Scramble Competition Mating System Research Articles

Exploration and social environment affect inbreeding avoidance in a small mammal

Abstract Individual exploration types are based on the cognitive speed-accuracy trade-off, which suggests that a higher speed of information acquisition is done by sacrificing information quality. In a mating context, fast exploration could thus increase the probability of finding mates at the cost of mating with kin or suboptimal partners. We tested this hypothesis by studying male mate choice patterns in a species with a scramble competition mating system. We used genotyping, localization by radio-collar, trapping, and repeated exploration measures from a long-term study on wild Eastern chipmunks (Tamias striatus). We predicted that, according to the speed-accuracy trade-off hypothesis, slower-thorough explorers should be choosier than faster-superficial ones, and thus avoid inbreeding. We found that slower males reproduced more often with less related females, but only on one site where variance in relatedness and female density were high. Males showed no preference for their mates’ exploration type. Our results suggest that superficial exploration decreases male choosiness and increases the risk of inbreeding, but only under decreased mate search costs due to high variance in relatedness among mates (at high density). Our findings reveal exploration-related, among-individual variance in inbreeding, highlighting the complexity of mate choice, and showing that many aspects of an individual’s life contribute to animal decision-making.

Circadian and seasonal flight activity differences between the sexes of the biocontrol agent Eadya daenerys (Hymenoptera: Braconidae) and the impact of host size on adult emergence

AbstractSince its accidental introduction into New Zealand in 1916, the invasive eucalypt leaf beetle Paropsis charybdis remains a pest of economic significance to Eucalyptus forestry. For this reason, the braconid parasitoid Eadya daenerys has been approved for release as a classical biological control agent. To aid in field monitoring using hand netting of biocontrol releases and laboratory rearing protocols, the flight activity over 6 a year field research program of E. daenerys and the relationships between the parasitoid and the size of its beetle hosts were evaluated in the country of origin. Wasps were observed flying over 32 days from mid‐November to mid‐December in Tasmania. E. daenerys was found to exhibit a morning‐based scramble competition mating system. Females flying in the early morning had lower mature egg loads (134.2 ± 27.6) than at other times of the day. In the first half of the season, male flight was most often seen in the early morning, whereas in the second half of the season, there was a 4.5‐fold decrease in any wasps flying. Female flight activity significantly increased to 70% female in the evening hours from 5:00 pm to 8:00 pm, which by the second half of the season was almost exclusively female. Wasps were caught flying across a wide range of air temperatures (10.7–23.8°C), humidity and light levels, but even light winds reduced flight when average wind speed exceeded 0.27 m/s for males and 0.73 m/s for females. Beetle prepupal weight was a predictor of E. daenerys parasitism success with mean beetle prepupal weights significantly higher for stung but unsuccessfully parasitised larva (62.09 mg) than those successfully parasitised (52.94 mg). The chance of an emerged wasp larva spinning a pupal cocoon was found to increase by 5% with every 1‐mg increase in its beetle prepupal weight. Heavier beetle prepupae produced bigger parasitoids. Field‐collected adult E. daenerys had larger (12%) head capsule widths (mean of 1.42 mm) than those reared in the laboratory (mean of 1.27 mm), suggesting that improving host nutrition and laboratory rearing conditions for increasing host size may help optimise E. daenerys rearing success.

Interaction between sexual communication functions leads to reproductive interference in two syntopic scorpion species

Sexual communication in mate choice scenarios involves nonmutually exclusive functions: species recognition, mate location and mate assessment. Most studies on sexual communication in mate choice have focused on female assessment of conspicuous male traits, such as visual and acoustic signals. Only a few studies have addressed male assessment of female traits, especially those that are inconspicuous to the human sensory system, such as chemical signals. Although chemical signals are important in sexual communication of many arachnids, there is a gap in our knowledge about how male scorpions use these signals to evaluate potential partners. Here we studied sexual communication functions in two scorpion species (Urophonius brachycentrus and Urophonius achalensis) with sympatric populations and synchronic reproduction, where males are under a scramble competition mating system. We addressed the volatile pheromones role in each of the sexual communication functions. We used Y-mazes to test volatile pheromones, exposing males to different female stimulus arrays in single-signal (conspecific only) and mixed-signal (conspecific and heterospecific) contexts. Males located conspecific females through volatile pheromones, and the time spent in proximity to the female was proportional to males' perception of the female's quality, reinforcing the pheromones' function in mate location and assessment in single-signal environments. However, these sexually selected functions were overridden in mixed-signal contexts, in which males could not differentiate conspecific from heterospecific females, suggesting that pheromones do not allow species recognition. This interaction between sexual communication functions can lead to reproductive interference in this sympatric zone. Male indiscriminateness mediated by scramble competition could attenuate pheromones' species recognition function during mate choice. We provide a comprehensive study of the pheromones' role for mate and species recognition in different social contexts in scorpions. We found support for nonindependent and interacting functions of sexual communication, indicating that mate choice is a complex process in scorpions.

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

BackgroundWhen 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.ResultsMales 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.ConclusionsWe 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.

Sexual dimorphism divergence between sister species is associated with a switch in habitat use and mating system in thorny devil stick insects

The habitat and resource use of females critically affects their pattern of distribution and consequently their monoposibility by males and the mating system of a species. Shifts in habitat use are therefore likely to be associated with changes in mating system and sexual selection acting on males’ phenotypes, consequently affecting patterns of sexual dimorphism. Although sexual dimorphism is often correlated with shifts in habitat use at the macroevolutionary scale, the underlying microevolutionary processes involved are typically unclear. Here, we used the New Guinean stick insect genus Eurycantha to investigate how changes in habitat use and mating system were associated with a change in sexual dimorphism seen specifically in the thorny devil stick insects (Eurycantha calcarata and Eurycantha horrida). Male thorny devils display sexually dimorphic and enlarged hindlegs endowed with a sharp spine. Sexual size dimorphism is also very reduced in these species relative to other phasmids. Using field observations, morphological measurements and radiotelemetry, we investigated changes in mating system associated with the reduction of sexual dimorphism and tested predictions from the hypothesis that sexual selection drove the evolution of this unusual male morphology. We found that thorny devils switched from solitary roosting in the canopy during the day to communal roosting inside cavities of a few host trees, shifting the distribution of females from scattered to clumped. Male thorny devils used their large hindlegs to fight with rivals for positions on the tree close to cavities containing females, and larger males were associated with cavities containing relatively more females. In contrast, the sister species, Eurycantha insularis, displays relatively small and unarmoured males (ancestral state). Adult female E. insularis were always scattered in the canopy, and this species displayed a scramble competition mating system typical of other phasmids, where mobility, rather than fighting ability, is probably critical to males’ reproductive success. Overall, our study illustrates how a drastic change in sexual dimorphism can be associated with a switch from solitary to communal roosting and from a scramble competition to a defense-based polygyny mating system.

Parthenogenetic Females of the Stick Insect Clitarchus hookeri Maintain Sexual Traits.

The New Zealand stick insect Clitarchus hookeri has both sexual and parthenogenetic (all-female) populations. Sexual populations exhibit a scramble competition mating system with distinctive sex roles, where females are signalers and males are searchers, which may lead to differences in the chemical and morphological traits between sexes. Evidence from a range of insect species has shown a decay of sexual traits is common in parthenogenetic lineages, especially those traits related to mate attraction and location, presumably due to their high cost. However, in some cases, sexual traits remain functional, either due to the recent evolution of the parthenogenetic lineage, low cost of maintenance, or because there might be an advantage in maintaining them. We measured morphological and chemical traits of C. hookeri to identify differences between males and females and between females from sexual and parthenogenetic populations. We also tested the ability of males to discriminate between sexual and parthenogenetic females in a laboratory bioassay. Our results show that male C. hookeri has morphological traits that facilitate mobility (smaller body with disproportionately longer legs) and mate detection (disproportionately longer antennae), and adult females release significantly higher amounts of volatile organic compounds than males when this species is sexually active, in accordance with their distinctive sex roles. Although some differences were detected between sexual and parthenogenetic females, the latter appear to maintain copulatory behaviors and chemical signaling. Males were unable to distinguish between sexual and parthenogenetic females, suggesting that there has been little decay in the sexual traits in the parthenogenetic lineage of C. hookeri.

Home range overlap and its genetic correlates in an avian brood parasite, the lesser cuckoo Cuculus poliocephalus

The absence of parental care is of importance in the evolution of mating and breeding systems. However, its importance in the evolution of these systems in higher vertebrates, such as birds and mammals, has not been well defined since nearly all of them exhibit some form of parental care. Avian brood parasites thus provide a unique opportunity to explore the effect of a lack of parental care in higher vertebrates because they shift care for their progeny onto other species. We investigated the social system of an avian brood parasite, the lesser cuckoo (Cuculus poliocephalus), by using a comprehensive approach that included field observation, radio-tracking, and genetic analyses. We found that genetically unrelated, multiple males and females coexist in a single area; radio-tracking confirmed that this co-occurrence was neither temporary nor arbitrary. Instead, sympatric cuckoos appeared to share their home ranges and often chased each other for mating without antagonistic physical interaction. We conclude that the lesser cuckoo may employ a scramble competition mating system without territoriality, and that this mating system, which is unusual in higher vertebrates, may have arisen due to the absence of parental care. Future studies assessing the parentage of nestlings are needed to establish the genetic basis of this social system.

Vocal Behavior of the Explosively Breeding Neotropical Yellow Toad,Incilius luetkenii

Abstract Frogs and toads exhibit complex vocal behavior, often in the context of mating and resource defense. Although over 6,500 species of frogs and toads have been described, detailed accounts of vocal behavior exist for very few species. In this study, we provide the first detailed investigation of the vocal behavior of Neotropical Yellow Toads, Incilius luetkenii, based on recordings collected from animals in northwestern Costa Rica. We provide quantitative measurements of two call types, we test the hypothesis that male body size is related to the structure of male calls, we investigate the context in which males produce calls, and we discuss the importance of their vocal behavior in the framework of an aggressive scramble competition mating system. We found that male I. luetkenii produce two vocalizations during their explosive breeding aggregations: advertisement calls and release calls. Advertisement calls are 2.5 sec-long vocalizations consisting of a series of pulses. Release calls are 0.6 sec-...

Nonlinear scaling of foraging contacts with rodent population density

Density‐dependent shifts in population processes like territoriality, reproduction, dispersal, and parasite transmission are driven by changes in contacts between individuals. Despite this, surprisingly little is known about how contacts change with density, and thus the mechanisms driving density‐dependent processes. A simple linear contact–density function is often assumed, but this is not based on a sound basis of empirical data. We addressed this question using a replicated, semi‐natural experiment in which we measured contacts at feeding stations between multimammate mice,Mastomys natalensis, across ten distinct, linearly increasing densities between 10 and 272 animals/ha. Unexpectedly, unique contacts increased not linearly but sigmoidally with density, which we attribute to the species’ scramble competition mating system, small‐scale dominance/avoidance and absence of territoriality. These results provide new insights into how species’ characteristics can relate to density‐dependent changes in contacts, and the unexpected shape of the contact–density function warrants that density‐dependence in ecological models, such as parasite transmission models, must be parameterized with care.

Mate detection and seasonal variation in stick insect mating behaviour (Phamatodea: Clitarchus hookeri)

For animals that exhibit a scramble competition mating system, sexual selection pressures on mate searching ability are expected to be strong. Scramble competition mating systems evolve when populations provide females with equal accessibility to all male competitors, yet sex ratio and population density influences mating systems and varies seasonally. The stick insect species,Clitarchus hookeri, is frequently found in copula, yet very little is known about it’s mating behaviour. We preformed behavioural tests and assayed antennal sensory morphology to determine whether males used chemosensory cues to detect females. Through natural field observations we found populations to be significantly male-biased earlier in the season, while later, populations began to display equal sex ratios. With increasing female availability mating pair proportions steadily increased, while copulation duration declined. These results supportC. hookerias a scramble competitor, and demonstrate males may alter their behaviour in response to the seasonal variation in female density.

Scramble competition by males of the velvet ant Nemka viduata (Hymenoptera: Mutillidae)

The mating systems of mutillid wasps have rarely been studied. Here we present information on the mating system of Nemka viduata. At a site in southern Spain, many males of this species were seen flying over host (digger wasp) nest aggregations while searching for females. Male activity was greatest in the early morning and late afternoon, when females were more active searching for hosts, and on days when relatively large numbers of females were active. Males were not territorial but instead attempted to find emerging females before their competitors. As many as six males might arrive at a receptive female more or less simultaneously. Struggles to control access to females continued until one male copulated with the female on the ground or carried it off in flight to a location away from rival males. Male size seems to affect the patrolling behaviour (number of patrolled sites), but there is little evidence of an advantage for larger males, as expected in a scramble competition mating system. Scramble competition mating systems often evolve in species in which large numbers of males compete for scarce receptive females, a factor that makes male territorial defence of large areas highly costly.

Factors affecting male and female reproductive success in a chipmunk (Tamias sibiricus) with a scramble competition mating system

While sexual selection in mammals with female defense systems has historically received considerable attention, much less is known about the factors affecting reproductive success in mammalian species with scramble competition mating systems. Using mark–recapture techniques and DNA microsatellite loci to assign parentage, we examined the mating system and factors affecting the variation of the annual reproductive success in an introduced population of the Siberian chipmunk, Tamias sibiricus, a small, monomorphic, solitary squirrel. Our analyses showed that breeding females were spatially dispersed, setting the stage for a scramble competition mating system. Male reproductive success was positively associated with the size of the home range. The strength of sexual selection on this behavioral trait was very strong, equaling previous estimates for morphological traits in female defense mating systems. These findings suggest that a behavioral trait, space use, strongly influences the annual reproductive success in males, which is consistent with expectations in a scramble competition mating system. In both sexes, reproductive success was influenced by habitat, with twice as many juveniles produced in semi-open than in closed habitat, possibly due to differences in food availability between the two habitats.

Energetic costs of male reproduction in a scramble competition mating system

1. The assumption that the primary limitations on reproductive success differ between the sexes is inherent in traditional sexual selection theory. Although the energy that can be allocated to reproduction is assumed to be the main limitation to females, the ability to attract and defend oestrous females is assumed to be the primary limitation to males. 2. Estimates of the energetic costs of reproduction in male mammals are, however, limited and have largely been obtained from sexually dimorphic species exhibiting female defence mating systems. These studies often reveal that the energetic cost of male reproduction is similar to or even exceeds that of females, and therefore challenge long-held assumptions of inter-sexual reproductive limitations, but their generality is little known. 3. We coupled measurements of energy expenditure with detailed behavioural observations of reproductive male North American red squirrels (Tamiasciurus hudsonicus). This species displays minimal sexual dimorphism and exhibits a scramble competition mating system, under which sexual selection favours enhanced mate searching effort by males. 4. We conducted the study over 2 years characterized by a substantial variation in upcoming natural food availability and across two study populations that experienced either natural food abundance or an ad libitum food-supplementation to investigate the influence of resource availability on male reproductive energy expenditure. 5. Under natural conditions, mean energy expenditure of males across the 2 years was high, approximating that of females during lactation. Furthermore, in the anticipation of high upcoming natural food availability and resultant offspring survival, expenditure approximately doubled (from 290 +/- 7 to 579 +/- 73 kJ day(-1)). When current food availability (and consequently the density of receptive females) was experimentally elevated, males displayed the highest levels of energy expenditure we recorded (873 +/- 98 kJ day(-1)). 6. Our results provide compelling evidence that the energy available for reproductive allocation places a strong limitation on reproduction in male North American red squirrels and contribute to previous work suggesting that high and limiting energetic costs of male reproduction may be a general feature of mammalian reproduction.

Sexually selected behaviour: red squirrel males search for reproductive success

1. Differential male reproductive success is commonplace in mammals and frequently attributed to variation in morphological traits that provide individuals with a competitive advantage in female defence mating systems. Other mammalian mating systems, however, have received comparatively little attention and correlates of male reproductive success in them are less well understood. 2. We studied a free-ranging population of North American red squirrels (Tamiasciurus hudsonicus Erxleben) exhibiting year-round individual territoriality. Males must temporarily vacate their territories to locate spatially dispersed receptive females, thereby setting the stage for a scramble competition mating system. 3. We predicted that both male annual mating success (measured as the number of females copulated with) and annual reproductive success (measured as the number of offspring sired) would be positively correlated with both search ability (measured as the number of oestrous females located over the mating season) and effort (measured as mating season home range size), generating directional sexual selection on these two metrics. 4. Mating season home ranges of males showed, on average, an almost 10-fold increase relative to those measured during the nonmating season, while those of females showed a more moderate twofold increase and both annual mating and reproductive success of males was positively correlated with search ability and search effort. 5. The spatial dispersion of females, resulting from the strict territorial social structure of red squirrels, gave rise to a predicted scramble competition mating system. Furthermore, the strength of sexual selection on behavioural traits in this mating system equalled previous estimates for morphological traits in female defence mating systems.

Mate location, antennal morphology, and ecology in two praying mantids (Insecta: Mantodea)

The sensory systems employed by animals to locate potential mates are diverse. Among insects, chemical and acoustic signals are commonly used over long distances, with visual signals playing a role in close-range orientation and courtship. Within groups that exhibit a scramble competition mating system, selection on mate searching ability will be particularly strong. Clearly, aspects of the species ecology, such as habitat complexity and population density, will be crucial in the evolution of mate searching systems and sexual signals. Praying mantids exhibit both chemical and visual sexual signalling behaviour, and also vary in their ecology. This study employs scanning electron microscopy of antennal sensory morphology and behavioural assays to investigate the relative importance of chemical and visual signalling in two Australian praying mantid species: Pseudomantis albofimbriata and Ciulfina biseriata. As predicted, the high level of habitat complexity, low population density and strong male dispersal capability of P. albofimbriata corresponded to the use of airborne sex pheromones. Conversely, the open habitat, high population density, and poor dispersal of C. biseriata corresponded to a greater reliance on short-range visual cues for mate location.

The scramble competition mating system of the sphecid wasp Palmodes praestans (Kohl)

Males of the wasp Palmodes praestans engage in non‐territorial patrolling behaviour within a scramble competition mating system, showing great fidelity to their wide‐ranging patrolling routes without attempting to monopolize the areas covered. Scramble competition appears adaptive given that mating opportunities are not spatially aggregated in this species, whose females build isolated, single‐celled nests. Because of the female distribution pattern, a male's reproductive success in P. praestans probably depends upon the ability to find highly scattered, unmated females. The females of some close relatives of this sphecid wasp form fairly dense nesting aggregations. As mating system theory predicts, the males of some (but not all) of these species focus their searching at these concentrated nesting/emergence sites where potential mates are clustered spatially.

Interactions between the sexually deceptive orchid Spiculaea ciliata and its wasp pollinator Thynnoturneria sp. (Hymenoptera: Thynninae)

Males of the thynnine wasp Thynnoturneria sp. attempt to mate with female decoys in the flowers of the elbow orchid Spiculaea ciliata. Experimentally shifted orchids usually attract male wasps quickly, often within 2 minutes of presentation of the ‘bait’ orchids in appropriate habitat. Although the orchid effectively exploits the scramble competition mating system of the wasp, the insect is not totally at the mercy of the deceptive orchid. Fewer than half of all arriving males contact the column of the orchid flower, as required for orchid pollination. Moreover, the number of deceived visitors falls sharply over a short period and the number of wasp visitors does not rebound with the replacement of one bait orchid by another at that location. These observations suggest that patrolling wasp pollinators can discriminate to some extent between orchid decoys and female wasps, especially by learning to avoid particular locations that are associated with unrewarding flower decoys.

Resource distribution and its effect on the mating system of a longhorned beetle, Perarthrus linsleyi (Coleoptera: Cerambycidae).

Adults of Perarthrus linsleyi feed on flowers of creosote bush, Larrea tridentata. Mating also occurs on the flowers and foliage of this plant. This food resource is widely and evenly distributed in space, and is usually abundant. The spatial distribution of the beetles bore no relationship to the spatial distribution of flowers among creosote bushes, nor were female beetles distributed predictably with respect to flower distribution. Males moved widely over the area under study, and moved much more frequently than females. Males actively searched for females, and mounted and began copulation attempts without preliminary courtship. Males did not engage in aggressive defense of females, creosote flowers, or creosote bushes. This species exhibits a scramble competition mating system. The spatial distribution of the food resource is a primary factor in the evolution of the mating system of Perarthrus linsleyi.