Masaru Hasegawa

Experimentally reduced male ornamentation increased paternal care in the Barn Swallow

AbstractIt has recently been demonstrated that male ornaments feedback on their physiological state and this is expected to be followed by behavioral alterations in paternal care. However, only a few studies have shown the effects of experimental manipulation of ornamentation on paternal care behavior. The present study investigated the effects of experimental manipulation of the size of a male ornament, the red throat patch, on the feeding rates of nestlings by male and female Barn Swallows (Hirundo rustica gutturalis). First, we observed a dynamic change in male throat patch height during the breeding season, indicating that behavioral alterations in response to changes in throat patch height may be beneficial for male Swallows. Second, males with an experimentally reduced throat patch, but not their mates, fed their nestlings significantly more often compared with control individuals. In addition, we found a significant interaction between treatment and male body size: i.e., the effects of a reduced throat patch height varied depending on male body size. Among males with a reduced throat patch, male feeding rate increased with body size, but this was not the case in control males. This interaction is consistent with the idea that small males have low behavioral flexibility, and might contribute on positive correlation between throat ornamentation and male body size, given that behavioral flexibility is adaptive. The current finding indicates that males can increase paternal care in response to a seasonal decrease in ornamentation and this differential behavioral flexibility among males may facilitate the honesty of ornamentation.ZusammenfassungExperimentelle Verkleinerung männlicher Gefiederornamente führt bei Rauchschwalben zu erhöhter väterlicher Brutfürsorge Es konnte kürzlich gezeigt werden, dass die Gefiederornamente der Männchen deren physiologischen Zustand widerspiegeln, und es ist zu erwarten, dass dies zu Verhaltensänderungen bei der väterlichen Fürsorge führt. Allerdings belegen nur wenige Untersuchungen den Einfluss experimenteller Manipulation dieser Gefiederornamente auf das väterliche Brutpflegeverhalten. Die vorliegende Studie beschäftigte sich mit den Auswirkungen experimenteller Manipulation der Größe des roten Kehlflecks bei den Männchen auf die Fütterungsraten bei männlichen und weiblichen Rauchschwalben Hirundo rustica gutturalis. Zunächst beobachteten wir bei den Männchen eine dynamische Veränderung der Kehlfleckhöhe im Verlauf der Brutsaison, was darauf hindeutet, dass Verhaltensänderungen als Reaktion auf die veränderte Kehlfleckhöhe von Vorteil für die Rauchschwalbenmännchen sein könnten. Zweitens fütterten Männchen mit experimentell verkleinertem Kehlfleck, jedoch nicht deren Partnerinnen, ihre Nestlinge signifikant häufiger als die Individuen der Kontrollgruppe. Zusätzlich fanden wir eine signifikante Wechselbeziehung zwischen experimenteller Behandlung und der Körpergröße der Männchen: Das heißt, der Effekt einer verringerten Kehlfleckhöhe variierte in Abhängigkeit von der Körpergröße der Männchen. Bei Männchen mit verkleinertem Kehlfleck nahm deren Fütterungsrate mit ihrer Körpergröße zu, bei den Männchen der Kontrollgruppe war dies nicht der Fall. Diese Wechselwirkung steht im Einklang mit der Ansicht, dass kleine Männchen in ihrem Verhalten weniger flexibel sind und könnte zu einem positiven Zusammenhang zwischen Kehlmuster und männlicher Körpergröße beitragen, falls eine Verhaltensflexibilität sich adaptiv auswirkt. Diese neuen Erkenntnisse deuten an, dass Männchen ihre väterliche Fürsorge in Reaktion auf eine jahreszeitlich bedingte Abnahme de Ornamentation steigern können und dass diese differenzielle Verhaltensflexibilität unter den Männchen die Ehrlichkeit dieses Merkmals begünstigt.

Evolution of tail fork depth in genus Hirundo

Abstract A classic example of a sexually selected trait, the deep fork tail of the barn swallow Hirundo rustica is now claimed to have evolved and be maintained mainly via aerodynamic advantage rather than sexually selected advantage. However, this aerodynamic advantage hypothesis does not clarify which flight habits select for/against deep fork tails, causing diversity of tail fork depth in hirundines. Here, by focusing on the genus Hirundo, we investigated whether the large variation in tail fork depth could be explained by the differential flight habits. Using a phylogenetic comparative approach, we found that migrant species had deeper fork tails, but less colorful plumage, than the other species, indicating that migration favors a specific trait, deep fork tails. At the same time, tail fork depth but not plumage coloration decreased with increasing bill size – a proxy of prey size, suggesting that foraging on larger prey items favors shallower fork tails. Variation of tail fork depth in the genus Hirundo may be explained by differential flight habits, even without assuming sexual selection.

Bayesian competitiveness estimation predicts dominance turnover among wild male chimpanzees

It is predicted that asymmetries in competitiveness (or resource holding potential (RHP)) would determine the outcomes of animal contests, but it has been difficult to estimate RHP in practice. In long-living animals, individual RHP changes as they age, making it necessary to quantify the individual trajectories of RHP rather than estimate a single representative value. Based on a Bayesian approach and a literature review, we estimated the age-related RHP trajectories of wild male chimpanzees, Pan troglodytes, and studied whether estimated RHP predicted the turnover of alpha males. A model containing a quadratic relationship between age and RHP fitted to the longitudinal changes in male dominance rankings well. When alpha male turnover occurred, the estimated RHP of the new alpha male was higher than were those of previous alpha males, which was explained by age. An exception to this pattern involved cases in which males formed coalitions. In these cases, a temporary increase in the RHP of the new alpha male, which was not attributed to age, was necessary for turnover. Cross-validation analysis showed that the turnover of alpha males was predictable except when males achieved alpha status via coalition formation; this exception is explainable by the instability of coalition partners in this species. This study provides a predictive framework for the occurrence of dominance turnover using incomparable long-term data and sophisticated statistical operations.

Beauty alone is insufficient: female mate choice in the barn swallow

Abstract The barn swallow, Hirundo rustica, is a model species for studying sexual selection, particularly female mate choice. Although there have already been several reviews of female mate choice and its geographic variation in this species, all of them have focused on secondary sexual characteristics. Here, for better understanding of the general pattern of female mate choice and their influence on male phenotype, I review all of the female mate choice criteria ever reported in the barn swallow, emphasizing the importance of relatively inconspicuous male traits. These include resources defended or provided by males, such as territory and paternal investment. In addition, females prefer a nestling-like vocalization, enticement call, which is particularly noteworthy because females prefer immature calls. This pattern contrasts with female choice based on secondary sexual characteristics, in which more mature, elaborate male traits are almost always favored. Nestling-like male traits are widespread, and thus female avoidance of, rather than preference for, mature forms might be common. In addition to selection on the target trait itself, these resources and nestling-like male traits would also matter in understanding the evolution of the overall male phenotype and its geographic variation, due to the interrelationships among male target traits and those among female mate preferences. Female preferences for inconspicuous traits are highly dependent on ecological factors such as nest predation pressure, and thus overall male phenotype including secondary sexual characteristics might be more predictable than previously thought. Future studies should focus on not only conspicuous secondary sexual characteristics but inconspicuous male traits.

Negative interplay of tail and throat ornaments at pair formation in male barn swallows

Pair formation is indispensable for breeding in monogamous species, generating selection for male traits that increase the probability of pairing success. Male ornamentation is one such trait, and several empirical studies have shown the importance of each of multiple ornaments. Still, it remains unclear how multiple ornaments in combination affect the probability of pair formation. Using the Japanese barn swallow, Hirundo rustica gutturalis , we studied the interplay of two sexually selected male traits, tail length and throat coloration, during pair formation. Independent of other morphological, ornamental and abiotic variables, the probability of pair formation was predicted by the negative interplay between tail length and plumage colour saturation: males possessing more-colourful plumage with shorter tails or males possessing less-colourful plumage with longer tails had a higher probability of pair formation than others. The current findings may explain spatiotemporal variation in ornamentation and sexual selection in this model species.

Rufous coloration associated with terrestrial locomotion in the adaptive radiation of Malagasy Couas

Adaptive radiation is a suitable system for the study of the (co)evolution of traits because phylogeographic constraints should be of minor importance. Adaptive radiation of beak morphology in birds is a classic example, but the evolution of other traits remains unclear. Here, using a comparative phylogenetic analysis, we studied plumage coloration in relation to terrestrial:aerial locomotion in Malagasy Couas. We hypothesized that rufous coloration would be beneficial for crypsis on the ground, and found that rufous coloration increased with the relative investment in terrestrial:aerial locomotion, measured as tarsus:wing length ratio. Terrestrial:aerial locomotion may explain the diversity of plumage coloration, at least in part.ZusammenfassungBraunrote Färbung, Fortbewegung am Boden und adaptive Radiation beim Seidenkuckuck (Madakaskar) Adaptive Radiation eignet sich besonders gut für die Untersuchung der (Ko)Evolution von Eigenschaften, weil dabei phylogeographische Einschränkungen vermutlich weniger wichtig sind. Ein klassisches Beispiel hierfür ist die adaptive Radiation bei der Schnabelmorphologie von Vögeln; die Evolution anderer Eigenschaften ist jedoch nach wie vor unklar. Anhand einer vergleichenden phylogenetischen Analyse untersuchten wir beim Seidenkuckuck (Madakaskar) die Gefiederfärbung in Verbindung mit der Fortbewegung am Boden und in der Luft. Wir nahmen an, die braunrote Färbung wäre vorteilhaft für die Tarnung am Boden und stellten fest, dass die braunrote Färbung mit wachsendem relativem Investment in terrestrischer zu „geflogener“ Fortbewegung, gemessen als das Verhältnis von Tarsus- zur Flügellänge, intensiver wurde. Diese beiden Fortbewegungsarten können eventuell, zumindest zum Teil, die Unterschiede in den Gefiederfärbungen erklären.

Bayesian estimation of competitiveness in male house finches: small-billed males are more competitive

A common method for assessing the competitiveness of animals, especially in birds, is to pit pairs of unfamiliar individuals against each other in contests for limited resources under controlled conditions. Although this approach can clarify dominant–subordinate relationships within dyads, it is often difficult to determine competitiveness for a large group of individuals. Here, by using Bayesian statistical inference and ‘hypothetical competition groups’, which are formed when individuals experience a series of paired contests, we estimated social competitiveness of male house finches, Haemorhous mexicanus. First, Bayesian competitiveness estimates from paired contests successfully predicted future contest outcomes among four unfamiliar individuals (i.e. social dominance). When data of all rank combinations were pooled, future dominant males had, on average, higher competitiveness estimates than future subordinate males. Similarly, Bayesian statistical inference and hypothetical competition groups identified accurately the relative competitiveness of four subgroups of males (i.e. colourful and drab males from urban and rural sites), which matched the result of direct contests when they were all put into the same cage. This consistency reinforces the validity of Bayesian competitiveness estimation based on hypothetical competition groups. Moreover, we found that the competitiveness estimate was negatively linked to male beak size in the Bayesian framework. Males with smaller bills were more competitive than those with larger bills, perhaps due to their elevated foraging motivation (i.e. limited ability to consume or husk large, valuable seeds). We argue that Bayesian competitiveness estimations, together with a series of paired contests, is a sophisticated approach for acquiring a broad understanding of social and individual competitiveness.

Sexual Selection Mechanisms for Male Plumage Ornaments in Japanese Barn Swallows

Abstract The Barn Swallow Hirundo rustica is a model species for studying sexual selection. After its long tail was shown to be the target of female mate choice, its plumage characteristics, including the red throat patch and the size of white tail spots in addition to tail length, have attracted the attention of many researchers. Although there have been several reviews of sexual selection in this species, these reviews have focused mostly on general patterns of sexual selection within and among subspecies, rather than on the functional differences between plumage characteristics in sexual selection. Here, I review mechanisms of sexual selection for male plumage ornaments in Japanese Barn Swallows H. r. gutturalis. In the last decade, studies of sexual selection in Japanese Barn Swallows have focussed on tail length, the size of the white tail spots, and the size and coloration of red throat patches. Each ornamental trait appears to be involved in different sexual selection mechanisms: the size of the white tail spots is important for several mechanisms of intersexual selection, including the Darwin-Fisher mechanism and differential allocation, whereas throat coloration may be intra-sexually selected, at least in part. The size of the throat patch, in contrast, is the target of an intersexual selection mechanism, differential access, indicating a partially independent evolution of the two components of the red throat patch. Multiple mechanisms of sexual selection would explain the multiple ornaments and their geographical variation, and even the probability of speciation and extinction.

Imperfect nestling-mimicry attracts high-quality females in the Asian barn swallow

The sensory trap hypothesis predicts that male signals mimic stimuli to which females respond in other contexts, yet these signals may not elicit equivalent responses from all females. Female barn swallows Hirundo rustica are attracted to male enticement calls, particularly high-pitched ones that resemble the food-begging calls of nestlings to a greater extent. However, male enticement calls are still distinguishable from nestling food-begging calls, denoting “imperfect mimicry”. It remains unknown why perfect mimicry is prevented given that nestling-like calls effectively attract females. Here, using Asian subspecies (H. r. gutturalis), we examined female preference for male enticement calls to nestling food-begging calls during the mating period in relation to the ornamentation of her previous mate and trial date, a proxy of female arrival date, in this experiment with the expectation that females vary their response depending on these variables. We found that female reaction to the playback of male enticement calls, relative to the playback of nestling food-begging calls, increased with increasing plumage coloration of her previous mate, and decreased with increasing trial date (and hence late arrival). This observation is predictable, because colorful males emit lower-pitched, i.e. less nestling-like, enticement calls, and because early-arriving females, which are more experienced females, would have higher discriminability than others. Imperfect mimicry to nestling food-begging calls would attract relatively high-quality females, and thus males should not emit enticement calls that closely resemble nestling food-begging calls except when high-quality females are not available.

Sex allocation based on maternal body size in Japanese barn swallows

Sex allocation is the parental resource allocation to sons versus daughters. Given the different benefits and costs of investments in sons and daughters, females should adjust the brood sex ratio according to their own quality to maximize their fitness. In the barn swallow Hirundo rustica, as in many other passerines, females should increase the proportion of sons in their broods with increasing maternal body size, because rearing sons is particularly costly to small females with low feeding capacity, and because reproductive success in the barn swallow, which may depend on rearing conditions, varies more in males. However, evidence supporting this prediction is lacking, even in this model species of sex allocation. Using the Asian subspecies of the barn swallow H. r. gutturalis, we confirmed this prediction. Larger mothers with a longer keel had more sons in their brood. We found that male nestlings invested more in pheomelanin pigmentation compared with female nestlings, while they had shorter wing lengths, indicating that male nestlings are costly to rear compared with female nestlings. Smaller mothers that are at a disadvantage in provisioning may not retard the nestling period by producing daughters, which reduces their parental investment and thus saves their residual reproductive value. This breeding strategy might be particularly beneficial to Japanese barn swallows that breed in environments with high nest predation.