Downloaded from rsbl.royalsocietypublishing.org on August 20, 2012
Remote copulation: male adaptation to female cannibalism
Daiqin Li, Joelyn Oh, Simona Kralj-Fiser and Matjaz Kuntner
Biol. Lett. 2012 8, 512-515 first published online 1 February 2012
doi: 10.1098/rsbl.2011.1202
Supplementary data
"Data Supplement"
http://rsbl.royalsocietypublishing.org/content/suppl/2012/01/27/rsbl.2011.1202.DC1.ht
ml
References
This article cites 22 articles, 3 of which can be accessed free
http://rsbl.royalsocietypublishing.org/content/8/4/512.full.html#ref-list-1
Article cited in:
http://rsbl.royalsocietypublishing.org/content/8/4/512.full.html#related-urls
Subject collections
Articles on similar topics can be found in the following collections
behaviour (537 articles)
evolution (557 articles)
Email alerting service
Receive free email alerts when new articles cite this article - sign up in the box at the top
right-hand corner of the article or click here
To subscribe to Biol. Lett. go to: http://rsbl.royalsocietypublishing.org/subscriptions
Downloaded from rsbl.royalsocietypublishing.org on August 20, 2012
the male palp [6]. However, some spider species further
exhibit the ‘eunuch phenomenon’, whereby males
break off the entire palp [8–12] (figure 1). While this
seems maladaptive because eunuchs are functionally
sterile, several adaptive hypotheses have been suggested
[10–12]. The broken male parts may effectively plug
the female genitals (epigynum) leading to paternity protection—the mating-plug hypothesis [9,11,12]. Yet,
entire emasculation is not necessary for effective plugging, and so additional functions of emasculation have
been suggested, including the better-fighter hypothesis;
indeed, eunuchs are more aggressive and agile compared
with intact males [12]. Finally, the remote copulation
hypothesis postulates that eunuchs secure continuing
sperm transfer through the severed palp attached to
the epigynum [12,13], but to date this has remained
untested. Here, we test for continual sperm transfer
after genital severance in an extremely sexually dimorphic
and cannibalistic nephilid orb-web spider, Nephilengys
malabarensis (Araneae: Nephilidae).
Biol. Lett. (2012) 8, 512–515
doi:10.1098/rsbl.2011.1202
Published online 1 February 2012
Animal behaviour
Remote copulation: male
adaptation to female
cannibalism
Daiqin Li1,2,*, Joelyn Oh1, Simona Kralj-Fišer3
and Matjaž Kuntner2,3,4
1
Department of Biological Sciences, National University of Singapore,
14 Science Drive 4, Singapore 117543, Republic of Singapore
2
College of Life Sciences, Hubei University, Wuhan 430062, Hubei,
China
3
Institute of Biology, Scientific Research Centre, Slovenian Academy of
Sciences and Arts, Ljubljana, Slovenia
4
National Museum of Natural History, Smithsonian Institution,
Washington, DC, USA
*Author for correspondence (dbslidq@nus.edu.sg).
Sexual cannibalism by females and associated
male behaviours may be driven by sexual conflict. One such male behaviour is the eunuch
phenomenon in spiders, caused by total genital
emasculation, which is a seemingly maladaptive
behaviour. Here, we provide the first empirical
testing of an adaptive hypothesis to explain this
behaviour, the remote copulation, in a highly
sexually cannibalistic orb-web spider Nephilengys
malabarensis. We demonstrate that sperm transfer continues from the severed male organ into
female genitals after the male has been detached
from copula. Remote copulation increases the
total amount of sperm transferred, and thus
probably enhances paternity. We conclude that
the mechanism may have evolved in response to
sexual cannibalism and female-controlled short
copulation duration.
2. MATERIAL AND METHODS
(a) Study subjects
Juvenile and subadult N. malabarensis were collected in the field, housed
in controlled environmental conditions [11,12], and monitored until
adulthood, so that their mating history and precise age (days elapsed
from the final moult) were known. Only virgin spiders were used in trials.
(b) Mating trials
We staged each mating trial by introducing a virgin male onto a virgin
female web (n ¼ 25). All trials ended with copulation and genital
damage, either through entire palp severance or only partial palp
damage. We recorded the copulation duration as the time elapsed
from palp insertion until breakage. To estimate the pattern of
sperm transfer after copulation, the broken palp was allowed to
remain in the epigynum (figure 1) for varying times (0–1255 s)
before we experimentally removed it.
(c) Sperm quantification
Immediately after palp removal from the epigynum, we placed it in a
2 ml tube to prevent the loss of remaining sperm. The female’s genitals
were dissected, and all reproductive parts placed in a separate tube.
The number of sperm in each of the reproductive parts was quantified separately using a modified sperm-counting procedure [14–17]
(see the electronic supplementary material for details). Samples containing either the used palp or isolated female reproductive organs
were extracted and placed on a haemocytometer. Sperm were counted
under a compound microscope for each sample. Calculations were
then performed to obtain the average number of sperm present in
the used palp or female reproductive organ. Our sperm quantification
was deemed reliable, judging by rare sperm clumping in the samples
and high correlations between the sperm counts [14–17].
Keywords: sexual conflict; sexual cannibalism;
coevolution; genital emasculation
1. INTRODUCTION
Sexual conflict, divergence between the reproductive
interests of males and females, can lead to antagonistic
coevolution in which adaptations in each sex select for
counter-adaptations in other [1]. Sexual cannibalism
by females and the associated male behaviours are
examples of antagonistic coevolution driven by sexual
conflict [1 – 5]. In many highly sexually cannibalistic
spiders, males adopt a monogynous mating strategy,
typically mating with one or two females (spiders
have paired genitals) [4]. However, in spite of monoor bigyny, polyandry may continue to serve interests
of females [5]. To counter female polyandry, however,
males have evolved mate guarding, male sacrifice to
a cannibalistic female, genital mutilation resulting
in mating plugs, and mechanisms that increase the
quantity of sperm transferred [3,4,6,7].
In spiders, male genital mutilation, the breakage of
a male’s palp within the female genitals, is common in
mono/bigynous and highly sexually cannibalistic species,
and is typically characterized by breakage of the tip of
(d) Female- and male-initiated palp breakages
The inserted palps could be broken by either sex. In female-initiated
palp breakages (n ¼ 14), the female terminated copulation by cannibalization (figure 1) or by pulling the male away from her epigynum.
In male-initiated palp breakages, the male deliberately severed his
palp (n ¼ 8). In all cases, copulation termination results in palp
breakage and its lodging in the epigynum. Copulation durations of
female- and male-initiated palp breakages were recorded.
3. RESULTS
All copulations (n ¼ 25) led to palp damage either
through entire palp severance (88%, n ¼ 22) or partial
severance leading only to palp damage (12%, n ¼ 3).
There was no significant difference in the occurrence
of female versus male-initiated palp breakage frequencies (female-initiated: 63.6%; x21 ¼ 1:636, p ¼ 0.201,
n ¼ 22). However, female-initiated severance resulted
in significantly shorter copulation durations than did
male-initiated palp breakages (t20 ¼ 22.746, n ¼ 22,
p ¼ 0.012; figure 2a).
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rsbl.2011.1202 or via http://rsbl.royalsocietypublishing.org.
Received 8 December 2011
Accepted 11 January 2012
512
This journal is q 2012 The Royal Society
Downloaded from rsbl.royalsocietypublishing.org on August 20, 2012
Remote copulation in eunuch spiders
D. Li et al.
513
Table 1. Results from a generalized linear model testing the
effects of five explorative factors (the time interval since palp
breakage until its experimental removal from the epigynum,
the sex initiated palp breakage, copulation duration, male size
(carapace width: CW), and the number of sperm remaining in
the palp) on the number of sperm transferred to the female’s
spermatheca. The data for the dependent variable and all the
continuous predictors were log-transformed. The maximal
model that including all five factors and all possible
interactions was fitted using normal with identity link and
gamma with log link error. Akaike information criterion (AIC)
was used to select the best model. Goodness of fit: AIC ¼
4.154; Omnibus test: x 2 ¼ 117.4, d.f. ¼ 8, p , 0.0001.
Figure 1. Nephilengys malabarensis female with a severed male
palp (red box) lodged in her epigynum after copulation, and
a half-cannibalized male at her side.
copulation duration (s)
(a)
16
12
8
4
female-initiated
percentage (%) of sperm
transferred to female
log (no. sperm
transferred)
Wald x 2
d.f.
p
time interval from palp breakage
sex initiated palp breakage
copulation duration
number of sperm remaining in
the emasculated palp
CW
sex initiated palp breakage time from palp breakage
sex initiated palp breakage copulation duration
11.315
11.406
0.188
1.170
1
2
1
1
0.001
0.003
0.665
0.279
1.805
4.584
1
1
0.179
0.032
2.669
1
0.102
0
male-initiated
(b)
(c)
exploratory factors
40
30
20
10
0
female-initiated
male-initiated
4.0
3.5
3.0
log (y) = 2.65 + 0.27 log (x)
R2 = 0.46, n = 22, p = 0.001
2.5
2.0
0
1.0
2.0
3.0
4.0
log (time (s) before experimental palp removal)
Figure 2. Copulation duration and sperm transfer pattern in
Nephilengys malabarensis. (a) Mean (+ s.e.m.) copulation durations resulted from female- (n ¼ 14) and male- (n ¼ 8)
initiated entire palp breakages. (b) Mean (+ s.e.m.) percentage
(%) of sperm transferred to females during copulation through
female- and male-initiated palp breakages. (c) A linear relationship between the log number of sperm transferred to female
epigynum and the log time interval (s) elapsed between palp
breakage and experimental palp removal (n ¼ 22).
There was a mean number of 3309 (+271) sperm
(range: 1300–7400; n ¼ 22) present originally in each
copulating palp. Female-initiated palp breakages resulted
in significantly fewer sperm transferred into spermatheca
during copulation (t20 ¼ 23.031, n ¼ 22, p ¼ 0.007;
figure 2b). About 67–72% of sperm were still left in the
broken palp lodged in the epigynum when the male and
Biol. Lett. (2012)
female separated. However, GLM results showed that
sperm transfer continued after copulation termination:
the longer the broken palp was left lodged in the female
before its experimental removal, the more sperm were
transferred (table 1 and figure 2c). Female-initiated
palp breakage significantly increased the number of
sperm transferred to female since the palp breakage
(table 1), suggesting that female-initiated palp breakage
induced faster sperm transfer in a given time. Moreover,
a significant two-way interaction between the sex-initiating palp breakage and the time before experimental palp
removal showed that, after female-initiated breakage,
a significantly shorter time was needed to transfer
equal sperm quantity from the emasculated palp to the
female (before experimental palp removal) compared
with male-initiated palp breakage (table 1). Other
explorative variables (copulation duration, number of
sperm remaining in the palp and male size) did not
have significant effects on the number of sperm
transferred (table 1).
4. DISCUSSION
Our results clearly show that remote copulation enables
continual sperm transfer from severed palp to female genitalia even after male detachment from copula. It not only
directly prolongs the duration of potential sperm transfer,
but also in fact increases the amount of transferred sperm.
This may reduce sperm competition by increasing the
eunuch’s fertilization success and paternity.
Remote copulation probably evolved as a male counter-adaptation to female aggression towards mates,
female-enforced short copulation durations and, ultimately, the high prevalence of female sexual cannibalism
(reaching 75%, [12]). Partial palp breakage, common in
spiders (here only in 12% of copulations), functions as
mate plugging [6]. However, internal plugs do not necessarily prevent female remating [10–12,18], and cannot
Downloaded from rsbl.royalsocietypublishing.org on August 20, 2012
514 D. Li et al.
Remote copulation in eunuch spiders
lead to continuous sperm transfer. Remote copulation is
hence a function exclusive to complete palp emasculation, a taxonomically scattered phenomenon known in
nephilids [8] and the theridiids [13,19]. A totally severed
palp facilitates both plugging and continuous sperm
transfer after the eunuch’s detachment from copula.
Females may control copulation duration by sexual
cannibalism: amounts of transferred sperm are thus
seen as the ultimate manifestation of sexual conflict
[20– 21]. The frequency of palp breakage initiations
did not differ between males and females; however,
female-initiated cases resulted in shorter copulation
durations and lower numbers of transferred sperm.
This implies that copulation duration in N. malabarensis, while already short (average 8.3 + 1.1 s) compared
with other spiders [22 – 25], is subject to female choice
[20]. With only about 28– 33% sperm transferred
before palp severance, the male’s reproductive capacity
is greatly reduced. However, sexual conflict commonly
leads to sexually antagonistic coevolution, where
male persistence strategies evolve to counter female
resistance to monopolization [10,12]. Mate plugging
to monopolize females is a common male response to
sexual conflict [26], but our study suggests that remote
copulation is an additional male adaptation to sexual
cannibalism and to female control of copulation duration. Remote copulation enables the male to
maximize his reproductive potential through (i) continuous sperm transfer after detachment from copula and
(ii) prolonged female monopolization, during which
the female is unavailable to rival males.
Another important finding is that the males
transferred sperm even faster via broken palps after
female-terminated copulations (table 1). This suggests
that eunuch males may not, ultimately, suffer a paternity cost. However, it is unclear why the rates of sperm
transfer differ between male- and female-terminated
copulations. Presumably, there is an advantage of rapid
‘remote’ transfer after female-initiated breakages, but
less so after male-initiated ones. However, there might
have been a difference in the rate of sperm transfer if
the duration of remote transfer had not been manipulated. Further studies are needed to test these hypotheses.
In sum, the adaptive significance of eunuchs seems
to be justified judging from their evolutionary persistence
[10]. Perhaps, there are other functions of palp severance
in addition to plugging [9], remote copulation (this
paper) and enhanced eunuch aggressiveness [12,26],
which may further justify the high costs of male sterility.
The study was supported by the Ministry of Education
Academic Research Fund (AcRF) grant to D.L. (R-154000-435-112), in part by the Slovenian Research Agency
(ARRS) grants to M.K. (J1-2063 and 1000-10-720023)
and Humboldt’s grant to S.K.F. We thank Poh Moi Goh
and Chun Hong Cheung for providing fruitflies and house
flies. Spiders were collected under research permit (NP/
RP10-036) granted by Singapore NParks.
1 Arnqvist, G. & Rowe, L. 2005 Sexual conflict. Princeton,
NJ: Princeton University Press.
2 Elgar, M. A. & Schneider, J. M. 2004 The evolutionary
significance of sexual cannibalism. Adv. Study. Behav.
34, 135 –163. (doi:10.1016/S0065-3454(04)34004-0)
Biol. Lett. (2012)
3 Miller, J. A. 2007 Repeated evolution of male sacrifice
behavior in spiders correlated with genital mutilation.
Evolution 61, 1301– 1315. (doi:10.1111/j.1558-5646.
2007.00115.x)
4 Schneider, J. M. & Fromhage, L. 2010 Mongynous
mating strategies in spiders. In Animal behaviour: evolution
and mechanisms (ed. P. M. Kappeler), pp. 441 –464.
Berlin, Germany: Springer.
5 Slatyer, R. A., Mautz, B. S., Blackwell, P. R. Y. & Jennions,
M. D. 2011 Estimating genetic benefits of polyandry from
experimental studies: a meta-analysis. Biol. Rev. 87, 1–33.
(doi:10.1111/j.1469-185X.2011.00182.x)
6 Uhl, G., Nessler, S. & Schneider, J. M. 2010 Securing
paternity in spiders? A review on occurrence and effects
of mating plugs and male genital mutilation. Genetica
138, 75–104. (doi:10.1007/s10709-009-9388-5)
7 Simmons, L. W. 2001 Sperm competition and its evolutionary consequences in the insects. Princeton, NJ: Princeton
University Press.
8 Kuntner, M., Coddington, J. A. & Hormiga, G.
2008 Phylogeny of extant nephilid orb-weaving spiders
(Araneae, Nephilidae): testing morphological and ethological homologies. Cladistics 24, 147–217. (doi:10.1111/j.
1096-0031.2007.00176.x)
9 Kuntner, M., Agnarsson, I. & Gregorič, M. 2009
Nephilid spider eunuch phenomenon induced by
female or rival male aggressiveness. J. Arachnol. 37,
266– 271. (doi:10.1636/St08-67.1)
10 Kuntner, M., Coddington, J. A. & Schneider, J. M.
2009 Intersexual arms race? Genital coevolution in
nephilid spiders (Araneae, Nephilidae). Evolution 63,
1451–1463. (doi:10.1111/j.1558-5646.2009.00634.x)
11 Kuntner, M., Kralj-Fišer, S., Schneider, J. M. & Li, D.
2009 Mate plugging via genital mutilation in nephilid spiders: an evolutionary hypothesis. J. Zool. 277, 257 –266.
(doi:10.1111/j.1469-7998.2008.00533.x)
12 Kralj-Fišer, S., Gregorič, M., Zhang, S., Li, D. & Kuntner,
M. 2011 Eunuchs are better fighters. Anim. Behav. 81,
933–939. (doi:10.1016/j.anbehav.2011.02.010)
13 Knoflach, B. & van Harten, A. 2001 Tidarren argo
sp. nov. (Araneae: Theridiidae) and its exceptional copulatory behaviour: emasculation, male palp organ as a
mating plug and sexual cannibalism. J. Zool. 254,
449 –459. (doi:10.1017/S0952836901000954)
14 Bukowski, T. C. & Christenson, T. E. 1997 Determinants
of sperm release and storage in a spiny orbweaving spider.
Anim. Behav. 53, 381 –395. (doi:10.1006/anbe.1996.
0329)
15 Bukowski, T. C., Linn, C. D. & Christenson, T. E. 2001
Copulation and sperm release in Gasteracantha canfiformis (Araneae: Araneidae): diffential male behavior based
on female mating history. Anim. Behav. 62, 887 –895.
(doi:10.1006/anbe.2001.1834)
16 Snow, L. S. E. & Andrade, M. C. B. 2004 Pattern of
sperm transfer in redback spiders: implications for
sperm competition and male sacrifice. Behav. Ecol. 15,
785 –792. (doi:10.1093/beheco/arh080)
17 Herberstein, M. E. et al. 2011 Sperm storage and copulation duration in a sexually cannibalistic spider. J. Ethol.
29, 9 –15. (doi:10.1007/s10164-010-0213-5)
18 Schneider, J. M., Thomas, M. L. & Elgar, M. A. 2001
Ectomised conductors in the golden orb-web spider,
Nephila plumipes (Araneoidea): a male adaptation to
sexual conflict? Behav. Ecol. Sociobiol. 49, 410 –415.
(doi:10.1007/s002650000307)
19 Knoflach, B. 2002 Copulation and emasculation in
Echinotheridion gibberosum (Kulczynski, 1899) (Araneae,
Theridiidae). In European arachnology 2000 (eds S. Toft &
N. Scharff), pp. 139–144. Aarhus, Denmark: Aarhus
University Press.
Downloaded from rsbl.royalsocietypublishing.org on August 20, 2012
Remote copulation in eunuch spiders
20 Elgar, M. A., Schneider, J. M. & Herberstein, M. E.
2000 Female control of paternity in the sexually cannibalistic spider Argiope keyserlingi. Proc. R. Soc. Lond. B 267,
2439 –2443. (doi:10.1098/rspb.2000.1303)
21 Schneider, J. M., Gilberg, S., Fromhage, L. & Uhl, G.
2006 Sexual conflict over copulation duration in a cannibalistic spider. Anim. Behav. 71, 781– 788. (doi:10.1016/
j.anbehav.2005.05.012)
22 Andrade, M. C. B. 1996 Sexual selection for male sacrifice in the Australian redback spider. Science 271, 70–72.
(doi:10.1126/science.271.5245.70)
23 Schneider, J. M., Herberstein, M. E., De-Crespigny,
F. C., Ramamurthy, S. & Elgar, M. A. 2000 Sperm
Biol. Lett. (2012)
D. Li et al.
515
competition and small size advantage for males of the
golden orb-web spider Nephila edulis. J. Evol. Biol. 13,
939 –946. (doi:10.1046/j.1420-9101.2000.00238.x)
24 Elgar, M. A. 2005 The duration of copulation in spiders:
comparative patterns. Rec. West. Aust. Mus. Suppl. 52, 1–11.
25 Schneider, J. M. & Elgar, M. A. 2001 Sexual cannibalism
and sperm competition in the golden orb-web spider
Nephila plumipes (Araneoidea): female and male perspectives. Behav. Ecol. 5, 547 –552. (doi:10.1093/beheco/12.
5.547)
26 Kralj-Fišer, S. & Kuntner, M. 2011 Eunuchs as better
fighters? Naturwissenschaften. (doi:10.1007/s00114-0110873-1)