Data from: Forecasting the future? Differential allocation of maternal hormones in function of experimentally manipulated social contexts in blue tits Cyanistes caeruleus

Description

Across the tree of life, there is a significant variability in sociability and, interestingly, this also holds at the intra-specific level, with some individuals being consistently more sociable than others. This variation may be maintained because in some contexts, e.g., more socially dense conditions, individuals with a higher sociability are likely to thrive better, while it might be more beneficial to be less social if there is a high prevalence of socially transmitted diseases. As mothers know about their offspring’s postnatal social environment, they might benefit from conveying that information by transferring non-genetic compounds (such as hormones) via the placenta or via allocation into the eggs. Such maternal hormones could prepare the offspring to match the social context that they will experience after birth.  To test this hypothesis, we experimentally manipulated the social density as perceived by blue tit (Cyanistes caeruleus) females before egg laying. We then collected one egg per clutch to measure the maternal yolk testosterone concentrations. Subsequently, we investigated the social interactions of the nestlings with their siblings (by means of social network analyses) and parents (i.e., begging intensity, feeding rates).

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We studied a wild blue tit population breeding in nest-boxes. The study area was divided into four plots of similar-sized area, two high-density and two control, which included. Almost three weeks prior to the laying of the first egg in the population, we exposed blue tit adults daily to either a recording of social stimuli simulating high-social density or to control playback. For the high-social density treatment, we broadcasted vocalizations of male and female blue tits. For the control plots, we broadcasted vocalizations of male and female common chaffinches, common in the study area, but do not breed in nest-boxes and do not compete for food with blue tits. Once laying started, nests were visited daily to collect the third egg of each clutch on the day it was laid. Collected eggs were kept cool during transport and stored at -80 °C within the same day of collection until analysis. We calculated the concentration of testosterone in the yolk by means of an enzyme immunoassay according to protocol (Testosterone ELISA; ref. EIA-1559; DRG, Germany). A Synergy HT Multi-Mode Microplate Reader at 450 nm was used to measure changes in absorbance. We multiplied the obtained concentrations by the yolk weight of each sample to calculate the absolute values contained in each egg. In the second week after hatching, we trapped both adults, weighed them, and determined their sex. Thus, we could distinguish male and female parents during video observations. On day 12, nestlings were ringed, weighed, and marked individually on the head or wings with a white permanent marker. We also collected a blood sample from the brachial vein for molecular sexing. On day 12, we replaced the original nest-box of each nest with a recording nest-box to familiarize blue tit breeding adults with it prior to video recordings. On day 13, we placed a night-vision video camera on the recording nest-box, approximately 10 cm above the nest. We recorded the behaviour of all family members during 30 min. egging intensity was rated on a 5-point scale: 0 = calm, 1 = weak gaping, 2 = gaping and neck stretched, 3 = gaping, neck stretched, and standing, and 4 = gaping, neck stretched, standing, and wing flapping. For each nestling, we calculated the average overall begging intensity across all feeding events directed to both parents together, and separately. Also, for each parent, we extracted the number of feeding events and the average time spent in the nest.  We scored the social behaviour of the nestlings in 10 events between the parental visits. We considered a social interaction as the direct physical contact between two nestlings. With this information, we created an NxN matrix where N is the number of individuals, and each cell indicates the existence of an interaction between two individuals (1) or the absence of interaction (0). As a proxy of sociability, we calculated network degree, that is, the average number of physical interactions among all nestlings within a nest.

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