Worthless donations: male deception and female counter play in a nuptial gift-giving spider

Stressful environments favor deceptive alternative mating tactics to become dominant

BACKGROUND

Deceptive alternative mating tactics are commonly maintained at low frequencies within populations because males using them are less competitive and acquire lower fitness than those using dominant tactics. However, the successful invasion of a male deceptive tactic is plausible if deception carries no fitness cost to females. Among populations of the gift-giving spider Paratrechalea ornata, males very often offer females a deceptive worthless gift, rather than a nutritive gift. We tested the degree to which deceptive worthless gifts can occur in natural populations living under divergent environmental conditions (moderate and stressful). We examined the plasticity of morphological and behavioral traits and analyzed the fitness of females in relation to the gift type, also examining the paternity acquired by males offering either gift type.

RESULTS

We demonstrated that worthless gifts can become dominant under highly stressful environmental conditions (84-100%). Individuals in such environment reach smaller sizes than those in moderate conditions. We suggest that the size reduction probably favors low metabolic demands in both sexes and may reduce the costs associated with receiving deceptive worthless gifts for females. In contrast, males living under moderate conditions varied the use of the deceptive tactic (0-95%), and worthless gifts negatively influenced female fecundity. Furthermore, male size, rather than gift content, positively impacted paternity success in the moderate but not in the stressful environment.

CONCLUSIONS

Overall, this is the first empirical evidence that a reversible deceptive tactic can become dominant when the environment becomes harsh and mate choice becomes limited.

Dead rats, dopamine, performance metrics, and peacock tails: Proxy failure is an inherent risk in goal-oriented systems

When a measure becomes a target, it ceases to be a good measure. For example, when standardized test scores in education become targets, teachers may start "teaching to the test," leading to breakdown of the relationship between the measure - test performance - and the underlying goal - quality education. Similar phenomena have been named and described across a broad range of contexts, such as economics, academia, machine learning, and ecology. Yet it remains unclear whether these phenomena bear only superficial similarities, or if they derive from some fundamental unifying mechanism. Here, we propose such a unifying mechanism, which we label proxy failure. We first review illustrative examples and their labels, such as the "cobra effect," "Goodhart's law," and "Campbell's law." Second, we identify central prerequisites and constraints of proxy failure, noting that it is often only a partial failure or divergence. We argue that whenever incentivization or selection is based on an imperfect proxy measure of the underlying goal, a pressure arises that tends to make the proxy a worse approximation of the goal. Third, we develop this perspective for three concrete contexts, namely neuroscience, economics, and ecology, highlighting similarities and differences. Fourth, we outline consequences of proxy failure, suggesting it is key to understanding the structure and evolution of goal-oriented systems. Our account draws on a broad range of disciplines, but we can only scratch the surface within each. We thus hope the present account elicits a collaborative enterprise, entailing both critical discussion as well as extensions in contexts we have missed.

Sperm competition intensity affects sperm precedence patterns in a polyandrous gift-giving spider

Sperm competition drives traits that enhance fertilization success. The amount of sperm transferred relative to competitors is key for attaining paternity. Female reproductive morphology and male mating order may also influence fertilization, however the outcome for sperm precedence under intense sperm competition remains poorly understood. In the polyandrous spider Pisaura mirabilis, males offer nuptial gifts which prolong copulation and increase sperm transfer, factors proposed to alter sperm precedence patterns under strong sperm competition. First, we assessed the degree of female polyandry by genotyping wild broods. A conservative analysis identified up to 4 sires, with a mean of 2 sires per brood, consistent with an optimal mating female rate. Then we asked whether intense sperm competition shifts sperm precedence patterns from first male priority, as expected from female morphology, to last male advantage. We varied sexual selection intensity experimentally and determined competitive fertilization outcome by genotyping broods. In double matings, one male monopolised paternity regardless of mating order. A mating order effect with first male priority was revealed when females were mated to 4 males, however this effect disappeared when females were mated to 6 males, likely due to increased sperm mixing. The proportion of males that successfully sired offspring drastically decreased with the number of competitors. Longer copulations translated into higher paternity shares independently of mating order, reinforcing the advantage of traits that prolong copulation duration under intense competition, such as the nuptial gift. Sperm competition intensity enhances the impact of competitive sexual traits and imposes multiple effects on paternity.

Beyond the prey: male spiders highly invest in silk when producing worthless gifts

In the spider Paratrechalea ornata, males have two gift-giving mating tactics, offering either a nutritive (prey) or a worthless (prey leftovers) silk wrapped gift to females. Both gift types confer similar mating success and duration and afford males a higher success rate than when they offer no gift. If this lack of difference in the reproductive benefits is true, we would expect all males to offer a gift but some males to offer a worthless gift even if prey are available. To test this, we allowed 18 males to court multiple females over five consecutive trials. In each trial, a male was able to produce a nutritive gift (a live housefly) or a worthless gift (mealworm exuviae). We found that, in line with our predictions, 20% of the males produced worthless gifts even when they had the opportunity to produce a nutritive one. However, rather than worthless gifts being a cheap tactic, they were related to a higher investment in silk wrapping. This latter result was replicated for worthless gifts produced in both the presence and absence of a live prey item. We propose that variation in gift-giving tactics likely evolved initially as a conditional strategy related to prey availability and male condition in P. ornata. Selection may then have favoured silk wrapping as a trait involved in female attraction, leading worthless gift-giving to invade.

Adult sex ratio and male body condition affect alternative reproductive tactics in a spider

Biases in adult sex ratios can alter the intensity of sexual selection by enhancing competition for mates. Under intense competition males increase their investment in behaviors to outcompete rivals (e.g., fighting). Yet, given that in male-biased environments mating opportunities are rare males may alternatively reduce costly courtship and/or adopt alternative reproductive tactics (ARTs). Males of the spider Pisaura mirabilis adopt different mating tactics, offering females genuine nuptial gifts (prey), nutritionally worthless gifts (prey leftovers), or no gifts. To test whether behavioral shifts between gift tactics are triggered by changes in the competitive environment, we established replicate spider populations under natural conditions at varying adult sex ratios (male-biased, female-biased and equal) and sampled gift tactics repeatedly over time. We additionally explored how male individual traits, such as body size and condition, affect the expression of ARTs. In male-biased populations males produced more gifts but of low quality, suggesting competition to trigger increased mating effort to ensure mate acquisition and fertilizations, but through a worthless gift tactic. Production of gifts and of genuine gifts was favored by high body condition, pointing to energetic limitations as being central for male reproductive capacity. We hence highlight two co-existing mechanisms at play to explain ARTs in this system, the competitive social environment where expression of gift tactics is based on optimal-decision making to overcome competition, and a conditional strategy linked to the individual’s energetic state.

Silk-borne chemicals of spider nuptial gifts elicit female gift acceptance

Chemical communication is important in a reproductive context for conveying information used for mate recognition and/or assessment during courtship and mating. Spider silk is a common vehicle for chemical communication between the sexes. However, despite being well described in females, male silk-borne chemicals remain largely unexplored. Males of the spider Pisaura mirabilis silk-wrap prey (i.e. nuptial gifts) that is offered to females during courtship and eaten by the female during mating. Interestingly, rejected males often add more silk to their gift which leads to successful mating, suggesting the presence of silk-borne chemicals that facilitate female gift acceptance. To test this hypothesis, we offered females standardized gifts covered with male silk that was either washed in solvents or unwashed, respectively, to remove or not any chemically active components. We scored female gift acceptance, and as expected in the case chemicals that mediate female mating behaviour are present in male silk, females were more likely to accept gifts covered with unwashed silk. Our findings suggest that silk-borne chemicals of nuptial gifts prime female responses, potentially signalling male quality or manipulating females into mating beyond their interests given the occurrence of male cheating behaviour via nutritionally worthless gifts in this system.

Differential allocation in a gift-giving spider: males adjust their reproductive investment in response to female condition

Background

When males are selective, they can either reject low-quality females or adjust their reproductive investment in response to traits that indicate female quality (e.g., body size or condition). According to the differential allocation hypothesis, males increase their reproductive investment when paired with high-quality females (positive differential allocation) or increase their reproductive investment when paired with low-quality females (negative differential allocation). This hypothesis has been proposed for monogamous species with biparental care, and most empirical studies focus on birds. Here we used the polygamous spider Paratrechalea ornata, in which males offer prey wrapped in silk as nuptial gifts, to test whether males adjust their reproductive investment in gift size, pre-copulatory and copulatory courtship, and sperm transfer in response to female body condition.

Results

Males exposed to females in good body condition added more flies to the gift, stimulated these females longer with abdominal touches during pre-copulatory courtship, and had longer pedipalp insertions than males exposed to females in poor body condition. Female condition affected neither silk investment in nuptial gift wrapping nor the quantity of sperm transferred by males. Finally, females in good body condition oviposited faster after copulation and laid more eggs than females in poor body condition.

Conclusions

We provide experimental evidence that males of a gift-giving spider exhibit positive differential allocation in three key aspects of their reproductive investment: the size of the nutritious gift, duration of pre-copulatory courtship, and duration of pedipalp insertions, which is regarded as a form of copulatory courtship in spiders. This positive differential allocation is likely associated with the benefits of copulating with females in good body condition. These females are more fecund and oviposit faster after copulation than females in poor body condition, which under natural field conditions probably reduces the risk of multiple matings and thus the level of sperm competition faced by the males. As a final remark, our findings indicate that the hypothesis of differential allocation also applies to species with a scramble competition mating system, in which males heavily invest in nuptial gift construction, but not in parental care.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01870-1.

Embryo Selection and Mate Choice: Can 'Honest Signals' Be Trusted?

When a measure becomes a target, it often ceases to be a good measure - an effect familiar from the declining usefulness of standardized testing in schools. This economic principle also applies to mate choice and, perhaps surprisingly, pregnancy. Just as females screen potential mates under many metrics, human mothers unconsciously screen embryos for quality. 'Examinees' are under intense selection to improve test performance by exaggerating formerly 'honest' signals of quality. Examiners must change their screening criteria to maintain useful information (but cannot abandon old criteria unilaterally). By the resulting 'proxy treadmill', new honest indicators arise while old degraded indicators linger, resulting in trait elaboration and exaggeration. Hormone signals during pregnancy show extreme evolutionary escalation (akin to elaborate mating displays).

Enough for all: no mating effort adjustment to varying mate availability in a gift-giving spider

Males of a gift-giving spider do not modify their allocation to reproduction when mating opportunities vary. Due to their costly courtship via provision of food gifts to females, with high female availability males should reduce their reproductive investment per partner to avoid exhausting their energetic budget too early. Our findings suggest instead that males may be able to enlarge their total reproductive budget, possibly drawing resources from their food gifts by partially feeding on them.

Maintenance of deceptive gifts in a natural spider population: ecological and demographic factors

Alternative mating tactics are expected to occur predominantly when mate competition is intense, resources are in short supply, or as a result of asymmetric power relationships between individuals. Males of the nuptial gift-giving spider Pisaura mirabilis use a prevailing tactic of offering a nutritive gift (insect prey) and a deceptive tactic of offering a worthless gift (consumed prey) to prospective mates. If the male’s tactic depends on precopulatory male–male competition, worthless gifts should occur primarily late in the season, when the operational sex ratio (OSR) becomes male-biased. If it depends on resource availability and/or postcopulatory sexual selection (sperm competition), worthless gifts should occur mostly early in the mating season, when prey availability is low and most females are unmated (i.e., postcopulatory sexual selection is weak). Nuptial gift construction correlated positively with prey availability and negatively with OSR, suggesting that males increase reproductive effort when resource and mate availability increase. We did not find evidence for body condition affecting male tactic use. Male size had a marked effect on the reproductive tactic employed. Males that matured early in the season were very small and employed mostly the nutritive gift tactic during their short life. Among the males that matured later and persisted through the season, relatively small males employed the worthless gift tactic whereas large males employed the nutritive gift tactic. We suggest that the existence of 2 distinct life-history strategies among males (early small and late large size) interacts with environmental and demographic conditions to maintain the deceptive tactic.

The shield effect: nuptial gifts protect males against pre-copulatory sexual cannibalism

Several not mutually exclusive functions have been ascribed to nuptial gifts across different taxa. Although the idea that a nuptial prey gift may protect the male from pre-copulatory sexual cannibalism is attractive, it has previously been considered of no importance based on indirect evidence and rejected by experimental tests. We reinvestigated whether nuptial gifts may function as a shield against female attacks during mating encounters in the spider Pisaura mirabilis and whether female hunger influences the likelihood of cannibalistic attacks. The results showed that pre-copulatory sexual cannibalism was enhanced when males courted without a gift and this was independent of female hunger. We propose that the nuptial gift trait has evolved partly as a counteradaptation to female aggression in this spider species.

Females of a gift-giving spider do not trade sex for food gifts: a consequence of male deception?

Background

Polyandry is commonly maintained by direct benefits in gift-giving species, so females may remate as an adaptive foraging strategy. However, the assumption of a direct benefit fades in mating systems where male gift-giving behaviour has evolved from offering nutritive to worthless (non-nutritive) items. In the spider Paratrechalea ornata, 70% of gifts in nature are worthless. We therefore predicted female receptivity to be independent of hunger in this species. We exposed poorly-fed and well-fed females to multiple males offering nutritive gifts and well-fed females to males offering worthless gifts.

Results

Though the treatments strongly affected fecundity, females of all groups had similar number of matings. This confirms that female receptivity is independent of their nutritional state, i.e. polyandry does not prevail as a foraging strategy.

Conclusions

In the spider Pisaura mirabilis, in which the majority (62%) of gifts in nature are nutritive, female receptivity depends on hunger. We therefore propose that the dependence of female receptivity on hunger state may have evolved in species with predominantly nutritive gifts but is absent in species with predominantly worthless gifts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0953-8) contains supplementary material, which is available to authorized users.

Emerging issues in the evolution of animal nuptial gifts

Uniquely positioned at the intersection of sexual selection, nutritional ecology and life-history theory, nuptial gifts are widespread and diverse. Despite extensive empirical study, we still have only a rudimentary understanding of gift evolution because we lack a unified conceptual framework for considering these traits. In this opinion piece, we tackle several issues that we believe have substantively hindered progress in this area. Here, we: (i) present a comprehensive definition and classification scheme for nuptial gifts (including those transferred by simultaneous hermaphrodites), (ii) outline evolutionary predictions for different gift types, and (iii) highlight some research directions to help facilitate progress in this field.

The evolutionary ecology of deception

Through dishonest signals or actions, individuals often misinform others to their own benefit. We review recent literature to explore the evolutionary and ecological conditions for deception to be more likely to evolve and be maintained. We identify four conditions: (1) high misinformation potential through perceptual constraints of perceiver; (2) costs and benefits of responding to deception; (3) asymmetric power relationships between individuals and (4) exploitation of common goods. We discuss behavioural and physiological mechanisms that form a deception continuum from secrecy to overt signals. Deceptive tactics usually succeed by being rare and are often evolving under co-evolutionary arms races, sometimes leading to the evolution of polymorphism. The degree of deception can also vary depending on the environmental conditions. Finally, we suggest a conceptual framework for studying deception and highlight important questions for future studies.

Worthless and Nutritive Nuptial Gifts: Mating Duration, Sperm Stored and Potential Female Decisions in Spiders

In nuptial gift-giving species females sometimes select their potential mates based on the presence and size of the gift. But in some species, such as the Neotropical polyandrous spider Paratrechalea ornate male gifts vary in quality, from nutritive to worthless, and this male strategy can be in conflict with female nutritional benefits. In this species, males without gifts experience a reduction in mating success and duration, while males that offer worthless or genuine nutritive gifts mate with similar frequencies and durations. The female apparently controls the duration of copulation. Thus, there is scope for females to favour males offering gifts and further if these are nutritious, via post-copulatory processes. We first tested whether females differentially store sperm from males that offer the highest nutritional benefits by experimentally presenting females with males that offer either nutritive or worthless gifts (uninterrupted matings). Second, we carried out another set of experiments to examine whether females can select sperm based only on gift presence. This time we interrupted matings after the first pedipalp insertion, thus matching number of insertions and mating duration for males that: offered and did not offer gift. Our results showed that the amount of sperm stored is positive related to mating duration in all groups, except in matings with worthless gifts. Gift presence itself did not affect the sperm stored by females, while they store similar number of sperm in matings with males offering either nutritive or worthless gifts. We discuss whether females prefer males with gifts regardless, if content, because it represents an attractive and/or reliable signal. Or alternatively, they prefer nutritive nuptial gifts, as they are an important source of food supply and/or signal of male donor ability.

Sexual deception in a cannibalistic mating system? Testing the Femme Fatale hypothesis

Animal communication theory holds that in order to be evolutionarily stable, signals must be honest on average, but significant dishonesty (i.e. deception) by a subset of the population may also evolve. A typical praying mantid mating system involves active mate searching by males, which is guided by airborne sex pheromones in most species for which mate-searching cues have been studied. The Femme Fatale hypothesis suggests that female mantids may be selected to exploit conspecific males as prey if they benefit nutritionally from cannibalism. Such a benefit exists in the false garden mantid Pseudomantis albofimbriata-females use the resources gained from male consumption to significantly increase their body condition and reproductive output. This study aimed to examine the potential for chemical deception among the subset of females most likely to benefit from cannibalism (poorly fed females). Females were placed into one of four feeding treatments ('Very Poor', 'Poor', 'Medium' and 'Good'), and males were given the opportunity to choose between visually obscured females in each of the treatments. Female body condition and fecundity varied linearly with food quantity; however, female attractiveness did not. That is, Very Poor females attracted significantly more males than any of the other female treatments, even though these females were in significantly poorer condition, less fecund (in this study) and more likely to cannibalise (in a previous study). In addition, there was a positive correlation between fecundity and attractiveness if Very Poor females were removed from the analysis, suggesting an inherently honest signalling system with a subset of dishonest individuals. This is the first empirical study to provide evidence of sexual deception via chemical cues, and the first to provide support for the Femme Fatale hypothesis.

Optimal numbers of matings: the conditional balance between benefits and costs of mating for females of a nuptial gift-giving spider

In species where females gain a nutritious nuptial gift during mating, the balance between benefits and costs of mating may depend on access to food. This means that there is not one optimal number of matings for the female but a range of optimal mating numbers. With increasing food availability, the optimal number of matings for a female should vary from the number necessary only for fertilization of her eggs to the number needed also for producing these eggs. In three experimental series, the average number of matings for females of the nuptial gift-giving spider Pisaura mirabilis before egg sac construction varied from 2 to 16 with food-limited females generally accepting more matings than well-fed females. Minimal level of optimal mating number for females at satiation feeding conditions was predicted to be 2-3; in an experimental test, the median number was 2 (range 0-4). Multiple mating gave benefits in terms of increased fecundity and increased egg hatching success up to the third mating, and it had costs in terms of reduced fecundity, reduced egg hatching success after the third mating, and lower offspring size. The level of polyandry seems to vary with the female optimum, regulated by a satiation-dependent resistance to mating, potentially leaving satiated females in lifelong virginity.

Sperm storage mediated by cryptic female choice for nuptial gifts

Polyandrous females are expected to discriminate among males through postcopulatory cryptic mate choice. Yet, there is surprisingly little unequivocal evidence for female-mediated cryptic sperm choice. In species in which nuptial gifts facilitate mating, females may gain indirect benefits through preferential storage of sperm from gift-giving males if the gift signals male quality. We tested this hypothesis in the spider Pisaura mirabilis by quantifying the number of sperm stored in response to copulation with males with or without a nuptial gift, while experimentally controlling copulation duration. We further assessed the effect of gift presence and copulation duration on egg-hatching success in matings with uninterrupted copulations with gift-giving males. We show that females mated to gift-giving males stored more sperm and experienced 17% higher egg-hatching success, compared with those mated to no-gift males, despite matched copulation durations. Uninterrupted copulations resulted in both increased sperm storage and egg-hatching success. Our study confirms the prediction that the nuptial gift as a male signal is under positive sexual selection by females through cryptic sperm storage. In addition, the gift facilitates longer copulations and increased sperm transfer providing two different types of advantage to gift-giving in males.

Silk wrapping of nuptial gifts as visual signal for female attraction in a crepuscular spider

An extensive diversity of nuptial gifts is known in invertebrates, but prey wrapped in silk is a unique type of gift present in few insects and spiders. Females from spider species prefer males offering a gift accepting more and longer matings than when males offered no gift. Silk wrapping of the gift is not essential to obtain a mating, but appears to increase the chance of a mating evidencing a particularly intriguing function of this trait. Consequently, as other secondary sexual traits, silk wrapping may be an important trait under sexual selection, if it is used by females as a signal providing information on male quality. We aimed to understand whether the white color of wrapped gifts is used as visual signal during courtship in the spider Paratrechalea ornata. We studied if a patch of white paint on the males' chelicerae is attractive to females by exposing females to males: with their chelicerae painted white; without paint; and with the sternum painted white (paint control). Females contacted males with white chelicerae more often and those males obtained higher mating success than other males. Thereafter, we explored whether silk wrapping is a condition-dependent trait and drives female visual attraction. We exposed good and poor condition males, carrying a prey, to the female silk. Males in poor condition added less silk to the prey than males in good condition, indicating that gift wrapping is an indicator of male quality and may be used by females to acquire information of the potential mate.

Evidence for tactical concealment in a wild primate

Theory predicts that cheating individuals should alter their behaviour to avoid detection, yet empirical data for such 'deceptive' behaviour (and its putative consequence-punishment) is almost entirely absent from the literature. This dearth of evidence, particularly among primates, limits our understanding of the evolution of deception and punishment. Here, we quantify deception and punishment in a reproductive context in wild geladas (Theropithecus gelada). Individuals involved in extra-pair copulations (9% of observed copulations) exhibited behaviour consistent with tactical deception: they were less likely to vocalize and more likely to copulate when the cuckolded male was a sizable distance away (>20 m). Further, many extra-pair copulations (∼20%) elicited post-copulatory aggression-likely, punishment-from cuckolded males. This rare empirical evidence of both tactical concealment and retaliatory aggression offers a potential model system for examining the co-occurrence of deception and punishment in natural settings.

Polyandrous females acquire indirect benefits in a nuptial feeding species

The relative force of direct and indirect selection underlying the evolution of polyandry is contentious. When females acquire direct benefits during mating, indirect benefits are often considered negligible. Although direct benefits are likely to play a prominent role in the evolution of polyandry, post-mating selection for indirect benefits may subsequently evolve. We examined whether polyandrous females acquire indirect benefits and quantified direct and indirect effects of multiple mating on female fitness in a nuptial gift-giving spider (Pisaura mirabilis). In this system, the food item donated by males during mating predicts direct benefits of polyandry. We compared fecundity, fertility and survival of singly mated females to that of females mated three times with the same (monogamy) or different (polyandry) males in a two-factorial design where females were kept under high and low feeding conditions. Greater access to nutrients and sperm had surprisingly little positive effect on fitness, apart from shortening the time until oviposition. In contrast, polyandry increased female reproductive success by increasing the probability of oviposition, and egg hatching success indicating that indirect benefits arise from mating with several different mating partners rather than resources transferred by males. The evolution of polyandry in a male-resource-based mating system may result from exploitation of the female foraging motivation and that indirect genetic benefits are subsequently derived resulting from co-evolutionary post-mating processes to gain a reproductive advantage or to counter costs of mating. Importantly, indirect benefits may represent an additional explanation for the maintenance of polyandry.