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Sexual size dimorphism (SSD) is caused by differences in selection pressures and life‐history trade‐offs faced by males and females. Proximate causes of SSD may involve sex‐specific mortality, energy acquisition, and energy expenditure for maintenance, reproductive tissues, and reproductive behavior. Using a quantitative, individual‐based, eco‐genetic model parameterized for North Sea plaice, we explore the importance of these mechanisms for female‐biased SSD, under which males are smaller and reach sexual maturity earlier than females (common among fish, but also arising in arthropods and mammals). We consider two mechanisms potentially serving as ultimate causes: (a) Male investments in male reproductive behavior might evolve to detract energy resources that would otherwise be available for somatic growth, and (b) diminishing returns on male reproductive investments might evolve to reduce energy acquisition. In general, both of these can bring about smaller male body sizes. We report the following findings. First, higher investments in male reproductive behavior alone cannot explain the North Sea plaice SSD. This is because such higher reproductive investments require increased energy acquisition, which would cause a delay in maturation, leading to male‐biased SSD contrary to observations. When accounting for the observed differential (lower) male mortality, maturation is postponed even further, leading to even larger males. Second, diminishing returns on male reproductive investments alone can qualitatively account for the North Sea plaice SSD, even though the quantitative match is imperfect. Third, both mechanisms can be reconciled with, and thus provide a mechanistic basis for, the previously advanced Ghiselin–Reiss hypothesis, according to which smaller males will evolve if their reproductive success is dominated by scramble competition for fertilizing females, as males would consequently invest more in reproduction than growth, potentially implying lower survival rates, and thus relaxing male–male competition. Fourth, a good quantitative fit with the North Sea plaice SSD is achieved by combining both mechanisms while accounting for sex‐specific costs males incur during their spawning season. Fifth, evolution caused by fishing is likely to have modified the North Sea plaice SSD. The paper presents a model to evolutionarily explain the sexual size dimorphism with an eco‐genetic model adapted to detailed empirical data of North Sea plaice. Behavioral investments and diminishing fitness returns are considered as alternative explanations with its implication in individual energy allocation, and put into context with the ecology of the species.

Reproductive males face a trade-off between expenditure on precopulatory male—male competition—increasing the number of females that they secure as mates—and sperm competition—increasing their fertilization success with those females. Previous sperm allocation models have focused on scramble competition in which males compete by searching for mates and the number of matings rises linearly with precopulatory expenditure. However, recent studies have emphasized contest competition involving precopulatory expenditure on armaments, where winning contests may be highly dependent on marginal increases in relative armament level. Here, we develop a general model of sperm allocation that allows us to examine the effect of all forms of precopulatory competition on sperm allocation patterns. The model predicts that sperm allocation decreases if either the \"mate-competition loading,\" a, or the number of males competing for each mating, M, increases. Other predictions remain unchanged from previous models: (i) expenditure per ejaculate should increase and then decrease, and (ii) total postcopulatory expenditure should increase, as the level of sperm competition increases. A negative correlation between a and M is biologically plausible, and may buffer deviations from the previous models. There is some support for our predictions from comparative analyses across dung beetle species and frog populations.

Gopher tortoises spend most of their time in burrows from which they emerge to forage and perform behaviors such as courtship and mating. Previous literature is divided regarding the mating system of this species; some assert that gopher tortoises conform to female defense polygyny, and others assert that scramble competition polygyny is more likely. Here, telemetry data were used to record the frequency with which pairs of tortoises shared burrows and the frequency with which they apparently chased each other from burrows. Additionally, telemetry locations were used to estimate patterns of dispersion of individuals. If gopher tortoises conformed to female defense polygyny, then males should have: (1) moved frequently to share burrows with females, (2) rarely shared burrows with males, (3) infrequently displaced females from burrows, and (4) frequently displaced males from burrows. Similarly, females should have: (1) infrequently moved to share burrows with either sex, and (2) infrequently chased either sex. Also, females should have shown an aggregated dispersion relative to other females. On the contrary, we found that males moved equally frequently to share burrows with adults of both sexes and chased females from burrows more frequently than they chased other males. Females moved more frequently to share burrows with males than with females and chased males more often than they chased other females. Females did not have an aggregated pattern of dispersion relative to other females. These data were most consistent with scramble competition polygyny.

Food competition is a major cost to group living. Resources vary in quality, distribution, and handling times, exerting differing competitive regimes and varied effects on individual food intake depending on dominance rank. To investigate this interplay and the tipping points between purely contest and purely scramble scenarios, we conducted a field experiment on wild vervet monkeys ( ), a species with linear, nepotistic intragroup dominance hierarchies. We baited a multi-destination foraging array with a mixture of clumped, preferred and less clumped, less preferred rewards to observe how individuals' foraging decisions and route choices were affected by the presence and proximity of competitors. In contrast to previous experiments conducted with this group, rewards had minimal handling times and greater quantities to create a mix of scramble and contest competition. We found that neither an individual's dominance rank nor the frequency with which they faced competition from a dominant competitor significantly affected their overall foraging success, suggesting that we were successful in invoking scramble competition. All individuals, regardless of rank, generally chose to prioritize the best reward at the cost of a less efficient route and increased travel time. Nonetheless, encountering dominant competitors in a higher proportion of trials made focal individuals more likely to begin trials at the nearest, less preferred reward, rather than face contest competition for the preferred, more distant platform. Our findings suggest that though greater scramble competition minimizes differences in food intake, risk avoidance still exerts powerful effects on the foraging route choices of those experiencing competition.

1 Root competition is defined as a reduction in the availability of a soil resource to roots that is caused by other roots. Resource availability to competitors can be affected through resource depletion (scramble competition) and by mechanisms that inhibit access of other roots to resources (contest competition, such as allelopathy). 2 It has been proposed that soil heterogeneity can cause size-asymmetric root competition. Support for this hypothesis is limited and contradictory, possibly because resource uptake is affected more by the amount and spatial distribution of resource-acquiring organs, relative to the spatial distribution of resources, than by root system size per se. 3 Root competition intensity between individual plants generally decreases as resource availability (but not necessarily habitat productivity) increases, but the importance of root competition relative to other factors that structure communities may increase with resource availability. 4 Soil organisms play important, and often species-specific, roles in root interactions. 5 The findings that some roots can detect other roots, or inert objects, before they are contacted and can distinguish between self and non-self roots create experimental challenges for those attempting to untangle the effects of self/non-self root recognition, self-inhibition and root segregation or proliferation in response to competition. Recent studies suggesting that root competition may represent a 'tragedy-of-the-commons' may have failed to account for this complexity. 6 Theories about potential effects of root competition on plant diversity (and vice versa) appear to be ahead of the experimental evidence, with only one study documenting different effects of root competition on plant diversity under different levels of resource availability. 7 Roots can interact with their biotic and abiotic environments using a large variety of often species-specific mechanisms, far beyond the traditional view that plants interact mainly through resource depletion. Research on root interactions between exotic invasives and native species holds great promise for a better understanding of the way in which root competition may affect community structure and plant diversity, and may create new insights into coevolution of plants, their competitors and the soil community.

Abstract Multilevel societies (MLSs) are present in several animal clades and our understanding of them is growing at a time with increasing habitat loss. The ways individuals in these complex societies respond to habitat restriction and fragmentation are unknown. We examined ranging and feeding competition in a Ruwenzori Angolan colobus MLS in a forest fragment at Nabugabo, Uganda. We analyzed GPS points collected over 2 years and scan samples collected over 10 months on 12 core units in one band to compare sex- and core unit-specific activity budgets and near-neighbor distances. The 95% home range size estimate of this band was 1.75 km2, much smaller than the ranges of similar-sized bands of primate MLS in continuous habitats. Day range length was also shorter. The colobus rarely utilized the matrix and over the two-year period core units’ home ranges overlapped by 93%. Indicators of scramble competition included more time spent feeding in larger core units, near-neighbors at greater distances when feeding relative to resting, and greater feeding for females compared to males. A quadratic relationship was found between core unit size and home range and core area size, where intermediate-sized units showed the smallest ranges. All units showed similarly tortuous paths; however, the smallest core units had the longest day range lengths and the fastest rates of travel. We conclude that while large core units suffer some food competition, small units may be displaced more in inter-unit contests because they appear to suffer the highest energetic costs searching for food within this constricted area.Significance statementWildlife worldwide is faced with increasing habitat loss and fragmentation. How this impacts animals that live in large, complex societies (e.g., multilevel societies) is an important question to investigate for future conservation. We examined ranging behavior and food competition for a multilevel society of Rwenzori Angolan colobus monkeys (12 core units, 139 monkeys) in Uganda that occupies a relatively small forest fragment. We found scramble competition for food, especially in the largest core units. However, membership in the smallest core units seemed to be the most energetically costly. These units moved the most per day and the fastest, perhaps because they were unable to displace larger core units from food patches. Overall, this multilevel society occupied an area many times smaller than similar-sized primate multilevel societies in continuous habitat and moved far less, suggesting that fragmentation is constraining their range.

Mate-searching success is a critical precursor to mating, but there is a dearth of research on traits and tactics that confer a competitive advantage in finding potential mates. Theory and available empirical evidence suggest that males locate mates using mate-attraction signals produced by receptive females (personal information) and avoid inadvertently produced cues from rival males (social information) that indicate a female has probably already mated. Here, we show that western black widow males use both kinds of information to find females efficiently, parasitizing the searching effort of rivals in a way that guarantees competition over mating after reaching a female's web. This tactic may be adaptive because female receptivity is transient, and we show that (i) mate searching is risky (88% mortality) and (ii) a strongly male-biased operational sex ratio (from 1.2 : 1 to more than 10 : 1) makes competition inevitable. Males with access to rivals' silk trails moved at higher speeds than those with only personal information, and located females even when personal information was unreliable or absent. We show that following rivals can increase the potential for sexual selection on females as well as males and argue it may be more widespread in nature than is currently realized.

Gull chicks beg for food from their parents. Peacocks spread their tails to attract potential mates. Meerkats alert family members of the approach of predators. But are these--and other animals--sometimes dishonest? That's what William Searcy and Stephen Nowicki ask inThe Evolution of Animal Communication. They take on the fascinating yet perplexing question of the dependability of animal signaling systems. The book probes such phenomena as the begging of nesting birds, alarm calls in squirrels and primates, carotenoid coloration in fish and birds, the calls of frogs and toads, and weapon displays in crustaceans. Do these signals convey accurate information about the signaler, its future behavior, or its environment? Or do they mislead receivers in a way that benefits the signaler? For example, is the begging chick really hungry as its cries indicate or is it lobbying to get more food than its brothers and sisters? Searcy and Nowicki take on these and other questions by developing clear definitions of key issues, by reviewing the most relevant empirical data and game theory models available, and by asking how well theory matches data. They find that animal communication is largely reliable--but that this basic reliability also allows the clever deceiver to flourish. Well researched and clearly written, their book provides new insight into animal communication, behavior, and evolution.

For male glow‐worms to achieve a mating, they must detect the bioluminescent glow emitted by females at night and subsequently reach them. Several aspects of male behaviour suggest they engage in scramble competition, with many males striving simultaneously to reach a female first and acquire a mating opportunity. Although male glow‐worms fly during the initial stages of their search, little is known about the final stages of their approach, despite this potentially involving the most intense competition. In a laboratory arena, males combine walking with flying short distances to reach a dummy female (DF; green LED), though walking predominates. To determine if this was representative of behaviour in the field, we used infrared videography coupled with DF traps. Most males landed near a DF, thereafter walking or making short bounding flights to reach it. The time taken by males to reach the DF after their initial landing increased with increasing numbers of males in the vicinity. Combining a DF attached to a wire stand with infrared videography showed that during the final approach, males engage in frequent, typically brief interactions. Greater numbers of males in the vicinity reduced the rate at which they reached the DF. After reaching the DF, males frequently fell in clusters, ending their mating opportunity. Our results show that the final stage of the males' approach to a DF (and we infer to females) is dominated by walking and influenced by interactions with other males, consistent with features of scramble competition found in other polygynous insects, including fireflies. Our findings offer novel insights into the life history of male glow‐worms and, more generally, features of scramble competition in insects. Using an observational approach in field and laboratory settings we investigated male glow‐worms' approach to females to determine whether their behaviour is consistent with scramble competition. Our results show that males combine flying and walking, favouring the latter during the final stage of their approach to females. Our data also show that males frequently interact with one another, reducing the rate at which they reach the female.

Aedes aegpyti mosquitoes are vectors of several viruses of major public health importance, and many new control strategies target mating behaviour. Mating in this species occurs in swarms characterised by male scramble competition and female choice. These mating swarms have a male‐biased operational sex ratio, which is expected to generate intense competition among males for mating opportunities. However, it is not known what proportion of swarming males successfully mate with females, how many females each male is able to mate with, and to what extent any variation in the male mating success phenotype can be explained by genetic variation. Here, we describe a novel assay to quantify individual male mating success in the presence of operational sex ratios characteristic of Ae. aegypti. Our results demonstrate that male mating success is skewed. Most males do not mate despite multiple opportunities, and very few males mate with multiple females. We compared measures of male mating success between fathers and sons and between full siblings to estimate the heritability of the trait in the narrow h 2$$ \\left({h}^2\\right) $$and broad H 2$$ \\left({H}^2\\right) $$sense, respectively. We found significant broad sense heritability estimates but little evidence for additive genetic effects, suggesting a role for dominance or epistatic effects and/or larval rearing environment in male mating success. These findings enhance our understanding of sexual selection in this species and have important implications for mass‐release programmes that rely on the release of competitive males.