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Male-male competition for females can significantly affect a male's reproductive success and hence his fitness. Game theory predicts that an individual should avoid fighting when its future reproductive potential is high, but should fight forcefully when its future reproductive potential is insignificant. When mates are scarce, extreme competition and fatal fighting is expected. We recently showed that Nephilengys malabarensis eunuchs, i.e. sterile spider males that lost their genitals during copulation, become more aggressive during male-male contests. Here, we add crucial comparative data by exploring eunuch fighting behaviour in Nephilengys livida from Madagascar, specifically by testing the 'better fighter hypotheses' in a laboratory setting. Similar to N. malabarensis, N. livida copulations resulted in total male castration with the severed palp plugging the female genitals in 70.83% cases, which mostly (63.63%) prevented subsequent copulations. Unexpectedly, however, N. livida eunuchs exhibited lower aggressiveness than virgin males. We interpret these results in the light of different mating biology between the so far studied species known for the eunuch phenomenon, which might reflect differing plug effectiveness due to variation in genital anatomy in N. livida, N. malabarensis and Herennia multipuncta. However, detected differences in aggressive behaviour of N. livida versus N. malabarensis eunuchs might also be explained by the species' ecology, with lower population densities resulting in a relaxed male-male competition making excessive aggression and mate guarding redundant. This study thus questions the generality of overt aggressiveness in mated males with no reproductive value, and highlights the importance of understanding the natural history of species in the question.
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Eunuchs as better fighters?
Simona Kralj-Fišer &MatjažKuntner
Received: 26 October 2011 /Revised: 27 November 2011 /Accepted: 30 November 2011 / Published online: 14 December 2011
#Springer-Verlag 2011
Abstract Malemale competition for females can signifi-
cantly affect a males reproductive success and hence his
fitness. Game theory predicts that an individual should
avoid fighting when its future reproductive potential is high,
but should fight forcefully when its future reproductive
potential is insignificant. When mates are scarce, extreme
competition and fatal fighting is expected. We recently
showed that Nephilengys malabarensis eunuchs, i.e. sterile
spider males that lost their genitals during copulation, be-
come more aggressive during malemale contests. Here, we
add crucial comparative data by exploring eunuch fighting
behaviour in Nephilengys livida from Madagascar, specifi-
cally by testing the better fighter hypothesesin a labora-
tory setting. Similar to N. malabarensis,N. livida
copulations resulted in total male castration with the severed
palp plugging the female genitals in 70.83% cases, which
mostly (63.63%) prevented subsequent copulations. Unex-
pectedly, however, N. livida eunuchs exhibited lower ag-
gressiveness than virgin males. We interpret these results in
the light of different mating biology between the so far
studied species known for the eunuch phenomenon, which
might reflect differing plug effectiveness due to variation in
genital anatomy in N. livida,N. malabarensis and Herennia
multipuncta. However, detected differences in aggressive
behaviour of N. livida versus N. malabarensis eunuchs
might also be explained by the speciesecology, with lower
population densities resulting in a relaxed malemale com-
petition making excessive aggression and mate guarding
redundant. This study thus questions the generality of overt
aggressiveness in mated males with no reproductive value,
and highlights the importance of understanding the natural
history of species in the question.
Keywords Aggression .Cannibalism .Emasculation .
Guarding .Plugging .Nephilidae .Nephilengys livida
Malemale competition over females and related resources
can significantly affect a males reproductive success and
hence his fitness (Huntingford and Turner 1987). Yet, costs
associated with competition favour the assessment of a
resource value relative to the contestants fighting ability
(also termed resource-holding value, e.g. weapons, size)
over confronting all rivals indiscriminately (Maynard Smith
and Parker 1976; Parker 1970; but see Moore et al. 2008).
An important factor is an individuals expected future re-
productive value (Enquist and Leimar 1990). When its
future reproductive potential is high, an animal should avoid
Communicated by: Sven Thatje
Electronic supplementary material The online version of this article
(doi:10.1007/s00114-011-0873-1) contains supplementary material,
which is available to authorized users.
S. Kralj-Fišer :M. Kuntner
Institute of Biology, Scientific Research Centre,
Slovenian Academy of Sciences and Arts,
Ljubljana, Slovenia
S. Kralj-Fišer
Biozentrum Grindel, University of Hamburg,
Martin-Luther-King Platz 3,
20146, Hamburg, Germany
M. Kuntner
National Museum of Natural History, Smithsonian Institution,
Washington, DC, USA
M. Kuntner (*)
College of Life Sciences, Hubei University,
Wuhan 430062 Hubei, China
Naturwissenschaften (2012) 99:95101
DOI 10.1007/s00114-011-0873-1
fighting and hence injury, but should fight forcefully when its
future reproductive potential is insignificant (e.g. Fromhage
and Schneider 2005a; Innocent et al. 2007). When females are
limited, extreme competition and fatal fighting is expected
(Bean and Cook 2001; Enquist and Leimar 1990; Maynard
Smith and Price 1973).
Male competitive behaviour for females often varies,
being dependent on the density of rivals, the availability
and value of the contestant resource, as well as the related-
ness between competitors (Elias et al. 2010; Enquist and
Leimar 1987; Innocent et al. 2011; Kasumovic et al. 2008;
Kokko and Rankin 2006; Moore et al. 2008; Murray and
Gerrard 1985; Reinhold 2003; West et al. 2001,2002). Male
encounter rates with females depend on female density
within a patch, on patch size and on the distance between
patches (Schneider and Lubin 1998). When maleschances
to encounter more females are high, they are expected to
avoid the risk of intensive fighting. On the other hand, if
maleschances of encountering more than one female are
slim, they are predicted to maximize their reproductive
success by investing all of their resources in the first female
they encounter (Buskirk et al. 1984). However, the situation
also depends on the density of the males, and in the cases
where the density of males is as low as that of rare females,
there might not be a need for the male to defend his paternity
(Fromhage et al. 2005,2008).
In many sexually dimorphic spider species, males break
their copulatory organs, the palps, during copulation to plug
up a female (Fromhage and Schneider 2006; Kuntner 2005,
2007; Kuntner et al. 2008; Kuntner et al. 2009a; Uhl et al.
2010). A mating plughypothesis postulates that these
palpal leftovers function as physical barriers to future cop-
ulations (Kralj-Fišer et al. 2011a). In most species, the males
with damaged or missing palps are functionally sterile and
have no further reproductive value. Despite high costs re-
lated to mating plugs and sterility, these do not always
effectively prevent female remating, and this may depend
on both male and/or female genital morphology (Kuntner et
al. 2009b; Uhl et al. 2010). Due to incomplete plug effec-
tiveness, those emasculated males that survive copulation
commonly practice post-copulatory mate guarding (Fromhage
and Schneider 2005b). In accordance with game theory
(Enquist and Leimar 1990), Nephilengys malabarensis males,
which entirely emasculate their palps during copulation (such
sterile males are termed eunuchs), commonly escalate fierce
fighting and win in contests against rivalsabetter fighter
phenomenon (Kralj-Fišer et al. 2011a). To elucidate the gen-
erality of detected patterns in eunuch behaviour, we here
expand our research to include the second Nephilengys
Nephilengys livida is a highly sexually size dimorphic
and synanthropic species (Kuntner 2007;Kuntnerand
Agnarsson 2011), which exhibits a combination of inviting
sexual traits such as complex genitalia, male genital damage
resulting in plugging and eunuchs, as well as sexual canni-
balism and post-copulatory mate guarding (Kralj-Fišer et al.
2011b; Kuntner 2007; Kuntner et al. 2009c). The interaction
between current resource value, future resource value, in-
vestment in sperm plugs and malemale competition for a
female, make this species an interesting system for testing
the predictions of game theory models. We explored male
fighting behaviour in N. livida, by conducting a series of
laboratory malemale contests on female webs. We com-
pared contests between two virgin rivals with those between
a virgin male and a eunuch. Since plug efficiency likely
affects male fighting behaviour, e.g. effective plugs reduce
sperm competition, we staged laboratory mating trials to test
if plugged female genitalia could be reused by another male.
In accordance with the better fighter hypothesis (Kralj-Fišer
et al. 2011a), we predicted that eunuchs would escalate
fighting intensity to outcompete virgin rivals and that
eunuchs would be more aggressive in a species with lower
plug efficiency.
Materials and methods
Study animals
We collected N. livida (formerly known as Nephilengys bor-
bonica, Kuntner 2007) spiders in Andasibe-Mantadia (Toa-
masina Province) and Ranomafana (Fianarantsoa Province) in
Madagascar, between 24 February 2010 and 4 April 2010. To
examine remating, we collected subadults and reared them to
adulthood in the laboratory (females019, males033). We
housed females in glass frames (50×50×10 cm), and males
in smaller plastic cups (250 ml). We provided water to spiders
daily and fed them fruit flies, crickets and mealworms twice a
week. As previously, we define eunuchs as those adult males
lacking bothpalps, and half-eunuchs as those lacking one palp
(Kralj-Fišer et al. 2011a). We compared N. livida with N.
malabarensis from Southeast Asia (for details see Kralj-
Fišer et al. 2011a).
Experimental protocol
We tested the better fighterand the mating plug
hypotheses in series of laboratory tests. The spiders
reached maturation at various times, thus we had to
continuously adapt our ongoing experiments to the
available spiders (see work flow table in ESM). Due
to the small number of wild-caught spiders, several
spiders were reused (Nreused males024; Nreused
females019; see also ESM), however, we considered
this in our analyses.
96 Naturwissenschaften (2012) 99:95101
Better fighter hypothesis
To establish if palpal severance during copulation triggers
mate guarding, particularly fighting as known in N. mala-
barensis (Kralj-Fišer et al. 2011a), eunuchs were placed on a
random female web, immediately followed by the introduc-
tion of the second, virgin male (N012 trials). The control
group staged contests between two virgin males (N012
trials). During malemale contests, we noted the frequencies
of guarding behaviours, i.e. walking towards female, touch-
ing female and exploring (walking back and forth), and
estimated the distance between males and a female every
five minutes. Malemale fighting behaviour was scored as
frequencies of walking towards another male (score 01),
web shaking (score 02), chasing (score03) and attacking
(score04). Aggressiveness intensity levels were estimated
as sums of scores (Kralj-Fišer et al. 2011a,b). We also noted
frequencies of escaping and web plucking (defined as male
tapping on web threads). We noted whether copulations
took place. Trials lasted 60 min. After a trial the spiders
were given at least a 24-h rest.
Mating plug hypothesis
To test for genital reuse, our aim was to document at least
two successful insertions in the same copulatory opening
(CO). We took 11 females that received plugs and males that
could only insert their palp(s) in the plugged CO (insertions
were always ipsilateral; N013, see ESM). If mating into the
used CO was attempted but was unsuccessful in two subse-
quent mating trials, we assumed that plugging prevented
successful access to the previously used CO (Kralj-Fišer et
al. 2011a). Observations lasted for 60 min. After a trial the
spiders were given at least a 24-h rest. In the analysis, we
considered also rematings into the used CO that occurred
during malemale contests.
Morphological examination
At the end of all trials, the females were euthanized and
preserved in 70% ethanol. The epigyna of 17 females (two
females escaped) were excised and macerated in concentrated
KOH for 24 h, cleaned in distilled water, then further treated
with methyl salicilate (Kuntner et al. 2009b). Examinations of
epigynal microscopic preparations were done under a Leica
MZ16 stereomicroscope.
Statistical analyses
We compared plugging efficiency between N. livida and
N. malabarensis using a chi square test. To detect the
effect of emasculation on fighting and guarding behav-
iours of N. livida eunuchs, we employed generalized
linear mixed models (GLMMs). The fixed factors were
malesandrivals reproductive status (10eunuch, 20
virgin). We introduced male identities as random fac-
tors, because some males were reused. Response varia-
bles were behavioural scores. We sequentially deleted
fixed terms in order of decreasing significance; only
terms with P0.1 remained in the final model. Excluded
terms were re-entered one by one into the final model to
confirm that they did not explain a significant part of the
variation (Poesel et al. 2006). We present Wald statistics for
final models including fixed terms with P0.1 only. The
differences of behavioural measureswhere P0.1were
interpreted using the graphs. The analyses were done in
PASW version 18.
Better fighter hypothesis
Virgin males were generally more aggressive than
eunuchs in malemale contests: virgin males shook the
web more often, walked more frequently towards a rival
and had higher general aggressiveness intensity than eu-
nuch males (for probabilities see Table 1; Fig. 1a). Virgin
males were more aggressive when confronted with an-
other virgin male than when opposed with a eunuch
(frequency of walking towards a rival, aggressiveness
intensity; Table 1). Interestingly, virgin males exhibited
escape behaviour more frequently than eunuchs (Table 1).
Virgin males signalled (web plucking) more frequently
than eunuchs, again more often so when confronted with
Table 1 Statistical results of the final GLMM model
State of 1 State of 2
Signal (pluck web) 0.037 0.006
Touch female >0.1 >0.1
Explore web >0.1 >0.1
Walk towards female 0.064 >0.1
Distance to female 0.003 0.004
Shake 0.034 >0.1
Walk towards rival 0.002 0.01
Attack >0.1 >0.1
Chase >0.1 >0.1
Intensity of aggressiveness 0.009 0.015
Escape 0.1 >0.1
Probabilities that fixed factors, e.g. number of palps in male 1, number of
palps in the rival male (male 2) affect the occurrences of behaviours in
male 1. Statistical probabilities P0.05 bolded, 0.05<P<0.1 italicized.
N024 contests, Nof trials virgin vs virgin012; Nof trials virgin vs
Naturwissenschaften (2012) 99:95101 97
another virgin rival (Table 1). Eunuchs did not employ
guarding behaviour: eunuchs and virgin males did not
differ in frequencies of touching a female and exploring
a web; however, virgin males more often walked towards
a female and stayed closer to her than eunuchs (Table 1;
Fig. 1b). Again, virgin males stayed closer to a female
when opposed with a virgin rival than when opposed
with a eunuch male (Table 1, Fig. 1b).
Mating plug hypothesis
In total we observed 31 copulations (Nfemales019; N
males021). A palp insertion always ended with total emas-
culation, resulting in whole-palpal mating plug in the used
CO. Mating plugs remained in the used female CO after
copulation termination, but were usually absent (externally)
several hours thereafter. In two cases, a plug was observed
Fig. 1 Male behaviours
during malemale contests.
aaggressiveness intensity;
baverage distance to the
female (cm) of the male 1
in the contest with male 2
98 Naturwissenschaften (2012) 99:95101
externally even a day after copulation. We observed four
females removing their plugs using the third pair of legs.
Plugging mostly prevented subsequent copulation into the
used female CO: in seven out of 11 females with a plugged
CO (63.63%) no further copulations occurred, whereas in
36.36% (four out of eleven) of cases a subsequent male used
the already plugged CO and produced the second plug seen
externally. Plugging efficiency by means of unsuccessful in-
sertion attempts in N. livida did not significantly differ from
plugging efficiency in N. malabarensis (χ
00.21, df01, p0
0.647, N022).
Our morphological examination revealed broken parts of
embolic conductor (EC) within the internal female genital
tract (within copulatory duct and/or spermathecae) in 17 out
of 24 cases (70.83%, Nfemales017). Eleven females were
subjected to males that could only copulate in the used CO.
We found a double plug in one such female, i.e. two parts of
EC in the remated organ. The other remated female had no
internal plug despite the previous occurrence of two external
palpal plugs. In seven females that did not remate, we found
parts of EC in the used genitals.
Our study found significantly different results to those pre-
dicted by theory, where mated (and functionally sterile)
males with no residual reproductive value (eunuchs) are
predicted to fight with maximal force when facing an
intruder (e.g. Fromhage and Schneider 2005a; Kralj-Fišer
et al. 2011a). Unexpectedly, eunuch behaviour in N. livida
differed strikingly from that in N. malabarensis, despite
these species being congeneric and exhibiting a similar
sexual biology (Kralj-Fišer et al. 2011a). In both species,
copulation leads to total emasculation of the used palp and
plugging of the female copulatory opening; about 70% of
males fall victim to sexual cannibalism, and about 30% of
females can remate using the same CO (this study; Kralj-
Fišer et al. 2011a). Contrary to the theory predicting that
surviving males with no reproductive value will escalate
fatal fighting (Enquist and Leimar 1990; Maynard Smith
and Price 1973), this was never observed in N. livida and
only rarely in N. malabarensis (this study; Kralj-Fišer et al.
2011a). Nevertheless, N. malabarensis eunuchs exhibited
higher aggressiveness levels than their virgin rivals in the
malemale contests on the female webs as predicted by
game theory (Kralj-Fišer et al. 2011a), while N. livida
eunuchs behaved rather meekly in comparison with virgin
males. A study on parasitoid wasps Mellittobia similarly found
mated and virgin males exhibiting similar levels of aggression
(Innocent et al. 2011). The contrasting results between the
studies suggest that a male fighting behaviour depends on
additional factors, not only future reproductive value. Potential
benefits of fighting may vary with plug efficiency, sperm
precedence, ability to assess resource value, and/or resource
density (e.g. Innocent et al. 2011; Reinhold 2003).
Perhaps unexpectedly, this study invalidates the general-
ity of eunuch-enhanced fighting abilities in spiders, but it
may provide some new insights into the N. livida mating
system in particular and into the eunuch biology in general.
In both studied Nephilengys species and in Herennia, eu-
nuch contests were observed at least 1 day after copulation:
after the male had lost a palp(s), it was separated from the
female for a day, then placed on a random female web for a
malemale contest (Kralj-Fišer et al. 2011a; Kuntner et al.
2009b). Despite a time lag and an unfamiliar female, N.
malabarensis or Herennia eunuchs aggressively fought a
virgin rival off, while the N. livida eunuchs in this study
did not. A possible explanation is that in nature, N. livida
eunuchs guard their mates only shortly after copulation, but
leave the female web, or even offer themselves as prey to the
female thereafter. Such strategy is known in other orbweav-
ing spiders, e.g. Argiope keyserlingi (Herberstein et al.
2005), and hints at a pronounced first sperm priority, where
mate guarding is only adaptive until the female fertilizes the
eggs. That N. malabarensis and Herennia eunuchs are more
persistent in mate-guarding might suggest a fiercer sperm
competition and perhaps longer sperm storage.
A possible reason for discrepancies between both Nephi-
lengys studies may also be a difference in efficiency of sperm
plugs to prevent paternity of subsequent suitors. This is be-
cause a mere reuse of a plugged CO does not necessarily
Fig. 2 Morphological comparison in palpal structure between N. livida
(short, wide and broad tip), N. malabarensis (long and thin tip) and H.
multipuncta (long and thin tip). Redrawn from Kuntner (2005,2007).
Scale lines00.1 mm. CB cymbium, EC embolic conductor, ST subtegu-
lum, Ttegulum
Naturwissenschaften (2012) 99:95101 99
imply paternity. Although the ultimate test of this would be
paternity analysis, which was beyond our scope here, genital
morphology suggests plug efficiency by means of protecting
paternity (Kuntner et al. 2009b). N. malabarensis and Here-
nnia multipuncta males have longer, thinner and more hooked
embolic conductors compared with short, wide and broad-
tipped ones in N. livida (Kuntner 2005;2007; Fig. 2). We
speculate that broader embolic plugs better shield the female
CO and thus prevent subsequent sperm transfer into sperma-
thecae than thinner plugs. Although the shift to more complex
and wider palps (N. livida) may have coevolved with
corresponding counter adaptations in females (Kuntner et al.
2009b), we find it nevertheless plausible that plug efficiency is
higher in N. livida with more complex palps than in N.
malabarensis and H. multipuncta with relatively simpler ones.
If true, more aggressive behaviour of N. malabarensis and H.
multipuncta compared to N. livida eunuchs might be means of
compensating for plug inefficiency.
Our results might also imply that in contrast to N. mala-
barensis and Herennia, N. livida males might be able to assess
whether a given female has in fact been their mate. Further-
more, N. livida males might also be able to assess their rivals
status, i.e. virgin versus sterile male, in the female web. Virgin
males signalled, tried to approach the female and engaged in
agonistic interactions more often when confronted with an-
other virgin male than when opposed by a eunuch. Such
behaviour of virgin males might be enhanced by females,
which significantly more often cannibalized non-aggressive
than aggressive males (Kralj-Fišer et al. 2011b;e.g.
Stoltz et al. 2008). Such female choice strategy might
explain the differences in aggressiveness in N. livida and N.
malabarensis virgin males. Interestingly, despite their higher
aggressiveness, virgin males nevertheless retreated more often
than eunuchs. Such higher risk aversion in virgin males com-
pared with mated ones is in accordance with game theory
(Fromhage and Schneider 2005a; Kralj-Fišer et al. 2011a).
Finally, competitive and mating behavioursmay depend on
ecological factors. Males may adapt their mating strategies
according to female density, levels of male competition, patch
size, etc. (Innocent et al. 2011; Kasumovic et al. 2008; Kokko
and Rankin 2006; Reece et al. 2007). Population density, in
particular, is important for mate guarding behaviour; if popu-
lation densities intensify malemale competition through
male-biased operational sex ratio, the intensity of mate guard-
ing is expected to increase (Davis and Brown 1999; Hardling
2004; Jormalainen 1998). According to our observations in
nature, N. livida occur at lower local abundances compared to
N. malabarensis. In populations with lower densities, male
survival during mate searching might be lower than in pop-
ulations with high densities, resulting in further reduced
numbers of rival males. If so, sperm competition in N. livida
is reduced, which makes paternity protection additional to
sperm plugs unnecessary (Fromhage et al. 2005,2008).
In conclusion, our study found no support for the better
fighter hypothesis in N. livida eunuchs. Differing mating
biology between the so-far-studied species known for the
eunuch phenomenon may account for this result. However,
non-aggressive behaviour of N. livida eunuchs might also be
explained by population ecology, with lower population
densities perhaps resulting in relaxed malemale competi-
tion, which makes excessive aggression and mate guarding
Acknowledgements We thank three anonymous reviewers and Jutta
Schneider for comments on the manuscript, MatjažGregoričfor the
spider collection and laboratory help; Živa Justinek for help with
experiments and lab support; and Ingi Agnarsson, Sahondra Lalao
Rahanitriniaina and Honore Rabarison for their help in field. This work
was funded by the Slovenian Research Agency (grant J1-2063 to MK)
and the National Geographic Society (grant 8655-09 to I. Agnarsson,
M. Kuntner and T. Blackledge). SKF was supported by Humboldt
fellowship for postdoctoral researchers and Humboldt return
Bean D, Cook JM (2001) Male mating tactics and lethal combat in the
nonpollinating fig wasp Sycoscapter australis.AnimBehav
Buskirk RE, Frohlich C, Ross KE (1984) The natural selection of
sexual cannibalism. Am Nat 123:612625
Elias DO, Botero CA, Andrade MCB, Mason AC, Kasumovic MM
(2010) High resource valuation fuels desperadofighting tactics
in female jumping spiders. Behav Ecol 21:868875
Davis JA, Brown CR (1999) Costs of coloniality and the effect of
colony size on reproductive success in purple martins. Condor
Enquist M, Leimar O (1987) Evolution of fighting behaviorthe
effect of variation in resource value. J Theor Biol 127:187205
Enquist M, Leimar O (1990) The evolution of fatal fighting. Anim
Behav 39:19
Fromhage L, Schneider JM (2005a) Virgin doves and mated hawks:
contest behaviour in a spider. Anim Behav 70:10991104
Fromhage L, Schneider JM (2005b) Safer sex with feeding females:
sexual conflict in a cannibalistic spider. Behav Ecol 16:377382
Fromhage L, Schneider JM (2006) Emasculation to plug up females:
the significance of pedipalp damage in Nephila fenestrata. Behav
Ecol 17:353357
Fromhage L, Elgar MA, Schneider JM (2005) Faithful without care:
the evolution of monogyny. Evolution 59:14001405
Fromhage L, Houston AI, McNamara JM (2008) A model for the
evolutionary maintenance of monogyny in spiders. J Theor Biol
Hardling KA (2004) Male brood care without paternity increases
mating success. Behav Ecol 15:715772
Herberstein ME, Barry KL, Turoczy MA, Wills E, Youssef C, Elgar
MA (2005) Post-copulation mate guarding in the sexually canni-
balistic St Andrew's Cross spider (Araneae Araneidae). Ethol Ecol
Evol 17:1726
Huntingford FA, Turner AK (1987) Animal Conflict. Chapman and
Hall, London
Innocent TM, Savage J, West SA, Reece SE (2007) Lethal combat and
sex ratio evolution in a parasitoid wasp. Behav Ecol 8:709715
100 Naturwissenschaften (2012) 99:95101
Innocent TM, West SA, Sanderson JL, Hyrkkanen N, Reece SE (2011)
Lethal combat over limited resources: testing the importance of
competitors and kin. Behav Ecol 22:923931
Jormalainen V (1998) Precopulatory mate guarding in crustaceans:
male competitive strategy and intersexual conflict. Q Rev Biol
Kasumovic MM, Bruce MB, Andrade MCB, Herberstein ME (2008)
Spatial and temporal demographic variation drives within-season
fluctuations in sexual selection. Evolution 62:23162325
Kokko H, Rankin DJ (2006) Lonely hearts or sex in the city? Density-
dependent effects in mating systems. Phil Trans R Soc B
Kralj-Fišer S, GregoričM, Zhang SC, Li D, Kuntner M (2011a)
Eunuchs are better fighters. Anim Behav 81:933939
Kralj-Fišer S, Schneider JM, Justinek Ž, Kalin S, GregoričM, Kuntner
M (2011b) Mate quality, not aggressive spillover, explains sexual
cannibalism in a size dimorphic spider. Behav Ecol Sociobiol
Kuntner M (2005) A revision of Herennia (Araneae: Nephilidae:
Nephilinae), the Australasian 'coin spiders'. Invertebr Syst
Kuntner M (2007) A monograph of Nephilengys, the pantropical
'hermit spiders' (Araneae, Nephilidae, Nephilinae). Syst Entomol
Kuntner M, Coddington JA, Hormiga G (2008) Phylogeny of extant
nephilid orb-weaving spiders (Araneae, Nephilidae): testing mor-
phological and ethological homologies. Cladistics 24:147217
Kuntner M, Kralj-Fišer S, Schneider JM, Li D (2009a) Mate plugging
via genital mutilation in nephilid spiders: an evolutionary hypoth-
esis. J Zool 277:257266
Kuntner M, Coddington JA, Schneider JM (2009b) Intersexual arms
race? Genital coevolution in nephilid spiders (Araneae, Nephilidae).
Evolution 63:14511463
Kuntner M, Agnarsson I, GregoričM (2009c) Nephilid spider eunuch
phenomenon induced by female or rival male aggressiveness. J
Arachnol 37:266271
Kuntner M, Agnarsson I (2011) Biogeography and diversification of
hermit spiders on Indian Ocean islands (Nephilidae: Nephi-
lengys). Mol Phylogenet Evol 59:477488
Maynard Smith J, Price GR (1973) Logic of animal conflict. Nature
Maynard Smith J, Parker GA (1976) The logic of asymmetric contests.
Anim Behav 24:159175
Moore JC, Obbard DJ, Reuter C, West SA, Cook JM (2008) Fighting
strategies in two species of fig wasp. Anim Behav 76:315322
Murray MG, Gerrard R (1985) Putting the challenge into resource
exploitationa model of contest competition. J Theor Biol
Parker GA (1970) Sperm competition and its evolutionary consequen-
ces in insects. Biol Rev Camb Philos Soc 45:525
Poesel A, Kunc HP, Foerster K, Johnsen A, Kempenaers B (2006)
Early birds are sexy: male age, dawn song and extrapair paternity
in blue tits, Cyanistes (formerly Parus)caeruleus. Anim Behav
Reece SE, Innocent TM, West SA (2007) Lethal malemale combat in
the parasitoid Melittobia acasta: are size and competitive envi-
ronment important? Anim Behav 74:11631169
Reinhold K (2003) Influence of male relatedness on lethal combat in
fig wasps: a theoretical analysis. Proc R Soc Lond Ser B Biol Sci
Schneider JM, Lubin Y (1998) Intersexual conflict in spider. Oikos
Stoltz JA, Elias DO, Andrade MCB (2008) Females reward courtship by
competing males in a cannibalistic spider. Behav Ecol 62:689697
Uhl G, Nessler S, Schneider J (2010) Securing paternity in spiders? A
review on occurrence and effects of mating plugs and male genital
mutilation. Genetica 138:75104
West SA, Murray MG, Machado CA, Griffin AS, Herre EA (2001)
Testing Hamiltons rule with competition between relatives. Nature
West SA, Pen I, Griffin AS (2002) Conflict and cooperationcooper-
ation and competition between relatives. Science 296:7275
Naturwissenschaften (2012) 99:95101 101
... In general support of the plugging hypothesis, numerous studies demonstrate paternity benefits of plugs made of male genital parts. For example, empirical research has shown adaptive function of lesser genital damage in Nephila (Fromhage and Schneider 2006), Argiope (Nessler et al. 2007(Nessler et al. , 2009Ghione and Costa 2011;Herberstein et al. 2012), and Latrodectus (Snow et al. 2006) and of emasculation in Tidarren (Knoflach and van Harten 2001;Ramos et al. 2004) and in two nephilid species, the SE Asian Nephilengys malabarensis and the Malagasy Nephilingis livida (Kralj-Fišer et al. 2011;Kralj-Fišer and Kuntner 2012;Lee et al. 2012;Li et al. 2012). However, mating plugs consisting of male spider genital parts are not always fully effective in preventing female remating (Schneider and Elgar 2001;Schneider and Elgar 2002;Kralj-Fišer et al. 2011;Fromhage and Schneider 2012) and thus the plugging spider male may increase his monopolization of the female through postcopulatory mate guarding (Robinson and Robinson 1980;Fromhage and Schneider 2006;Kralj-Fišer et al. 2011). ...
... This remote-copulation hypothesis predicts continuous sperm transfer into female spermatheca after palpal removal (Kralj-Fišer et al. 2011;Li et al. 2012). Recent empirical research has demonstrated adaptive function of emasculation in the South-East Asian N. malabarensis, with support for the above hypotheses (haemolymph leakage not tested), and in part in the Malagasy N. livida, with support for the plugging hypothesis, but not for better-fighter hypothesis (Kralj-Fišer et al. 2011;Kralj-Fišer and Kuntner 2012;Lee et al. 2012;Li et al. 2012). Another emasculating nephilid spider, Herennia, remains untested as do the araneids Caerostris and Argiope, and with the exception of the gloves-off hypothesis, so do the theridiids Tidarren and Echinotheridion (Kuntner et al. 2014). ...
... We showed that H. multipuncta eunuchs exhibit higher aggressiveness levels during male-male contests and remain closer to their mates compared with their virgin rivals, resembling N. malabarensis (Kralj-Fišer et al. 2011). This mirrors the predictions from the theory: A male with high future reproductive potential should not escalate fights to avoid injury, but when his residual reproductive potential is insignificant, he should fight forcefully (Enquist et al. 1990;Clark 1994;Fromhage and Schneider 2005;Kralj-Fišer and Kuntner 2012). Although half eunuchs have another chance to mate, full eunuchs are effectively sterile with no chances for further mating. ...
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Emasculation—males becoming effectively sterile by self-removing their genitals—has long been considered a peculiar evolutionary phenomenon with unknown function, taxonomically restricted to few spiders and flies. In spiders, emasculation results in half or full eunuchs when males sever one or both sperm transferring organs, palps. Three types of emasculation, pre-maturation, mating, and post-mating are known in spiders, all having evolved multiple times. Males practicing pre-maturation emasculation sever one of their palps while still immature, then engage in strict monogyny via genital plugging and spontaneous death. Emasculation during mating also results in genital plugs, but half eunuchs have another chance to mate. So far, the behavior of those males that become eunuchs post-mating by self-removing disfigured palps has not been investigated empirically. We test the mechanism and adaptive significance of post-mating emasculation in coin spiders (Herennia multipuncta) and use phylogenetic reconstruction to understand its evolutionary history. Our laboratory assays corroborate three hypotheses related to mate monopolization: (1) The plugging hypothe-sis—predicting genital plugs to prevent female remating; (2) The better-fighter hypothesis—predicting enhanced eunuch aggressiveness toward rivals; and (3) The gloves-off hypoth-esis—predicting increased eunuch endurance. The support for these hypotheses in spiders practicing emasculation during and after mating reinforces recent phylogenetic interpretations of these two emasculation types being evolutionarily linked in the family Nephilidae. We weigh the evidence in support of three different, but equally parsimonious scenarios of nephilid emasculation evolution. We conclude that emasculation is an adaptive, sexually selected trait that calls for further compar-ative and experimental research.
... Relatively small nephilid males of certain species engage in extreme mating strategies, including severing terminal parts of their pedipalps (sperm transferring appendages), which are used to plug female copulatory openings [39,40]. Experimental studies on selected species found that plugs from males with complex genitals commonly prevent female polyandry, whereas plugs from simple genitals do not [39,41]. Assuming that male strategies to monopolize paternity with a single female via genital plugging are not in the interest of the female [20], females ought to evolve counter-adaptations. ...
... Our morphological examinations on the prevalence of genital plugs, consisting of palpal parts from a single versus multiple males [43,44], helped score for male genital damage presence or absence for most taxa in the phylogeny (Additional file 1: Table S1). Additional evidence comes from detailed species level experimental studies [39,41,[48][49][50][51][52][53]. ...
... We define polygyny as male mating with more than one female, whereas monogynous males invest into repeated mating with the same female in an attempt to plug both of her copulatory openings. We based the inferred mating rates in nephilids and outgroups (Additional file 1: Table S1) on available experimental studies [13,[39][40][41] and on genital damage data where single versus multiple male mating plugs per female copulatory opening predict monandry and polyandry, respectively [26,35,40,41,53]. Most Nephila species, and Phonognatha graeffei, are polyandrous [49,61]. ...
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Background Genital diversity may arise through sexual conflict over polyandry, where male genital features function to manipulate female mating frequency against her interest. Correlated genital evolution across animal groups is consistent with this view, but a link between genital complexity and mating rates remains to be established. In sexually size dimorphic spiders, golden orbweaving spiders (Nephilidae) males mutilate their genitals to form genital plugs, but these plugs do not always prevent female polyandry. In a comparative framework, we test whether male and female genital complexity coevolve, and how these morphologies, as well as sexual cannibalism, relate to the evolution of mating systems. Results Using a combination of comparative tests, we show that male genital complexity negatively correlates with female mating rates, and that levels of sexual cannibalism negatively correlate with male mating rates. We also confirm a positive correlation between male and female genital complexity. The macroevolutionary trajectory is consistent with a repeated evolution from polyandry to monandry coinciding with the evolution towards more complex male genitals. Conclusions These results are consistent with the predictions from sexual conflict theory, although sexual conflict may not be the only mechanism responsible for the evolution of genital complexity and mating systems. Nevertheless, our comparative evidence suggests that in golden orbweavers, male genital complexity limits female mating rates, and sexual cannibalism by females coincides with monogyny. Electronic supplementary material The online version of this article (doi:10.1186/s12862-016-0821-y) contains supplementary material, which is available to authorized users.
... Experimental studies in Trichonephila and Phonognatha confirm that larger males win competitions for access to females, but not in N. pilipes and Nephilingis livida (30,72,83). On the whole, direct male-male antagonism overwhelmingly selects for larger male size (Figure 2) and tends to decrease, not increase, eSSD (83). ...
... Postcopulatory mate guarding (Figure 4c-e) is one way to monopolize a female, but plugging of the female genital tract, by male genital breakage or even full emasculation, is another (82). Coin and hermit spider males-all approximately four times smaller than their females-use their severed genitalia physically to block females from remating (71,72,88,89). Curiously, N. pilipes males have hair-like genitalic termini that are ineffective plugs, even though they break off inside the female (88). ...
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Sexual size dimorphism is one of the most striking animal traits, and among terrestrial animals, it is most extreme in certain spider lineages. The most extreme sexual size dimorphism (eSSD) is female biased. eSSD itself is probably an epiphenomenon of gendered evolutionary drivers whose strengths and directions are diverse. We demonstrate that eSSD spider clades are aberrant by sampling randomly across all spiders to establish overall averages for female (6.9 mm) and male (5.6 mm) size. At least 16 spider eSSD clades exist. We explore why the literature does not converge on an overall explanation for eSSD and propose an equilibrium model featuring clade- and context-specific drivers of gender size variation. eSSD affects other traits such as sexual cannibalism, genital damage, emasculation, and monogyny with terminal investment. Coevolution with these extreme sexual phenotypes is termed eSSD mating syndrome. Finally, as costs of female gigantism increase with size, eSSD may represent an evolutionary dead end. Expected final online publication date for the Annual Review of Entomology, Volume 65 is January 7, 2020. Please see for revised estimates.
... males maturing earlier than females), and therefore, for the evolution of female-biased sexual size dimorphism (SSD) (Danielson-François et al. 2012). Males, however, often experience direct male-male competition as they accumulate on female webs, and this sexual selection mechanism favours larger male sizes (Robinson and Robinson 1976;Elgar et al. 2003;Schneider and Elgar 2005), albeit with exceptions (Fromhage and Schneider 2005;Kralj-Fišer and Kuntner 2012). Theory therefore predicts to find balancing selection pressures (e.g. ...
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The role of developmental plasticity in the evolution and maintenance of sexual size dimorphism (SSD) has recently received more attention. We experimentally investigated the effects of genetics (pedigree), social cues, and food availability on developmental time and adult male size in Nephilingis cruentata, an extremely female-biased sexually size dimorphic spider with notable male size variation. In a split-brood design, we exposed spiderlings of known pedigrees to either a high or low feeding regime. We tested the males’ ability to match the sub-adult growth and time of maturation to the perceived female availability and male competition by exposing them to silk cues of either males or females during the subadult stage. We recorded male size at maturation and total developmental time, the duration of the sub-adult stage, and the growth during the sub-adult stage. Poorly fed males had a longer development and matured at extremely small sizes compared to well-fed males. The social cues did not influence the duration of the sub-adult stage nor the male size at maturation. However, males exposed to male cues grew more and were heavier at reaching maturity than those exposed to female cues, which implies that sub-adult males respond to perceived male–male competition by investing more in growth. Furthermore, variation in male size has been explained by low additive genetic variability but high maternal effects. Our results highlight the role of maternal effects and/or common environment in shaping male body size. Future studies with a scope for maternal effects on SSD are warranted.
... grasped by a predator or a conspecific during fight, in order to escape more easily (Punzo 1997;Foelix 2011). Furthermore, they self-amputate injured appendages (Kralj-Fišer et al. 2011;Kralj-Fišer and Kuntner 2012;Kuntner et al. 2014), and they "lick" or rub their wounds. Missing appendages may negatively affect development, web building, foraging success, competitive abilities and mating success in some species, whereas in several species it has no apparent costs (reviewed in Fleming et al. 2007). ...
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Spiders with around 48,000 recorded species are major terrestrial predators and thus crucially important for ecosystem functioning. They are widely used as research models and for biodiversity displays and sometimes also kept as pets. Nevertheless, we are not aware of any legal ethical rules bound to spider welfare during rearing or research. To set ethical standards, we first need to detect and assess how spiders “perceive” the external world. Based on the current knowledge of spiders’ sensory and nervous system, it is difficult to judge whether spiders feel pain, distress and suffering, although their behaviours like thanatosis, “bailing out”, autotomy and associative avoidance learning imply so. As is now known, arthropods are not simply mini-robots as traditionally believed. Thus, spider welfare deserves more research effort, and the ethical standards for rearing or using spiders in research need to be set. Here, we describe the variety of spider physiological and behavioural characteristics and how they apply to their rearing, housing, handling and experimental use. We hope reporting these methods will help ensuring welfare and well-being of spiders in captivity.
... . On the other hand, mating systems in other species range from almost complete monogamy (e.g. Herennia, Nephilengys, Nephilingis17,28,34 ) to polygamy (e.g. some Latrodectus and Nephila 28,31,32,35,36 ). ...
... Considering only the literature since 2000, spider phylogenetic studies are increasingly used to test evolutionary hypotheses. Examples from the spider literature of the use of phylogenies to pose and test phylogeographic (Bidegaray-Batista et al., 2007;Starrett & Hedin, 2007;Su et al., 2007;Cooper et al., 2011;Kuntner & Agnarsson, 2011b), biogeographic (Crews & Hedin, 2006;Garb & Gillespie, 2006;Hendrixson & Bond, 2007;Wood et al., 2007;Dimitrov et al., 2008;Macias-Hernandez et al., 2008;Wang et al., 2008;Bidegaray-Batista & Arnedo, 2011;Kuntner & Agnarsson, 2011a;Su et al., 2011;, ecological (Arnedo & Gillespie, 2006;Opell, 2006;De Busschere et al., 2010;Spagna et al., 2010;Satler et al., 2011), behavioural Agnarsson, 2012;Kralj-Fišer & Kuntner, 2012), palaeontologi- cal ( Penney et al., 2003;Penney, 2004) and evolutionary hypotheses and sce- narios ( Garb & Hayashi, 2005;Stratton, 2005;Agnarsson, 2006a;Agnarsson et al., , 2010Johannesen et al., 2007;Blackledge et al., 2009b;Hedin & Lowder, 2009;Cheng et al., 2010;Higgins et al., 2011;Dimitrov et al., 2012;Ledford et al., 2012). Additionally, several recent studies have used molecular or combined data in order to discover and describe unknown diversity, or delimit species (Griffiths et al., 2005;Hendrixson & Bond, 2005;Johannesen et al., 2005;Bond & Stockman, 2008;Vink et al., 2008b;Duncan et al., 2010;Macias-Hernandez et al., 2010;Hamilton et al., 2011;Hedin & Carlson, 2011;Kuntner & Agnarsson, 2011a). ...
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Adult body size, development time, and growth rates are components of organismal life histories, which crucially influence fitness and are subject to trade-offs. If selection is sex-specific, male and female developments can eventually lead to different optimal sizes. This can be achieved through developmental plasticity and sex-specific developmental trajectories. Spiders present suitable animals to study differences in developmental plasticity and life history trade-offs between the sexes, because of their pronounced sexual dimorphism. Here, we examine variation in life histories in the extremely sexually size dimorphic African hermit spider (Nephilingis cruentata) reared under standardized laboratory conditions. Females average 70 times greater body mass (and greater body size) at maturity than males, which they achieve by developing longer and growing faster. We find a small to moderate amount of variability in life history traits to be caused by family effects, comprising genetic, maternal, and early common environmental effects, suggesting considerable plasticity in life histories. Remarkably, family effects explain a higher variance in male compared to female life histories, implying that female developmental trajectories may be more responsive to environment. We also find sex differences in life history trade-offs and show that males with longer development times grow larger but exhibit shorter adult longevity. Female developmental time also correlates positively with adult body mass, but the trade-offs between female adult mass, reproduction, and longevity are less clear. We discuss the implications of these findings in the light of evolutionary trade-offs between life history traits.
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Integrating the insights derived from both phylogenetic and experimental approaches offers a more complete understanding of evolutionary patterns and processes, yet it is rarely a feature of investigations of the evolutionary significance of trait variation. We combine these approaches to reinterpret the patterns and processes in the evolution of female biased sexual size dimorphism in Nephilidae, a spider lineage characterized by the most extreme sexual size dimorphism among terrestrial animals. We use a molecular phylogeny to reconstruct the size evolution for each sex and reveal a case of "sexually dimorphic gigantism": both sexes steadily outgrow their ancestral sizes, but the female and male slopes differ, and hence sexual size dimorphism steadily increases. A review of the experimental evidence reveals a predominant net selection for large size in both sexes, consistent with the phylogenetic pattern for females but not for males. Thus, while sexual size dimorphism in spiders most likely originates and is maintained by fecundity selection on females, it is unclear what selection pressures prevent males from becoming as large as females. This integrated approach highlights the dangers of inferring evolutionary significance from experimental studies that isolate the effects of single selection pressures.
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We measured how fighting, mate-guarding, ectoparasitism, and annual reproductive success varied with colony size in Purple Martins (Progne subis) in Tulsa County, Oklahoma, in 1997. Fights bird(-1) hr(-1) changed significantly with colony size, peaking in intermediate-sized colonies. Intensity of mate-guarding by males increased significantly with colony size. Ectoparasitism of nestlings by the martin mite (Dermanyssus prognephilus) increased significantly with colony size, but nestling body mass declined with mite load only for broods of six nestlings. Reproductive success did not vary significantly with colony size. Purple Martin colonies probably do not routinely reach large enough sizes for the costs of fighting or ectoparasitism to be important. These birds apparently do not experience direct benefits of coloniality, and groups likely form in response to limited nesting sites. The pattern in reproductive success suggests that Purple Martin colonies represent ideal free distributions of birds with respect to resource availability in each habitat patch, and mean fitness of birds in different patches is similar.
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Although most animals employ strategies to avoid costly escalation of conflict, the limitation of critical resources may lead to extreme contests and fatal fighting. Evolutionary theories predict that the occurrence and intensity of fights can be explained by resource value and the density and relatedness of competitors. However, the interaction between these factors and their relative importance often remains unclear; moreover, few systems allow all variables to be experimentally investigated, making tests of these theoretical predictions rare. Here, we use the parasitoid wasp Melittobia to test the importance of all these factors. In contrast to predictions, variation in contested resource value (female mates) and the relatedness of competitors do not influence levels of aggression. However, as predicted, fight intensity increased with competitor density and was not influenced by the greater cost of fighting at high density. Our results suggest that in the absence of kin recognition, indirectly altruistic behavior (spite) is unlikely to evolve, and in such circumstances, the scale of competition will strongly influence the amount of kin discrimination in the form of level of aggression as observed in Melittobia species.
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The nephilid 'coin spiders' (Herennia Thorell) are known for their arboricolous ladder webs, extreme sexual size dimorphism and peculiar sexual biology. This paper revises Herennia taxonomy, systematics, biology and biogeography. The widespread Asian Herennia multipuncta (Doleschall) (= H. sampitana Karsch, new synonymy; = H. mollis Thorell, new synonymy) is synanthropic and invasive, whereas the other 10 species are narrowly distributed Australasian island endemics: H. agnarssoni, sp. nov. is known from Solomon Islands; H. deelemanae, sp. nov. from northern Borneo; H. etruscilla, sp. nov. from Java; H. gagamba, sp. nov. from the Philippines; H. jernej, sp. nov. from Sumatra; H. milleri, sp. nov. from New Britain; H. oz, sp. nov. from Australia; H. papuana Thorell from New Guinea; H. sonja, sp. nov. from Kalimantan and Sulawesi; and H. tone, sp. nov. from the Philippines. A phylogenetic analysis of seven species of Herennia, six nephilid species and 15 outgroup taxa scored for 190 morphological and behavioural characters resulted in 10 equally parsimonious trees supporting the monophyly of Nephilidae, Herennia, Nephila, Nephilengys and Clitaetra, but the sister-clade to the nephilids is ambiguous. Coin spiders do not fit well established biogeographic lines (Wallace, Huxley) dividing Asian and Australian biotas, but the newly drawn 'Herennia line' suggests an all-Australasian speciation in Herennia. To explain the peculiar male sexual behaviour (palpal mutilation and severance) known in Herennia and Nephilengys, three specific hypotheses based on morphological and behavioural data are proposed: (1) broken embolic conductors function as mating plugs; (2) bulb severance following mutilation is advantageous for the male to avoid hemolymph leakage; and (3) the eunuch protects his parental investment by fighting off rival males.
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Plugging of female genitals via male sexual mutilation is a common sexual repertoire in some nephilid spiders (Herennia, Nephila, Nephilengys), but the behavioral pathways leading to emasculation are poorly understood. Recent work suggests that copulating Herennia males damage their reproductive organs during copulation and then voluntarily, and stereotypically, remove their pedipalps to become eunuchs. Presumably, such emasculation increases agility allowing the male to better fend off rival males. However, through our observation of male antagonism in Nephilengys borbonica (Vinson 1863) in La Reunion (Indian Ocean), we discovered that genital severance involving the entire male palp is induced by a rival eunuch. Additionally, laboratory matings of the same species from Mayotte provide the first observations of female sexual cannibalism in this species, one such forceful copulation termination leading to emasculation of the entire palp. These novel behaviors suggest that mate plugging and the eunuch phenomenon are more plastic repertoires than hitherto thought, and thus our observations add to possible pathways leading to them. Based on our examination of 791 samples of Nephilengys spp. from museum collections and of a freshly collected representative sample of N. borbonica, we conclude that i) palpal severance is common (50% of males from the wild were eunuchs lacking both palps), but ii) the females (or perhaps subsequent males) must possess a mechanism for removing severed palps from the epigyna (none had a whole palpal bulb), leaving behind only partial, embolic plugs, and iii) the disparity between male palpal damage (50%) and visible mating plugs in females (21%) merits further research as the relative numbers of severed males and plugged females can offer insight into which sex may have the upper hand in an evolutionary arms race.
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Sexual cannibalism, in which a male is eaten by his mate following copulation, is expected to convey a selective advantage to the male under certain conditions. As shown quantitatively, the phenomenon is expected when 1) a male can mate only a few times in his lifetime and 2) the cannibalism significantly increases the number and/or viability of eggs fertilized by his own sperm. The expected number of male matings appears to be the more important of these 2 conditions. Phenomena selecting for sexual cannibalism appear more closely related to paternal investment strategies than to ecological factors associated with other forms of cannibalism. -from Authors
Conflicts between animals of the same species usually are of ``limited war'' type, not causing serious injury. This is often explained as due to group or species selection for behaviour benefiting the species rather than individuals. Game theory and computer simulation analyses show, however, that a ``limited war'' strategy benefits individual animals as well as the species.
Sex allocation theory provides excellent opportunities for testing how behavior and life histories are adjusted in response to environmental variation. One of the most successful areas from this respect is Hamilton's local mate competition theory. As predicted by theory, a large number of animal species have been shown to adjust their offspring sex ratios (proportion male) conditionally, laying less female-biased sex ratios as the number of females that lay eggs on a patch increases. However, recent studies have shown that this predicted pattern is not followed by 2 parasitoid species in the genus Melittobia, which always produce extremely female-biased sex ratios. A possible explanation for this is that males fight fatally and that males produced by the first female to lay eggs on a patch have a competitive advantage over later emerging males. This scenario would negate the advantage of later females producing a less female-biased sex ratio. Here we examine fatal fighting and sex ratio evolution in another species, Melittobia acasta. We show that females of this species also fail to adjust their offspring sex ratio in response to the number of females laying eggs on a patch. We then show that although earlier emerging males do have an advantage in winning fights, this advantage 1) can be reduced by an interaction with body size, with larger males more likely to win fights and 2) only holds for a brief period around the time at which the younger males emerge from their pupae. This suggests that lethal male combat cannot fully explain the lack of sex ratio shift observed in Melittobia species. We discuss alternative explanations.
Fatal fights are rare in the majority of animal species but are a common component of mate competition between wingless males of some species of fig-associated wasps. We investigated fatal fighting in Sycoscapter australis, a nonpollinating fig wasp found in the syconia (inflorescences) of the Moreton Bay fig, Ficus macrophylla. Overall, about 25% of males sustained fatal injuries during the mate competition period. Measurement and analysis of 349 males revealed a sevenfold difference in mandible size between the largest and smallest individuals, as well as evidence for dimorphism in the mandibles, which showed positive allometry in relation to other body parts. Small and large males did not differ significantly in their injury levels. Mandibles are the main weapons used in fights and injury levels were highest in syconia where males on average had larger mandibles. Injury levels also increased with the estimated operational sex ratio in a syconium, although this variable explained little of the variation. In contrast to the usual assumptions about wingless male fig wasps, a significant number (17%) of males emerged from their natal syconia. Females emerged at a fairly constant rate over many hours, consistent with contest competition between males for access to matings. In contrast, the more numerous females of the pollinating wasp speciesPleistodontes froggatti showed a strong peak in emergence, consistent with scramble competition between males for access to mates.