Discussion
Here we examine the impact of multiple cues on the mating behaviour of
spider mites. We found that the number of mating attempts was only
influenced by cues left on the patch prior to the mating sessions, being
higher in patches with cues of virgins. In turn, female acceptance and
the number of mating events of the couple were affected by both
substrate cues and the mating status of the females, being the highest
in patches with virgins and with substrate cues of virgin females and
the lowest on patches with mated females and with substrate cues of
mated females. Once copulation started, its duration seemed to depend
mainly on the mating status of the female being fertilized, with the
overall amount of time invested in mating being higher in matings with
virgins than with mated females. Ultimately, male survival costs
mirrored the reproductive investment of males, with patches in which
there was a mismatch between cues showing an intermediate number of
mating events, cumulative copulation duration and survival costs.
In species with first male sperm precedence, like spider mites and many
spiders, the sperm from the first insemination sires most of the
offspring, thus virgins are more valuable mates than mated females. It
is thus expected that males exhibit a preference for virgin females. The
intensity of male eagerness observed here when males were presented with
matching cues is aligned with such expectations and with previous
studies done in this and other species with the same pattern of sperm
precedence . When exposed to discordant cues, males disregarded the cues
emitted by the female present, basing their pre-copulatory mating
behaviour solely on the substrate cues on the patch. These cues are
obviously less reliable determinants of the status of the female on
those patches than cues emanating from the female itself. Although we
are unaware of studies investigating the duration of substrate cues,
results shown here, together with other studies done on mites (Rodrigues
et al. 2017, Magalhães et al. 2005) clearly indicate that cues remain on
patches at least 24h after the individuals move (or are removed). The
use of more unreliable cues in pre-copulatory mating behaviour might
help explain why, in spider mites and perhaps in species with a similar
pattern of sperm precedence, matings with mated females are frequently
observed, despite their weak reproductive value.
Unlike male eagerness, female acceptance depended both on the substrate
cues and on the females’ own mating status, being weaker not only in
mated females, but also in patches with substrate cues of mated females.
That virgins accept more matings than mated females goes in line with
the expectations for species with this pattern of sperm precedence and
with what is known about spider mites reproduction: first, only virgin
females receive genetic benefits by mating ; second, multiple mating
does not provide females with any non-genetic benefit and can even lead
to reduced fecundity . The effect of the substrate cues on female
acceptance might seem more cryptic. Yet, one could speculate that
females accept more matings in patches with substrate cues of virgins
because those are the patches in which the number of mating attempts is
higher and thus, resistance is expected to be more costly. This
strategy, called “convenience polyandry”, should occur under intense
harassment, when by accepting more mates than their optima, females
suffer fewer costs than by resisting them . Such is the case for
instance in female water striders that modify their mating rate based on
the relative costs of mating and of resisting mating attempts .
The combination of male eagerness to mate and the frequency of female
acceptance is reflected in the number of mating events observed here.
When there was no mismatch between cues, and given the pattern of sperm
precedence of this species, the interest of males and females were
aligned: in patches with cues of virgins, both sexes are willing to
mate, thus the number of mating events was the highest; in patches with
cues of mated females, none of the sexes directly benefits by mating ,
so the number of mating events was the lowest. In the other two
treatments where there was a mismatch between cues, because males only
used substrate cues but females responded according to their own mating
status and to the substrate cues, the response of the two sexes was not
aligned, resulting in intermediate number of mating events.
Copulation duration was shorter in matings with mated females than in
matings with virgins, which in several species has been suggested to
reflect a lower investment by males towards females of lower
reproductive value . In the case of spider mites, shorter copulations
should correspond to reduced investment in post-copulatory guarding ,
that is typically used as a strategy to guarantee sperm precedence. This
trait seems to be more affected by the mating status of the female
mating than by substrate cues present in the environment, with
copulation duration across mating events decreasing faster in patches
with virgin females, regardless of the substrate cues present. Thus,
although the substrate cues left in a patch are important for mate
acquisition, they seem to play a less significant role in
post-copulatory strategies in spider mites. Evidence of adjustments in
the use of cues during an individual lifetime is manyfold . For
instance, in the bushcricket, Ephippiger diurnus , young males
adjust their investment in spermatophore production based on social
(acoustic) experience, while old male invest equally across social
environments .
The response of males to multiple cues, including both pre- and
post-copulatory behaviours, should come at some costs. Previously, it
was shown that, in male spider mites, survival is affected differently
depending on the mating status of their reproductive partners : matings
with virgin females result in high offspring yield but reduced male
survival, while matings with mated females lead to no offspring but also
fewer survival costs. Again, being exposed to discordant cues influenced
this trait. First, we saw that, in patches occupied by mated females,
males had lower survival when substrate cues were from virgins than when
they were from mated females only. It seems that in these cases, the
existence of a mismatch leads to an over-investment in ineffective
matings. Still, this behaviour could be maintained not to risk rejecting
mating opportunities with suitable females, as proposed by Reeve . In
his model, Reeve shows that males are expected to exhibit more
permissive mating acceptance thresholds as the value of the desirable
female increases and the costs of accepting a wrong female decreases,
which are the exact conditions we find in this system. Indeed, virgin
females are highly valuable compared to mated females and the costs for
males of mating with mated females is quite low . An equivalent decrease
of the acceptance threshold would be expected if assessing multiple cues
was too costly, in which case one would expect individuals to neglect
the least reliable cue , that is the cues left on the substrate by mated
females.
Male survival in patches with mismatches between cues is higher than in
patches with cues of virgins only. In these patches, the number of
mating attempts is similar to that in patches with virgins, but the
total number of matings and the total amount of time spent copulating is
significantly lower. This suggests that the number of mating events
and/or postcopulatory events are important determinants of male mating
costs, ensuring a reduction in the costs of reproduction in mating with
less valuable females. Moreover, male survival was higher in patches
with virgin females but substrate cues of mated females, than in patches
with cues of virgins only. Therefore, it seems that in these conditions,
males invest less in effective matings, possibly via a reduction in the
number of male mating attempts and in the total amount of time spent
mating. However, we have not tested whether the observed reduction in
copulation duration is translated into reduced mating success and
previous results suggest copulation duration does not correlate
positively with offspring production .
We did not measure the composition of the cues that males were exposed
to, but we can make a few inferences from the patterns observed in male
behaviour upon exposure. For example, we do not know whether the cues of
the females themselves have the same composition as those left in the
substrate. This is however not very likely, as different components of
male mating behaviour react differently to the different combinations of
mating cues from virgins and/or mated females. Another possibility is
that the different treatments result only in a different quantity,
rather than quality, of cues. For example, it may be that only virgin
females produce cues. Our results are compatible with this possibility.
Still, this would mean that males are exposed to situations in which the
information stemming from the females themselves and the substrate they
occupy are either concordant or discordant.
The optimal use of cues and corresponding behaviour should depend on the
balance between the costs of acceptance and rejection errors and this,
in turn, should vary with the dynamics of the social and ecological
environment. In spider mite populations, individuals disperse among
patches after a variable number of generations in the same patch,
following a subdivided haystack population structure . Such cycles of
colonization-expansion foster the conditions for cue mismatch within a
patch. Indeed, while the cues emitted by females will change
simultaneously with the shift in mating status, the cues left on the
patch should remain unaltered for some time after this shift. While
these cues seem to be less reliable than the cues emitted by females
themselves, they are probably accessible at a larger scale than those of
the female itself, allowing males to move in the direction of areas with
suitable mates (i.e., virgins) before their competitors. This should be
highly advantageous in species with first male sperm precedence. These
findings could thus have important implications for mating system
evolution, potentially helping to explain why female multiple mating is
maintained in species with first male sperm precedence. Still, the
benefit of using multiple, sometimes discordant, cues will hinge upon
the frequency of discordance among cues, which itself should vary with
the dynamics of populations.
References
Figure 1. Male and female pre-copulatory mating behaviour and
the corresponding number of mating events in response to substrate cues
and female mating status. a) Number of male mating attempts, b)
proportion of mating attempts accepted by females and c) number of
mating events. Males were exposed for 1 hour to 5 virgin or mated
females in patches impregnated with cues of virgin or mated females.
Circles represent individual replicates. Black circles –patches with
mated females; grey circles – patches with virgin females; open circles
– patches with cues of virgin females; full circles – patches with
cues of mated females.