Search

Males often exercise mate choice when mating frequency is constrained, costs of choice are low and variation in female quality and/or expected paternity can be reliably detected. Across invertebrates, males use sex pheromones to discern female mating status, but there are few demonstrations that information about expected fecundity (‘quality’) is encoded in pheromones alone. Here we examine whether females' sex pheromones allow males to detect differences in female food intake and mass in two species of widow spiders ( Latrodectus hesperus and Latrodectus hasselti ) in which chemicals are deposited by females in silk. Recent work shows that male L. hesperus prefer well-fed females, and that these females produce more silk than hungry females. Thus, changes in diet could be mechanistically linked to changes in silk-bound pheromonal signals. We show that unmated females of both species lose more than half of their mass when food is withheld, and silk production is reduced by 48% ( L. hesperus ) to 67% ( L. hasselti ). Males had a significant sexual response to pheromones extracted from the females' silk in both species, although this response was not directly correlated with silk or female mass. In L. hesperus , but not in L. hasselti , males were less responsive to sex pheromones from food-deprived females compared to well-fed females. While females on good diets provide the benefit of higher fecundity in both species, the risk of being cannibalized by hungry females during courtship exists only in L. hesperus . We conclude that sex pheromones alone can provide information about recent female feeding history, possibly reducing the costs of males expressing choice in the field. The species difference in male response also suggests that male preferences in these spiders may depend less on the benefit of seeking a highly fecund female and more on avoiding the cost of risky mating attempts with a likely cannibal. [ABSTRACT FROM AUTHOR]

Strong male mate preferences have been documented in species with otherwise conventional sex roles in the laboratory, and across taxa, male preferences generally focus on indicators of female fecundity and sperm competition risk. However, the few field studies of male choice rarely show equally strong effects. This suggests that costs of choice in nature may make the expression of preferences unlikely, or that interacting assessment cues may lead to unpredictable and variable outcomes in the wild. Field studies are therefore critical for testing whether or not male mate choice is sufficiently strong to exert sexual selection on females. Here we examine male choice in nature in response to two experimentally manipulated cues of female reproductive value. We recorded the attraction of male black widow spiders, Latrodectus hesperus, to females in a field enclosure and in a unconstrained natural setting. Stimulus females varied in two ways: sperm competition risk (low–unmated; high–mated) and fecundity (high–fed; low–unfed). Females and their silk were caged to prevent behavioural interactions while allowing the spread of airborne pheromones that attract males. Males in both experiments showed strong, nearly unanimous choice for unmated, well-fed females. In field enclosures, 80% of males made this choice, and wild males made the same choice in 94% of trials. We conclude that male choice in L. hesperus is mediated by detection of airborne pheromones, is strong and consistent despite natural costs of mate searching, and is not affected by natural interactions between (sometimes conflicting) cues of female reproductive value. Thus, universal mate choice may exist among males in nature. We propose general features of systems in which this may be likely and discuss implications for the study of sexual selection on females under conventional sex roles. [ABSTRACT FROM AUTHOR]

Male mate choice is predicted to evolve as male investment in mating increases. Thus, taxa in which males pay a high cost for mating offer researchers an ideal system to test traditional sex roles. Males courting potentially sexually cannibalistic females may be under strong selection to bias their courtship efforts away from hungry females that may be more likely to attack. Here we tested the prediction that web-based chemotactile cues of recent female foraging success influence male courtship in the black widow spider, Latrodectus hesperus. In addition, we tested the underlying assumption that previous female foraging success reduces precopulatory sexually cannibalistic attacks. We found that males courted well-fed females significantly more than they did starved females, and that this bias was maintained in the absence of the female when males encountered only (1) cues from female webs and (2) cues from female silk after web architecture had been eliminated. A nonsignificant courtship bias in favour of well-fed females was also evident when these females were transplanted to the webs of starved females. Male courtship biases in favour of well-fed females appear adaptive, as starved females were significantly more likely to attack males before mating. We conclude by noting that sexual cannibalism offers a model system wherein the exploitation of cues of predation risk have unique implications such as the effects on male courtship preferences seen here. [Copyright &y& Elsevier]

Abstract: In this article, we show the potential for utilizing proton-detected heteronuclear single quantum correlation (HSQC) NMR in rigid solids under ultra-fast magic angle spinning (MAS) conditions. The indirect detection of carbon-13 from coupled neighboring hydrogen nuclei provides a sensitivity enhancement of 3- to 4-fold in crystalline amino acids over direct-detected versions. Furthermore, the sensitivity enhancement is shown to be significantly larger for disordered solids that display inhomogeneously broadened carbon-13 spectra. Latrodectus hesperus (Black Widow) dragline silk is given as an example where the sample is mass-limited and the sensitivity enhancement for the proton-detected experiment is 8- to 13-fold. The ultra-fast MAS proton-detected HSQC solid-state NMR technique has the added advantage that no proton homonuclear decoupling is applied during the experiment. Further, well-resolved, indirectly observed carbon-13 spectra can be obtained in some cases without heteronuclear proton decoupling. [Copyright &y& Elsevier]

Animals use behavioural plasticity to cope with conflicting selective pressures. We investigated how prey availability affects the spinning of cobwebs, whose architecture influences both prey capture and defence. Fed western black widows spun cobwebs containing more silk than did fasted spiders. However, fed spiders invested relatively less silk in the sheets and sticky gumfooted threads of webs and relatively more silk in supporting threads than did fasted spiders. The material properties of silk spun by fed and fasted spiders were relatively similar, but silk threads spun by fed spiders were twice as thick as those of fasted spiders, increasing web strength by 225%. Finally, spiders fed late in the experiment did not reproduce, even though they received the same amount of food as spiders fed earlier, 43% of which reproduced. The reallocation of silk in cobwebs away from sheets and sticky gumfooted threads, which function in prey capture, to supporting threads, which surround spiders with defensive clouds of silk, is consistent with reduced foraging effort and increased predation risk avoidance by fed spiders. Furthermore, this behavioural shift occurs as spiders are reproducing, when spiders can most afford the costs of reduced prey capture and would most benefit from enhanced protection. Our results contrast with those of previous studies on orb-weaving spiders, which found reductions in the sizes of webs as spiders caught more food. Thus, the architectures of spider webs reflect many selective pressures, the effects of which may vary with physiological state. [Copyright &y& Elsevier]

The reproductive success of a male is thought to be a function of the number of females he fertilizes. Nevertheless, various naturally occurring male behaviours seem to reduce dramatically the probability of obtaining additional matings. Such behaviours are expected when mating opportunities are limited for males and when males compete strongly for females or fertilizations. In widow spiders, males cohabit in females'' webs, engage in long courtship displays, often lose the tip of one or both emboli (copulatory organs) inside the female''s genital tract during copulation and are occasionally cannibalized by the female. We investigated conditions that may favour the evolution of a high-investment strategy of mating in white widow, Latrodectus pallidus, males. We followed male movements between females in a patchily distributed population of white widows and estimated the minimum number of males with which field-caught females mated from the number of embolus tips found in their genital tracts. The encounter rate of males with females was low with less than 20% of the searching males reaching a female. Females, in contrast, frequently mated with more than one male (up to six). We suggest that low encounter rates with females, coupled with high intrasexual competition, drives the evolution of a strategy of high investment by males in a single mating, as was observed in the white widow spider. [Copyright &y& Elsevier]

The spider genus Latrodectus includes the widely known black widows, notorious because of the extreme potency of their neurotoxic venom. The genus has a worldwide distribution and comprises 30 currently recognized species, the phylogenetic relationships of which were previously unknown. Several members of the genus are synanthropic, and are increasingly being detected in new localities, an occurrence attributed to human mediated movement. In particular, the nearly cosmopolitan range of the brown widow, Latrodectus geometricus, is a suspected consequence of human transport. Although the taxonomy of the genus has been examined repeatedly, the recognition of taxa within Latrodectus has long been considered problematic due to the difficulty associated with identifying morphological features exhibiting discrete geographic boundaries. This paper presents, to our knowledge, the first phylogenetic hypothesis for the Latrodectus genus and is generated from DNA sequences of the mitochondrial gene cytochrome c oxidase subunit I. We recover two well-supported reciprocally monophyletic clades within the genus: (1) the geometricus clade, consisting of Latrodectus rhodesiensis from Africa, and its is sister species, the cosmopolitan L. geometricus, and (2) the mactans clade containing all other Latrodectus species sampled, including taxa occurring in Africa, the Middle East, Iberian Peninsula, Australia, New Zealand, and North and South America. Recovery of the geometricus and mactans clades is consistent with previous designations of species groups within the genus based on female genitalic morphology. All L. geometricus sampled, consisting of specimens from Africa, Argentina, North America, and Hawaii, were recovered as a strongly supported monophyletic group with minimal amounts of genetic divergence, corroborating the hypothesis that human transport has recently expanded the range of this species. [Copyright &y& Elsevier]