Fighting success and attractiveness as predictors of male mating success in the black field cricket, Teleogryllus commodus: the effectiveness of no-choice tests

Females are generally assumed to prefer larger, more dominant males. However, a growing number of studies that control for male-male competition have shown no correlation between dominance and attractiveness. Aggressive males can interfere with female mate preference either by physically coercing females into mating or by driving submissive males away and restricting mate choice. The most common method of assessing female mate choice is by using simultaneous two-choice tests. These control for male-male interactions, but usually interfere with physical and chemical cues involved in mate selection or alter male behaviour. They are therefore unsuitable for many study species, especially insects. Another method is the no-choice test that measures a females latency to mating when placed with a single male as an indication of male attractiveness. No-choice tests control for male-male aggression while allowing full contact between pairs (they allow actual mating to be directly observed rather than to occur based on a correlated behaviour). So far, however, no study has confirmed that males that entice females to mate sooner actually enjoy increased longer-term mating success. As such, the accuracy of no-choice tests as a method of examining mate choice remains untested. Here, we used no-choice tests on the black field cricket, Teleogryllus commodus, to show that (1) females did not prefer males that won fights (dominant males), and (2) latency to mating predicts actual mating success. We have clearly demonstrated the usefulness of no-choice tests and, considering the advantages of this method, they should be more often considered for a wider variety of taxa.Communicated by D. Gwynne     

Costs and consequences of variation in the size of ruff leks

Summary We studied 13 ruff leks in a small region on the island of Gotland (Sweden) to investigate the effect of lek size on the costs and benefits of lekking for individual males. Male ruffs occur in two behaviourally and morphologically distinct forms, independents (residents plus marginals) and satellites, whose costs and benefits we have assessed separately. These ruff leks had from 1–10 resident (territory-holding) males and were visited daily by satellites, marginals and females from 5–25 May, when most copulations occurred. We used the average number of independent males, counted during censuses taken every 5 min during 2-h observation periods at each lek, as an index of mean lek size. Per independent male, the numbers of both satellites and females increased significantly with mean lek size. Female arrival rate and attendance (total female-minutes) also increased significantly with mean lek size as did the average per capita rate of mating success for resident males (that of satellites was not quite significant). Thus, the dispersion of both of these male categories did not appear to fit an ideal free distribution with respect to mating success. In addition, the number of independent-independent fights per independent and the rate of satellite-resident dyad formation per resident increased significantly with mean lek size. These results suggest that ruffs on larger leks enjoy higher mating success than those on smaller leks but also that costs increase with lek size. We suggest that independent males distribute themselves so as to maximize their own net benefits and that this factor can account for both the occurrence of ruff leks and the variation in their size. Correspondence to: J. Höglund     

Correlates of male mating success in the ruff Philomachus pugnax,a lekking shorebird

Summary Male ruffs (Philomachus pugnax), a lekking shorebird, can generally be divided into two morphs according to behavior and plumage coloration. Predominantly dark-colored, independent male ruffs defend small territories on a lek, whereas lighter colored satellite males are non-territorial and less site-faithful to a particular lek. The traits associated with the mating success of independent male ruffs were studied at two leks during two seasons on the island of Öland, Sweden in May and June of 1985 and 1987. Using multivariate analyses, three characteristics were found to be significantly related to mating success: high frequency of visits by satellites to an independent male's residence, consistency of lek attendance, and low rates of aggressive behavior. In contrast, mating success was unrelated to the degree of darkness of the independent male nuptial plumage, territory location on the lek, or rate of courtship displays. The use of multivariate analyses helped to resolve conflicting conclusions from previous studies which employed simple statistical analyses, or none at all. Experimental manipulations are suggested which will help to further determine the influence of female mate choice in this lekking system.     

Dispersal-generated sexual selection in a beetle-riding pseudoscorpion

Summary After several generations within a decaying tree (Ficus spp.), populations of the pseudoscorpion Cordylochernes scorpioides disperse by climbing under the elytra of harlequin beetles (Acrocinus longimanus) eclosing from the tree. Because the beetles then fly to newly-decaying Ficus for mating and oviposition, they act as effective dispersal agents. Field experiments and observations indicate that this dispersal mode has been exploited by males who compete to remain on a beetle as a strategic site for inseminating females dispersing on it and on other beetles. Whereas beetles just eclosed from old trees carried large, female-biased groups of dispersing pseudoscorpions, beetles captured after their maiden flight generally carried a single large male. Multivariate morphometric analyses indicated that these beetle-riding males were much larger than individuals randomly sampled from trees, with fighting traits exhibiting the greatest potential for selection (highest phoretic differentials). In females, sexual receptivity was significantly higher at the beginning of dispersal than at the end, suggesting that mating occurs on beetles. Field experiments confirmed that on-beetle insemination does take place and that small males are displaced from the subelytral space by larger rivals. By contrast, laboratory experiments suggest that large male size may not confer high mating success under the low-density conditions which characterize populations within trees. Thus, in addition to indicating a novel role for dispersal in the evolution of exaggerated male traits, this study suggests that oscillating sexual selection may be important in the maintenance of the extreme phenotypic variation exhibited by males of this species.     

Correlates of male mating success in two species of Anthidium bees (Hymenoptera : Megachilidae)

Summary Field observations were made on the mating behavior of two congeneric species of solitary bees, Anthidium porterae in an arid grassland and A. palliventre in a coastal sand dune habitat. Males of both species exhibited resource defence polygyny and defended hostplants to gain access to females foraging for nectar and pollen. The mating frequencies of marked and measured resident (territorial) males were monitored during periods of continuous observation, following which measurements of territory size and floral resources were obtained. Mating success of A. palliventre males was strongly influenced by territory characteristics: Males that defended small areas with a few rich hostplant patches mated more often than males that held larger territories containing many hostplant patches of low floral density. Large males generally held high-quality sites and thus had a mating advantage over smaller individuals. In A. porterae, on the other hand, male mating success was unrelated to any measure of territory quality. Copulation frequency and male size were positively correlated, however, apparently due to the increased ability of large males to seize and hold females for mating. The two species also differed in the incidence of non-territorial, sneaky males. While absent in A. palliventre, sneaky males accounted for 12% of all mating observed in A. porterae. Males of A. porterae that displayed sneaky tactics mated, on average, as often as resident males. Offprint requests to: E.M. Villalobos     

Size-assortative mating and non-reciprocal copulation in a hermaphroditic intertidal limpet: test of the mate availability hypothesis

Mating behaviour and mating patterns are affected to a large extent by body size in both hermaphrodites and gonochorists. Detailed research on mating patterns, mate choice, pre- and post-copulatory sexual selection in marine pulmonates is wanting, thus warranting more attention as a study system in the future. The simultaneous hermaphroditic limpet Siphonaria capensis (Pulmonata: Basommatophora) shows size-dependent fecundity, and acts as a suitable organism to test the effects of body size on mating patterns, mating success, gender expression and reciprocity in hermaphroditic marine gastropods. We mainly used bootstrap resampling techniques to estimate the effects of different factors on mating patterns. In the populations studied, a strong size-assortative mating pattern was observed, where small-scale spatial distribution of potential mating partners (the mate availability hypothesis) could explain 65% of this pattern, while mate choice and mating constraints explaining the rest. No significant difference in mating success between limpets with different body size was found, even though in one population, the sperm recipients were larger than the non-copulating limpets. Interestingly, we found that intromission was non-reciprocal during copulations. This may mean that this species does not conform to the common rule of reciprocity predicted for hermaphrodites, unless there is sex-role alternation between individuals in a mating pair. The mating partners consisted of similar sized, acting males and females, thus without any indication of the body size determining the gender expression. The matings took place in early mornings only at spring tides and the animals were observed laying egg masses only during neap tides. Since siphonariid limpets possess both a spermatheca and a seminal vesicle, which may have either a sperm storing and/or digestive function, post-copulatory sexual selection (e.g., sperm competition) cannot be dismissed.     

Parental care of nestlings by male red-winged blackbirds

Summary Nestling feeding by males is less common among birds with polygynous mating systems than in monogamous species, because of the pronounced trade-off between parental behavior and the attraction of additional mates. In this study, however, we found that male red-winged blackbirds (Agelaius phoeniceus) commonly assisted females in feeding nestlings in several Ontario marshes. Male parental care was additional to that provided by females, and it significantly enhanced the fledging success of nests (Table 2). Male redwings did not help to feed all nests on their territories: primary and secondary nests were much more likely to receive male parental care than tertiary and later nests. Contrary to expectation, male parental care was not restricted to the nests of primary females: a greater proportion of secondary than primary nests were assisted (Tables 4a and b). The presence of new females settling on the territory at the same time that a resident female had nestlings, resulted in males deferring parental care until a later brood. This suggests that males trade off the recruitment of females against parental care to an existing brood. Although the number of nestlings fledging from a male's territory was strongly influenced by the number of females in the harem, males could additionally increase their reproductive success by feeding nestlings in one or more nests on their territories (Fig. 2). The reproductive success of females was significantly enhanced by male assistance in feeding nestlings (Table 3). However, those females not receiving male assistance on territories of feeding males did not suffer a significantly reduced reproductive success in comparison to females on territories of non-feeding males (Table 2). Males varied considerably in the quality of brood care given. We therefore suggest that the quality of male parental care may be a factor considered by females in choosing a breeding situation.     

Polyandry and its effect on female reproduction in the green-veined white butterfly (Pieris napi L.)

Summary In many insects nutrients transferred by the male to the female at mating are later incorporated into both the eggs and soma of the mated females. Accordingly, it has been suggested that female insects can use these male-derived nutrients both for somatic maintenance and to increase both the number and quality of their offspring. Moreover, much discussion is presently devoted to whether the male nuptial gift represents paternal investment, defined as any increase in given male's total surviving progeny by increasing the reproductive output by a given female, or mating effort which obtains if a male gains by increasing the proportion of eggs he fertilizes from a given female (Parker and Simmons 1989). If the male nuptial gift represents parental investment it should be expected to benefit predominantly the offspring sired by the donor, whereas the physiological fate of the male nuptial gift is somewhat irrelevant under the mating effort explanation. In this paper we test these issues by studying the lifetime fecundity, egg weights and longevity of two groups of females of the polyandrous green-veined white butterfly, Pieris napi, one group of which was allowed to mate only once and the other of which was allowed to mate at liberty, the latter group of females mating on average 2.28 times. Moreover, to test the incorporation rate of male-derived nutrients, we performed a second set of experiments where females were allowed to mate with radioactively labelled males. The results showed that polyandrous females had higher lifetime fecundity compared to monandrous females, laying on average 1.61 as many eggs, and that the difference in cumulative fecundity between the two groups was statistically significant from the 5th day of egg-laying onwards. Polyandrous females also lived longer and maintained egg weight at a high level for longer than monandrous females. Largely concomitant with egg-laying rate, incorporation rate of male-derived nutrients peaked 3–4 days after mating, subsequently tapering off to stabilize at about 40% of the maximum. Given the opportunity, female P. napi remated after 3–5 days, the duration of the refractory period being positively correlated with ejaculate mass. Hence, although the nutrient investment of the first male to mate with a female subsidizes the progeny of later-mating males, the male nuptial gift in P. napi clearly qualifies as both paternal investment and mating effort. Correspondence to: C. Wiklund     

“Alternative” mating behavior in the swordtails Xiphophorus nigrensis and Xiphophorus pygmaeus (Pisces: Poeciliidae)

Summary The swordtail Xiphophorus nigrensis exhibits three relatively discrete male body-size classes that derive from allelic variation at the Y-linked pituitary (P) locus. Previous studies have shown that larger males have greater relative reproductive success, and that females prefer large males. We describe the mating behavior utilized by males of each size class during individual encounters with females. Small males rely on chase behavior, similar to the alternative mating behavior classified as sneaker in small males of other species. Large males court and intermediate-sized males court or chase, depending on their body size. There is a strong correlation between P alleles for small size (s) and large size (L) with chasing and courting, respectively. The relationship between mating behavior of males of the genotype I is ambiguous. In the closely related species X. pygmaeus, males are of size similar to smaller X. nigrensis males. Paradoxically, these males do not show the courtship display that typifies larger male X. nigrensis and many other species of swordtails, but instead often employ chase behavior identical to the alternative mating behavior in small male X. nigrensis. We suggest that historical and genetic constraints, in addition to current selection forces, might be important factors in explaining the existence of alternative mating behavior in X. pygmaeus.     

Sperm competition in the damselfly Enallagma hageni Walsh (Odonata: Coenagrionidae): benefits of multiple mating to males and females

Summary Sperm competition was investigated in the non-territorial damselfly, Enallagma hageni. Using irradiated (sterile) male techniques, the last male to mate was found to fertilize up to 95% of the eggs of the first clutch laid after mating. Dissection of females collected before, during, and after copula showed that a male removes a maximum of 87% of the sperm by volume of a previous mate. These data verify an earlier estimate of lifetime reproductive success in this species which was based on mating success, and suggest that indirect dissection methods offer minimum estimates of sperm precedence. Male E. hageni have ample opportunity to benefit from sperm precedence, since at least 10% of the receptive females encountered had already mated once that day, but still contained complete or partial clutches of eggs. Female E. hageni benefit directly from high sperm precedence because it allows them to exchange matings for guarding service by males during oviposition bouts under water.     

Mate choice and fitness in a hybrid frog: Rana esculenta females prefer Rana lessonae males over their own

Summary The evolution and maintenance of female choice based on purely genetic differences is still a controversial issue, not only for theoretical reasons, but also because of the practical difficulty of demonstrating the fitness consequences of preferences and heritability of and genetic variability in the chosen traits. We argue that hybrid systems (broadly defined) offer suitable models for studying mate choice according to genetic differences. We present such a study for European water-frogs of the hybridogenetic Rana lessonae/Rana esculenta complex (L/E complex). R. esculenta, originally a hybrid between R. lessonae and R. ridibunda, eliminates the L genome premeiotically and only produces eggs and sperm containing only the R. ridibunda (R) genome. Consequently, the hybrid will only persist when it lives and mates with R. lessonae in mixed populations where it can regain the lost L genome. In such mixed populations, there is strong selection against E x E matings. because these will produce no viable offspring. We tested whether females of the hybrid R. esculenta do indeed avoid their own R. exculenta males and choose males of the parental species R. lessonae instead. Eleven E females were offered a simultaenous choice between one L and one E male. Females exhibited a significant preference for L males that was determined by the type of male, rather than by its size or activity. This choice is in the direction predicted from genetics. The question of why L males agree to mate with E females, but L females only rarely mate with E males, is answered by a sexual asymmetry in the cost/benefit ratios of mating with the wrong type and the right size. Our results are consistent with the mating pattern found in natural populations, but further studies are needed to show that female choice really causes this pattern.Correspondence to: H.-U. Reyer     

Sex ratio,monogamy and breeding success in the Midas cichlid (Cichlasoma citrinellum)

Summary The Midas cichlid (Cichlasoma citrinellum) is a large, aggressive monogamous fish from the Great Lakes of Nicaragua. It lives in an ecosystem where breeding sites are at a premium and where breeding success can be low. I tested the strength of the pair bond by manipulating the sex ratios in captive groups, using ratios of 1:1, 2:1, 4:1, 1:2 and 1:4 (:). No polygamy was seen at any of the ratios. Departure from a ratio of 1:1 decreased breeding success for those pairs that formed. Both sexes appeared to enforce the pair bond but did so in different ways: females attacked males and females equally but males attacked other males significantly more than they did females. These results, coupled with field observations, indicate the Midas cichlid is obligated to monogamy by the demands of competition for breeding sites, the need for two parents to protect the fry and by energetic constraints.     

Sterols in ocean sediments: novel tracers to examine habitats of cetaceans,pinnipeds, penguins and humans

This paper contributes to the understanding of the evolution of alternative mating strategies by comparing morphological, ecological and ethological aspects of the reproductive systems of two closely related fish species studied near the marine biological station STARESO at Calvi, Corsica, between March 1982 and May 1985. The Mediterranean breeding areas of Tripterygion tripteronotus and T. delaisi partly overlap. The red territorial males of T. tripteronotus defend territories in the upper 6 m of the water column and the yellow territorial males of T. delaisi breed between a depth of 3 and 40 m. Small male T. tripteronotus possess relatively large gonads exhibit sneaking behaviour, while small male T. delaisi do not participate in mating. T. tripteronotus expends more effort in reproduction than T. delaisi. This is expressed in larger gonads, a higher density of and stronger competition for nest sites, and a longer breeding season and shorter spawning bouts with a higher fertilization rate in the former species. We hypothesize that the sneaking strategy of non-territorial male T. tripteronotus evolved in response to competition for nest sites in the shallow, upper water layers, which are limited in depth, but contain a good food supply. The depth restriction is imposed by the light-reflecting properties of the red territorial males. We suggest that the evolution of T. tripteronotus began with the appearance of a red morph of T. delaisi after the invasion of this latter species into the tideless Mediterranean Sea.     

Mate choice in the yellow-shouldered widowbird: correlates of male attractiveness

Recent investigations of male ornaments in sexual selection have used experimental manipulation of tail length in three widowbird species, but only for one of these have correlates of male reproductive success been reported. I examined correlates of male attractiveness to nesting females over two breeding seasons for the polygynous yellow-shouldered widowbird, Euplectes macrourus, in order to discover which cues females may be using to select mates. The black, long-tailed ( 10 cm) males defend large territories and build nest frames, or cock's nests, which females then line and use for nesting. I examined various aspects of male morphology, five behavioral displays, territory characteristics, and the number of cock's nests that males built. Few correlates of mating success were found. The best predictor was the number of cock's nests that a male builds, though one courtship display also correlated with male mating success in 1 year, as did average grass height. Tail length did not correlate with male mating success. A partial correlation analysis confirmed that cock's nests and, in 1 year, grass height, were the primary contributers to male success. Females may choose where to nest primarily on the availability of suitable nesting sites. Long tails may be used by females seeking extra-pair copulations or in male-male competition for territories.     

Experimental studies on water strider mating dynamics: spatial variation in density and sex ratio

Summary We used field surveys, field experiments and experiments in artificial pools to study the effects of variation in sex ratio and density on mating dynamics of a stream water strider, Aquarius remigis. Our field survey documented the existence of hot spots, sites of higher than average total gerrid density, a male-biased sex ratio, and higher than average female mating activity. Female gerrids frequently changed sites, usually moving upstream, perhaps to spread their eggs among many sites. Male gerrids showed two movement strategies: some males frequently changed sites, while other males were stationary at hot spots. Surprisingly, smaller males tended to be stationary at hot spots. A field manipulation of the availability of refuges for females to avoid harassment by males supported the notion that males prefer hot spots because they are sites where a scarcity of refuge for females makes it relatively easy for males to intercept females. Experiments in plastic pools compared sites with 20 males: 5 females (simulating hot spots) to pools with 5 males: 5 females. The rate of male harassment of females was higher in 20:5 pools as compared to 5:5 pools. In response to increased male harassment, females reduced their activity on the water and increased their time spent out of the water and thus unable to forage. Males showed a large male mating advantage (LMMA) in 5:5 pools, but, surprisingly, not in 20: 5 pools. This pattern can explain the field observation that small males prefer hot spots. A behavioral mechanism that can explain the LMMA is as follows. Mating occurs when males overcome female resistance. Larger males have a mating advantage over smaller males if females resist heavily. Increased harassment (e.g., in 20:5 pools as compared to 5:5 pools) might result in reduced female resistance to males and thus a reduced LMMA. Females also showed some non-random mating by size that might reflect an interplay between male preference for large females and the avoidance of males by large females. Correspondence to: A. Sih     

Protandry and mate assessment in the wartbiter Decticus verrucivorus (Orthoptera : Tettigoniidae)

Summary Protandry, or early male emergence, is prevalent in the majority of insects. There are several explanations, both adaptive and incidental for this widespread phenomenon. Here I report the occurrence of protandry in the wartbiter, Decticus verrucivorus, and argue that the explanation for the evolution of protandry in this species is the result of selection acting on males in relation to sperm competition. It has previously been shown that sperm mixing occurs in this species, leading to a reduction in confidence of paternity with number of matings. Emerging early and thereby having a higher probability of mating with virgin females benefits males in two ways: both through a 100% assurance of paternity and because female egglaying rate decreases over time. A singly mated female can lay up to 30% of her lifetime egg production during her first refractory period, during which time the eggs are exclusively fertilized by the first male. Wartbiter males seem also to be able to assess female mating status and transfer larger spermatophores when mating with virgin females than when mating with already mated females. This holds true both for previously mated and virgin males. Thus, it seems that male wartbiters allocate their resources differentially depending on female quality.     

Clutch loss affects the operational sex ratio in the strawberry poison frog Dendrobates pumilio

Both modelling and field data from three breeding seasons show that an environmental factor, clutch loss (CL), affects the operational sex ratio (OSR) and therefore male mating frequency in strawberry poison frogs. Clutch loss affects the length of reproductive cycles of both sexes: with increasing clutch losses, males spend proportionately more time than females in parental investment activities. Because of this, males spend relatively less time in the mating pool, i.e. exhibit proportionately more time-out than females in comparison to a situation with low or no clutch loss. Hence, clutch loss leads to a less male-biased OSR, coupled with a decrease in the opportunity for sexual selection. Furthermore, this study resolves an apparent paradox, the negative correlation between mating frequency and reproductive success (=number of produced tadpoles) of individual males in one breeding season. Clutch loss decouples the correlation between mating frequency and reproductive success because females re-enter the mating pool when they lose their offspring. However, clutch loss diminishes the reproductive output. Similar consequences of clutch loss on the OSR may be true for many species where both sexes reproduce frequently in one breeding season.Communicated by J. Christensen-Dalsgaard     

Active mate choice at cock-of-the-rock leks: tactics of sampling and comparison

Summary The importance of active mate choice as a component of sexual selection remains controversial, despite recent studies identifying traits that correlate with male mating success. We analyzed the sampling sequences of individual female Guianan cock-of-the-rock, providing quantitative documentation of active female choice in this lekbreeding species. We compared observed sampling patterns with null models of random female movement and of movement weighted by the observed distribution of male success. The courts actually visited by individual females were significantly more clumped and centrally located than predicted by computer simulations of either random or weighted movement. Contrary to simplified hypotheses of female choice, hen cock-of-the-rock were not limited either to sequential comparisons or to threshold decision rules when selecting mates. Instead, they repeatedly sampled a subset of potential mates, using the pool-comparison tactic. Within this pool, females preferentially mated with the males ranked highest in terms of total courtship visits received, and rejected low-ranking males. Our results suggest complex interactions between patterns of female sampling and male dispersion in lek mating systems, and also provide insights into the constraints on female sampling in lek species.     

The role of learning in the mating biology of a sweat bee Lasioglossum zephyrum (Hymenoptera: Halictidae)

Summary Results from experiments on the role of learning in the mating biology of a sweat bee, Lasioglossum zephyrum (Hymenoptera: Halictidae), are described in this paper. Male learning of individual female odors is important in natural populations (Table 1), as has been shown in the laboratory. Four other hypotheses are unlikely to account for the observed behavior: (1) Female odors dissipate rapidly; (2) Males learn and then avoid the study area; (3) Males or females produce repellents which are effective against other males; or (4) males recognize their own odor on previously-contacted females, which they subsequently avoid. Regarding questions of male preference and optimal outbreeding, Tables 2 and 3 show there are no consistent preferences for more novel or less novel female odors.     

Extra-pair copulations in a lek: the secondary mating system of monogamous razorbills

Summary I studied the mating strategies of razorbills Alca torda, a monogamous colonial seabird, during 1987–1989 in Wales, U.K. The outstanding feature of the study population's mating system was the performance of most copulations in mating arenas outside the colony. Females visited the arenas and sometimes accepted extra-pair copulations (EPCs), and males aggressively competed for EPCs. During the fertilizable period of most females, the arenas resembled leks because they contained no resources except mates, and males contributed only sperm to extra-pair females. Typical of leks, the operational sex ratio was strongly male-biased, and male mating success was highly skewed. To examine the potential for sexual selection to operate in this system, I correlated male EPC success with variables that could be associated with male-male competition and/or female choice. The frequencies of three behaviors (interference of copulation attempts, fights won against other males, and EPC attempts), accounted for 62% of the variance in EPC success and were strongly intercorrelated, with interference explaining most of the variance in a multivariate test. EPC success was not correlated with body size, age, paired status or either of two estimates of the time spent in the arena. The hypothesis that male EPC success was determined by active female choice was tested by examining the relative success (EPCs per attempt) of males. Relative success was not correlated with any of the three behaviors associated with absolute success, or with age, body size or attendance, suggesting that male-male competition, mainly in the form of copulation interference, is the principal correlate of EPC success. These findings in a monogamous species illustrate that EPC is a secondary mating system of razorbills in which sexual selection operates. The existence of this system outside the colony raises the possibility that lekking for EPCs may also occur within the nesting territories of other monogamous, colonial species but is hidden by competition for nests and breeding partners.