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Female chronotype and aggression covary on different hierarchical levels in a songbird

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Abstract

Individual variation in the timing of activities is increasingly being reported for a wide variety of species, often measured as the timing of activity onset in the morning. However, so far, the adaptive significance of consistent variation in temporal phenotypes (i.e. the chronotype) remains largely elusive. Potentially, differences in timing of activities may arise as a result of competition among individuals for resources. Less aggressive individuals may try to avoid competition by becoming active earlier during the day when other individuals are still inactive, leading to a positive correlation between chronotype and aggressive personality type (i.e. a behavioural syndrome). To investigate this, we assessed the chronotype of female great tits, Parus major, by measuring emergence time from the nestbox in the morning and experimentally tested their levels of same-sex aggression through simulated territorial intrusion tests. Contradicting our initial hypothesis, consistently more aggressive females became active earlier during the day compared to less aggressive females, which could be caused by shared underlying mechanisms, like pleiotropic effects of sex hormones or gene pleiotropy, which potentially impose constraints on the independent evolution of both traits. Surprisingly, on the within-individual level we found an opposing correlation between emergence time and aggression. Our findings highlight the need for further investigations into the interplay between chronotype and aggression that take the underlying mechanisms into account in order to understand the adaptive significance of this trait association.

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Nearly all organisms alter physiological and behavioral activities across the twenty-four-hour day. Endogenous timekeeping mechanisms, which are responsive to environmental and internal cues, allow organisms to anticipate predictable environmental changes and time their daily activities. Among-individual variation in the chronotype, or phenotypic output of these timekeeping mechanisms (i.e. timing of daily behaviors), is often observed in organisms studied under naturalistic environmental conditions. The neuroendocrine system, including sex steroids, has been implicated in the regulation and modulation of endogenous clocks and their behavioral outputs. Numerous studies have found clear evidence that sex steroids modulate circadian and daily timing of activities in captive animals under controlled conditions. However, little is known about how sex steroids influence daily behavioral rhythms in wild organisms or what, if any, implication this may have for survival and reproductive fitness. Here we review the evidence that sex steroids modulate daily timing in vertebrates under controlled conditions. We then discuss how this relationship may be relevant for the reproductive success and fitness of wild organisms and discuss the limited evidence that sex steroids modulate circadian rhythms in wild organisms.
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Central theories explaining the maintenance of individual differences in behavior build on the assumption that behavior mediates life-history trade-offs between current and future reproduction. However, current empirical evidence does not robustly support this assumption. This mismatch might be because current theory is not clear about the role of behavior in individual allocation versus acquisition of resources, hindering empirical testing. The relative importance of allocation compared to acquisition is a key feature of classic life-history theory, but appears to have been lost in translation in recent developments of life-history theory involving behavior. We argue that determining the relative balance between variation in resource allocation and acquisition, and the role of behavior in this process, will help to build more robust and precise predictions.
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Social interactions can shape daily activity patterns, and this is an area of growing research interest. The propensity for individuals to be active at certain times of day may structure interactions with competitors and potential mates, influencing fitness outcomes. Aspects of daily activity such as timing of activity onset and offset exhibit within-individual repeatability, which raises the possibility that they may be subject to natural and sexual selection. This study used a biologging approach in free-living Arctic ground squirrels, Urocitellus parryii, to test the hypothesis that interactions between the sexes shape temporal patterns of daily activity. We predicted that males would become active earlier than females during the mating period, consistent with sexual selection on activity timing. We also examined repeatability in activity timing and how repeatability estimates are affected by sampling methodology. We deployed collar-mounted light loggers on ground squirrels and used timing of light transitions to determine daily onset and offset of above-ground activity. We observed 87 animals over 3 years, collecting over 7500 observations of daily activity. We found that daily activity timing had moderate (onset) to low (offset) adjusted repeatability. The difference in daily activity timing between the sexes varied seasonally: males remained active later than females during mating, while females initiated activity earlier and extended activity later during lactation. Adjusted repeatability estimates were sensitive to sampling methodology: they varied seasonally and declined with increased behavioural sampling. These results suggest that offset, not onset, may be shaped by sexual selection and that life history differences shape sex-dependent activity patterns in ground squirrels. We propose that activity onset may be under more strict circadian control than offset. This study suggests that researchers must consider seasonality and sampling design when estimating behavioural repeatability. Biologging could transform studies of individual variation by enabling high-frequency sampling of free-living animals.
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We used an automated radiotelemetry system to determine diurnal patterns of activity and temporal phenotype (onset and cessation of activity) in female European starlings during breeding. Parental care is thought to be the most ‘costly’ part of reproduction, with high rates of intense activity due to foraging and provisioning for chicks, so we predicted that variation in timing of activity should be closely related to breeding success. Diurnal variation in activity varied systematically with breeding stage in a way consistent with specific demands of each phase of parental care: incubating females were more active late in the day (1600–1800 hours), while chick-rearing females were more active early in the morning (0700–1100 hours). There was marked individual variation in timing of onset, and to a lesser extent cessation, of activity, e.g. chick-rearing females first became active 7–127 min after morning civil twilight, with low to moderate repeatability within and among breeding stages (individual explained 2–62% of total variation). On average, females were active later, and ceased being active earlier, during chick rearing compared with incubation. Chick-rearing birds had a longer active day, but only by 2.3% (36% of the seasonal increase in total available daylength). Thus, chick-rearing females were relatively less active (‘lazier’), which is consistent with the idea that parents work more efficiently rather than simply working harder. We found little evidence that chick-rearing activity was associated with variation in measures of current reproduction (provisioning rate, number and quality of chicks), future fecundity (initiating a second brood, cumulative 2-year productivity) or survival (local return rate). Our study demonstrates that time-keeping mechanisms show plasticity in response to reproductive state and can be modulated by ‘biotic’ (e.g. prey availability) or ‘social’ time (demands of parental care).
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Research in captive birds and mammals has demonstrated that circadian (i.e., daily) behavioral rhythms are altered in response to increases in sex-steroid hormones. Recently we and others have demonstrated a high degree of individual repeatability in peak (GnRH-induced sex) steroid levels, and we have found that these GnRH-induced levels are highly correlated with their daily (night-time) endogenous peak. Whether or not individual variation in organization and activity of the reproductive endocrine axis is related to daily timing in wild animals is not well known. To begin to explore these possible links, we tested the hypothesis that maximal levels of the sex steroid hormone estradiol and onset of daily activity are related in a female songbird, the dark-eyed junco (Junco hyemalis). We found that females with higher levels of GnRH-induced estradiol departed from their nest in the morning significantly earlier than females with lower stimulated levels. We did not observe a relationship between testosterone and this measure of onset of activity. Our findings suggest an interaction between an individual's reproductive endocrine axis and the circadian system and variation observed in an individuals' daily activity onset. We suggest future studies examine the relationship between maximal sex-steroid hormones and timing of daily activity onset.
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17β-estradiol (E2) has numerous rapid effects on the brain and behavior. This review focuses on the rapid effects of E2on aggression, an important social behavior, in songbirds. First, we highlight the contributions of studies on song sparrows, which reveal that seasonal changes in the environment profoundly influence the capacity of E2to rapidly alter aggressive behavior. E2administration to male song sparrows increases aggression within 20 min in the non-breeding season, but not in the breeding season. Furthermore, E2rapidly modulates several phosphoproteins in the song sparrow brain. In particular, E2rapidly affects pCREB in the medial preoptic nucleus, in the non-breeding season only. Second, we describe studies of the white-throated sparrow, which reveal how a genetic polymorphism may influence the rapid effects of E2on aggression. In this species, a chromosomal rearrangement that includes ESR1, which encodes estrogen receptor α (ERα), affects ERα expression in the brain and the ability of E2to rapidly promote aggression. Third, we summarize studies showing that aggressive interactions rapidly affect levels of E2and other steroids, both in the blood and in specific brain regions, and the emerging potential for steroid profiling by liquid chromatography tandem mass spectrometry (LC-MS/MS). Such studies of songbirds demonstrate the value of an ethologically informed approach, in order to reveal how steroids act rapidly on the brain to alter naturally-occurring behavior.
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Vertebrates use environmental cues to time reproduction to optimal breeding conditions. Numerous laboratory studies have revealed that light experienced during a critical window of the circadian (daily) rhythm can influence reproductive physiology. However, whether these relationships observed in captivity hold true under natural conditions and how they relate to observed variation in timing of reproductive output remains largely unexplored. Here we test the hypothesis that individual variation in daily timing recorded in nature (i.e. chronotype) is linked with variation in timing of breeding. To address this hypothesis and its generality across species, we recorded incubation behavior data to identify individual patterns in daily onset of activity for 2 temperate-breeding songbird species, the dark-eyed junco (Junco hyemalis aikeni) and the great tit (Parus major). We found that females who first departed from their nest earlier in the morning (earlier chronotype) also initiated nests earlier in the year. Date of data collection and ambient temperature had no effect, but stage of incubation influenced daily onset of activity in great tits. Our findings suggest a role for daily rhythms as one mechanism underlying the observed variation in seasonal timing of breeding.
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Individual animals show consistent differences in behavioural responses when coping with environmental challenges. Consistency over time and across context in a behavioural trait is an indication for animal personality. Chronotypes refer to consistent inter-individual differences in diurnal rhythmicity driven by underlying variation in circadian clock processes. Personality traits and chronotype may relate to a single behavioural syndrome, but few studies have investigated such a link explicitly. Here, we explored zebrafish larvae for the presence of consistency in activity levels and timing, and their correlation with and without external cues (Zeitgeber: light–dark cycle versus constant light). We found individual consistency in activity level and timing, and their correlation independent of the presence of Zeitgeber: early-active individuals were less active overall than late-active individuals. Our study suggests a link between personality and chronotype and provides new insights into the early development of individual variation in behavioural tendencies of zebrafish.
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Individual differences in personality affect behavior in novel or challenging situations. Personality traits may be subject to selection because they affect the ability to dominate others. We investigated whether dominance rank at feeding tables in winter correlated with a heritable personality trait (as measured by exploratory behavior in a novel environment) in a natural population of great tits, Parus major. We provided clumped resources at feeding tables and calculated linear dominance hierarchies on the basis of observations between dyads of color-ringed individuals, and we used an experimental procedure to measure individual exploratory behavior of these birds. We show that fast-exploring territorial males had higher dominance ranks than did slow-exploring territorial males in two out of three samples, and that dominance related negatively to the distance between the site of observation and the territory. In contrast, fast-exploring nonterritorial juveniles had lower dominance ranks than did slow-exploring nonterritorial juveniles, implying that the relation between dominance and personality is context-dependent in the wild. We discuss how these patterns in dominance can explain earlier reported effects of avian personality on natal dispersal and fitness. Copyright 2004.