Divorce in the barn owl: securing a compatible or better mate entails the cost of re-pairing with a less ornamented female mate.

Two nonmutually exclusive hypotheses can explain why divorce is an adaptive strategy to improve reproductive success. Under the 'better option hypothesis', only one of the two partners initiates divorce to secure a higher-quality partner and increases reproductive success after divorce. Under the 'incompatibility hypothesis', partners are incompatible and hence they may both increase reproductive success after divorce. In a long-term study of the barn owl (Tyto alba), we address the question of whether one or the two partners derive fitness benefits by divorcing. Our results support the hypothesis that divorce is adaptive: after a poor reproductive season, at least one of the two divorcees increase breeding success up to the level of faithful pairs. By breeding more often together, faithful pairs improve coordination and thereby gain in their efficiency to produce successful fledglings. Males would divorce to obtain a compatible mate rather than a mate of higher quality: a heritable melanin-based signal of female quality did not predict divorce (indicating that female absolute quality may not be the cause of divorce), but the new mate of divorced males was less melanic than their previous mate. This suggests that, at least for males, a cost of divorce may be to secure a lower-quality but compatible mate. The better option hypothesis could not be formally rejected, as only one of the two divorcing partners commonly succeeded in obtaining a higher reproductive success after divorce. In conclusion, incompatible partners divorce to restore reproductive success, and by breeding more often together, faithful partners improve coordination

Social rules govern vocal competition in the barn owl

To resolve the share of limited resources, animals often compete through exchange of signals about their relative motivation to compete. When two competitors are similarly motivated, the resolution of conflicts may be achieved in the course of an interactive process. In barn owls, Tyto alba, in which siblings vocally compete during the prolonged absence of parents over access to the next delivered food item, we investigated what governs the decision to leave or enter a contest, and at which level. Siblings alternated periods during which one of the two individuals vocalized more than the other. Individuals followed turn-taking rules to interrupt each other and momentarily dominate the vocal competition. These social rules were weakly sensitive to hunger level and age hierarchy. Hence, the investment in a conflict is determined not only by need and resource-holding potential, but also by social interactions. The use of turn-taking rules governing individual vocal investment has rarely been shown in a competitive context. We hypothesized that these rules would allow individuals to remain alert to one another's motivation while maintaining the cost of vocalizing at the lowest level

Information retention during competitive interactions : siblings need to constantly repeat vocal displays

Memory is essential to adjust behaviour according to past experience. In societies where animals interact on numerous occasions, memory of previous social interactions may help optimise investment in competition. How long information about the resource holding potential and motivation to compete of conspecifics is retained depends on how fast the value of this information fades, but also on the cost and benefit of retaining information. Information retention has never been investigated in the context of interactions prevailing within the family and more specifically sibling competition. In the absence of parents, barn owl (Tyto alba) nestlings vocally compete for priority of access to the next indivisible food item brought by a parent. The finding that owlets eavesdrop on vocal interactions between siblings to adjust investment in vocalization once competing with them suggests that they memorize siblings' vocal interactions. Playback experiments showed that owlets take into account the past siblings' vocal performance that signals hunger for at least 15 min, but only if the performance was witnessed during a sufficiently long period of time (30 min). Moreover, using natural vocal exchanges in another set of individuals, we showed that sibling signalling was no more taken into account after a few minutes. This suggests that young barn owls need to continuously display their motivation to trigger siblings' withdrawal from the current competition. Repeating a vocal display may ensure its honesty. Studying the extent to which individuals retain past information is important to understand how individuals adjust their competitive investment over resources

Prosody Predicts Contest Outcome in Non-Verbal Dialogs.

Non-verbal communication has important implications for inter-individual relationships and negotiation success. However, to what extent humans can spontaneously use rhythm and prosody as a sole communication tool is largely unknown. We analysed human ability to resolve a conflict without verbal dialogs, independently of semantics. We invited pairs of subjects to communicate non-verbally using whistle sounds. Along with the production of more whistles, participants unwittingly used a subtle prosodic feature to compete over a resource (ice-cream scoops). Winners can be identified by their propensity to accentuate the first whistles blown when replying to their partner, compared to the following whistles. Naive listeners correctly identified this prosodic feature as a key determinant of which whistler won the interaction. These results suggest that in the absence of other communication channels, individuals spontaneously use a subtle variation of sound accentuation (prosody), instead of merely producing exuberant sounds, to impose themselves in a conflict of interest. We discuss the biological and cultural bases of this ability and their link with verbal communication. Our results highlight the human ability to use non-verbal communication in a negotiation process

Sleep and vigilance linked to melanism in wild barn owls.

Understanding the function of variation in sleep requires studies in the natural ecological conditions in which sleep evolved. Sleep has an impact on individual performance and hence may integrate the costs and benefits of investing in processes that are sensitive to sleep, such as immunity or coping with stress. Because dark and pale melanic animals differentially regulate energy homeostasis, immunity and stress hormone levels, the amount and/or organization of sleep may covary with melanin-based colour. We show here that wild, cross-fostered nestling barn owls (Tyto alba) born from mothers displaying more black spots had shorter non-REM (rapid eye movement) sleep bouts, a shorter latency until the occurrence of REM sleep after a bout of wakefulness and more wakefulness bouts. In male nestlings, the same sleep traits also correlated with their own level of spotting. Because heavily spotted male nestlings and the offspring of heavily spotted biological mothers switched sleep-wakefulness states more frequently, we propose the hypothesis that they could be also behaviourally more vigilant. Accordingly, nestlings from mothers displaying many black spots looked more often towards the nest entrance where their parents bring food and towards their sibling against whom they compete. Owlets from heavily spotted mothers might invest more in vigilance, thereby possibly increasing associated costs due to sleep fragmentation. We conclude that different strategies of the regulation of brain activity have evolved and are correlated with melanin-based coloration

Age- and sex-specific response to population density and sex ratio

Both population density and sex ratio shape competition for mates, resources and mating costs. Thus they may critically affect the intensity of sexual selection in the populations. Susceptibility to inter- and intrasexual competition, which changes with age in a large number of species, may additionally influence population response to these demographic factors. In this study, we monitored 16 seminatural populations of common lizards (Lacerta vivipara) to determine whether the reproductive output varied with male and female densities as a function of the individual sex and age. Our results suggest that the intensity of sexual selection was weaker in male-biased populations, supporting new theoretical models. In populations with a male-biased sex ratio, reproductive success was more equally distributed between males and, unlike female-biased populations, the choosiest females (middle-aged) did not obtain sires of higher quality than low-performance females. Our results also suggest that age may influence the intensity of sexual conflict. Middle-aged females (the class with the best performance) produced offspring with a lower body condition in male-biased populations, suggesting that they may be the preferred target of male harassment. By contrast, a male-biased sex ratio appeared to be beneficial for low-quality females, allowing these females to obtain higher quality sires and to produce offspring with a better body condition. These age- and sex-dependent responses to population density and sex ratio have important implications for population ecology and sexual selectio

Corticosterone promotes scramble competition over sibling negotiation in Barn Owl nestlings (Tyto alba)

In species with parental care, siblings compete for access to food resources. Typically, they vocally signal their level of need to each other and to parents, and jostle for the position in the nest where parents deliver food. Although food shortage and social interactions are stressful, little is known about the effect of stress on the way siblings resolve the conflict over how food is shared among them. Because glucocorticoid hormones mediate physiological and behavioral responses to stressors, we tested whether corticosterone, the main glucocorticoid in birds, modulates physical and vocal signaling used by barn owl siblings (Tyto alba) to compete for food. Although corticosterone-implanted (cort-) nestlings and placebo-nestlings were similarly successful to monopolize food, they employed different behavioral strategies. Compared to placebo-nestlings, cort-individuals reduced the rate of vocally communicating with their siblings (but not with their parents) but were positioned closer to the nest-box entrance where parents predictably deliver food. Therefore, corticosterone induced nestlings to increase their effort in physical competition for the best nest position at the expense of investment in sib-sib communication without modifying vocal begging signals directed to parents. This suggests that in the barn owl stress alters nestlings' behavior and corticosterone could mediate the trade-off between scramble competition and vocal sib-sib communication. We conclude that stressful environments may prevent the evolution of sib-sib communication as a way to resolve family conflicts peacefully

Vocal communication regulates sibling competition over food stock

Animals resolve conflicts over the share of resources by competing physically or signalling motivation with honest signals of need. In some species, young siblings vocally signal to each other their hunger level and the most vocal individual deters its siblings from competing for the non-divisible food item delivered at the next parental visit. This so-called sibling negotiation for forthcoming food has been studied only in this context. It therefore remains unclear whether siblings could also negotiate access to a pool of divisible resources, a situation that is similar to a group of individuals competing for an accessible food resource. To tackle this issue, we placed barn owl (Tyto alba) nestlings singly in artificial nests containing several mice, and we simulated the presence of a sibling calling at low or high rate using playback experiments. If nestling barn owls vocally negotiate over a divisible food stock, we propose the following two predictions. First, nestlings would vocally signal before eating from this stock of food, and second, numerous playback vocalizations would inhibit feeding. Accordingly, singleton nestlings vocalized just before consuming food stored in their artificial nest and they delayed the consumption of the food stock if hearing many playback calls. The production of such food-associated vocalizations has been observed in foraging adults in various birds and mammals, but never in young animals and when resource is divisible and easily accessible. Our study raises the possibility that vocal communication could evolve in a variety of competitive contexts. We present here the first experimental evidence that sibling barn owls use food-associated vocalizations to compete over the preys stored in the nest. Owlets emit calls just before consuming an available food item and broadcasting calls induces nestlings to temporarily refrain from eating from the food stock. This raises the possibility that vocal communication can mediate the share of a food stock accessible to all competitors

Reciprocal preening and food sharing in colour-polymorphic nestling barn owls.

Barn owl (Tyto alba) siblings preen and offer food items to one another, behaviours that can be considered prosocial because they benefit a conspecific by relieving distress or need. In experimental broods, we analysed whether such behaviours were reciprocated, preferentially exchanged between specific phenotypes, performed to avoid harassment and food theft or signals of hierarchy status. Three of the results are consistent with the hypothesis of direct reciprocity. First, food sharing was reciprocated in three-chick broods but not in pairs of siblings, that is when nestlings could choose a partner with whom to develop a reciprocating interaction. Second, a nestling was more likely to give a prey item to its sibling if the latter individual had preened the former. Third, siblings matched their investment in preening each other. Manipulation of age hierarchy showed that food stealing was directed towards older siblings but was not performed to compensate for a low level of cooperation received. Social behaviours were related to melanin-based coloration, suggesting that animals may signal their propensity to interact socially. The most prosocial phenotype (darker reddish) was also the phenotype that stole more food, and the effect of coloration on prosocial behaviour depended upon rank and sex, suggesting that colour-related prosociality is state dependent

Social huddling and physiological thermoregulation are related to melanism in the nocturnal barn owl.

Endothermic animals vary in their physiological ability to maintain a constant body temperature. Since melanin-based coloration is related to thermoregulation and energy homeostasis, we predict that dark and pale melanic individuals adopt different behaviours to regulate their body temperature. Young animals are particularly sensitive to a decrease in ambient temperature because their physiological system is not yet mature and growth may be traded-off against thermoregulation. To reduce energy loss, offspring huddle during periods of cold weather. We investigated in nestling barn owls (Tyto alba) whether body temperature, oxygen consumption and huddling were associated with melanin-based coloration. Isolated owlets displaying more black feather spots had a lower body temperature and consumed more oxygen than those with fewer black spots. This suggests that highly melanic individuals display a different thermoregulation strategy. This interpretation is also supported by the finding that, at relatively low ambient temperature, owlets displaying more black spots huddled more rapidly and more often than those displaying fewer spots. Assuming that spot number is associated with the ability to thermoregulate not only in Swiss barn owls but also in other Tytonidae, our results could explain geographic variation in the degree of melanism. Indeed, in the northern hemisphere, barn owls and allies are less spotted polewards than close to the equator, and in the northern American continent, barn owls are also less spotted in colder regions. If melanic spots themselves helped thermoregulation, we would have expected the opposite results. We therefore suggest that some melanogenic genes pleiotropically regulate thermoregulatory processes