There is pronounced spatial and temporal variation in habitat quality on Cousin Brouwer et al. During each season, the quality of every territory was calculated as a function of foliage density, insect abundance, and territory size following Komdeur and Brouwer et al. Insect availability across the island also varies annually, so for each season we calculated food availability as the mean number of insects counted across the whole island during each breeding season following Brouwer et al.
We determined the sex of all offspring using the PCR method developed by Griffiths et al. We used quantitative PCR qPCR to obtain relative telomere length henceforth telomere length measurements as described for the Seychelles warbler in full detail elsewhere Barrett et al. Threshold values Nq were set in the center of the window-of-linearity per amplicon for all samples.
We corrected for variation across plates using a golden sample inter-plate calibrator and then calculated telomere length for each sample as the amount of telomere DNA relative to that of a constantly expressed reference gene GAPDH that was simultaneously amplified on the same plate, following equation 1 in Pfaffl We examined the costs of sibling rivalry using a total of nestling and juvenile Seychelles warblers.
Unless stated otherwise, all analyses were conducted using a mixed modeling procedure in the lme4 Bates et al. All models included year of birth to account for variation in island density, climate and resources across years.
In models using data from two individuals from the same nest we also included nest identity to account for nonindependence between nestmates. Stepwise elimination of nonsignificant variables can increase the likelihood of type I error Mundry and Nunn , but can be appropriate in cases of specific hypothesis testing with a small number of variables Bolker et al.
We report estimates from the final model including only significant terms and fixed effects; we obtained estimates for nonsignificant terms by reintroducing these terms individually to the final minimum adequate model. To test for differences in resource availability , we first tested for inherent differences in the physical and social environment between nests containing 1 and 2 nestlings.
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We modelled brood size as a binomial response and tested for relationships with territory quality, food availability, and group size. In our investigation of variation in per-capita provisioning rate, we first determined how well per-capita provisioning rate reflects general resource availability at a given nest. Using the 20 nests for which a Day 3 provisioning watch was also performed, we built a linear model with Day 10 provisioning rate as the response variable and tested the strength of relationship with Day 3 provisioning rate.
Using each nest as a single data point, we then examined whether per-capita provisioning rate on Day 10 response variable was related to brood size. We also tested whether helper presence, territory quality, and food availability interacted with brood size. We examined physiological condition separately in nestlings and juveniles by testing the relationship between size rank and 2 Gaussian response variables: body mass and telomere length.
THE HUMAN CONDITION / SIBLING RELATIONSHIPS : Sisterhood Is Powerful
In nestlings, we created separate models for high-quality A-offspring and high-quality single offspring and low-quality B-offspring and low-quality single offspring categories. In juveniles, we compared all A-, B-, and single offspring together to maximize power under limited sample sizes. We tested whether body mass was related to competitor presence and size rank. We included time classified as above and month of capture, the interaction between tarsus length and sex to account for sex-specific scaling of mass and tarsus , territory quality, and food availability which may affect offspring body mass through maternal effects [ Richardson et al.
For nestlings, we also included helper presence to account for varying food acquisition and for juveniles we included sampling age dependent or independent and used the per-capita measure of territory quality to account for group-size mediated postfledging competition Brouwer et al ; Ridley and Raihani To investigate telomere length, we used the same additional predictors as for body mass. For nestlings, we also added tarsus length to control for variation in growth rates between nestlings. In all models, we tested for interactions between competitor presence or size rank and food availability and territory quality; and in nestlings, we also tested the interaction with helper presence.
To analyze survival to adulthood of nestlings and juveniles, we used a generalized linear mixed model with a binomial error structure and survival to adulthood as a binary response. In nestlings, we performed the quality-based comparisons described above: A-offspring versus higher-quality single offspring and B-offspring versus lower-quality single offspring.
In juveniles, we compared all A-, B-, and single offspring. We did not include food availability or territory quality based on a prior study reporting no effect of these variables on juvenile survival Brouwer et al. Among individuals that survived to adulthood, we compared the reproductive potential and life span of A- and B-offspring with that of their single counterparts as described above.
Acquisition of a breeding position was modeled as a binomial response in a standard generalized linear model, excluding 3 individuals who were still alive at the time of analysis but had not yet gained a breeding position 2 single offspring and 1 B-offspring. Because some individuals were still alive at the time of analysis, our data were left-censored: each individual was classified as either dead or alive in the model. The assumption of proportional hazards were met in all models Cox We included sex and group size number of independent birds in the territory as additional predictors in all models to account for potential sex differences in breeding performance and group-size—mediated differences in reproductive opportunities.
We also tested the interactions between these 2 predictors and competitor presence.
Model estimates for all other nonsignificant additional predictors and nonsignificant interaction terms are available in Supplementary Appendix A. This repeatability suggests that our Day 10 measures of per-capita provisioning rate reflect general resource availability at a given nest. Boxplot showing median horizontal line per-capita provisioning rate to nestlings with and without a competitor. Numbers on each box denote sample sizes per group. In nestlings, the body mass of both A- and B-offspring was lower than that of their single counterparts Figure 2a , Table 2. Territory quality, food availability, and helper presence had no effect on nestling mass and were not significant in interactions with size rank Supplementary Table 2.
Nestling telomere length did not vary with size rank Table 2 but declined with increasing tarsus length in low-quality individuals, likely as a function of increasing nestling age Table 2. Food availability, territory quality, and helper presence had no effect on nestling telomere length and did not significantly interact with size rank Supplementary Table 2.
Sibling Relationships Across the Life Span | Victor G. Cicirelli | Springer
Early life body condition and recruitment costs of sibling rivalry. In nestlings, high-quality refers to A-offspring and single offspring with greater than average body condition, and low-quality refers to B-offspring and single offspring with lower than average body condition see Methods. In juveniles, A- and B-offspring are compared with all single offspring. Different letters between groups denote significant differences.
Throughout, numbers denote sample sizes per group, boxplots display median values per group, and bar plots display mean values per group. Significant terms are in bold. Juvenile body mass was not related to nestling size rank Figure 2b , Table 2 but the sample size for B-offspring was very low. None of the additional predictors were related to juvenile body mass Supplementary Table 2 , nor were present in interactions Supplementary Table 2.
Juvenile telomere length was not related to size rank Table 2 nor to any additional predictors Supplementary Table 2 and there was no interaction between size rank and any other predictor on juvenile telomere length Supplementary Table 2. Competitor presence Figure 3b , group size, and sex were also unrelated to age at first reproduction in both high- and low-quality offspring Table 3. A-offspring had longer breeding tenures than their singleton counterparts, as indicated by a lower hazard ratio Table 3 , but the breeding tenure of B-offspring did not differ from low-quality single offspring Table 3 , Figure 3c.
Among both low- and high-quality offspring, individuals from larger groups had lower breeding tenures, as indicated by a higher hazard ratio Table 3. A-offspring also had longer life spans than their single counterparts, whereas the life span of B-offspring and low-quality single offspring did not differ Table 3 , Figure 3d. In both high-and low-quality categories, individuals from larger groups had lower life spans, as indicated by a positive hazard ratio Table 3.
Sibling Relationships Across the Life Span
There were no interactions between competition and either sex or group size for any of the 3 reproductive components or life span for either high- or low-quality offspring Supplementary Table 3. The relationship between competitor presence and a proportion of individuals acquiring a breeding position, b age at which the breeding position was attained, c length of the breeding tenure, and d adult lifespan among individuals surviving to adulthood. High- and low-quality groups are defined as for Figure 2a see Methods.
Predictors of reproductive potential and life span among Seychelles warbler offspring that survived to adulthood. The analysis of whether individuals achieved breeding status was performed with a logistic regression: all other models were based on survival analyses. Hazard ratio describes the risk of the event becoming a breeder, ceasing to be a breeder or dying for an individual raised with a competitor relative to an individual raised alone, such that values below 1 indicate less risk to competing individuals.
In this study, we tested a suite of hypothesized mediators and costs of sibling rivalry Table 1. We found evidence for decreasing resource availability as a function of increased brood size, which translated into reduced physiological condition in both A- and B-nestlings when compared to competition-free, single nestlings of the same quality category. However, the survival cost imposed by having a competitor was asymmetric within broods: in nestlings, only B-offspring had lower survival than their single counterparts, and in juveniles, B-offspring were less likely to survive than single offspring.
Among individuals who survived to adulthood, the relationship between sibling rivalry and adult reproductive potential and life span was positive for A-offspring, who outperformed their single counterparts in terms of breeding tenure and life span, and neutral for B-offspring, who performed equally well as their single counterparts in all tested aspects of adult success.
We discuss these results in detail below. Individuals in larger broods may suffer from resource depletion as a function of the number or strength of competitors Forbes et al. In our dataset, we found no evidence that brood size was linked to territory quality or food availability, suggesting that resource depletion as a function of increased brood size is not mitigated by increased overall resource availability. We also found that nestlings with a competitor received substantially less food than those raised alone.
This suggests that the reduced body mass found in competing nestlings is, at least partly, the result of reduced food intake; but without quantifying nestling begging behavior, we cannot rule out additional energetic costs of behavioral competition. However, evidence for energetic costs of begging is limited e.
Intra-brood scramble competition Stamps et al. Preliminary evidence collected earlier in the Seychelles warbler long-term study also shows that provisioning rate to each nestling appears approximately equal Supplementary Table 4 ; although we acknowledge that we do not have sufficient data for a formal statistical analysis, taken together this anecdotal evidence is compatible with the hypothesis that resource-based rivalry costs should be relatively equal between the 2 competitors.
The fact that A-offspring have lower nestling body mass than the highest-quality single offspring Figure 2a suggests that A-offspring do indeed suffer a cost associated with the presence of the B-offspring, but whether or not the relative extent of this cost is greater for B-offspring is difficult to determine. Differences in juvenile body mass and telomere length between A- and B-offspring would have allowed us to better determine whether physiological condition does indeed differ between competitors, but we found no differences in telomere length according to size rank.
It may also be because telomeres lack the resolution to reflect differences in condition at the scale at which it was considered here. It would be interesting to test for differences in other physiological characteristics, such as immune function, between A- and B-offspring to determine whether either, or both, competitors suffer with respect to physiological condition more generally.
Although physiological condition was reduced among nestling competitors regardless of size rank, only B-offspring had lower nestling survival to adulthood than their single counterparts. Together these results suggests that the physiological costs of sibling rivalry in early life have a disproportionately large impact on the survival of weaker competitors. We believe this to be unlikely for several reasons.
First, B-offspring fledge as often as those raised alone in all but three of the nests in the nestling analysis, the entire brood fledged and we found no interaction between food availability and competitive ability on offspring condition Supplementary Table 2.
Second, approximately half of all nests containing 2 nestlings are the result of communal breeding of 2 females Richardson et al. We therefore suggest that variation in brood size in this species is likely to reflect variation in parental perception of the likelihood of success of the whole brood.