Ornamentation predicts reproductive success in female pipefish

In the sex-role reversed pipefish Syngnathus typhle females compete for access to males and males are choosy. Females develop a temporary ornament when competing over mates with other females and when performing nuptial dances with males. This ornament is an amplification of the normal striped pattern in these fishes. We here show experimentally that (1) the contrast of this normal pattern forecasts the extent to which the ornament is shown, (2) contrast and ornamentation honestly signal female quality (egg numbers), (3) contrast and ornamentation accurately predict female mating success, (4) contrast is a phenotypically plastic trait specifically exaggerated under situations of female – female competition, and (5) neither contrast nor ornament are energetically expensive to the females (i.e., they are independent of short-term nutritional status). Hence, as predicted in sex-role reversed species, ornament design is constrained by costs to female fecundity: an energetically demanding ornament would impair on a female's ability to produce eggs. The type of ornament described here is the expected one, costly for reasons other than being energetically expensive to produce. Received: 4 April 1996 / Accepted after revision: 27 October 1996     

Female mate choice in the non-territorial lizard Ameiva plei (Teiidae)

Mate choice by females has been documented in a variety of taxa. Female mate choice in species lacking male resource control or paternal care might occur if preferred males provide protection from harassment. Female mate choice was investigated in a natural population of the non-territorial lizard Ameiva plei (Teiidae). Consort pairs were allowed to form naturally. Consort males were significantly larger than non-consort males. After removal of consort males, the “abandoned” female's reaction to the first male who approached her was recorded. Females rejected all small males. Female preference for large males was significantly higher than preference for small males. Large males may be better equipped to guard the females from harassment and behavior of large males is less harassing than behavior of small males, thereby affording the female increased foraging time. Received: 21 June 1996 / Accepted after revision: 28 December 1996     

Lack of detectable nepotism in multiple-queen colonies of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Multiple-queen (polygyne) colonies of the introduced fire ant Solenopsis invicta present a paradox for kin selection theory. Egg-laying queens within these societies are, on average, unrelated to one another, and the numbers of queens per colony are high, so that workers appear to raise new sexuals that are no more closely related to them than are random individuals in the population. This paradox could be resolved if workers discriminate between related and unrelated nestmate sexuals in important fitness-related contexts. This study examines the possibility of such nepotism using methods that combine the following features: (1) multiple relevant behavioral assays, (2) colonies with an unmanipulated family structure, (3) multiple genetic markers with no known phenotypic effects, and (4) a statistical technique for distinguishing between nepotism and potentially confounding phenomena. We estimated relatedness between interactants in polygyne S. invicta colonies in two situations, workers tending egg-laying queens and workers feeding maturing winged queens. In neither case did we detect a significant positive value of relatedness that would implicate nepotism. We argue that the non-nepotistic strategies displayed by these ants reflect historical selection pressures experienced by native populations, in which nestmate queens are highly related to one another. The markedly different genetic structure in native populations may favor the operation of stronger higher-level selection that effectively opposes weaker individual-level selection for nepotistic interactions within nests. Received: 28 June 1996 / Accepted after revision: 6 October 1996     

Variation in foraging effort by lactating Antarctic fur seals: response to simulated increased foraging costs

Seasonally breeding predators, which are limited in the time available for provisioning young at a central location, and by the fasting abilities of the young, are likely to maximize energy delivery to the young by maximizing the rate of energy delivery averaged over the whole period of investment. Reduction in food availability or increased foraging costs will alter the optimal behavior of individuals. This study examined the behavioral adaptations of a diving predator, the Antarctic fur seal, to increased foraging costs during lactation. One group of mothers (n=5, treatment) was fitted with additional drag to increase the cost of transport in comparison with a control group (n=8). At the scales of the individual dives, the treatment group made more shorter, shallower (< 30 m) dives. Compensation for slower swimming speeds was achieved by diving at a steeper angle. Overall, diving behavior conformed to several specific theoretical predictions but there were also departures from theory, particularly concerning swimming speed during diving. Diving behavior appears to be adjusted to maximize the proportion of time spent at the bottom of dives. At the scale of diving bouts, no difference was observed between the treatment and control groups in terms of the frequency and duration of bouts and there was also no difference between the two groups in terms of the proportion of time spent diving. At the scale of complete foraging cycles, time taken to return to the pup was significantly longer in the treatment group but there was no difference in the rate of delivery of energy (measured from pup growth rate) to the pups in each group. Since mothers in the treatment group did not use significantly more body reserves, we conclude that behavioral adjustments at the scale of individual dives allowed mothers in the treatment group to compensate for the additional foraging costs. Pup growth rate appears to be less sensitive to the foraging conditions experienced by mothers than foraging trip duration. Received: 14 June 1996 / Accepted after revision: 16 November 1996     

Sexual cohabitation as mate-guarding in the leaf-curling spider Phonognatha graeffei Keyserling (Araneoidea, Araneae)

The leaf-curling spider Phonognatha graeffei incorporates a twisted leaf into the central hub of its orb-web that is used as a retreat. This species is unusual among orb-weaving spiders because males cohabit in the leaf retreat with both immature and mature females, mating with the former shortly after the female molts. Cohabitation appears to be a form of mate-guarding because cohabiting males respond agonistically to rival males that venture onto the web, and their behaviour depends upon the reproductive status of the female; males defending immature females are more aggressive than those defending virgin, adult females. Males copulate with previously mated females for significantly longer than with virgin females. Females may cannibalise cohabiting males, which occurs independently of whether the female has been deprived of food. Females that cannibalise a single male do not have a higher fecundity than non-cannibalistic females. Received: 2 February 1996 / Accepted after revision: 27 October 1996     

Correlates of extra-pair fertilization success in hooded warblers

We examined correlates of extra-pair fertilization (EPF) success in the hooded warbler (Wilsonia citrina), a species where females pursue extra-pair matings. The good genes hypothesis predicts that females choose extra-pair mates with morphological or behavioral traits that reflect differences in male genetic quality. EPFs were common, as 35.3% (42/119) of broods contained extra-pair young (EPY) and 26.7% (95/356) of nestlings were the result of EPFs. There was a strong skew in male EPF success which increased the variance in annual male mating success 2–3 fold. However, male morphology did not predict EPF success, as extra-pair males were not older or larger than the males they cuckolded. Likewise, there were no significant correlations between the proportion of extra-pair young in a brood and male size or age. The good genes hypothesis predicts that high-quality males will be consistently preferred as genetic mates, but the number of young sired by a male with his social mate was not consistent from one year to the next. There was a significant negative correlation between female age and proportion of EPY produced, which could result if older females obtain higher-quality social mates. We found no strong evidence that females choose extra-pair mates for good genes, but females may use behavioral rather than morphological cues to assess relative male quality. Received: 31 May 1996 / Accepted after revision: 17 October 1996     

Magpies' tails: damage as an indicator of quality

Quality-indicating sexually selected traits may have their honesty maintained by their costs or by an inherent “revealing” nature. Long tails in birds are usually considered to be costly “handicaps”, but may have additional potential as revealing indicators through the incidence of breakage. Magpies Pica pica with unbroken and less abraded tails paired earlier, but did not nest or fledge young earlier than pairs with tails in poorer condition. Pairs mated assortatively by tail quality, and magpies with very broken tails remained unmated. Pairs in which both members had almost undamaged tails fledged more offspring than pairs with poorer tails. Tail quality did not correlate with the extent of any habitat type in the territory. Tail damage thus honestly indicated a magpie's reproductive potential, and the data are consistent with its having a role in mate choice, as a revealing element of tail morphology. Received: 28 March 1996 / Accepted after revision: 9 December 1996     

Control of reproduction in social insect colonies: individual and collective relatedness preferences in the paper wasp, Polistes annularis

Social insect colonies often have one or a few queens. How these queens maintain their reproductive monopoly, when other colony members could gain by sharing in the reproduction, is not generally known. DNA microsatellite genotyping is used to determine reproductive interests of various classes of colony members in the paper wasp, Polistes annularis. The relatedness estimates show that the best outcome for most individuals is to be the reproductive egg-layer. For workers, this depends on the sex of offspring: they should prefer to lay their own male eggs, but are indifferent if the queen lays the female eggs. The next-best choice is usually to support the current queen. As a rule, subordinates and workers should prefer the current queen to reproduce over other candidates (though subordinates have no strong preference for the queen over other subordinates, and workers may prefer other workers as a source of male eggs). This result supports the theory that reproductive monopoly stems from the collective preferences of non-reproductives, who suppress each other in favor of the queen. However, we reject the general hypothesis of collective worker control in this species because its predictions about who should succeed after the death of the present queen are not upheld. The first successor is a subordinate foundress even though workers should generally prefer a worker successor. If all foundresses have died, an older worker succeeds as queen, in spite of a collective worker preference for a young worker. The results support the previous suggestion that age serves as a conventional cue serving to reduce conflict over queen succession. Received: 3 May 1996 / Accepted after revision: 22 September 1996     

Non-adaptive adoptions of nestlings in the colonial lesser kestrel: proximate causes and fitness consequences

We examined the causes, costs and benefits of adoption in the altricial lesser kestrel Falco naumanni. Specifically, we tested the intergenerational conflict hypothesis, proposed to explain adoption in some birds. Adoptions involved 76% of the nests and 51% of the nestlings at a mean age of 25 days (12 days before fledging). Nest-switching nestlings were not in poorer body condition, more parasitized or younger than their siblings, and body condition and prey delivery rates of their parents did not differ from those of other parents. In the foster nest, adopted nestlings did not benefit from higher feeding rates or a prolongation of the nestling period. They did not have fewer nest-mates or achieve higher rank within the new brood. Thus, adopted nestlings did not improve their body condition and survival. Adult lesser kestrels seemed unable to finely discriminate beween their own and alien chicks. Foster parents bore the cost of an increase of prey delivery rates, although it did not affect their survival or subsequent reproductive performance. Therefore, our results do not support the intergenerational conflict hypothesis, and suggest that adoption in this species is non-adaptive. Traditionally, the lesser kestrel bred in cliffs where movement among nest-sites was restricted. Nowadays, about half of the colonies are in tiled roofs which facilitate nest-switching by nestlings. The high rate of adoptions may thus be explained as reproductive errors associated with the recent occupation of a new breeding habitat. Received: 3 May 1996 / Accepted after revision: 19 January 1997     

Dominance and the dynamics of phenotype-limited distribution in common cranes

We studied the behavior of 13 radiotagged cranes dispersing from a communal roost over days when they changed their main daily foraging area between consecutive days during two winter seasons. Individuals went to a new foraging zone when on the previous day their morning food intake had fallen below their mean morning food intake measured over the whole winter. Food intake on the day before a change in foraging area was positively correlated with dominance rank. Dominant cranes changed to new zones with higher numbers of birds and food density, while subordinate cranes went to new zones with lower numbers of birds. As a result, all birds increased their food intake over that of the previous day. Dominant cranes remained more faithful to their most preferred foraging zone, where they spent 69% of the mornings, while subordinate birds were more mobile, switching among zones frequently. Dominant birds left the roost later than subordinate birds on the days they changed to a new zone, which could be used to track the main departing flows. The results suggest that the dynamics that led to a truncated phenotype-limited distribution were determined by social dominance and food abundance, with dominant cranes shifting to a new zone to maintain their high intake levels and subordinates changing more frequently whenever their daily intake did not reach the minimum metabolic requirements. Received: 16 December 1996 / Accepted after revision: 22 February 1997     

Graded recruitment and hunting strategies linked to prey weight and size in the ponerine ant Ectatomma ruidum

According to the weight and size of their prey, Ectatomma ruidum workers can employ different recruitment systems (solitary hunting, cooperative hunting and group hunting with recruitment) when mastering and retrieving prey items from short distances from the nest. Prey size determined the backwards entry typically adopted by this species, while prey weight determined the predatory strategy selected. After a common initial sequence (search for prey, detection, localization), predatory sequences varied in terms of the type of approach, the site of seizure, the reaction after stinging and the type of transport. Nevertheless, irrespective of prey weight and size, seizure was preferentially oriented towards the head and prey were always stung. Short-range recruitment and mass recruitment without trail laying were elicited by a large range of heavy prey (> 2.5 times the weight of an individual worker). According to the mortality risk associated with each prey, hunters exhibited a “prudent” stinging posture associated with an increase in the duration of the subsequent phase of waiting for prey immobilization. The overall time of capture was positively correlated with the weight of the prey. When collective hunting strategies were involved, E. ruidum colonies matched the number of recruited hunters to the size and weight of the prey. Compared to solitary hunting strategies, for short food–nest distances, this graded recruitment appeared to enhance the energetic benefits derived by this species from the use of recruitment systems: the higher the number of workers involved in the recruitment process, the greater the energetic benefits obtained. The exhibition or absence of trail laying behavior in the recruitment responses displayed by E. ruidum workers is discussed in relation to their involvement in scavenging or predatory behavior. Received: 27 June 1996 / Accepted after revision: 3 March 1997     

The relative roles of predation and sperm competition on the duration of the post-copulatory association between the sexes in the blue crab, Callinectes sapidus

In many species, post-copulatory mate guarding prevents other males from mating with the guarded female. In crabs, males stay with their mates to protect the female from predators because, in some species, mating occurs when she is soft and vulnerable after molting. I tested the relative roles of sperm competition and predation on the duration of the post-copulatory association in the blue crab, Callinectes sapidus. Unpaired females suffered greater predation mortality than paired females and males stayed with the female longer in the presence of predators than in their absence, suggesting that the post-copulatory association protects females during their vulnerable period. However, the association may also occur in blue crabs because of sperm competition since spermathecal contents of females in the field indicate that 12.4% mated twice. Females experimentally mated with two males contained both males ejaculates and each ejaculate had access to the unfertilized eggs, suggesting that the size of a male's ejaculate influences his fertilization rate in a multiply-mated female. Males stayed longest in response to a high risk of sperm competition. Longer post-copulatory associations allowed the first male's ejaculate to harden into a type of sperm plug, which limited the size of a second inseminator's ejaculate in a non-virgin female as compared with a virgin. Males passed larger ejaculates in the presence of rivals and when previous ejaculates were in the female spermathecae, another response to sperm competition. Larger ejaculates may need longer post-copulatory associations before a more effective sperm plug forms. Large males stayed with the female longer, which is consistent with their ability to pass larger ejaculates than small males and suggests that there may be costs to minimizing the duration of the post-copulatory association. In the field, associations last long enough to protect the female during her vulnerable phase and may ensure that the guarding male fertilizes the most eggs in the female, even if she remates. Thus, the post-copulatory association protects female blue crabs from additional inseminators as well as from predators. Received: 23 January 1996 / Accepted after revision: 9 November 1996     

Reproductive dominance and differential ovicide in the communally breeding burying beetle Nicrophorus tomentosus

Communal breeding can be characterized by the division of reproduction among cooperating individuals and, if the distribution of reproduction is inequitable, by the mechanisms for achieving skewed reproductive success. The burying beetle (Nicrophorus tomentosus) is a facultative communal breeder. Unrelated adults, especially females, provide extensive parental care to broods of mixed parentage. The frequency and degree of reproductive skew between two females were examined experimentally. On medium-size carcasses, the proportion of eggs attributed to each female was not significantly different from random in 42% of the broods, skewed in 42% and not shared in 16%. Although reproduction was usually skewed in favor of the larger female, the relative sizes of the two females did not predict the degree of skew. On large carcasses, the proportion of eggs attributed to each female was not different from random in 87% of the broods and weakly skewed in 13%. Several mechanisms for biasing reproductive success were investigated. Females increase the proportion of their offspring in the brood by committing differential ovicide. Secondly, burying and preparing a carcass cooperatively stimulates ovarian development of the larger female and slows it for the smaller female, reducing or delaying oviposition by the subordinate. Thirdly, larger females are more likely to be dominant and are more fecund than smaller females. Received: 20 July 1996 / Accepted after revision: 30 November 1996     

The structural significance of a vocalization that stimulates gaping in black-capped chickadees

In a variety of avian species, vocalizations used to stimulate gaping in nestlings during feeding share similar features such as broad frequency range, multiple spectral components, and sudden onset. It is currently not understood what functional significance, if any, these acoustic features have for close-range communication. One example of such a vocalization is the “squawk” of the black-capped chickadees (Parus atricapillus). The present study analyzes the significant features of the squawk by observing the response of chickadee nestlings to field playbacks of natural and computer-modified squawks. Of the features tested, frequency range appears most critical, but responses to frequency ranges change with age, such that low frequencies are most stimulating around the time of hatching but cease to elicit gaping after 5 days posthatch. Aperiodic stimuli and computer-modified squawks with gradual onsets tend to be less stimulating than natural squawks at some ages, but these differences are not significant at any age. The functional significance of the broad frequency range of the squawk is discussed, including the possible role of accommodating shifting frequency preferences related to ontogenetic changes in auditory sensitivity and elaboration of the vocal repertoire. Received: 16 January 1996 / Accepted after revision: 28 December 1996     

A molecular genetic analysis of social structure, dispersal, and interpack relationships of the African wild dog (Lycaon pictus )

The African wild dog is a highly social, pack-living predator of the African woodland and savannah. The archetypal wild dog pack consists of a single dominant breeding pair, their offspring, and non-breeding adults who are either offspring or siblings of one of the breeding pair. Non-breeding adults cooperate in hunting, provisioning and the protection of young. From these observations follows the prediction that the genetic structure of wild dogs packs should resemble that of a multigenerational family, with all same-sexed adults and offspring within a pack related as sibs or half-sibs. Additionally, a higher kinship between females from neighboring packs should be evident if females tend to have small dispersal distances relative to males. We test these predictions through analysis of mitochondrial DNA control region sequences and 14 microsatellite loci in nine wild dog packs from Kruger National Park, Republic of South Africa. We show that as predicted, African wild dog packs generally consist of an unrelated alpha male and female, subdominant close relatives, and offspring of the breeding pair. Sub-dominant wild dogs occasionally reproduce but their offspring rarely survive to 1 year of age. Relatedness influences the timing and location of dispersal events as dispersal events frequently coincide with a change in pack dominance hierarchy and dispersers often move to areas with a high proportion of close relatives. Received: 22 February 1996 / Accepted after revision: 16 November 1996     

Relatedness, polyandry and extra-group paternity in the cooperatively-breeding white-browed scrubwren (Sericornis frontalis )

We used DNA fingerprinting to examine the genetic parentage and mating system of the cooperatively breeding white-browed scrubwren, Sericornis frontalis, in Canberra, Australia. Our analyses revealed a remarkable variety of mating tactics and social organization. Scrubwrens bred in pairs or multi-male groups that consisted of a female and two or more males. Females were always unrelated to the pair male or alpha (dominant) male. Among multi-male groups we found three different mating tactics. Firstly, when alpha and beta (subordinate) males were unrelated, they usually shared paternity in the brood. This resulted in both males gaining reproductive benefits directly. Secondly, when beta males were not related to the female but were related to the alpha males, beta males sired offspring in some broods. In this situation, beta males gained reproductive benefits both directly and potentially indirectly (through the related alpha male). Thirdly, when beta males were related to the female or both the female and alpha male, they remained on their natal territory and did not sire any offspring. Thus beta males gained only indirect reproductive benefits. Overall, when group members were related closely, the dominant male monopolized reproductive success, whereas when the members were not related closely the two males shared paternity equally. This positive association between monopolization of reproduction and relatedness is predicted by models of reproductive skew, but has not been reported previously within a single population of birds. Other cooperatively breeding birds with both closely related and unrelated helpers may show a similar variety of mating tactics. Finally, we found that extra-group paternity was more common in pairs (24% of young) than in multi-male groups (6%), and we discuss three possible reasons for this difference. Received: 21 May 1996 / Accepted after revision: 14 December 1996     

Weapon size versus body size as a predictor of winning in fights between shore crabs, Carcinus maenas (L.)

Relative body size (carapace width) and weapon size (chela length) were used as indicators of resource holding potential (RHP) in the agonistic behaviour of male shore crabs, Carcinus maenas (L.). Weapon size was found to be a more reliable predictor of the outcome of pairwise fights than body size. Crabs with longer chelae than their opponents were more likely to win fights than crabs with relatively larger bodies. Body size had less influence on the outcome of fights. Relative body and weapon size did not influence initiation of contests but did affect the likelihood of winning; however, this was significant only for weapon size. Winning crabs had heavier claws with greater surface area than losing crabs. There was no relationship between relative size and fight duration. The frequency of cheliped display increased with chela length and win- ners performed significantly more displays than losers. Received: 5 February 1997 / Accepted after revision: 20 May 1997     

Lack of kin recognition in swarming honeybees ( Apis mellifera )

Honeybee colonies reproduce by colony fission and swarming. The primary swarm leaves the nest with the mated mother queen. Further “after-swarms” can leave the nest. These are composed of virgin queens and sister workers. Since all workers in the primary swarm have the same relationship to the mother queen, kin recognition cannot have any effect on the worker distribution in the swarm. Because of polyandry of the mother queen, the after-swarm is composed of super- and halfsister workers of the virgin queen. In this case kin recognition might affect swarm composition if workers increase their inclusive fitness by preferentially investing in a supersister queen. The distribution of workers in the mother colony, the primary and the after-swarm was analyzed using single-locus DNA fingerprinting in two colonies of the honeybee (Apis mellifera). The colonies were composed of 21 and 24 worker subfamilies because of multiple mating of the queen. The subfamily distribution in the mother colonies before swarming was significantly different from the subfamily frequencies in the primary swarm. This indicates different propensities for swarming in the various subfamilies. The subfamily distribution was also significantly different between the mother colony and the after-swarm. There was however no significant difference between the subfamily composition of the primary and the after-swarm. The average effects of kin recognition on the distribution of the subfamilies in the two after-swarms were less than 2%. We conclude that colony-level selection sets the evolutionary framework for swarming behaviour. Received: 22 May 1996 / Accepted after revision: 2 November 1996     

The role of turbidity as a constraint on predator-prey interactions in aquatic environments

Many of the world's most productive aquatic ecosystems usually contain turbid water. Paradoxically, many fish species that live in these habitats are also those that often rely on vision to detect their predators and their prey. For these fish, turbidity will reduce the distance at which predator-prey interactions occur, and there should be a reduction in the opportunity for behavioural modification to control the risk of predation. Under these conditions, we predict that most antipredator behaviour will become ineffective and that predator-prey interactions in turbid water will be primarily characterised by the direct effects of predator consumption of prey, rather than behavioural modification reducing the growth rates of prey. This hypothesis was tested in two laboratory experiments. The first experiment investigated how water turbidity, risk of predation, and their interaction affect habitat choice decisions by fathead minnows (Pimephales promelas). These data demonstrate that fathead minnows reduce their use of dangerous habitats, but that this effect is diminished in turbid water. A second experiment examined mortality patterns when these fish were preyed upon by yellow perch (Perca flavescens) in clear and turbid water. Absolute mortality rates were unaffected by visibility, but patterns of mortality were random in turbid water and skewed towards smaller individuals in clear water. Combined, these results support our hypothesis and suggest that the impact of predation risk will be reduced in turbid aquatic ecosystems. Received: 23 May 1996 / Accepted after revision: 18 November 1996     

Testosterone stress does not increase asymmetry of a hormonally mediated sexual ornament in a lizard

The theory that fluctuating asymmetry is sensitive to both environmental and genetic stress is gaining acceptance among evolutionary biologists. Most empirical work has focused on ornamental traits on the assumption that they are more susceptible to stress. In tegumentary coloration is a common ornament in nature, and frequently has a hormonal basis. Earlier studies in the lizard Psammodromus algirus indicate that testosterone induces the development of head nuptial coloration in large males and, at the same time, produces an increase of the ectoparasite load and higher mortality. Hence, the manipulation of testosterone levels may be a way to increase ornament expression and simultaneously create conditions that may make symmetrical development difficult. This positive covariation between character size and symmetry is opposite to that expected in theory for sexually selected traits, so the predicted elevation of asymmetry due to the treatment cannot be confounded by any intrinsic association between symmetry and character size. We firstly consider the effect that testosterone supplementation has on two variables that reflect the symmetry of bilateral throat nuptial coloration in large P. algirus males. Also, we examine whether ornament symmetry is positively associated with reproductive success, a prediction of theory of sexually selected symmetry. Testosterone treatment did not increase the fluctuating asymmetry of throat coloration. Size asymmetry increased with character size in individuals with a fragmented colour pattern, but changed suddenly to a highly symmetric pattern in individuals with non-fragmented coloration. Mirror asymmetry decreased steadily with character size. These results suggest that the development of coloration on both sides of the throat midline follows a random pattern. Asymmetry did not correlate with variables that estimated reproductive success, suggesting that asymmetry is not affected by physiological stress and that this trait is not a sexually selected signal in P. algirus. Received: 1 July 1996 / Accepted after revision: 4 May 1997