# Mate Choice and Copulation Frequency in the Burying BeetleNicrophorus quadripunctatus (Coleoptera: Silphidae): Effect of Male Body Size and Presence of a Rival

Authors: Seizi Suzuki
Journal: Psyche
 (2009)

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## Abstract

It is widely assumed that there exists a  competition between males for mating and that females prefer males with elaborate male traits. Further, such traits are considered to be synonymous with high quality in terms of benefits to females. The number and duration of copulations and the frequency of mate refusal between large and smallNicrophorus quadripunctatus males were examined both for single males and for two males competing. The number of copulations was not affected by the size of the male or by the presence of a rival, but there was a significant interaction such that large males increased their number of copulations when a small rival was present. Copulation duration was not affected by male size but was shortened by a rival male. Females rejected copulation attempts of small males more often than of large males, whether the males were alone or paired with a rival. These results suggest that large males have two advantages: they win contests between males and are preferred by females.


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## Body

## 1. Introduction

Species which  provide biparental
care often exhibit monogamous mating behaviors since assistance from the male is essential for
successful breeding [1]. Monogamous males fertilize most of the brood, but
extrapair fertilizations (EPFs) have been observed in various species [2].
Since EPFs decrease the fitness of male partners, the frequency and/or
influence of EPFs is reduced by male partners. Thus, male partners react to
EPFs by guarding their mates and frequent copulations [3]. In contrast, females
prefer males with some specific traits because fertilization with high-quality
males brings about direct and/or indirect benefits [4]. Thus, the success of
EPFs depends on both the competition between males and the mate preferences by
the female; however, the interaction between these factors is poorly
understood.The complex parental behavior of burying
beetles (Silphidae: Nicrophorinae:Nicrophorus)
has been well studied (reviewed in [5, 6]). Nicrophorus exploits small vertebrate carrions as food for its young. Typically, a male-female
pair prepares a carcass by burying, removing hair, and rounding it into a ball
[7]. Eggs are laid in the soil adjacent to the carrion ball. After hatching, the
larvae crawl to the carrion ball, where they are fed by parental
regurgitations.Nicrophorus is generally monogamous [8–11] and displays
intense intrasexual competition in both sexes [12, 13]. More than one
individual of both sexes often locates a carcass, and usually a single,
dominant male-female pair occupies the carcass. Both the males and females
defend their carcass and brood even after the larvae hatch by attacking the intruders
cooperatively [14–17]. Inferior individuals are occasionally present
around the carcass as satellite males [18] and brood parasites [19]. Larger
individuals usually win the contest among conspecifics [12, 20]. Because
females occasionally reject copulation with males [21], we hypothesize that
females prefer to copulate with larger males and that they reject copulation
with smaller males. In this study, we examined the number and duration of
copulation and the frequency of mate refusal in N.quadripunctatus. In
addition, we also examined the differences in the frequency of copulations in
the presence and absence of other males.


## 2. Materials and Methods

All beetles  were
caught in the field using hanging traps baited with rotten meat. For the
experiments,N. quadripunctatus individuals
were sorted into large- (pronotal width  >  5.5 mm), medium-(5.5  >  pronotal
width  >  4.5 mm), or small- (<4.5 mm) sized classes. The beetles were
placed along with a small piece of chicken meat (approx. 15 g) in a plastic
arena (50  ×  250  ×  50 mm). The arenas were maintained under standard laboratory
conditions of light and ambient temperatures. The treatments were as follows.Treatment 1
A large or small male and
a medium female were introduced into the arena (large:N  =  22, small: N  =  23).Treatment 2
A medium
female and 2 males (a large and a small male) were introduced into the arena (N  =  21).The following behavioral interactions
between the sexes were recorded for 1 hour. A copulation attempt was recorded when a
male attempted to mount the female, and copulation success was recorded when a
male mounted the female and successfully inserted his aedeagus. Using a
stop-watch, the copulation duration was recorded from the time when the male
beetle inserted his aedeagus to the time when he ceased mounting.


## 3. Results

The frequency of
copulation between large and small males was not significantly different
(F  =  0.42, P  =  .52). In contrast, significant interaction between male size
and the presence of rivals suggests that large males repeatedly copulated in
the presence of small males  than that of a single large male (F  =  8.29, P  <  .01,
d.f.  =  1, two-way ANOVA, Figure 1). The copulation duration
of single males was longer significantly than pair males (F  =  6.57, P  <  .01, d.f.  =  1, ANOVA, Figure 2).Figure 1
Number of copulation attempts (grey bar) and copulation
rejections (black bar) for each treatment (mean  ±  SE). Sample size of the observed copulations is shown at the upper part of each bar.Figure 2
(Mean  ±  SE) copulation duration for each treatment. Sample
size of the observed copulations is shown at the base of each bar.The females displayed
two rejection behaviors: holding the abdomen down and moving away from the male
[22]. The females tended to accept large males and reject the small males (F  =  36.28, P  <  .01, d.f.  =  1), regardless of the
presence of rival males (F  =  0.28, P  =  .59, d.f.  =  1, two-way ANOVA, Figure 1).


## 4. Discussion

Often two or more male
burying beetles are found near a carcass [21]. Thus, intense competition
between males has been reported [12, 13, 23]. The dominant male achieves higher
paternity, but inferior males often stay around the carcass and copulate in N.vespilloides [18, 19].Mate rejection is reported
inN. vespilloides [22], but the
reason for rejection is still unknown. This study revealed that females
accepted larger males but rejected smaller males. In burying beetles, conspecific
dominant-subordinate relationships are determined by the body size, and larger
individuals usually win contests both between males and females [12]. However, even
in the presence of only one male, the females prefer larger males over smaller
ones as evidenced by the probability of copulation rejection. Thus, the outcome
of the contest itself does not affect mate preferences. Since females often
equate superior fighting ability with high quality in terms of fitness [24], they
are generally attracted to dominant males in many animal species [25], except for
a few (e.g., females of Pacific blue-eye fish do not use traits correlated with
fighting ability to choose males [26]). The
result of N. quadripunctatus coincides
with the assumption that females generally prefer dominant males. Females
maximize reproductive success by optimizing the “quality” of their mating
partners [27], they are assumed to benefit by mating with dominant males.In male burying
beetles, our study revealed that the dominant males copulated more frequently in
the presence of inferior males. Territorial (larger) males repeatedly copulated
in the presence of other males (Figure1). Dominant males of N. vespilloides require large number of
repeated copulations to achieve sperm replacement for high paternity [18]. The
act of repeated copulation was reported only for sperm displacement, and
smaller males did not reduce their frequency of copulation in the presence of
larger males (Figure 1). The territorial males have the method only with
repeated copulations to prevent EPFs in N.
vespilloides [18, 28]. Territorial males sire a much larger proportion of
the brood (60–100%, [18, 19, 29]),
and repeated copulations can largely prevent EPFs. In contrast, smaller males are
at a disadvantage with regard to fertilization because their possibility of
winning contests is low and their rejection as mates is high. Smaller males face
a higher possibility of mate rejection, but the number of successful copulations
by smaller males is not different from that by larger males without rival males.
This demonstrated that smaller males attempted to copulate more frequently than
larger males; this also suggested that the smaller males acquired more EPFs by
repeated copulation attempts.It is often stated
that males that win contests are the preferred mates because they are of higher
quality [27]. It is assumed that females prefer higher quality males for the direct
benefits that these males provide, such as paternal care and indirect benefits
such as “good gene” [4]. Though large body size has not been confirmed an
indicator of good genes in burying beetles, larger individuals usually win the
intrasexual contest [12] and acquire higher paternity in burying beetle species
(N. vespilloides: [29]). There are many
conspecific and congeneric intruders even after the larvae have hatched [9], and
the parents must guard the brood against them. Larger males usually win contests
[12, 13], and pairs can guard their brood more effectively than a single female
(Nicrophorus
pustulatus: [17]). Intruders occasionally take over the owner’s brood
and kill all the larvae (Nicrophorus orbicollis,
[30]). Thus, defence against intruders directly affects the reproductive success
of the residents. Biparental care improves the protection against intruders
provided to the brood by burying beetle parents (N.pustulatus: [31]). Trumbo
[17] hypothesized that in burying beetles, the threat of infanticide is the
primary reason for extended biparental care. In the field condition, sometimes
reproductive pairs were determined without intrasexual contest [21]. In such
situation, females must select whether they breed with the males presented now
or wait for arriving another larger male. Although the relative importance of
female choice is still unknown, females will have advantage on selecting larger
males for pairs to prevent infanticide effectively. Females can acquire direct
benefit by paternal care whether male body size is an indicator of genetic
quality or not. In addition, it is said that parental investment is related to
certainty of parentage [32], female may insure the paternity of larger male by
the rejection of the copulation of smaller males to increase paternal care. Anyway,
the effectiveness of mate rejection for females needs further investigation.This study revealed
that females rejected copulation with smaller males, smaller males attempted to
copulate more frequently than larger males, and larger males attempted to
copulate more frequently in the presence of otherN.quadripunctatus males.
Large-sized males have two advantages: they win contests between males and are preferred
by females. Since the adult body size of burying beetles has important effects
on reproductive success, the results suggest that the two above-mentioned
factors prevent EPFs by smaller males.



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*Source: 904604-2008-12-28.xml*