Neighbor–stranger discrimination in Audubon's shearwater (Puffinus l. lherminieri) explained by a “real enemy” effect

Neighbor–stranger discrimination (NSD) occurs when animals respond with more aggression to strangers than to territorial neighbors. NSD has been reported in many species that defend multi-purpose territories for breeding and foraging, but it is rare among species that defend other types of territories. For birds that defend only their nest sites, there is no experimental evidence for NSD, and observational studies have provided mixed results. In a colony of Audubon's shearwaters (Puffinus l. lherminieri), I played back the calls of a neighbor and a stranger to males defending nest sites. Subjects responded with longer calls to playbacks of strangers than to those of neighbors. In shearwater colonies, strangers are often birds looking for future breeding sites. In contrast, there is no evidence that established breeders compete with their neighbors for any resources. Shearwaters should benefit from NSD because strangers represent a “real enemy” and established neighbors do not.     

Sex-specific nest defense in house sparrows (Passer domesticus) varies with badge size of males

According to indicator models of sexual selection, females can benefit from choosing males with above average epigamic traits, but empirical evidence for such benefits is scarce. Here, we report results from an experiment with 29 pairs of house sparrows (Passer domesticus) where the intensity of nest defense against a mounted mustelid predator was related to the size of the black throat and breast patch (“badge”) in males. Using principal components analysis (PCA), original response variables of both sexes were reduced to two factors: “Approach” to the predator and “Distant warning”. “Approach”, the more risky behavior, increased from small- through medium- to large-badged males and decreased in their females. Since large-badged males have a higher certainty of paternity (i.e. greater benefits from defense) and may be older and more experienced (i.e. incur lower costs), the most likely explanation for male defense intensity increasing with badge size is an improving benefit/cost ratio. The resulting optimal response of their females and evolutionarily stable participation in joint parental care is illustrated by a graphical model. It shows that females would, indeed, benefit directly from choosing large-badged males. This, however, is no proof of a direct evolutionary tie between badge size and paternal behavior, as assumed by indicator models of sexual selection. It may simply represent a spurious relationship, originating from the correlation of badge size and defense with confidence of paternity. Received: 22 September 1997 / Accepted after revision: 3 November 1997     

Synchrony in egg-laying and reproductive success of neighboring common murres,Uria aalge

Common murres (Uria aalge) are highly colonial; pairs often breed at the highest possible densities, in bodily contact with neighbors. At Bluff, a colony in western Alaska, we tested for synchrony in egg laying at various spatial scales and found little evidence for higher synchrony, either within study plots of 15–195 pairs, or within subplots containing several pairs, than among plots in a 5-year study. Egg laying of neighbors generally was more synchronous than expected based on overall frequency distributions in laying dates, however. Breeding success was positively correlated with the number of breeding neighbors and the number of neighbors tending eggs or nestlings at the time of egg laying. Breeding success of pairs with neighbors was positively related to the breeding success of neighbors. Pairs that produced eggs synchronously with at least one neighboring pair had higher success than those that began breeding either before or after their neighbors. Most reproductive failures at Bluff are due to accidental egg loss and predation on eggs by common ravens, Corvus corax, soon after laying. By occupying space where a raven might otherwise land and defending their own eggs, active breeding neighbors locally reduce the probability of egg predation. Active breeding neighbors also are less likely to flush and accidentally dislodge nearby eggs when disturbed than are nonbreeders. Murres breeding synchronously with neighbors have the highest assurance of the presence of active breeding neighbors both at the time of egg laying and throughout their reproductive attempts. Groups of neighboring murres can be considered small “selfish herds,” demonstrating by-product mutualism through their continued presence and defense of their own eggs and nestlings. Despite the advantages of breeding synchronously with neighbors, early breeding may often be favored, however. Received: 22 January 1996/Accepted after revision: 16 June 1996     

Brood defense and age of young: a test of the vulnerability hypothesis

Summary In many altricial species including the great tit (Parus major) the intensity of brood defense against predators has been shown to increase with the age of the offspring. This effect has been ascribed amongst others to the young becoming more vulnerable as they age (vulnerability hypothesis). In a great tit population suffering heavy losses from brood depredation by the great spotted woodpecker (Dendrocopus major), we rendered first and second broods more vulnerable by artificially enlarging the entrance of the nest hole. Contrary to the vulnerability hypothesis, 16 experimental pairs defended their brood against a dummy great spotted woodpecker less vigorously than did 16 control pairs. Nest concealment behavior potentially compromising active defense was minimized by simultaneous playback of nestling distress calls, thus simulating the act of nest predation. This leaves the brood value hypothesis as an alternative functional explanation of the defense level — age effect. It predicts that parents should defend their brood in proportion to the reproductive value (or some more suitable cohortal equivalent measure) of their offspring. At present, this explanation pertains to one predator species. In first broods, but not in second broods, males defended them more vigorously than did their females. While this parallels previous experiments on brood defense against predators posing a much greater risk to the parents, two functional explanations previously put forward can hardly apply.     

Offspring reproductive value and nest defense in the magpie (Pica pica)

Summary Magpie (Pica pica) brood defense against a human at the nest was studied in a Mediterranean population with low renesting potential. Variations in two defense measures recorded during 106 trials at 41 different nests were positively correlated with brood age. Ineremental effects due to the number of successive visits to nests by us, brood size, and the time in the breeding season were not significant. Partial correlation analyses showed that visit rate was not an important determinant of nest defense, which thus favors an adaptive explanation of nest defense patterns. Two functional hypotheses to account for the increase in defense intensity with brood age were tested: whether (1) increased parental defense serves to compensate the higher predation risk of older nests or (2) increased parental defense reflects the increasing reproductive value of nestlings as they grow older. Daily mortality and incidende of predation (estimated from contribution of whole-brood losses to total mortality) was higher early in the nestling period, hence providing weak evidence for the assumption on which hypothesis (1) is based. The timing of parental defense intensity did not mirror variations in predation risk for the nest but variations in reproductive value of the brood, as can be estimated from daily mortality, thus supporting hypothesis (2). Magpie parents increased defense intensity in response to premature escaping by almost fully-developed nestlings. Since such a response lowers predation risk for the offspring and increases their probability of survival, this finding supports hypothesis (2), but runs contrary to hypothesis (1). Parents also increased defense in response to play-backs of alarm calls uttered by nestlings during escaping episodes. It is argued that parents should continuously monitor the degree of offspring development in order to assess their reproductive value and that, by alarm calling, chicks honestly make their parents aware of the gain in reproductive value that results from enhancement in locomotory abilities that occur at the end of the nestling period.     

Tit for tat among neighboring hooded warblers

Summary The dear-enemy relationship of territorial songbirds could be mutually beneficial to neighbors, as males who recognize neighbors and reduce their responses to these neighbors would require less time and energy for territorial defense. In order for this relationship to be evolutionarily stable, this reduction in response to a neighbor must be conditional on reciprocal restraint by that neighbor. This study examined the possibility of such conditional responses in hooded warblers (Wilsonia citrina). Responses of territorial hooded warblers to playbacks of neighbors' songs from shared boundaries were measured before and after playbacks that simulated intrusions of those same neighbors (NNNN treatment) or strange birds (NSSN treatment) into the subjects' territories. Each male received both treatments separated by at least 8 days. Males increased their responses to playbacks of a neighbor's songs at the boundary after simulated intrusions of that same neighbor (NNNN) but did not increase their responses to such playbacks after simulated intrusions of strangers (NSSN). This increased response to a defecting neighbor suggests that the relationship between neighboring territorial hooded warblers is based on a conditional strategy like tit-for-tat.     

Consistent personality differences in house-hunting behavior but not decision speed in swarms of honey bees (<Emphasis Type="BoldItalic">Apis mellifera</Emphasis>)

Speed-accuracy tradeoffs are a common feature of decision-making processes, both in individual animals and in groups of animals working together to reach a single collective decision. Individual organisms display consistent differences in their “impulsivity,” and vary in their tendency to make rapid, impulsive choices as opposed to slower, more accurate decisions. However, we do not yet know whether groups of animals consistently differ in their tendency to prioritize decision speed over accuracy. We challenged 17 swarms of honey bees (Apis mellifera) to simultaneously choose a new nest site in each of three locations, and measured their decision speeds in each trial. We found that swarms displayed consistent personality differences in the number of waggle dances and shaking signals they performed and in how actively they scouted for new nest sites. However, swarms did not consistently differ in how long they took to choose a nest site. We suggest that house-hunting A. mellifera swarms may place an especially high emphasis on decision accuracy when choosing a nest site, and that chance events—such as the time when each swarm discovers a sufficiently high-quality nest site—may consequently play a greater role in determining a swarm’s decision speed than intrinsic characteristics such as a swarm’s “impulsivity.”     

Why do snakes have eyes? The (non-)effect of blindness in island tiger snakes (Notechis scutatus)

Large (to >1 m), diurnally active tiger snakes (Notechis scutatus) are abundant on Carnac Island, near the coast of Western Australia. Our behavioural and mark-recapture studies provide the first ecological data on this population, and reveal a surprising phenomenon. Many adult tiger snakes have had their eyes destroyed, apparently during nest defence, by silver gulls (Larus novaehollandiae). This loss of vision did not reduce the snakes' body condition (mass relative to length), or their rates of growth or survival (measured over a 12-month period). Blind male snakes trail-followed females, and mated successfully. Thus, destruction of a major sensory modality had no detectable effect on these predators. This result is strongly counter-intuitive, but mirrors an earlier report of congenital blindness (without ill-effects) in American viperid snakes. Similarities between the two systems (island populations, highly venomous snakes, reliance on sessile prey) clarify the circumstances under which the loss of vision does not reduce an organism's viability. These natural experiments support Gans' hypothesis of “momentarily excessive construction” in that the snakes possess a complex organ system that they do not actually require for successful feeding, survival or reproduction. Received: 30 November 1998 / Received in revised form: 24 April 1999 / Accepted: 9 May 1999     

Nest-size variation reflecting anti-predator strategies in social spider mites of<Emphasis Type="Italic"> Stigmaeopsis</Emphasis> (Acari: Tetranychidae)

The social spider mites (Acari: Tetranychidae) of Stigmaeopsis weave dense nests on the underside of host leaves. Four species occur on the leaves of bamboo in Japan: Stigmaeopsis longus, S. celarius, S. takahashii and S. saharai. We initially reconfirmed the occurrence of distinct variation in nest size among the species. Based on the hypothesis that this variation plays a role in protecting the spider mites from predators, we looked at the behavior of the natural enemies that occur on the host plants along with members of Stigmaeopsis. We found considerable variation in the ability of nests to protect the spider-mite eggs. The smallest nests protected the eggs against three predators, whereas the largest nests protected the eggs against only one predator species. So, decreases in nest size increased egg defense. Thus we concluded that nest-size variation reflects a strategy for reducing predation.Communicated by D. Gwynne     

Costs of delayed dispersal and alloparental care in the fungus-cultivating ambrosia beetle <Emphasis Type="Italic">Xyleborus affinis</Emphasis> Eichhoff (Scolytinae: Curculionidae)

Body reserves may determine the reproductive output of animals, depending on their resource allocation strategy. In insects, an accumulation of reserves for reproduction is often obtained before dispersal by pre-emergence (or maturation) feeding. This has been assumed to be an important cause of delayed dispersal from the natal nest in scolytine beetles. In the cooperatively breeding ambrosia beetles, this is of special interest because in this group delayed dispersal could serve two alternative purposes: “selfish” maturation feeding or “altruistic” alloparental care. To distinguish between these two possibilities, we have experimentally studied the effect of delayed dispersal on future reproductive output in the xyleborine ambrosia beetle Xyleborus affinis. Females experimentally induced to disperse and delayed dispersing females did not differ in their body condition at dispersal and in their founding success afterwards, which indicates that females disperse independently of condition, and staying adult females are fully mature and would be able to breed. However, induced dispersers produced more offspring than delayed dispersers within a test period of 40 days. This suggests that delayed dispersal comes at a cost to females, which may result primarily from alloparental care and leads to a reduced reproductive output. Alternatively, females might have reproduced prior to dispersal. This is unlikely, however, for the majority of dispersing females because of the small numbers of offspring present in the gallery when females dispersed, suggesting that mainly the foundress had reproduced. In addition, “gallery of origin” was a strong predictor of the reproductive success of females, which may reflect variation in the microbial complex transmitted vertically from the natal nest to the daughter colony, or variation of genetic quality. These results have important implications for the understanding of proximate mechanisms selecting for philopatry and alloparental care in highly social ambrosia beetles and other cooperatively breeding arthropods.     

Within-group spatial position and vigilance: a role also for competition? The case of impalas (<Emphasis Type="Italic">Aepyceros melampus</Emphasis>) with a controlled food supply

Theory predicts that individuals at the periphery of a group should be at higher risk than their more central conspecifics since they would be the first to be encountered by an approaching terrestrial predator. As a result, it is expected that peripheral individuals display higher vigilance levels. However, the role of conspecifics in this “edge effect” may have been previously overlooked, and taking into account the possible role of within-group competition is needed. Vigilance behavior in relation to within-group spatial position was studied in impalas (Aepyceros melampus) feeding on standardized patches. We also controlled for food distribution in order to accurately define a “central” as opposed to a “peripheral” position. Our data clearly supported an edge effect, with peripheral individuals spending more time vigilant than their central conspecifics. Data on social interactions suggest that it was easier for a foraging individual to defend its feeding patch with its head lowered, and that more interactions occurred at the center of the group. Together, these results indicate that central foragers may reduce their vigilance rates in response to increased competition. Disentangling how the effects of competition and predation risk contribute to the edge effect requires further investigations.     

Risk taking during parental care: a test of three hypotheses applied to the pied flycatcher

According to life-history theory, there will often be a conflict between investment in current versus future reproduction. If a predator appears during breeding, parents must make a compromise between ensuring the growth and survival of offspring (nest defence, feeding and brooding of young), and reducing the risk of predation to ensure their own survival. We model three hypotheses for the outcome of this conflict which are particularly relevant for altricial birds. They are not mutually exclusive, but focus on different costs and benefits. (1) Parental investment is determined by the parents’ own risk of predation. This hypothesis predicts that a lone parent should take smaller risks than a parent that has a mate. (2) Parental investment is related to the reproductive value of the offspring: Parents are predicted to take greater risks for larger broods, larger-sized or older offspring. (3) Finally, we present the new hypothesis that parental investment is related to the harm that offspring would suffer during a period of no parental care (incubation, brooding, feeding). This hypothesis predicts that parents should take greater risks for younger offspring, or for offspring in poorer condition, because the marginal benefit of parental care is largest in such cases. Hence, one may also expect that lone parents should take greater risks than two parents because their offspring are more in need of care. We tested these hypotheses on the pied flycatcher (Ficedula hypoleuca) by presenting a stuffed predator of the parents (a sparrowhawk, Accipiter nisus) close to the nest when parents were feeding the young. Risk taking was measured as the time that elapsed until the first visit to the nest. Most support was found for the ‘‘harm to offspring’’ hypothesis. Previous studies have usually measured the intensity of nest defence against typical nest predators, and have found evidence for the ‘‘reproductive value of offspring’’ hypothesis. However, our model predicts that the importance of the reproductive value of the offspring should decrease relative to the harm that offspring would suffer if they were not cared for when the predator type changes from a nest predator to a predator of adults, and when conditions for breeding turn from good to bad. Received: 13 April 1995/Accepted after revision: 11 March 1996     

The evolution of female social relationships in nonhuman primates

Considerable interspecific variation in female social relationships occurs in gregarious primates, particularly with regard to agonism and cooperation between females and to the quality of female relationships with males. This variation exists alongside variation in female philopatry and dispersal. Socioecological theories have tried to explain variation in female-female social relationships from an evolutionary perspective focused on ecological factors, notably predation and food distribution. According to the current “ecological model”, predation risk forces females of most diurnal primate species to live in groups; the strength of the contest component of competition for resources within and between groups then largely determines social relationships between females. Social relationships among gregarious females are here characterized as Dispersal-Egalitarian, Resident-Nepotistic, Resident-Nepotistic-Tolerant, or Resident-Egalitarian. This ecological model has successfully explained differences in the occurrence of formal submission signals, decided dominance relationships, coalitions and female philopatry. Group size and female rank generally affect female reproduction success as the model predicts, and studies of closely related species in different ecological circumstances underscore the importance of the model. Some cases, however, can only be explained when we extend the model to incorporate the effects of infanticide risk and habitat saturation. We review evidence in support of the ecological model and test the power of alternative models that invoke between-group competition, forced female philopatry, demographic female recruitment, male interventions into female aggression, and male harassment. Not one of these models can replace the ecological model, which already encompasses the between-group competition. Currently the best model, which explains several phenomena that the ecological model does not, is a “socioecological model” based on the combined importance of ecological factors, habitat saturation and infanticide avoidance. We note some points of similarity and divergence with other mammalian taxa; these remain to be explored in detail. Received: 30 September 1996 / Accepted after revision: 20 July 1997     

Mechanism of egg recognition in defenses against conspecific brood parasitism: American coots (<Emphasis Type="Italic">Fulica americana</Emphasis>) know their own eggs

Hosts of avian brood parasites use a variety of defenses based on egg recognition to reduce the costs of parasitism; the most important of which is rejecting the parasitic eggs. Two basic recognition mechanisms are possible: “true recognition”, whereby hosts recognize their own eggs irrespective of their relative frequency in the clutch, and minority recognition (or “recognition by discordancy”), whereby hosts respond to the minority egg type. The mechanism of recognition has been experimentally studied in a handful of species parasitized by interspecific brood parasites, but the mechanism used in defenses against conspecific brood parasitism is unknown. I experimentally determined the mechanism of egg recognition in American coots (Fulica americana), a species with high levels of conspecific brood parasitism, egg recognition, and rejection. I swapped eggs between pairs of nests to alter frequencies of host and “parasite” eggs and then used two criteria for recognition: egg rejection and nonrandom incubation positions in the clutch. Eight of 12 nests (66%) given equal frequencies of host and parasite eggs showed evidence of true recognition. In contrast, only one of eight (12.5%) nests where host eggs were in the minority showed evidence of recognition by discordancy. The nonrandom incubation positions of parasitic eggs indicates that birds sometimes recognize parasitic eggs without rejecting them and provides a means of assessing recognition on a per nest basis in species with large clutches. Adaptive recognition without rejection may also be an important evolutionary stepping stone to the evolution of egg rejection in some taxa.     

Facultative pheromonal mimicry in snakes: “she-males” attract courtship only when it is useful

Males of many animal species mimic females, and thereby deceive rival males. Facultative shifts in posture, color, or movement allow a male using visually-based mimicry to adopt and terminate mimicry rapidly. Pheromonal mimicry is rare in vertebrates perhaps because it is difficult to redeploy pheromones rapidly enough to adjust male tactics to local conditions. In Manitoba garter snakes (Thamnophis sirtalis parietalis), female mimicry benefits males immediately after they have emerged from hibernation. While the snakes are cold and slow, courtship warms them and protects them against predatory crows. This benefit disappears as soon as the snakes are warm. We show that (unlike females) she-male garter snakes attract courting males only when they are cold. Low temperatures may suppress volatility of “less attractive” components of the pheromones (saturated methyl ketones) that she-males use to attract courtship, allowing male snakes to function as transvestites only when this tactic is beneficial.     

Multiple mating opportunities boost protandry in a pied flycatcher population

Protandry, the earlier arrival of males than females to breeding areas, is widespread in birds, but its underlying mechanisms are far from well understood. The two, not mutually exclusive most highly supported hypotheses to explain avian protandry postulate that it has evolved from intrasexual male competition to acquire the best territories (“rank advantage” hypothesis) and/or to maximize the number of mates (“mate opportunity” hypothesis). We studied for two consecutive years the relative importance of both hypotheses in a population of pied flycatchers (Ficedula hypoleuca), a territorial songbird with a mixed mating strategy. We measured territory quality using a long-term dataset on nest occupation and breeding output, and we used molecular techniques to assess male fitness across the range of social and genetic mating options. Territory quality was unrelated to breeding date and had no influence on extra-pair paternity or social polygynous events. However, males breeding early increased their chances of becoming socially polygynous and/or of attaining extra-pair paternity and, as a consequence, increased their total reproductive success. These results support the “mate opportunity” hypothesis, suggesting that sexual selection is the main mechanism driving protandry in this population.     

The effects of size-dependent predation risk on the interaction between behavioral and life history traits in a stream-dwelling isopod

In field surveys, laboratory observations and field-based assays of behavior, I examined the effects of size-dependent predation risk on the interaction between size at reproductive maturity and maternal care behavior in the stream-dwelling isopod, Lirceus fontinalis. L. fontinalis exhibit population-specific sizes at reproductive maturity which result in population differences in predation risk during the adult phase. Females from streams containing salamander larvae (that prefer small prey) mature at large sizes and then become relatively safe from predation. Females from streams containing fish (that consume all size classes of prey equally) mature at small sizes and remain at risk. I tested whether these differences in expected survival were reflected in the behavior of females during the maternal phase (i.e., the period during which females exhibit maternal care). Female L. fontinalis carry developing juveniles inside a brood pouch. I simulated predatory attacks on gravid female L. fontinalis from the different population types and found that female behavior correlated with population differences in risk. When “attacked”, females from streams with predatory fish (that experience high risk to adult females) released juveniles from the brood pouch, whereas females from populations with predatory salamander larvae (that pose relatively little risk to adult females) did not release juveniles. I discuss the results with reference to the joint evolution of behavioral and life history traits. Received: 6 March 1996 /Accepted after revision: 12 August 1996     

Production and perception of situationally variable alarm calls in wild tufted capuchin monkeys (Cebus apella nigritus)

Many mammalian and avian species produce conspicuous vocalizations upon encountering a predator, but vary their calling based on risk urgency and/or predator type. Calls falling into the latter category are termed “functionally referential” if they also elicit predator-appropriate reactions in listeners. Functionally referential alarm calling has been well documented in a number of Old World monkeys and lemurs, but evidence among Neotropical primates is limited. This study investigates the alarm call system of tufted capuchin monkeys (Cebus apella nigritus) by examining responses to predator and snake decoys encountered at various distances (reflecting differences in risk urgency). Observations in natural situations were conducted to determine if predator-associated calls were given in additional contexts. Results indicate the use of three call types. “Barks” are elicited exclusively by aerial threats, but the call most commonly given to terrestrial threats (the “hiccup”) is given in nonpredatory contexts. The rate in which this latter call is produced reflects risk urgency. Playbacks of these two call types indicate that each elicits appropriate antipredator behaviors. The third call type, the “peep,” seems to be specific to terrestrial threats, but it is unknown if the call elicits predator-specific responses. “Barks” are thus functionally referential aerial predator calls, while “hiccups” are better seen as generalized disturbance calls which reflect risk urgency. Further evidence is needed to draw conclusions regarding the “peep.” These results add to the evidence that functionally referential aerial predator alarm calls are ubiquitous in primates, but that noncatarrhine primates use generalized disturbance calls in response to terrestrial threats.     

Multiple mating affects offspring size in the opisthobranch Chelidonura sandrana

Offspring size can have pervasive effects throughout the life history stages of many marine invertebrates. Although maternal offspring investment is largely determined by the environmental conditions experienced by the mother, egg size might additionally vary in response to the number and quality of previous mating partners. Positive effects of mating multiply with several different males (polyandry) have been confirmed for a variety of species, whereas such investigations are lacking for marine invertebrates. Here we differentiated between the effects of ejaculate amount (repeatedly mated) and ejaculate diversity (polyandry) on maternal offspring investment in the simultaneously hermaphroditic sea slug Chelidonura sandrana. We found that focal “females” mated with four different “males” produced significantly larger egg capsules and larger veligers, while focal “females” mated four times with the same “male” suffered from reduced mid-term fecundity. We found no effect of veliger size on veliger survival. Our results show that female mating patterns are an important addition to understanding the variation in offspring size in internally fertilizing marine invertebrates.     

Do bumble bee queens choose nest sites to maximize foraging rate? Testing models of nest site selection

We proposed “foundress-max” hypothesis that a bumble bee foundress chooses her nest site to maximize her energy intake rate from nectar. To examine the hypothesis, we estimated the maximum energy intake rate at each site in the study area and compared the distribution of the maximum energy intake rates with those of actual nest sites. We also calculated rank correlations of the maximum energy intake rate with the number of nest-searching foundresses at 54 sites. The nest locations supported the foundress-max hypothesis, but the number of nest-searching foundresses did not. This could be attributed to the density of food sites: many food sites may attract many foundresses. Therefore, we subsequently proposed “foundress-sum” hypothesis that a foundress chooses her nest site to maximize the sum of energy intake rates. The nest locations supported the foundress-max hypothesis more than the foundress-sum hypothesis. A profitable food site would affect foundresses’ nest site selection.