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.