Failures of reproductive suppression in dwarf mongooses (Helogale parvula): accident or adaptation?

In many communally breeding species, only the dominant femalenormally breeds, while subordinates tolerate reproductive suppression(these are "despotic" species, in the terminology of Brown,1987; Macdonald and Moehlman, 1983; Vehrencamp, 1983). Yet inmany species for which reproductive suppression is the norm(across a wide variety of taxa), subordinates do occasionallybreed. Because reproduction by subordinates is atypical forthese species, it is often regarded as simple failure of thenormal mechanisms of suppression. An alternate hypothesis isthat subordinate pregnancies represent an evolutionary compromisebetween dominant and subordinate, in which dominants concedetheir monopoly on reproduction in order to retain helpers. Weuse data from a long-term study of dwarf mongooses (Helogaleparvula) in Serengeti National Park, Tanzania, to test thishypothesis, using an inclusive fitness model adapted from oneby Vehrencamp (1983). We find that the incidence of subordinatepregnancy closely matches that predicted by the model, suggestingthat the mechanisms that underlie reproductive suppression indwarf mongooses are finely adjusted to the social and demographicenvironment. [Behav Ecol 1991; 2: 7–15]     

Life-history analysis of the Trivers and Willard sex-ratio problem

Phenotypic quality, such as condition or size, often variesbetween individuals. For species with extensive maternal care,the quality of offspring may partially be determined by thequality of their mother. Trivers and Willard (1973) predictedthat high quality females should prefer offspring of the sexwhose reproductive success is most strongly influenced by maternalcare, which in many cases will be sons. Correspondingly, lowquality females should prefer daughters. However, this predictionis not based on a proper analysis of variation in reproductivevalue. Using state-dependent life-history theory, I show herethat high quality females should prefer offspring of the sexwhose reproductive value is most strongly influenced by maternalcare. I also show that when offspring quality is strongly determinedby their mother's quality, but not influenced by their father'squality, high quality females can have higher reproductive valuethan high quality males, even though their reproductive successmay be much lower. In such cases, high quality females shouldprefer daughters and, correspondingly, low quality females shouldprefer sons.[Behav Ecol 7: 316–325 (1996)]     

Assessment of local predation risk: the role of subthreshold concentrations of chemical alarm cues

The ability to accurately assess local predation risk is criticalto prey individuals, as it allows them to maximize threat-sensitivetrade-offs between predator avoidance and other fitness relatedactivities. A wide range of taxonomically diverse prey (includingmany freshwater fishes) relies on chemical alarm cues (alarmpheromones) as their primary information source for local riskassessment. However, the value of chemical alarm cues has beenquestioned due to the availability of additional sensory inputs(i.e., visual cues) and the lack of an overt antipredator responseunder conditions of low perceived risk. In this paper, we testthe hypothesis that chemical alarm cues at concentrations belowthe point at which they elicit an overt behavioral responsefunction to increase vigilance towards other sensory modalities(i.e., visual alarm cues). Shoals of glowlight tetras (Hemigrammuserythrozonus) exposed to the subthreshold concentration of hypoxanthine-3-N-oxide(the putative Ostariophysan alarm pheromone) did not exhibitan overt antipredator response in the absence of secondary visualcues (not different than the distilled water control). However,when exposed to the sight of a visually alarmed conspecific,they significantly increased the intensity of their antipredatorresponse (not different from shoals exposed to the suprathresholdalarm cue). This study demonstrates that prey may benefit fromresponding to low concentration alarm cues by increasing vigilancetowards secondary cues during local risk assessment, even inthe absence of an overt behavioral response. By increasing vigilancetowards secondary risk assessment cues in the presence of alow risk chemical cue, individuals are likely able to maximizethe threat-sensitive trade-offs between predator avoidance andother fitness related activities.     

Dart shooting influences paternal reproductive success in the snail Helix aspersa (Pulmonata, Stylommatophora)

Although animal courtship behaviors are generally understoodwithin the context of sexual selection, the relevance of manysexual behaviors to sexual selection, and vice versa, remainsunexplained. For example, the adaptive function of the "lovedart" used in the precopulatory behavior of hermaphroditicland snails is only now becoming apparent. Contrary to previous assumptions, dart shooting is unlikely to function as a stimulusfor copulation. In searching for a more ultimate explanationof the dart's function, we tested whether variation in dartshooting influences reproductive fitness in Helix aspersa.Individual mother snails were mated sequentially to two potentialfathers. Dart shooting was closely observed and quantifiedfor all pairings, and percentages of offspring sired by each potential father were determined using allozymes. The resultsindicate that snails that shoot darts effectively have significantlygreater paternal reproductive success than snails that shootpoorly. In contrast, there was no significant effect of matingorder on either dart shooting or paternal reproductive success.     

Collective detection in escape responses of temporary groups of Iberian green frogs

When confronted with a predator, prey are often in close proximityto conspecifics. This situation has generated several hypothesesregarding antipredator strategies adopted by individuals withingroups of gregarious species, such as the "risk dilution," "earlydetection," or "collective detection" effects. However, whethershort-term temporary aggregations of nongregarious animals arealso influenced in their escape decisions by nearby conspecificsremains little explored. We simulated predator approaches togreen frogs (Rana perezi) in the field while they were foragingat the edge of water, either alone or spatially aggregated intemporary clusters. "Flight initiation distances" of frogs (i.e.,the distance between the simulated predator and the frog atthe time it jumped) that escaped by jumping into the water wereinfluenced by microhabitat variables (vegetation at the edgeand in water and the initial distance of the frog to the closestwater edge) and also by the responses of nearby individuals.In clusters, risk dilution did not influence the first individualto respond to the predator simulation or the average responseof all frogs in the cluster as the frog's responses were independentof group size. Also, flight initiation distances of individualsthat first responded to the predator within clusters did notdiffer from those of solitary individuals, which is contraryto the predictions of the early detection hypothesis. However,the remaining frogs in the cluster had longer flight initiationdistances than expected from the comparison with solitary individuals.We suggest that this pattern originated because the responseof the first frog within a cluster triggered the sequentialresponse of the remaining frogs in the cluster, which agreeswith the expectations from the collective detection hypothesis.Our findings give insight into an early stage in the evolutionof grouping as they suggest that individual frogs may benefitfrom being part of a cluster, even for short periods of time.     

Size-based variation in antipredator behavior within a snake (Agkistrodon piscivorus) population

Variation in an animal's response to a predator likely reflectsthe complex interaction of factors that influence predationrisk. Due to their high degree of behavioral variation and simplifiedbauplan, snakes offer a unique model for investigating the influenceof sex and body size on antipredator behavior. We examined variationin antipredator behavior within a cottonmouth (Agkistrodon piscivorusleucostoma) population. Behavioral responses to human-inducedpredation risk were compared across a continuous scale of bodysize. Defensive responses significantly declined with increasingbody size. After controlling for body size, no differences betweenthe sexes were detected. Although this study suggests that variationin antipredator behavior is, in part, related to body size,some studies on snakes have not found this relationship. Likewise,some studies have demonstrated differences between sexes. Suchdisparate patterns of variation indicate a need for future comparativestudies examining the complex interaction of factors that mayinfluence predator–prey relationships.     

Risk-taking behavior predicts aggression and mating success in a fiddler crab

Evidence is growing that an individual's propensity to takerisks in the presence of a predator is correlated to behaviorsthat can affect individual fitness. We examined whether risk-takingbehavior predicts aggression, surface activity levels, and matingsuccess in male fiddler crabs, Uca mjoebergi. Risk-taking behaviorwas highly consistent among individuals, remained stable overtime, and was unrelated to male size. We found that males thattook greater risks in the presence of a potential predator alsobehaved more aggressively when searching for a new territory.In addition, bold males exhibited higher surface activity levelsand spent more time courting females compared with their shycounterparts. Although risk-taking behavior was independentof other sexually selected traits, it accurately predicted malemating success in U. mjoebergi. We suggest nonsexually selectedtraits, such as risk taking, may represent important behavioralpredictors of success in other species.     

Hissing females of great tits (Parus major) have lower breeding success than non‐hissing individuals

Antipredator strategies vary remarkably between individuals within populations. Parents tend to take greater risks when brood value is increased. Moreover, individuals consistently differ in a whole suite of correlated behaviours that may cause distinctive responses to predators. It is likely that individual differences in antipredator behaviour may co‐vary with proxies for fitness such as reproductive success. We used a 4‐year data from wild great tits (Parus major) to test whether passive and active antipredator strategies (females with no response vs. those giving hissing calls towards a nest predator) during the incubation stage can reflect variation in breeding success. Although clutch size did not depend on hissing behaviour, the number of surviving offspring from eggs and neonates to fledglings was higher for non‐hissing than hissing birds. We conclude that females with distinct antipredator strategies can prioritize different fitness components.     

A rare predator exploits prey escape behavior: the role of tail-fanning and plumage contrast in foraging of the painted redstart (Myioborus pictus)

Escape response, triggered by an approaching predator, is acommon antipredatory adaptation of arthropods against insectivores.The painted redstart, Myioborus pictus, represents insectivorousbirds that exploit such antipredatory behaviors by flushing,chasing, and preying upon flushed arthropods. In field experimentsI showed that redstarts evoke jump and flight in prey by spreadingwings and tail: this display increased frequency of aerial chasesby redstarts. Artificial models with spread tails also elicitedescape responses more often than models with closed tails and wings.The white patches on black wings and tails additionally help:the frequency of chases decreased when the white patches werecovered with black dye. Black models also tended to elicit escaperesponse less often than black-and-white models did, at leastin some situations. Hence, the prey's ability to detect birdsand to flee could cause the evolution of predators specializedat using conspicuous behavior and contrast in body colorationto elicit and exploit such antipredatory responses. Redstartsconstitute only a small proportion of the predatory guild, andtheir adaptations to exploit the prey's behavior illustratethe theoretically modeled "rare enemy" effect present in multispeciespredator-prey systems. This is the first experimental studyof morphological and behavioral adaptations of a rare predatorthat both elicits and exploits antipredator escape behaviorof its prey against more common predators. Hence, the studydocuments a behavior that could be evolutionarily explainedonly if indirect interactions in predator-prey communities aretaken into account.     

An experimental approach to altering mating tactics in male horseshoe crabs (Limulus polyphemus)

Alternative reproductive tactics are often correlated with phenotype, density, environment, or social context. Male horseshoe crabs(Limulus polyphemus) have two mating tactics that are associatedwith phenotype. Males in good condition arrive at the nestingbeach and spawn while attached to females, whereas those inpoorer condition come ashore unattached and crowd around thenesting couples as satellites, fertilizing eggs through sperm competition. The correlation between mating tactic and phenotypemay be due to males choosing tactics based on condition, orit may be that males that have not found a female choose tocome ashore as satellites. To distinguish between these twopossibilities, I conducted an experiment on male horseshoe crabsin the field at Seahorse Key on the northern Gulf coast ofFlorida. I prevented males from attaching to females by placingsmall plastic bags over the claws they use to attach. The resultsshowed that males in poor condition came ashore as satellites,whereas males in good condition remained at sea. This meansthat mating tactics are cued by information about the male'scondition and not about whether he found a female. The evolutionof phenotype-correlated mating tactics can be represented bya model in which the fitness of each tactic changes with conditionand fitness curves cross. I hypothesize that male horseshoecrabs in good condition have higher fitness when attached and that males in poorer condition to better when unattached.     

Predicted fitness consequences of threat-sensitive hiding behavior

In studies of refuge use as a form of antipredator behavior,where prey hide in response to a predator's approach, factorssuch as foraging costs and the perceived risk in a predator'sapproach have been shown to influence the hiding behavior ofprey. Because few studies of waiting games have focused on mammals,we studied the hiding behavior of the yellow-bellied marmot(Marmota flaviventris), a ground-dwelling rodent. We testedthe prediction that marmots vary hiding time as a function ofpredator approach speed and presence and absence of food outsidetheir refuge and that marmots hide differently depending ontheir relative condition. We conducted "fast approaches" and"slow approaches" in the presence and absence of extra foodand evaluated hiding times. Multiple regression analyses demonstratedthat the interaction between the approach speed and the presenceand absence of food influenced hiding behavior; body conditionhad a smaller, but nonsignificant effect. We then developeda state-dependent dynamic model to explore potential fitnessconsequences of these decisions. The model suggested that theoverall survival of a population is substantially reduced whenindividuals make suboptimal decisions. Our research builds onprevious studies, indicating that animals integrate both costsand benefits of hiding when determining their hiding times.     

高山松种实性状与生殖适应性

高山松(Pinus densata)是云南松(P. yunnanensis)与油松(P. tabulaeformis)自然杂交产生的二倍体杂种,是植物同倍体杂交物种形成的典型案例。高山松分布于青藏高原东南缘,占据了两个亲本种都不能正常生长的高海拔地带。良好生殖适应性的产生是物种形成的关键,但对高山松在高海拔生境下生殖适应性的变异与杂交起源遗传背景关系的研究尚未见报道。该研究通过对来自该种分布区内6个代表居群样本、13个反映高山松生殖适应性的种实性状的分析,探讨了其生殖性状在居群水平的变异模式,揭示了具有不同遗传组成和起源历史的高山松居群的生殖特性。研究结果表明:6个研究居群在种实性状上存在显著差异;可育种鳞数、总种鳞数、总种子数、可育种鳞率和可育种鳞密度等与纬度呈显著负相关关系;球果结种率与经度和生态梯度值呈显著的正相关关系;胚珠败育率则与经度和生态梯度值呈显著的负相关关系;球果长度也呈现了与经度的显著负相关。高山松球果结种率很高,平均为74%;球果的结种数、种子长和种翅长与云南松、油松及其它松属双维管亚属多数种接近;球果长度、球果总种鳞数接近油松的报道。这些结果表明,高山松在亲本种不能正常生长、繁衍的高原环境下具备正常的生殖能力;其种实特征在居群间分化显著并呈现一定的地理梯度变化。居群间的显著差异反映了其杂种起源特性,与高山松居群的遗传背景有关,也反映了处在异质生态环境中的高山松居群具有各自的生殖生态进化趋向。     

Do anuran larvae respond behaviourally to chemical cues from an invasive crayfish predator? A community-wide study

Antipredator behaviour is an important fitness component in most animals. A co-evolutionary history between predator and prey is important for prey to respond adaptively to predation threats. When non-native predator species invade new areas, native prey may not recognise them or may lack effective antipredator defences. However, responses to novel predators can be facilitated by chemical cues from the predators’ diet. The red swamp crayfish Procambarus clarkii is a widespread invasive predator in the Southwest of the Iberian Peninsula, where it preys upon native anuran tadpoles. In a laboratory experiment we studied behavioural antipredator defences (alterations in activity level and spatial avoidance of predator) of nine anurans in response to P. clarkii chemical cues, and compared them with the defences towards a native predator, the larval dragonfly Aeshna sp. To investigate how chemical cues from consumed conspecifics shape the responses, we raised tadpoles with either a tadpole-fed or starved crayfish, or dragonfly larva, or in the absence of a predator. Five species significantly altered their behaviour in the presence of crayfish, and this was largely mediated by chemical cues from consumed conspecifics. In the presence of dragonflies, most species exhibited behavioural defences and often these did not require the presence of cues from predation events. Responding to cues from consumed conspecifics seems to be a critical factor in facilitating certain behavioural responses to novel exotic predators. This finding can be useful for predicting antipredator responses to invasive predators and help directing conservation efforts to the species at highest risk.     

Rank and colony defense against conspecifics in a facultatively eusocial hover wasp

An important benefit of social living is increased capacityfor defense. Highly eusocial species have often evolved a fightingcaste for this purpose, but many facultatively eusocial insectsand cooperatively breeding vertebrates lack morphological castesand the decision to defend or not can depend on costs and benefitsto each individual. Defense by subordinates in a social groupcan be regarded as a form of helping, and helping input oftenvaries among subordinates of different age or size. Severalhypotheses attempt to explain variation in helping effort, includingthe effects of relatedness and differences in the costs of helping.Evidence for these hypotheses is mixed and often lacks dataon the rank of subordinates, an important determinant of expectedfuture fitness. We examined individual variation in propensityto defend the nest against conspecifics in the tropical hairy-facedhover wasp Liostenogaster flavolineata. Prior to experimentation,we determined the positions of all wasps in the age-based queueto inherit the single egg-laying position in each L. flavolineatagroup. Two approaches were then used: observations of defenseagainst natural intrusions by conspecifics and experimentaltrials where wasps were presented attached to a wire. Higherranks were more likely to defend the nest than lower ranks,opposite to the pattern previously documented for another formof helping: foraging effort. Possible explanations for thisresult are that higher ranked females are better defenders andthat they suffer a larger decrease in expected future fitnesswhen an intruder usurps their position in the inheritance queue.     

Using potential reproductive rates to predict mating competition among individuals qualified to mate

The potential reproductive rate (PRR), which is the offspringproduction per unit time each sex would achieve if unconstrainedby mate availability, often differs between the sexes. An increasingsexual difference in PRR predicts an intensified mating competitionamong the sex with the higher PRR. The use of PRR can providedetailed predictions of when, where, and how the intensityin mating competition and hence sexual selection will vary.Previous models have focused on the "time out" from mate searchingas a major component of PRR. Here, we suggest some improvementsand clarifications: in a population where individuals haveto compete for specific resources that are prerequisites formating (e.g., nest sites), individuals unable to obtain sucha resource will not qualify to mate. We suggest how a conceptof the ratio of males and females qualified to mate, Q, canimprove previous models designed to use the sexual differencein PRR to estimate the operational sex ratio (OSR). Further,when estimating the sexual difference in PRR of a population,it is important that each sex is given free access to matingpartners. Jointly, this provides an empirical approach basedon estimates of Q and the sexual difference in PRR.     

Predation risk and social interference as factors influencing habitat selection in two species of stream-dwelling waterstriders

We investigated how an aggressive species of waterstrider, Aquariusremigis, and potential predators, green sunfish (Lepomis cyanellus),affected the habitat use and mating behaviors of a less aggressivecongeneric species, A. conformis. Although these species sometimesco-occur, A. remigis typically lives in small streams with fewor no fish, whereas A. conformis are typically in medium- orlarge-sized streams with large populations of potentially predatoryfish. We tested in separate experiments in seminatural streams:1) the effect of fish on behaviors of A. conformis; 2) the effectof A. remigis on A. conformis; and 3) the habitat use of A.conformis when given a choice between pools with A. remigisor fish. The first experiment showed no effect of fish on eithermating behaviors or microhabitat use of A. conformis. This isin surprising contrast to the strong effects of fish alreadydocumented in A. remigis. The second experiment showed thatthe mating activity of A. conformis was reduced when A. remigiswere present; hence, A. conformis should avoid A. remigis. Finally,when A. conformis were presented with a choice between two pools,one containing A. remigis and the other containing fish, bothsingle males and pairs of A. conformis chose the pools withfish. In contrast, the habitat use of single female A. conformiswas not affected by either fish or A. remigis. Results fromthese experiments demonstrate that closely related species exhibitcontrasting social and antipredator behaviors and that aggressivesocial behavior is an important determinant of habitat partitioning     

Increasing vulnerability to predation increases preference for the scrounger foraging tactic

When animals forage in groups, individuals can search for foodthemselves (producer tactic) or they can search for and joinother individuals that have located food (scrounger tactic).The scrounger tactic may provide greater antipredator benefitsthan the producer tactic because "scroungers" hop with theirheads up and tend to occupy central positions in a group, whereas"producers" hop with their heads down and tend to occupy edgepositions. We tested whether increasing an individual's vulnerabilityto predation (using wing-loading manipulations) causes an increasedpreference for the scrounger tactic in zebra finches (Taeniopygiaguttata). Wing-loading manipulations were effective at increasingfocal individuals' perception of vulnerability to predation;treatment individuals increased their total time allocated tovigilance, whereas control individuals did not. Treatment individualsalso increased their use of the scrounger tactic (proportionof hops with head up) and scrounged a greater proportion ofpatches, whereas control individuals exhibited no changes. Ourresults are consistent with the hypothesis that the scroungertactic confers greater antipredator benefits than the producertactic, although whether antipredator benefits are achievedthrough differences in head orientation, spatial position, orboth, remains unclear. Our finding that individuals adjust theiruse of the scrounger tactic according to changes in their phenotypeprovides evidence for phenotype-limited allocation strategiesin producer–scrounger games.     

Defending the Weak: Parental Defense Peaks When Chick Vulnerability is Greatest in the Herring Gull (Larus argentatus)

Successful reproduction often depends upon parents providing offspring with resources and protection. In birds, reproductive success can often be enhanced by parents engaging in antipredator behaviors, but these behaviors can be costly. Theoretically, individuals should temporally modify the intensity of nest defense behavior to balance the costs and benefits of current and future reproductive success. More specifically, nest defense should vary throughout a nesting attempt to maximize fitness of the adults. Here, we consider the relationship between nest defense behavior and chick vulnerability in the herring gull (Larus argentatus), where chicks are under high predation risk. We estimated chick vulnerability by quantifying survival probabilities at different periods of the nestling stage. Simultaneously, we quantified changes in parental aggression throughout the nesting cycle by simulating predation attempts using a human predator model. We found that chick survival probabilities were lowest (i.e., vulnerability was highest) and parental aggression in nest defense was greatest during the first 10 days after hatching. Thus, we show that parents are most defensive when chicks are most vulnerable and that adults optimize nest defense behaviors in a way that maximizes their fitness.     

Male morphological variation and the determinants of body size in two Otiteselline fig wasps

The chalcid wasps (Hymenoptera) that develop in fig tree inflorescences(Ficus: Moraceae) have often been used to study alternativereproductive behaviors. However, recent work suggests that suchbehaviors are more complex than previously thought. We investigatedthis in Otitesella longicauda and O. rotunda. In addition toknown dimorphisms in the two species (each have "religiosa"males that use their mandibles to fight for mates in the fig,and "digitata" males that disperse from the fig to mate), wefound that religiosa males below species-specific body sizeswitch points have relatively larger mandibles and are lesssclerotized than those above. Thus, they are actually trimorphic.We suggest that the religiosa morph variation is linked to fighter/nonfighteralternative mating behaviors, with small (nonfighting) maleshaving relatively larger mandibles because they also use themto pull females out of their galls before mating. Also, we investigatedthe determinants of wasp body size, and whether females (foundresses)adjust their offspring allocation strategies according to expectedoffspring size. We found that wasp size is larger in ovariesnear the center of the fig, and more females and fewer religiosamales are laid in such ovaries than in those further away. Thisprobably indicates that foundresses lay females when they areexpected to be large because their fitness is more body size–dependentthan that of religiosa males. We then discuss the implicationsof our findings for the study of alternative reproductive behaviorsand foundress offspring allocation strategies.     

Insular tammar wallabies (Macropus eugenii) respond to visual but not acoustic cues from predators

We studied the way in which a population of tammar wallabies(Macropus eugenii), which have been isolated from mammalianpredators since the last ice age, responded to the sight andsound of historical and ontogenetically and evolutionarilynovel predators. Tammars were shown a range of visual stimuli,including taxidermic mounts of two evolutionarily novel predators,a red fox (Vulpes vulpes) and a cat (Felis catus), and a modelof an extinct predator, the thylacine (Thylacinus cynocephalus).Controls were a conspecific, the cart on which all mounts werepresented, and blank trials in which spontaneous change in behaviorwas measured. We played back recorded sounds to characterizeresponses to acoustic cues from predators and to a putativeconspecific antipredator signal. Treatments included the howlsof dingoes (Canis lupus dingo), an evolutionarily novel predator;calls of a wedge-tailed eagle (Aquila audax), a historicaland current predator; and wallaby foot thumps. Controls werethe song of an Australian magpie (Gymnorhina tibicen) and ablank trial. After seeing a fox, wallabies thumped their hindfeet in alarm, suppressed foraging, and increased looking.The sight of a cat similarly suppressed foraging and increasedlooking. The sounds of predators did not influence responsiveness,but wallabies foraged less and looked more after thump playbacks.Our results suggest that tammars respond to the sight, butnot the sounds, of predators. In contrast, the response to footthumps demonstrates that this particular sound functions asan antipredator signal. We suggest that responsiveness to visualcues has been preserved under relaxed selection because predatormorphology is convergent, but vocalizations are not.