How blood-derived odor influences mate-choice decisions by a mosquito-eating predator

The evolution of sanguivory in vampire bats: origins and convergences

Blood-feeding (sanguivory) has evolved more than two dozen times among birds, fishes, insects, arachnids, molluscs, crustaceans, and annelids; however, among mammals, it is restricted to the vampire bats. Here, the authors revisit the question of how it evolved in that group. Evidence to date suggests that the ancestors of phyllostomids were insectivorous, and that carnivory, omnivory, and nectarivory evolved among phyllostomids after vampire bats diverged. Frugivory likely also evolved after vampire bats diverged, but the phylogeny is ambiguous on that point. However, vampire bats lack any genetic evidence of a frugivorous past, and the behavioural progression from frugivory to sanguivory is difficult to envision. Thus, the most parsimonious scenario is that sanguivory evolved in an insectivorous ancestor to vampire bats via ectoparasite-eating, wound-feeding, or some combination of the two—all feeding habits found among blood-feeding birds today. Comparing vampire bats with other sanguivores, the authors find several remarkable examples of convergence. Further, it was found that blood-feeding has been ca. 50 times more likely to evolve in a vertebrate lineage than in an invertebrate one. The authors hypothesize that this difference exists because vertebrates are more likely than invertebrates to have the biochemical necessities required to assimilate the components of vertebrate blood.

Females of a cannibalistic spider control mutilation of their genitalia by males

When females can mate multiply, the interests of both sexes over female remating may not coincide, leading to selection for adaptations and counteradaptations in males and females. In several orb-weaving spiders, males damage external structures of the female genitalia during copulation, which hinders the female from remating. We investigated whether females have control over the mutilation of their genitalia in the orb-weaving spider Larinia jeskovi. We found that female sexual cannibalism during copulation reduced the number of insertions a male was able to perform and hence limited the probability of genital mutilation by the male. Genital mutilation did not differ between treatments in which females experienced different availabilities of other males before the mating trial: males absent, males near the female (“vicinity group”), and males in the female’s web (“web group”). However, traits of the mating male (size, condition) were significantly correlated with the occurrence of cannibalism during mating in “web” and “vicinity” treatments. These results suggest that females have control over mutilation by an early termination of mating, can respond to the availability of potential mates and can alter the probability of mutilation according to certain male traits. Female sexual cannibalism may represent a counteradaptation to genital mutilation allowing females to mate multiply.

Ontogenetic shift in plant-related cognitive specialization by a mosquito-eating predator

Evarcha culicivora, an East African salticid spider, is a mosquito specialist and it is also a plant specialist, with juveniles visiting plants primarily for acquiring nectar meals and adults visiting plants primarily as mating sites. The hypothesis we consider here is that there are ontogenetic shifts in cognition-related responses by E. culicivora to plant odour. Our experiments pertain to cross-modality priming effects in three specific contexts: executing behaviour that we call the 'visual inspection of plants' (Experiment 1), adopting selective visual attention to specific visual targets (Experiment 2) and becoming prepared to respond rapidly to specific visual targets (Experiment 3). Our findings appear not to be a consequence of salient odours in general elevating E. culicivora's motivation to respond to salient visual stimuli. Instead, effects were specific to particular odours paired with particular visual targets, with the salient volatile plant compounds being caryophyllene and humulene. We found evidence that prey odour primes juveniles and adults to respond to seeing specifically prey, mate odour primes adults to respond to seeing specifically mates and plant odour primes juveniles to respond to seeing specifically flowers. However, plant odour appears to prime adults to respond to seeing specifically a mate associated with a plant.

Frequent misdirected courtship in a natural community of colorful Habronattus jumping spiders

Male courtship display is common in many animals; in some cases, males engage in courtship indiscriminately, spending significant time and energy courting heterospecifics with whom they have no chance of mating or producing viable offspring. Due to high costs and few if any benefits, we might expect mechanisms to evolve to reduce such misdirected courtship (or ‘reproductive interference’). In Habronattus jumping spiders, males frequently court heterospecifics with whom they do not mate or hybridize; females are larger and are voracious predators, posing a severe risk to males who court indiscriminately. In this study, we examined patterns of misdirected courtship in a natural community of four sympatric Habronattus species (H. clypeatus, H. hallani, H. hirsutus, and H. pyrrithrix). We used direct field observations to weigh support for two hypotheses (differential microhabitat use and species recognition signaling) to explain how these species reduce the costs associated with misdirected courtship. We show that, while the four species of Habronattus do show some differences in microhabitat use, all four species still overlap substantially, and in three of the four species individuals equally encountered heterospecifics and conspecifics. Males courted females at every opportunity, regardless of species, and in some cases, this led to aggression and predation by the female. These results suggest that, while differences in microhabitat use might reduce misdirected courtship to some extent, co-existence of these four species may be possible due to complex communication (i.e. species-specific elements of a male’s courtship display). This study is the first to examine misdirected courtship in jumping spiders. Studies of misdirected courtship and its consequences in the field are limited and may broaden our understanding of how biodiversity is maintained within a community.

Cross-modality effects of prey odour during the intraspecific interactions of a mosquito-specialist predator

One of the predictions from evolutionary game theory is that individuals will increase their willingness (i.e., become primed) to escalate aggression when they detect the presence of a limiting resource. Here we test this prediction in the context of prey odour priming escalation decisions during vision-based encounters by Evarcha culicivora. This East African jumping spider (Salticidae) feeds indirectly on vertebrate blood by actively choosing blood-carrying female mosquitoes as preferred prey. Unlike many salticid species, it also expresses pronounced mutual mate choice. As predicted, we show here that, in the presence of odour from their preferred prey, both sexes of E. culicivora escalate during vision-based same-sex encounters. This is further evidence that the odour of blood-carrying mosquitoes is salient to this salticid. For both sexes of E. culicivora, this particular prey may be a resource that matters in the context of intrasexual selection.

Love is in the air: olfaction-based mate-odour identification by jumping spiders from the genus Cyrba

Jumping spiders (Salticidae) are known for having good eyesight, but the extent to which they rely on olfaction is poorly understood. Here we demonstrate for the first time that olfactory pheromones are used by two species from the salticid genus Cyrba (C. algerina and C. ocellata). Using a Y-shape olfactometer, we investigated the ability of adult males and females of both species to discriminate between mate and non-mate odour. A hidden spider or a spider’s draglines (no spider present) were used as odour sources. There was no evident response by females of either Cyrba species to any tested odour. Males of both species chose odour from conspecific females, or their draglines, significantly more often than the no-odour control, but there was no evident response by males to any of the other odours (conspecific male and heterospecific female). Our findings demonstrate that C. algerina and C. ocellata males can make sex- and species-specific discriminations even when restricted to using olfaction alone. Also, by showing that draglines can be a source of olfactory pheromones, our findings illustrate the difficulty of ruling out olfaction when attempting to test for chemotactile cues.

The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider

Evarcha culicivora is an East African jumping spider that feeds indirectly on vertebrate blood by choosing blood-fed female Anopheles mosquitoes as prey. Previous studies have shown that this predator can identify its preferred prey even when restricted to using only visual cues. Here, we used lures and virtual mosquitoes to investigate the optical cues underlying this predator's prey-choice behaviour. We made lures by dissecting and then reconstructing dead mosquitoes, combining the head plus thorax with different abdomens. Depending on the experiment, lures were either moving or motionless. Findings from the lure experiments suggested that, for E. culicivora, seeing a blood-fed female mosquito's abdomen on a lure was a necessary, but not sufficient, cue by which preferred prey was identified, as cues from the abdomen needed to be paired with cues from the head and thorax of a mosquito. Conversely, when abdomens were not visible or were identical, spiders based their decisions on the appearance of the head plus thorax of mosquitoes, choosing prey with female characteristics. Findings from a subsequent experiment using animated 3D virtual mosquitoes suggest that it is specifically the mosquito's antennae that influence E. culicivora's prey-choice decisions. Our results show that E. culicivora uses a complex process for prey classification.

Salticid predation as one potential driving force of ant mimicry in jumping spiders

Many spiders possess myrmecomorphy, and species of the jumping spider genus Myrmarachne exhibit nearly perfect ant mimicry. Most salticids are diurnal predators with unusually high visual acuity that prey on various arthropods, including conspecifics. In this study, we tested whether predation pressure from large jumping spiders is one possible driving force of perfect ant mimicry in jumping spiders. The results showed that small non-ant-mimicking jumping spiders were readily treated as prey by large ones (no matter whether heterospecific or conspecific) and suffered high attack and mortality rates. The size difference between small and large jumping spiders significantly affected the outcomes of predatory interactions between them: the smaller the juvenile jumping spiders, the higher the predation risk from large ones. The attack and mortality rates of ant-mimicking jumping spiders were significantly lower than those of non-ant-mimicking jumping spiders, indicating that a resemblance to ants could provide protection against salticid predation. However, results of multivariate behavioural analyses showed that the responses of large jumping spiders to ants and ant-mimicking salticids differed significantly. Results of this study indicate that predation pressure from large jumping spiders might be one selection force driving the evolution of nearly perfect myrmecomorphy in spiders and other arthropods.

Olfaction-based anthropophily in a mosquito-specialist predator

Evarcha culicivora is an unusual salticid spider because it feeds indirectly on vertebrate blood by choosing blood-carrying mosquitoes as preferred prey. Its preferred mosquitoes are Anopheles, the genus to which all human malaria vectors belong. Here, we show that human odour, which is known to be salient to malaria vectors, is also salient to the adults and juveniles of E. culicivora. Test spiders spent more time in the vicinity of a source of human odour (previously worn socks) when the alternative was unworn socks.

Olfactory search-image use by a mosquito-eating predator

By choosing blood-carrying mosquitoes as prey, Evarcha culicivora, an East African salticid spider, specializes at feeding indirectly on vertebrate blood. It also has an exceptionally complex mate-choice system. An earlier study revealed that search-image use assists E. culicivora in finding prey and mates when restricted to using vision alone. Here we show that search-image use assists E. culicivora in finding prey and mates when restricted to using olfaction alone. After being primed with prey odour or mate odour (control: not primed with odour), spiders were transferred to an olfactometer designed to test ability to find a prey-odour or mate-odour source that was either 'cryptic' (i.e. accompanied by a masking odour source, Lantana camara) or 'conspicuous' (no L. camara odour). When tested with conspicuous odour, the identity of the priming odour had no significant effect on how many spiders found the odour source. However, when tested with cryptic odour, significantly more spiders found the odour source when primed with congruent odour and significantly fewer spiders found the odour source when primed with incongruent odour.