of frequency of calling and searching were exceptionally high
under all the experimental conditions. Eggert
(1992) also found that the
repeatability of the frequency of pheromone release and frequency of searching
for N. vespilloides was 46% and 43%, respectively. Given the expected
levels of plasticity in behavior and the difficulties in maintaining constant
conditions across trials, repeatabilities greater than 60% are extraordinarily
high for behaviors, and repeatabilities around 20% suggest that the behaviors
we examined are fundamental (Arnold and
Bennett, 1984; Boake,
1989). Furthermore, there are several factors that we expect would
tend to lower repeatabilities. The consistently lower repeatabilities of
field-caught beetles might reflect differences in extent of social experience
that provides the beetles with information about their relative size and
therefore potential fighting ability. The relatively lower repeatabilities of
timing may reflect our gross measures as much as variation in an individual's
behaivor, or may reflect less importance of the timing of calling when no
carrion is present. Some support for this latter hypothesis comes from the
increase in repeatability for timing of calling when males hold carrion
resources. Finally, sequence effects, if present, are unavoidable and would
also lower repeatabilities.
High repeatabilities of our measures of behavior allow us to interpret
changes in behavior as reflecting changes in condition (presence or absence of
resources) rather than as changes within individuals (e.g., motivation, aging,
hunger). The behavioral acts performed by males are remarkably repeatable. Too
few behavioral studies use the power of repeated measurements
(Boake, 1989); yet, as our
results show, considerable information can be gained by such an analysis.
Furthermore, by using mean values in subsequent analyses of our behavioral
measures, we reduce measurement error and can have more confidence in our
results (Becker, 1992).
Alternative tactics based on intrinsic differences and extrinsic
resources
Calling by male N. orbicollis is related to whether he is
relatively large or small and to whether or not he holds carrion. Our data
show that without carrion, males that are smaller are more likely to be
attempting to pheromonally attract a mate than are larger males. It appears
that larger males preferentially search for carrion rather than call. In
contrast, once a male has obtained carrion, larger males call as frequently as
do smaller males. Thus it appears that male N. orbicollis exhibits
alternative reproductive tactics depending on both intrinsic and extrinsic
factors.
Burying beetles require carrion to rear young, so attracting a female
without providing her with resources may increase the likelihood of
fertilizing eggs, but it does not ensure reproduction. In N.
vespilloides, females will mate when there is no carrion available
(Eggert and
Müller, 1989), and we suspect this
is true for N. orbicollis as well. All Nicrophorus females
appear to store sperm (Eggert and
Müller, 1989;
Scott and Williams, 1993;
Trumbo, 1990b), so males other
than the resource holder can sire broods. The paternity of the
resource-holding males is high, however—as high as 90% for both N.
vespilloides
(Müller and
Eggert, 1989) and N. orbicollis
(Trumbo, 1990a,
b,
1991).
The pattern of change in frequency of calling with resources present
suggests a novel cost to pheromone release by N. orbicollis. Our data
suggest that not only do larger males increase their calling when they possess
carrion, but smaller males also decrease their calling. It is likely that, as
in European species of Nicrophorus
(Müller and
Eggert, 1987), N. orbicollis males as well as females are
attracted to calling males. Fights between males appear unlikely unless the
male that is calling holds carrion. Smaller males typically lose contests over
resources (Robertso, 1993;
Scott and Gladstein, 1993;
Trumbo, 1990a,
b,
1991;
Wilson and Fudge, 1984), so
for smaller males calling when defending carrion may be detrimental. On the
other hand, for larger males, calling without carrion does not carry as much
benefit as searching for carrion because larger males can both defend and
usurp carrion (Robertson,
1993; Scott and Gladstein,
1993; Trumbo,
1990a, b,
1991;
Wilson and Fudge, 1984).
The role of female mate discrimination in structuring the reproductive
success of males is unknown. Because of fitness differences associated with
different-sized males, it seems reasonable that females might discriminate
among males based on size. Our preliminary data suggest that females can use
pheromones to discriminate among different-sized males, but some females are
attracted to small males (Beeler et al., in preparation). It is also not clear
why females mate with small males that have no resources. Eggert
(1992) suggested that this
behavior ensures motile sperm. Perhaps females mate rather than struggle and
use postmating tactics (sperm precedence or replacement) to affect mate choice
(see Moore, 1989, for a
similar argument for dragonflies).
Our study was not an exhaustive examination of the various factors that
might influence male mate attraction behavior. Additional intrinsic and
extrinsic factors are likely to be important. Trumbo and Eggert
(1994) and Eggert and Sakaluk
(1995) show that the size of
the carrion and number of mates present influence pheromone calling. Scott
(1998b) found that different
social environments also influence parental care. Body size may also interact
with social environment because probability of aggression between males or
between males and females determines calling under different social
conditions.
Conclusions
The pattern of high repeatabilities for pheromone calling behavior,
particularly when males defend carrion, and the relationship between size and
calling suggest that male N. orbicollis use alternative tactics in
mate attraction. Furthermore, the number and complexity of the influences on
mate finding in N. orbicollis likely leads to the extreme plasticity
rather than discrete alternatives. Larger males spend their time searching,
probably for carrion. Smaller males may be less able to defend a carcass even
if they find one, and therefore spend more time calling. In addition to these
differences in behavior related to intrinsic differences among the males,
extrinsic (resource holding) also influences male behavior. This is consistent
with the expected costs (male-male competition where larger males win) and
benefits (mating) associated with calling by male N. orbicollis. This
pattern suggests the possibility of an unusual cost associated with pheromone
signaling in N. orbicollis. If calling males attract other males in
addition to females, pheromone signaling would result in fights among rival
males.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Bill Wallin provided outstanding technical help, and the Moore laboratory members provided helpful discussions and moral support. Berea College graciously provided access to Berea Forest to collect beetles. Anne-Katrin Eggert and Michelle Scott provided helpful discussions on burying beetle biology. We appreciate the comments of Laura Corley, Chris Grill, Richard Preziosi, and two anonymous reviewers on an earlier version of this manuscript. Our research has been financially supported by a Swiss National Science Foundation (NSF) postdoctoral fellowship awarded to C.M.R., an NSF REU supplement for A.E.B., a Howard Hughes Research Committee Grant and an Undergraduate Research and Creativity Grant awarded to A.E.B., and NSF grants awarded to A.J.M. (IBN-9514063, DEB-9521821, IBN-9616203, and IBN-9808629). State and federal Hatch funds also supported this research (KAES #98-08-187).
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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