Behavioral Ecology Vol. 12 No. 5: 612-618
© 2001 International Society for Behavioral Ecology
Precopulatory mate assessment in relation to body size in the earthworm Lumbricus terrestris: avoidance of dangerous liaisons?
Max-Planck-Institut for Behavioral Physiology, Seewiesen, Postfach 1564, D-82305 Starnberg, Germany
Address correspondence to N.K. Michiels, who is currently at the Department for Evolutionary Biology, Westfaelische Wilhelms-Universitaet, Huefferstrasse 1, D-48149 Muenster, Germany. E-mail: michiels{at}uni-muenster.de .
Received 29 March 2000; revised 3 November 2000; accepted 6 December 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In the earthworm Lumbricus terrestris L., mating occurs on the soil surface, but partners remain anchored in their burrow and mating is preceded by repeated mutual burrow visits between neighbors. This study focuses on body size as one possible trait that earthworms may assess during these burrow visits. Size-related mate choice is predicted to result in size-assortative mating, which we found in one field sample (n = 90 pairs), but not in a second (n = 102). We discovered that when mates separate, one of them can be pulled out of its burrow. This was more likely for small individuals or those mating across wide distances. In a subsequent greenhouse experiment, we allowed focal individuals to mate with two neighbors of different sizes. Relative size affected neither mating rate nor primary preference, but focals mated sooner with the same-sized neighbor than with a differently sized one. Small focals visited large neighbors more often than small ones. We conclude that size influences mate choice as well as the outcome of mating and discuss how the "tug-of-war" that ends a mating contributes to this result. Precopulatory visits may involve assessment as well as enticement to lure the partner closer to the individual's own burrow, in order to minimize the risk when mating with a partner that is large or far away.
Key words: assortative mating, cost of sex, sexual conflict, simultaneous hermaphroditism, size assessment.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In species with separate sexes, mating success typically contributes more to male lifetime reproductive success (LRS) than to female LRS (Arnold, 1994
;
Bateman, 1948; but see
Dewsbury 1982), and the same
logic applies to simultaneous hermaphrodites
(Arnold, 1994;
Charnov, 1979). Hence, the
basic expectation is that the male role of a hermaphrodite should be, in
principle, more eager to mate than the female role. This, however, holds only
for as long as the male role remains cheap. At elevated densities, promiscuity
favors increased sperm investment (Greef
and Michiels, 1999). More-over, allosperm digestion is widespread
(Michiels, 1998,
1999) and will favor even
larger ejaculates. As a result, allocation to sperm may become as expensive as
allocation to eggs (Greeff and Michiels,
1999; Pen and Weissing,
1999). Consequently, individuals able to donate sperm may reject
receptive mates when the expected gain in paternity does not outweigh the
costs. Leonard (1999) comes to
a similar conclusion by arguing that the male role is less preferred because
it has a higher variance in success rate.
Precopulatory assessment could take several forms. First, individuals may
prefer mates that reciprocate, as allosperm digestion will compensate for
their own investment (Greeff and Michiels,
1999). The resultant "sperm trading" is known from sea
slugs (Leonard and Lukowiak,
1984,
1991), free-living flat-worms
(Michiels and Bakovski, 2000;
Vreys and Michiels, 1998) and
possibly cestodes
(Schärer and
Wedekind, 1999) and may be widespread
(Leonard, 1990). Second, sperm
donors may prefer fecund (typically larger) mates. Size-related mate choice is
known from some hermaphrodites (Tomiyama,
1996; Vreys and Michiels,
1997; Yusa, 1996;
but see Baur, 1992;
Peters and Michiels, 1996).
Dewitt (1996) showed that in a
hermaphroditic snail in which only unilateral insemination is possible, small
individuals prefer the male role when encountering a large mate (but see
Wethington and Dillon, 1996).
In Aplysia, individuals donate more sperm to large partners
(Yusa, 1994). In the
hermaphroditic polychaete genus Ophryotrocha
(Premoli and Sella, 1995;
Sella and Lorenzini, 2000), large individuals play the female role and their
partners play the male role, but these roles alternate when sizes reverse due
to heavy investment in eggs.
Oligochaetes represent a large taxon of exclusively simultaneously
hermaphroditic animals with reciprocal insemination
(Edwards and Bohlen, 1996).
The earthworm Lumbricus terrestris L. lives solitarily in vertical
burrows 1-3 m deep (Sims and Gerard,
1985). It forages and mates on the surface at night. During
mating, partners remain anchored in their home burrow with their tail end,
allowing for instant retreat. Mating costs are presumed to be high. First,
mates are so tightly interlocked that withdrawal after disturbance is slow,
compared to single individuals (Michiels NK et al., personal observations).
Second, copulations start late, last long, and often end well after sunrise
(this study). Hence, pairs are more exposed to desiccation and predators than
are single individuals. Third, copulations involve physiological costs in the
form of sperm and mucus production, as well as large-scale damage caused by
the partner's copulatory bristles or setae
(Grove, 1925). Finally, the
sperm-receiving organs (spermathecae) resorb sperm
(Grove, 1925), which is known
to raise the optimal amount of sperm an individual needs to donate
(Greeff and Michiels,
1999).
Nuutinen and Butt (1997)
found that neighboring L. terrestris regularly stick their heads into
each other's burrows. When this happens, the visited individual may follow the
slowly retracting visitor to the visitor's burrow entrance and vice versa. An
irregular series of reciprocal visits like these was found to precede most
copulations and was interpreted as a kind of courtship
(Nuutinen and Butt, 1997).
In this study, we addressed one possible function of visiting behavior in
L. terrestris estimation of partner size. Because burrow diameter is
related to the occupant's body weight
(Shipitalo and Butt, 1999),
individuals may visit neighboring burrows to assess the occupant's size. Size
may indicate health or vigor or female fecundity. Although large earthworms do
not appear to produce more cocoons (Butt
and Nuutinen, 1998), they tend to produce heavier cocoons
(Nuutinen V, personal communication) and larger offspring (Solmsdorff K and
Michiels NK, unpublished data). An allometric correlation between individual
mass and clitellum size (cocoon-producing skin structure) probably explains
this relationship. Fecundity increases with size in other hermaphrodites as
well (Baur, 1992;
DeWitt, 1996;
Madec et al., 2000;
Schärer et al., 2001;
Trouvé et
al., 1999; Vreys and Michiels,
1997; Wedekind et al.,
1998; Weinzierl et al.,
1999). Although size may be a sign of quality, it may also be a
risk indicator, as explained below.
In the first part of the study, we investigated size-dependent mating in the field. We expected matings to be assortative by size, as favored individuals would reject suboptimal partners, leaving the latter no other choice than to accept another suboptimal partner. During trial observations preceding this study, we observed that when partners pull apart at the end of a mating, one individual is occasionally pulled out of its burrow. To determine the significance of this tug-of-war, we also estimated the likelihood of being on the surface in relation to body mass and distance between partners in the field.
In the second part of the study, we recorded sexual interactions and matings in experimental groups consisting of one focal individual and two neighbors: one of the same size as the focal individual, and one of a different size (larger or smaller). We expected to obtain indications whether size-related mate choice, if any, is inspired by risk reduction (preference for small partner when small) or a preference for fecundity (constant preference for the larger neighbor). In either case, one would predict assortative mating.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Biology of Lumbricus terrestris
L. terrestris is probably the best studied earthworm (Edwards and Bohlen, 1996
).
Natural densities are around 30-200 individuals/m2, and burrows can
go down to 2 m or more (Sims and Gerard,
1985). During the active season (spring to autumn) animals are
closer to the surface. Foraging on the soil surface is nocturnal
("nightcrawler") and strongly depends on the presence of dew
("dewworm"). The mating process has been described meticulously by
Grove (1925) and involves a
unique mechanism of reciprocal sperm exchange: individuals oppose their
genital regions (clitellum and segments 5-15) while attaining a typical
S-shaped posture. Slime and specialized setae are used to tighten the bond.
Matings typically take place after midnight and may last until well after
sunrise (max 1.5 h later; Michiels NK et al., personal observations). At the
end of a mating, individuals start to pull hard and separate with a tearing
sound (Michiels NK et al., personal observations).
Choice of a field site
Individuals were collected from a public golf course near Feldafing,
south-southwest of Munich. The density was visually estimated to be around 50
adult worms/m2. Standard sampling methods such as chemical or
electrical extraction or digging could not be used, but the short grass
allowed us to see pairs on the surface from a 5 m distance before disturbing
them. The homogeneity of the surface also reduced spatial structure within the
population, which is essential (see below). All individuals used in this study
were collected from a single lawn of approximately 30 x 100 m.
Collection in the field
After a series of trial visits, we collected two samples of mating pairs
between 0300 and 0715 h on 1 May and 9 June 1998. Worm pairs were located
using head-mounted torches covered with dark red foil while slowly walking in
a systematic pattern across the lawn. Two people were needed to grab and hold
on to one individual each. Worms were put in numbered vials. We then measured
the distance between burrows (± 5 mm) and the distance to two fixed
points at the edge of the lawn using a laser range finder (± 1 m; see
below). Incomplete pairs of which one partner escaped or was injured were not
collected, but position and burrow distance were recorded. We also noted
whether one mate was not properly anchored (tail tip at burrow entrance or
surfaced) when collected. Out of 309 attempts to collect a pair, 192 (62%)
were successful. Burrow distance could be obtained for 302 pairs. On 9 June,
we also collected all single individuals (n = 8) that were found
lying freely on the surface in the final hours of the night (0300-0500 h),
when single individuals are normally already below the surface. Swollen
genital regions and mucus remnants showed that they had mated recently, and we
assumed that they had been pulled out of their burrow by their partner. Such
individuals were also witnessed on 1 May, but not collected. In the laboratory
all animals were rinsed in tap water, dried on paper tissue, and weighed alive
(± 1 mg). Measurements were finished within 6 h after collection. As a
measure of size, we used weight rather than length. Length is impossible to
measure reliably in living animals, whereas fixed animals are unnaturally
contracted, which results in an underestimate of an animal's ability to
stretch over long distances.
Spatial heterogeneity
Heterogeneity of the lawn could lead to some patches housing smaller
individuals than others. This confounding effect would result in
size-assortative mating in the absence of mate choice. We checked for spatial
structure by determining the position of each pair on the lawn using the
distance measurements described above and simple triangulation. Using a
K-means cluster analysis (SPSS), we grouped pairs spatially and temporally
(sampling time). The analysis was repeated twice to define 6 and 10 clusters
for each sampling date (4 analyses). We then statistically compared individual
size between clusters. None of the comparisons revealed significant
differences (p values between.19 and.87). We therefore conclude that
local size variance did not differ from overall variance within the study
site, as expected for a homogeneous environment.
Effects of size in the greenhouse
After a series of trial runs, an experiment was designed such that one
focal individual could choose between two partners: one of the same size, and
one of a different size; the latter could either be smaller or larger, in a
size ratio of 1:1.5 (Figure 1).
Half of the trios had a large focal individual that was combined with one
equally large and one smaller neighbor (SLL trios), whereas the other half
consisted of small focal individuals paired with an equally small and a large
neighbor (SSL trios). Animals were taken from a sample of 73 pairs collected
on 26 April. We chose focal individuals randomly from this set. We then looked
for the closest matching "identical" and "different"
individual to make a trio. If this was not possible, another focal animal was
taken. This procedure was repeated until twelve trios of each kind (SSL and
SLL) were assembled. At the start of the experiment, equal individuals within
a trio differed by only 0.299 ± 0.298%, whereas unequal individuals
differed by 41.7 ± 0.98%. The average weight of small and large
individuals was 2.32 ± 0.23 g (range 1.84-2.80 g) and 3.56 ±
0.43 g (range 2.89-4.50 g), respectively.
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Trios were housed in a bucket mounted on three 40-cm PVC pipes (Figure 1). Pipes were filled with earth in which an artificial burrow of 5 mm diameter was made by means of a metal rod. The bottom of the bucket and the top of the pipes were covered with a 2-cm layer of soil mixed with decomposing leaves and some chopped vegetables (carrots, onions, and salad greens) that had been frozen to soften them. Buckets were sprayed with water vapor in the evening and morning. The greenhouse was shaded, and doors and windows were kept open to prevent overheating and to allow for cooling at night.
Sets of six buckets (3 SSL and 3 SLL trios) were put together in a single large container in an alternating pattern and recorded in infrared using a time-lapse video recorder. Four complete set-ups ran simultaneously (4 x 6 = 12 + 12 trios of each type). Recording commenced shortly before dusk and ended between 0800 and 0900 h. The experiment started on 29 April and ran until 13 May (14 nights). Some individuals did not accept the burrow in which they were put, moved out, and stayed in the soil layer in the bucket or made a new burrow in a neighboring pipe. Such trios were discarded for analysis, leaving 11 SLL and 10 SSL trios.
Data collection and analysis
The experiment was designed in such a way that the focal individual could
choose without interference. This, however, also allowed the two neighbors to
interact with each other. We cannot exclude that this may have affected the
differences between the SSL and SLL treatments. Such effects were considered
during the analysis. We therefore pay more attention to within-treatment
effects than between-treatment effects.
When analyzing precopulatory visits and mate choice, we only used the first
mating by the focal individual, which was the first mating in the bucket in
80% of the cases. We noted number, duration, and direction of visits between
the focal individual and its neighbors until its first copulation. For a
general analysis of mating activity, copulation start and duration, as well as
the individuals involved, were recorded for all matings throughout the
observation period. Statistical analyses were done using SPSS version 8.0 for
Windows. Parametric tests were used when the underlying assumptions were
fulfilled, or when an adjustment procedure was available. In SPSS, a corrected
t (with adjusted degrees of freedom) is calculated when variances
differ in a t test. Box-plots (Figures
4 and
5) represent median ± 1
quartile (box) and range (lines) excluding outliers. Averages are shown
± SDs. As discussed more extensively by Vreys and Michiels
(1997), traits of
hermaphroditic mates cannot be correlated with a regular correlation analysis,
but need to be compared using a one-way ANOVA with pair number as factor.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Size-assortative mating in the field?
We collected 90 and 102 intact pairs on each sampling occasion. The variance in body weight within pairs was lower than between pairs on 1 May (ANOVA F89,90 = 1.45, p =.040), but not on 9 June (ANOVA F101,102 = 0.99, p =.52), indicating assortative mating in the first, but not in the second sample. The maximum relative weight difference between individuals was 1:2.3. Differently sized partners were seen stretching or contracting their body between the genital region (segments 8-15) and the clitellum (segments 31-37). This suggests that physical incompatibility of small and large individuals cannot explain assortative mating. Average mass of mating individuals increased from 2.84 ± 0.62 g on 1 May to 3.23 ± 0.73 g on 6 June (t390 = -5.6, p <.001).
Distance between partners in the field
Despite the fact that animals were larger in the second sample, average
burrow distance between mating partners was longer in the first sample than in
the second (12.2 ± 4.8 cm and 10.1 ± 3.8 cm, respectively;
t278 = 4.09, p <.001, including escaped
pairs). The maximum distance across which individuals mated was 26.0 cm. In
collected pairs the average body weight of the two mates was not related to
burrow distance on 1 May (rp =.068, n = 90,
p =.53). Body weight was related to burrow distance, however, on 6
June (rp =.209, n = 102, p =.035); large
individuals mated across larger distances than small individuals in the second
sample.
Completely surfaced individuals in the field
We found 14 out of 90 (1 May) and 5 out of 102 (9 June) pairs in which one
partner was on or very close to the surface. This proportion was higher on 1
May (
21 = 6.09, Exact p =.016). The likelihood of
being on the surface was higher when mating with a partner far away
(Figure 2). Smaller individuals
were also more likely to be on the surface than large individuals
(Figure 3). This was
particularly true for individuals found singly on the surface, who were
significantly smaller than the 204 paired individuals collected during the
same night (2.13 ± 1.02 g versus 3.27 ± 0.68 g;
t7.3 = 3.13, p =.016).
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Greenhouse experiment: general results
Although all individuals were caught while mating 3 days earlier, the first
mating was observed during the second night of the experiment. A distribution
of copulation duration of all 53 observed matings showed a bimodal pattern:
short matings (minimum, median, and maximum: 2, 11, and 63 min) were
considered mating attempts (7 in SSL and 7 in SLL trios). Only those longer
than 2 h were considered true matings: they lasted 3.63 ± 0.57 h (range
2.6-5.5 h; n = 39; 19 in 10 SSL and 20 in 11 SLL trios). Matings
started between 2053 and 0510 h (mean = 0022 ± 0206 h). The latest time
at which a mating ended was 0845 h (sunrise around 0700 h). The total number
of matings per individual varied from 0 to 3 (9, 33, 18 and 3 individuals in
each category). The average rate was 1.24 ± 0.76 matings in 2 weeks or,
alternatively, once every 11 days. Considering that all animals were mating
when collected 3 days before the experiment, mating rate may be as high as
once per 7.6 days. Small individuals mated as often as large individuals
during the experiment (Mann-Whitney U test: Z = 1.381,
p =.167).
Precopulatory visits in relation to relative body size
To investigate the effect of relative body size (measured as weight), we
only considered behavioral interactions until the first mating of the focal
individual. Trios in which the focal worm did not mate or in which one of the
two neighbors was not involved in any visiting behavior until the first focal
copulation were ignored. We also removed one statistical outlier in which the
focal individual in an SLL trio made 107 visits in the course of 10 nights
before its first copulation. The second highest number was 34. This reduced
sample size to seven SSL and eight SLL trios. The first mating by focals did
not suggest a preference for the larger partner: Four out of seven SSL focals
mated with their larger neighbor, whereas five out of eight SLL focals mated
with the equal-sized (also large) individual. The overall ratio of 9:6 does
not diverge from 1:1 (21 = 0.60, Exact p
=.61). This suggests the absence of a general preference for large mates, but
due to small sample sizes this result should be considered with caution.
Copulations occurred earlier in same-sized pairs, suggesting assortative
mating. Small focals hesitated longer than large focals
(Figure 4).
There was no difference between small or large focal individuals in the total number of visits to their neighbors until their first mating (SSL: 9.86 ± 6.16; SLL 14.5 ± 11.4; t13 = 0.958, p =.35). However, small focals made more visits to their large neighbor than to the equal-sized alternative (Figure 5; Wilcoxon signed-ranks test Z = 2.03, p =.042). No such difference existed in SLL trios (Wilcoxon Z = 0.35, p =.73).
A visit series that ended with a copulation consisted of 10.0 ± 9.2 visits, which is more than for series not leading to copulation (1.91 ± 0.63; t14.1 = 3.40, p =.004). Treatment had no effect, and data were therefore pooled for this analysis. Visit bouts lasted for 26.1 ± 22.4 min (range 0-72.1 min).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We found assortative mating in one field sample early in the season, not in a second taken one month later. Small size and mating with a partner across a long distance increased the likelihood that a mating individual was not anchored in its burrow, indicating that small individuals have a higher likelihood of being pulled onto the surface after the tug-of-war that ends a mating. The small size of eight postmating individuals found singly on the surface confirmed this. In the green-house, there were no overall indications for size-assortative matings, possibly in part caused by reduced sample size. Yet, pairs of same-sized individuals formed earlier than pairs of differently sized individuals, and this can lead to size-assortative mating. On the other hand, small individuals visited their large neighbor more often than their small neighbor, suggesting a high interest, but maybe also hesitation as suggested by the longer delay until the first mating. Below, we first discuss whether it is possible to find mate choice in the field at all, and then evaluate the relevance of the tug-of-war for mate choice.
Assortative mating in the field may require a synchronizing
mechanism
How likely is it to find size-assortative mating in the field? Because
L. terrestris lives in a permanent burrow, mating is only possible
within the neighborhood, limiting the opportunity for choice.
Figure 6 suggests that for an
estimated density of 50 adults/m2, there may be only little
opportunity for choice, unless some external mechanism ensures synchronization
of mating activity. Weather strongly affected nocturnal activity, with calm,
clear, and cool nights being best (Michiels NK et al., personal observations).
Prolonged periods of unfavorable weather (e.g., rainy nights) followed by
weather improvement may therefore serve as a synchronizer. The lunar cycle
also affects earthworm activity: preliminary data suggest very low earthworm
activity around the full moon (Solmsdorf K, Michiels N, and Vorndran I,
personal observations).
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The tug-of-war: a unique and new mating cost
L. terrestris individuals are sometimes pulled out of their burrow
by their mate. Because pairs were still mating when collected in the field, it
is not possible to judge whether surfaced individuals would have found their
way back to their burrow. But eight single individuals found on the surface on
9 June suggest that many may not. This adds an important trade-off to the
benefits of choosiness: by increasing the radius within which potential
partners are assessed, the risk is increased as well, particularly for a small
individual. The tug-of-war predicts that individuals should prefer small
partners, whereas size-related fecundity predicts that they should prefer a
large partner. The optimal solution of this trade-off will be determined by
burrow distance: at short distances fecundity-related choice may be
predominant, whereas over long distances minimizing the risk may be more
important. In both cases, assortative mating is expected.
The function of multiple visits: assessment or attraction
Why do earthworms need repeated visits? If they only want to assess size, a
single visit would be sufficient. Anecdotal observations and pictures of
mating pairs in the field have documented that pairs often mate closer to the
burrow of one of the two partners. This offers a tentative explanation for
multiple visits: they may represent an attempt to mate closer to home, thus
reducing the risk of having to stretch out over a long distance. The
back-and-forth movements during a visit series would then represent a conflict
about the exact locality where they should attain the S-position. Both mate
enticement and mate assessment may therefore be at work. Visit frequency may
be a measure of the reluctance to mate, as eager mates should give in early
and mate close to the neighbor's home.
This view offers a tentative explanation for the difference between the two field samples. In the first, early in the season, the density of sexually active individuals was probably low, as suggested by the fact that individuals reached out farther for their mating partners in the first than in the second sample. Because risk increases with distance and individuals were smaller, choosiness should have been higher in the first sample, as was the case. There were also more nonanchored individuals in the first sample. The absence of choosiness in the second sample would then suggest that at a higher density of sexually active individuals, earthworms prefer the safety of mating close to home over the risks associated with increased choice opportunities. In the greenhouse, the large number of visits by small individuals to large partners is suggestive of the fact that small individuals have a bigger conflict with a larger neighbor over the location of the copulation. It also offers an explanation for why similarly sized pairs mated sooner than differently sized pairs.
Sexual conflict in hermaphrodites
This study adds another example to the multitude of hermaphrodite mating
conflicts, in which similarity of interests between identical partners results
in conflicts that may be difficult to solve
(Baur, 1998; Leonard
1990,
1991,
1999;
Michiels 1998;
Michiels and Newman 1998).
Crowley et al. (1998) describes
this problem as the "complementarity dilemma." Although their
model only allows an individual to play one out of two possible roles (which
limits its scope to unilaterally inseminating hermaphrodites), it may be
applied to L. terrestris. The "my place or yours" dilemma
may be regarded as a binary choice in which "my place" is the
preferred role, but "your place" is acceptable when the risk is
low and the benefits are high. Repeated visits may elucidate subtle
differences in interests allowing mates to find a compromise, in the same way
as unilaterally inseminating snails decide on who is allowed to be male
(DeWitt, 1996).
Mating behavior of Lumbricus terrestris
Our study provides the first quantitative data on mating behavior in
Lumbricus terrestris. Mating frequency is relatively high (once every
7-11 days). This exceeds what is needed for full fertility, as a single mating
can ensure full fertility for up to 6 months
(Butt and Nuutinen, 1998). The
fact that mating rate is still relatively low suggests that matings are
costly, an argument that is also used to explain long mating intervals in a
pulmonate snail (Locher and Baur,
1999). Matings can be subdivided into short attempts and long true
matings, very much in the same way as found in Schmidtea (Dugesia)
polychroa (Peters et al.,
1996). Short matings may represent failed attempts to align the
genitalia or, alternatively, indicate that mate assessment continues during
the first part of the mating, as is also known from S. polychroa
(Michiels and Streng,
1998).
Conclusions
We conclude that size affects mating behavior in Lumbricus
terrestris in an intricate way. Relative partner size, distance to
putative partners, the risk of being pulled onto the surface, and size-related
fecundity all appear to play key roles.
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ACKNOWLEDGEMENTS |
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Thanks to Bruno Baur, Anders Berglund, Visa Nuutinen, Martin Storhas, Jaco Greeff, and Hinrich Schulenburg for helpful comments on earlier drafts, to Jaco Greeff, Harald Huber, and Susi Gistl for assistance in the field, and to Visa Nuutinen for advice and stimulating conversations. Special thanks to Mr. Lange from the golf course in Feldafing for allowing us to collect worms during the night (rather than worshipping the moon as suspected by some of his gardeners).
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