Age-Related Variation in Mate-Guarding Intensity in the Bluethroat (Luscinia s. svecica)

Female extra‐pair copulations (EPCs) have selected for male paternity guarding strategies in many bird species. In the bluethroat, Luscinia s. svecica, males guard their mates closely during the last 2 d before the start of egg laying, but there is great individual variation in the intensity of mate guarding. Here we show that some of this variation is related to male age. Old males guarded their mates with much lower intensity and sang more than young males, although the latter difference was not statistically significant. Controlling for male age, male and female coloration and size were not significantly related to the intensity of mate guarding. We have previously shown that young and old males had a similar paternity loss in their own broods. On the other hand, old males were far more successful than young males in achieving extra‐pair fertilizations. These patterns suggest that young and old males have different trade‐offs between preventing paternity loss in own nest and gaining paternity in others, because male skills in obtaining EPCs improve with experience and/or because of female preferences for old males as copulation partners. There were no significant relationships between paternity and male mate‐guarding behaviour during the fertile period, indicating that mate guarding is not a very effective paternity‐assurance strategy in the bluethroat.     

Sex-specific patterns in body condition and testosterone level changes in a territorial migratory bird: the Bluethroat Luscinia svecica

The Bluethroat Luscinia svecica is a migratory passerine that exhibits a socially monogamous pair bond and a high level of parental care. Males are territorial both when wintering and breeding whereas females are territorial only in winter. We investigated changes in body condition and testosterone levels during successive life-history stages and determined their relationships. Sex-specific patterns were observed in the variation in body condition and testosterone level. Male body condition varied mainly during the winter. It peaked at the onset of the prenuptial moult and then decreased, whereas it remained stable throughout breeding. In contrast, female body condition varied mainly during the breeding season. It increased during the prelaying stage and then abruptly decreased until fledgling provisioning. As in other monogamous and territorial passerines, testosterone levels in Bluethroat males were low during winter, increased in late winter, peaked during the prelaying stage and then decreased when provisioning young. In wintering females, territorial competition caused testosterone levels to rise. These females were able to produce territorial vocalizations and exhibit aggressive postures. Females showed higher mean testosterone levels than males when wintering whereas the opposite was observed when breeding. Our data from wintering female Bluethroats support the 'challenge hypothesis' under which high testosterone levels are associated with periods of social instability, and testosterone can regulate female territorial behaviour during this period.     

Energy crops as a new bird habitat: utilization of oilseed rape fields by the rare Bluethroat (Luscinia svecica)

As part of the expanding cultivation of renewable primary products, oilseed rape has become a prominent part of the agricultural landscape of many European countries. However, the ecological impacts of increasing cultivation of oilseed rape have received little attention so far. In this study, we provide the first systematic investigation of the suitability of oilseed rape as a habitat for the Bluethroat (Luscinia svecica cyanecula), an endangered and highly specialized bird species. We conducted territorial mapping and a habitat analysis, revealing the distinct habitat requirements of L. s. cyanecula in oilseed rape fields over three breeding seasons. The soil type was identified as the crucial factor for habitat occupancy. It acts as a surrogate for soil moisture and this in turn decisively influences food availability. Ditches can further improve habitat quality, whereas reed is not a necessary structure, although it is a regular feature in the primary habitat of the Bluethroat. Occupied oilseed rape stands were structurally different from the semi-natural reed habitat, but provided similar essential habitat requirements, such as shelter from predation and moist, bare soil. Our results clearly demonstrate the utilization of oilseed rape by a rare and threatened wetland bird species and suggest that this crop may act as a secondary habitat.     

Parasite Infestation and Parental Care in the Barn Swallow Hirundo rustical a Test of the Resource-provisioning Model of Parasite-mediated Sexual Selection

Barn swallows, Hirundo rustica, are commonly infested by the haematophagous tropical fowl mite Omithonyssus bursa (Macronyssidae, Gamasida), which severely reduces various measures of reproductive success among the barn swallow hosts. Food provisioning rate by parent barn swallows, measured in terms of absolute feeding rate by males and females and relative feeding rate by males (percentage of food provided by the male parent), was not significantly related to natural levels of infestation of nests. Experimental manipulation of mite loads in nests during the egg-laying period of the first clutch, which also affected mite loads of parent barn swallows, significantly affected food-provisioning rates of single-brooded, but not of double-brooded barn swallows. These results suggest that effects of mites on the parenting ability of barn swallow hosts depend on host resistance towards parasites. This is consistent with the resource-provisioning hypothesis of parasite-mediated sexual selection, suggesting that females prefer parasite-free males because they are efficient parents, but also with the hypothesis that females prefer males with traits signalling genetic resistance to parasites.     

Ultraviolet plumage ornamentation affects social mate choice and sperm competition in bluethroats (Aves: Luscinia s. svecica): a field experiment

The blue throat feathers of male bluethroats (Luscinia s. svecica) show a reflectance peak in the ultraviolet (UV) waveband (320 to 400 nm). The throat is actively displayed during courtship, suggesting a role for sexual selection on an ultraviolet signal. Indeed, a recent aviary experiment demonstrated that females discriminated against males with artificially reduced UV reflectance (Andersson and Amundsen 1997). Here, we report the results of a similar experimental manipulation applied on free-ranging males. UV-reduced (UVR) males had a lower success in attracting mates, as judged from a significantly later start of egg laying, compared with control (C) males. UVR males also spent significantly less time advertising for additional mates when their own mate was fertile, and they had a lower success in achieving extra-pair fertilizations. Furthermore, UVR males tended to guard their mates more closely and lose more paternity in their own brood than C males did. We conclude that the treatment affected both social and extra-pair mate choice. This is the first experimental evidence that UV signalling influences male mating success in free-ranging birds.     

Childhood Association and Sexual Attraction: A Further Test of the Westermarck Hypothesis

A preliminary study of household registration records from Taiwan supports Edward Westermarck's contention that intimate childhood association promotes sexual aversion. Women who are forced to marry a childhood associate bear fewer children than those who marry a stranger. They are also more likely to leave their husband by divorce or avoid him in favor of other men. This evidence suggests that the incest taboo does not prohibit what men's feelings incline them to do, as Westermarck's critics argue, but that it is instead an expression of these feelings, socially unnecessary but psychologically inevitable. [incest taboo, law, China, Taiwan]     

Functional infertility among territorial males in two passerine species, the willow warbler Phylloscopus trochilus and the bluethroat Luscinia svecica

Information about male infertility in free-living birds is scarce, but anecdotal and circumstantial evidence suggests that it does occur regularly at a low frequency. In this paper we document three cases of azoospermia in two passerine species, the willow warbler Phylloscopus trochilus and the bluethroat Luscinia svecica at their breeding grounds in South Norway. In willow warblers, two males out of a sample of 50 territory holders had no sperm in their seminal glomera, the storage site of sperm ready for ejaculation. The two males also had very small testes. One out of 48 bluethroat males also had no sperm in the seminal glomera. This male had an extreme asymmetry of the testes, with the right testis being about twice as large as the left. He also failed to fertilize any eggs in his own nest, as well as in neighbouring nests, as revealed by microsatellite genotyping. Thus, the proportion of males without sperm seems to be at a magnitude of a few (2–4) percent in both species. These are among the first estimates of the frequency of azoospermia in wild birds. Our results indicate a significant risk for sexually monogamous females of laying unfertilized eggs, which could favour the evolution of extra-pair copulation as a fertility insurance strategy in females.     

Age-related variation in primary sexual characters in a passerine with male age-related fertilization success, the bluethroat Luscinia svecica

In passerine species with frequent extrapair mating, young (second calendar year) males often have a lower fertilization success than older (after second calendar year) males. This pattern might be explained by male- or female-driven mechanisms, such as female preference for older males or higher competitive ability of older males. In this study we measured the size of the testes, the seminal glomera and the cloacal protuberance as well as the size and motility of the sperm, in individual bluethroats Luscinia svecica . In this species, nearly all extrapair fertilizations (EPFs) are obtained by older males. We found that the mass of the testes and the seminal glomera were highly positively correlated and that older males had significantly larger testes (38%), seminal glomera (15%) and cloacal protuberance (23%) than young males. In contrast, there was no difference between age groups in average sperm size or sperm motility. Our results are consistent with the idea that higher fertilization success by older males in this species is due to their higher rate of sperm production, allowing larger ejaculates and/or more frequent copulations. Unequal sperm production capacities by young and older males have important implications for the interpretation of paternity patterns in extrapair mating systems.     

Infanticide in Propithecus diadema edwardsi: An Evaluation of the Sexual Selection Hypothesis

Infanticide might be described as a reproductive strategy employed by anthropoid primate males when they immigrate into new groups. But infanticide has rarely been observed in wild prosimian primates. For the Malagasy lemurs this may reflect one or more of the following: strict breeding seasons; relative monomorphism in canine tooth and body size; small group sizes; male–female dominance relations; and male–female dyads within groups. We addressed the following questions: Do prosimian males commit infanticide in circumstances similar to those in which anthropoids do? and Is there any reproductive advantage for a highly seasonal breeder to commit infanticide? To help answer these questions, we describe the death of a 24-hr-old infant male Propithecus diadema edwardsi from wounds received during a fight between his mother, her adult daughter, and a newly immigrant male. Interbirth intervals between surviving offspring are 2 years for Propithecus diadema edwardsi; therefore, a male could dramatically shorten the time between reproductive windows by killing an infant. Whether this tactic would be favored by sexual selection cannot be addressed until more information has been collected on the length of interbirth interval due to infanticide relative to that of infant death by other causes; how social factors such as stability of breeding relationships affect long-term male reproductive success; how effective female counterstrategies are to prevent infanticide and/or whether they choose to mate with males that commit infanticide; and how often males that kill infants subsequently sire infants, particularly in groups that contain a resident male.     

A Test for Male Parental Care in a Fundulid, the Bluefin Killifish, Lucania goodei

Under field conditions, breeding male bluefin killifish, Lucania goodei, have been observed aggressively defending territories from other breeding males, non-breeding females, and minnows (mainly Notropis harperi). We performed an aquarium experiment to test whether male aggression serves to protect newly deposited eggs from predation. We allowed a male and a female to spawn in a yarn mop, removed the female, and exposed the eggs to one of four treatments (spawning male present, two minnows present, spawning mal+two minnows present, no adult fish present). Mops were censused daily for seven days. Egg predation rates were highest in the male+minnows and male only treatments. Egg predation rates in the male+minnows treatment did not differ from the predicted predation rate (sum of male only and minnows only treatments). Hence, there is no evidence for male parental care in L. goodei. In addition, we compared the egg predation rates (filial cannibalism) between males of 3 different color morphs and found no evidence for differential egg cannibalism.     

Olfaction, Sexual Behavior, and the Pheromone Hypothesis in Rhesus Monkeys: A Critique

SYNOPSIS. Evidence in support of and contrary to the hypothesisthat rhesus monkeys possess a sexual pheromonal system is presentedand discussed. New evidence demonstrates that solicitationsof females given estradiol are more attractive to males in partbecause the females allow males to mount much more frequentlyonly when the females have initiated the sexual interaction.In addition, evidence is presented which demonstrates that anodorous control substance stimulates sexual activity just aswell as the purported "active component" of vaginal secretionsunder restricted conditions of evaluation. Finally, preliminaryevidence suggests the possibility that odorous products fromthe vagina, unrelated to short-chain aliphatic acids, presentqualitatively distinct cues to the male during the periovulatoryphase of the menstrual cycle. However, only sexually experiencedmales, and not young adult "virginal" males nor adult isosexually-rearedmales, selectively attended to midcycle vaginal products whenthese were placed on environmental surfaces. It is concludedthat male rhesus monkeys may utilize odorous cues from the femaleduring sexual interactions, but these cues are neither necessarynor sufficient for the coordination of fertile matings. Furthermore,olfactory communication in this species does not fit the accepteddefinition of pheromone: the odors are not specific for onespecies; the behavior elicited is not specific to sexual arousal;and volatile materials other than vaginal products can stimulatesexual activity.     

Maternal Investment in the Swordtail Fish Xiphophorus multilineatus: Support for the Differential Allocation Hypothesis

The differential allocation hypothesis predicts that reproductive investment will be influenced by mate attractiveness, given a cost to reproduction and a tradeoff between current and future reproduction. We tested the differential allocation hypothesis in the swordtail fish Xiphophorus multilineatus, where males have genetically influenced (patroclinous inheritance) alternative mating tactics (ARTs) maintained by a tradeoff between being more attractive to females (mature later as larger courting males) and a higher probability of reaching sexual maturity (mature earlier as smaller sneaker males). Males in X. multilineatus do not provide parental care or other resources to the offspring. Allelic variation and copy number of the Mc4R gene on the Y-chromosome influences the size differences between males, however there is no variation in this gene on the X-chromosome. Therefore, to determine if mothers invested more in offspring of the larger courter males, we examined age to sexual maturity for daughters. We confirmed a tradeoff between number of offspring and female offspring’s age to sexual maturity, corroborating that there is a cost to reproduction. In addition, the ART of their fathers significantly influenced the age at which daughters reached sexual maturity, suggesting increased maternal investment to daughters of courter males. The differential allocation we detected was influenced by how long the wild-caught mother had been in the laboratory, as there was a brood order by father genotype (ART) interaction. These results suggest that females can adjust their reproductive investment strategy, and that differential allocation is context specific. We hypothesize that one of two aspects of laboratory conditions produced this shift: increased female condition due to higher quality diet, and/or assessment of future mating opportunities due to isolation from males.     

Infant Killing in Primates: A Review of Observed Cases with Specific Reference to the Sexual Selection Hypothesis

Discussion of infant killing in free-ranging primates has focused on the sexual selection hypothesis developed by Hrdy during the mid-1970s. This hypothesis suggests that infant killing is a form of sexual competition whereby an infanticidal male gains a reproductive advantage by selectively killing the offspring of his male rivals. Despite criticisms that the evidence in support of the hypothesis is distorted by misinterpretation of data and observer bias, the sexual selection hypothesis, bolstered in part by additional reports of infanticide in a variety of specks, has become entrenched as the primary explanatory hypothesis for primate infanticide. However, the majority of reliably documented instances of infanticide in primates come from a very small number of species, and a careful examination of the specific context of each of these episodes fails to support the interpretation of infanticide as a primatewide adaptive complex. Most importantly, the atmosphere of generalized inter- and intrasexual aggression that surrounds the majority of infant killings obscures the evolutionary significance of this behavior.     

Frequency of Encounter with Risk and the Tradeoff between Pursuit and Antipredator Behaviors in Crayfish: A Test of the Risk Allocation Hypothesis

Prey animals encountering multiple stimuli must often make behavioral tradeoffs. Many environmental cues may influence the tradeoff observed, but recent theoretical work suggests that temporal variation in risk should influence how prey animals behave during any given period of risk. As time spent under risk of predation increases, prey animals will increase their allocation of foraging during periods of risk. This model is known as the risk allocation hypothesis (RAH) ( Lima & Bednekoff 1999 ). We tested the RAH using the crayfish Orconectes virilis. We selected two frequency regimes (exposure to risk every 6 or 12 h) and three cues suggestive of increasing risk (water, snapping turtle cue, and conspecific alarm cue). Test animals were exposed to one of the six frequency × risk combinations for 24 h, followed immediately by the simultaneous introduction of a food and a risk cue. Three behaviors (burrow use, non‐ambulatory motion, and locomotion) were then recorded for 5 min. Responses were significantly influenced by the interaction of risk and frequency. Further analysis indicated that responses were not consistently influenced by frequency alone. While our results do not support the predictions of the RAH for our frequency regimes, qualitative comparison with an earlier, similar study ( Hazlett 1999 ) suggests that risk allocation is occurring in this system. We recommend that frequency of encounter with risk be considered in future studies. Ignoring temporal variation may lead to over‐ or underestimation of the subject's natural responses.     

Maternal Care, Iteroparity and the Evolution of Social Behavior: A Critique of the Semelparity Hypothesis

The Semelparity Hypothesis (Tallamy and Brown in Animal Behav 57:727–730, 1999) predicts that among insects with parental care that iteroparity will be rare. It represents two important challenges. First, life history ecologists have sometimes linked extended parental care with iteroparity, not semelparity, as part of a suite of correlated characters associated with K-selective environments. Second, behavioral ecologists have developed theories for the evolution of eusociality that rely upon a subsocial species producing multiple cohorts of offspring, a precondition for offspring allocare and/or inheritance of a social unit. Using a database of invertebrates exhibiting maternal care in Costa (The other insect societies. Harvard University Press, Cambridge, 2006), the association between semelparity and maternal care was tested using a broad comparative analysis. Semelparity was found in only 24.5 % of the best-studied representative species. In addition, semelparity was more rare in species that form nests, burrows or galleries (12.1 %) than in species that guard offspring out in the open (45.0 %). Iteroparity was common both among nesting species with non-overlapping broods (serial nesting) and in species where a female produces broods of different aged offspring in the same nest (within-nest iteroparity). It is hypothesized that common factors, particularly rapid juvenile development on high quality resources, facilitated both serial nesting and parental care. Within-nest iteroparity is an essential stage in the evolution of eusociality that has often been overlooked. Recent models of sibling conflict and reproductive spacing suggest that parental care can be an indirect cause of within-nest iteroparity despite the fact that parental investment can lead directly to diminished future reproduction. The reversal of this life history correlation may occur as a result of the transition between asocial and subsocial nesting behavior; analogous reversals may be a frequent outcome of transitions between levels of social organization.     

Tropical Monodominance: A Preliminary Test of the Ectomycorrhizal Hypothesis1

This study reports results from the first explicit test of the ectomycorrhizal hypothesis for tropical monodominance in the Ituri Forest of the Democratic Republic of Congo (formerly Zaire), where the canopy tree Gilbertiodendron dewevrei forms large, monospecific stands. To test the hypothesis that ectomycorrhizae are important to the success of dominant species, we surveyed the mycorrhizal status of dominant species, as well as other common, but not dominant, species in the forest. The survey reveals that two dominant species, Gilbertiodendron dewevrei and Julbernardia seretii, form ectomycorrhizae and vesicular-arbuscular mycorrhizae, while Cynometra alexandri, another dominant, forms only vesicular-arbuscular mycorrhizae. These results, along with those of other species in this and other forests, are discussed within the context of the ectomycorrhizal hypothesis for tropical mondominance. This study demonstrates that the relationship between EM and tropical monodominance is more complex than has been previously recognized.     

The Sexual Cascade and the Rise of Pre-Ejaculatory (Darwinian) Sexual Selection,Sex Roles,and Sexual Conflict

After brief historic overviews of sexual selection and sexual conflict, I argue that pre-ejaculatory sexual selection (the form of sexual selection discussed by Darwin) arose at a late stage in an inevitable succession of transitions flowing from the early evolution of syngamy to the evolution of copulation and sex roles. If certain conditions were met, this “sexual cascade” progressed inevitably, if not, sexual strategy remained fixed at a given stage. Prolonged evolutionary history of intense sperm competition/selection under external fertilization preceded the rise of advanced mobility, which generated pre-ejaculatory sexual selection, followed on land by internal fertilization and reduced sperm competition in the form of postcopulatory sexual selection. I develop a prospective model of the early evolution of mobility, which, as Darwin realized, was the catalyst for pre-ejaculatory sexual selection. Stages in the cascade should be regarded as consequential rather than separate phenomena and, as such, invalidate much current opposition to Darwin–Bateman sex roles. Potential for sexual conflict occurs throughout, greatly increasing later in the cascade, reaching its peak under precopulatory sexual selection when sex roles become highly differentiated.Sexual selection and sexual conflict are vast fields in evolutionary biology; when possible, here, I refer to reviews. I begin with brief general historic overviews of sexual selection and sexual conflict; more detail can be found in Andersson (1994), Simmons (2001), Chapman et al. (2003), and Arnqvist and Rowe (2005). Much of the current state of the field of sexual conflict is covered in this collection.My principal aim, however, is to outline how sexual selection and sexual conflict have changed through evolutionary time, from mostly gamete competition in early unicellular eukaryotes, intense sperm competition in ancestral sessile and relatively immobile organisms, to both pre-ejaculatory (Darwinian) and postejaculatory sexual selection. These transitions in the evolution of sexual strategy arise as logical consequences whenever certain successive conditions are met, and together form what may be termed the “sexual cascade.”