Better explicating the strengths and shortcomings of these models will help refine projections and improve transparency in the years ahead.
Image credit: Witsawat.S.
Edited by Joan E. Strassmann, Washington University in St. Louis, St. Louis, MO, and approved September 23, 2016 (received for review April 22, 2016)
Article Figures & SI
Figures
- Fig. S1.
What is honesty? Species were considered honest if there was a strong relationship between offspring need and begging intensity, as measured by the correlation coefficient (37). (A) When the correlation coefficient approaches 1, begging is considered honest because it is almost perfectly informative. (B) When the correlation is weaker (e.g., r = 0.5), begging may provide some information about need, but it is less reliable. (C) When the correlation coefficient approaches 0, begging is considered dishonest because it provides no information about need. (D and E) Begging could also function as an honest signal of quality, rather than need, in which case the correlation coefficient would be negative rather than positive. Our analyses looked at whether differences in the relative strength of the correlation between need and begging across species can be explained by increases in within-family conflict as predicted by Hamilton’s rule.
- Fig. 1.
The presence of siblings is associated with a reduction in offspring honesty. Data points represent species’ mean correlation coefficients of long-term need and begging intensity for species where parents rear only one (n = 6 species) or more than one offspring per brood (n = 54 species). Positive correlations indicate that offspring in worse condition beg more intensely, providing honest information about need. Gray lines represent the 95% CIs from the model, run on the full dataset, controlling for phylogeny and repeated measures. Species with siblings present have a weaker correlation between need and begging, suggesting less honest signaling of need (Wald = 6.69, P = 0.0097).
- Fig. 2.
Conflict with future siblings is associated with a reduction in offspring honesty. Data points show species’ mean correlation coefficient of long-term need and begging intensity. The number of potential future broods is the adult life expectancy multiplied by the number of successful broods that can be reared each breeding season (n = 51 species). The gray line is the regression coefficient from the model, run on the full dataset, controlling for phylogeny and repeated measures. Positive correlations indicate that offspring in worse condition beg more intensely, providing honest information about need. The dashed line at zero indicates no relationship between condition and begging. The correlation between need and begging is weaker in species where parents can potentially produce more broods over their lifetimes (Wald = 7.22, P = 0.0087), suggesting that future reproduction selects for less honest signaling of need.
- Fig. 3.
Parental divorce and death is associated with a reduction in offspring honesty. Data points represent species’ mean correlation coefficients of long-term need and begging intensity for species. We divided species by whether there is a higher or lower than 50% chance that parents will breed together again in the next year, based on survival and divorce rates (n = 19 species where full siblings are expected; n = 30 species where half siblings are expected). Positive correlations indicate that offspring in worse condition beg more intensely, providing honest information about need. Gray lines represent the 95% CIs from our analyses. The correlation between need and begging is weaker in species with higher rates of divorce and lower rates of survival, where half siblings are expected (Wald = 5.98, P = 0.016), suggesting less honest signaling of need.
- Fig. S2.
Relatedness to the whole brood decreases as brood size increases. The average relatedness of a focal offspring to the whole brood decreases with each additional sibling, until reaching an asymptote r = 0.5 for full siblings (●) and r = 0.25 for half siblings (○). Whole brood relatedness is calculated by averaging a focal offspring’s relatedness to itself and each of its siblings. For example, r = 0.75 for two full siblings (1 + 0.5)/2, and r = 0.5 for three half siblings: (1 + 0.25 + 0.25)/3. Even without any extrapair paternity, the relatedness of a focal chick to the whole brood decreases rapidly as brood size increases: average relatedness to the whole brood is 75% with one full sibling, drops to 67% with two full siblings, and to 55% with ten full siblings.
- Fig. 4.
Family conflict is associated with a reduction in the honesty of offspring signals of need. Offspring in species with siblings present are less honest when signaling need (Wald statistic = 6.69, P = 0.0097). The probability that parents will breed again is also associated with more dishonest offspring, as the current brood competes against future broods for parental investment (Wald = 7.22, P = 0.0087). Finally, offspring are less honest in species where parents are likely to divorce or die before breeding again, and all future siblings will be half siblings (Wald = 5.98, P = 0.016).
- Fig. S3.
PRISMA flowchart of search results and the study selection process. See Dataset S2 for a list of papers excluded from the analysis. The final dataset for begging analyses contained 336 effect sizes from 108 studies on 60 species of birds. The final dataset for structural signal analyses contained 140 effect sizes from 33 studies on 18 species of birds. Overall, we found data in 136 studies on 72 species.
Tables
Model no. Fixed effects Results N species N study N observations 1 Siblings present y/n F1,71.8 = 6.69, P = 0.0097 60 108 336 2 Brood size F1,31.0 = 4.57, P = 0.033 3 Siblings present y/n F1,67.0 = 8.45, P = 0.0050 51 98 317 No. of future broods possible F1,87.9 = 3.13, P = 0.080 4 Brood size F1,82.2 = 11.87, P = 0.0009 No. of future broods possible F1,83.5 = 7.22, P = 0.0087 5 Siblings present y/n F1,84.2 = 0.42, P = 0.42 49 96 314 No. of future broods possible F1,95.4 = 9.39, P = 0.0028 Full vs. half siblings likelihood F1,112.5 = 6.94, P = 0.0096 6 Brood size F1,87.1 = 3.07, P = 0.083 No. of future broods possible F1,86.4 = 13.09, P = 0.0005 Full vs. half siblings likelihood F1,89.6 = 5.98, P = 0.016 7 Siblings present y/n F1,60.5 = 0, P = 1 31 68 230 No. of future broods possible F1,58.4 = 3.10, P = 0.083 Full vs. half siblings likelihood F1,76.0 = 3.77, P = 0.056 Extrapair paternity F1,58.5 = 0.04, P = 0.84 8 Brood size F1,66.2 = 3.87, P = 0.053 No. of future broods possible F1,58.3 = 4.94, P = 0.038 Full vs. half siblings likelihood F1,67.1 = 1.00, P = 0.32 Extrapair paternity F1,58.5 = 0.27, P = 0.61 Mean results (conditional Wald statistics) of 500 ASReml linear mixed models. Models controlled for phylogeny, repeated measures on studies, and species, and were weighted by study sample size (the number of broods used to calculate the original test statistic). Fixed effects in bold are significant at the P < 0.05 level and in italics at the P < 0.10 level. Models are grouped by the dataset used for analysis, as sample size decreased in later analyses due to unavailable life history data.
No. Log likelihood Total I2 Random effects Component SE N species N study N observations Notes Null (a) 123.5 12.8% Phylogeny 0.0086 0.0182 60 108 336 Full dataset Species 0.0005 0.0254 Study 0.1059 0.0265 Residual 0.0320 0.0077 1 124.3 12.0% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.1039 0.0224 Residual 0.0321 0.0077 2 121.7 12.0% Phylogeny 0.0000 NA Species 0.0056 0.0187 Study 0.0990 0.0277 Residual 0.0324 0.0078 Null (b) 117.5 12.7% Phylogeny 0.0060 0.0214 51 98 317 Excluding 9 species missing data on adult mortality or the broods possible each breeding season Species 0.0048 0.0266 Study 0.1014 0.0294 Residual 0.0330 0.0080 3 115.0 11.6% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.0986 0.0227 Residual 0.0330 0.0080 4 114.8 11.0% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.0896 0.0213 Residual 0.0335 0.0080 Null (c) 119.0 12.4% Phylogeny 0.0104 0.0189 49 96 314 Excluding 11 species missing data on adult mortality, the broods possible each breeding season, or mating system Species 0.0001 0.0245 Study 0.0984 0.0255 Residual 0.0328 0.0079 5 118.0 10.5% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.0850 0.0205 Residual 0.0329 0.0079 6 117.6 10.1% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.0787 0.0196 Residual 0.0333 0.0080 Null (d) 104.5 12.9% Phylogeny 0.0241 0.0274 31 68 230 Excluding 29 species missing data on adult mortality, the broods possible each breeding season, mating system, or extra pair paternity Species 0.0000 NA Study 0.0955 0.0283 Residual 0.0288 0.0079 7 95.8 11.3% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.0985 0.0262 Residual 0.0285 0.0079 8 95.9 10.6% Phylogeny 0.0000 NA Species 0.0000 NA Study 0.0900 0.0244 Residual 0.0289 0.0079 Mean results of 500 ASReml linear mixed models. Models were weighted by study sample size (the number of broods used to calculate the original test statistic). Models are grouped by the dataset used for analysis, and the notes column describes the loss in sample size due to missing life history data. Sample error variance was constrained to 1. Total heterogeneity (I2) is a measure of the proportion of observed variance due to true differences in effect sizes in the null model (34). I2 values of 25%, 50%, and 75% indicate low, moderate, and high levels ratios of signal to noise (34). This measure does not take effect size dispersion into account (34). Model numbers correspond to Table 1. NA indicates that models could not estimate the standard error of this variance component.
Life history variables Environmental variables N species Predictability Quality Siblings y/n −33.07 to −2.56 −4.24 to 23.28 60 Brood size −0.76 to 0.47 −0.28 to 0.59 Siblings y/n −19.37 to −9.42 −2.18 to 9.03 51 Brood size −0.72 to 0.56 −0.31 to 0.52 No. future broods possible −0.02 to 0.02 −0.01 to 0.01 Siblings y/n −33.19 to −13.06 −3.69 to 8.12 49 Brood size −0.89 to 0.41 −0.30 to 0.53 No. future broods possible −0.02 to 0.02 −0.01 to 0.01 Full vs. half siblings likelihood −5.50 to 2.81 −3.54 to 2.22 Siblings y/n −61.65 to −5.45 −6.66 to 25.08 33 Brood size −0.91 to 0.74 −0.64 to 0.43 No. future broods possible −0.03 to 0.03 −0.01 to 0.01 Full vs. half siblings likelihood −9.01 to 37.59 −80.76 to 7.21 Promiscuity −0.03 to 0.03 −0.01 to 0.01 Table reports the 95% CI for the relationship of environmental variables on each of the life history variables used with each subset of the overall dataset (n = 60 species corresponds to models 1 and 2; n = 51 species corresponds to models 3 and 4; n = 49 species corresponds to models 5 and 6; n = 33 species corresponds to models 8 and 7). Results are from MCMCglmm models, controlling for phylogeny. MCMCglmm was used instead of ASReml because ASReml models did not converge properly when accounting for phylogenetic variance in some models with smaller sample sizes. Cells in bold indicate the relationship is significant (the 95% CI does not include 0).
Model no. Fixed effects Variables included in model N Only life history Only environmental All variables Model 1 Intercept Coef = 0.10 ± 0.04 Coef = −0.09 ± 0.09 Coef = −0.09 ± 0.09 60 species, 108 studies, 336 effect sizes F1,89.9 = 13.57, P = 0.0005 F1,91.0 = 13.23, P = 0.0002 F1,88.9 = 13.57, P = 0.0005 Environmental predictability Coef = 0.20 ± 0.08 Coef = 0.14 ± 0.09 F1,91.7 = 6.73, P = 0.011* F1,92.7 = 2.66, P = 0.11 Environmental quality Coef = 0.32 ± 0.09 Coef = 0.32 ± 0.09 F2,191.8 = 7.13, P = 0.0010** F2,192.2 = 7.14, P = 0.0010** Siblings present y/n Coef = −0.28 ± 0.11 Coef = −0.20 ± 0.12 F1,71.8 = 6.69, P = 0.0097** F1,73.7 = 2.86, P = 0.095. Model 2 Intercept Coef = 0.31 ± 0.09 Coef = −0.09 ± 0.09 Coef = 0.05 ± 0.12 F1,34.1 = 12.61, P = 0.0002 F1,91.0 = 13.23, P = 0.0002 F1,89.6 = 13.45, P = 0.0004 Environmental predictability Coef = 0.20 ± 0.08 Coef = 0.18 ± 0.08 F1,91.7 = 6.73, P = 0.011* F1,92.6 = 4.86, P = 0.03* Environmental quality Coef = 0.32 ± 0.09 Coef = 0.32 ± 0.09 F2,191.8 = 7.13, P = 0.0010** F2,192.2 = 7.18, P = 0.0010** Brood size Coef = −0.04 ± 0.02 Coef = −0.04 ± 0.02 F1,31.0 = 4.57, P = 0.033* F1,96.1 = 3.07, P = 0.083. Model 3 Intercept Coef = 0.21 ± 0.07 Coef = −0.06 ± 0.09 Coef = 0.03 ± 0.11 51 species, 98 studies, 317 effect sizes F1,79.1 = 9.14, P = 0.0034 F1,81.7 = 15.70, P = 0.0002 F1,78.6 = 9.04, P = 0.0036 Environmental predictability Coef = 0.19 ± 0.08 Coef = 0.09 ± 0.09 F1,81.9 = 5.26, P = 0.024* F1,81.2 = 0.92, P = 0.34 Environmental quality Coef = 0.32 ± 0.09 Coef = 0.32 ± 0.09 F2,183.8 = 6.93, P = 0.0013** F2,182.4 = 6.86, P = 0.0013** Siblings present y/n Coef = −0.33 ± 0.11 Coef = -0.27 ± 0.13 F1,67.0 = 8.45, P = 0.0050** F1,69.3 = 4.14, P = 0.046* No. future broods possible Coef = -0.02 ± 0.01 Coef = −0.02 ± 0.01 F1,87.9 = 3.13, P = 0.080. F1,89.9 = 2.03, P = 0.16 Model 4 Intercept Coef = 0.65 ± 0.14 Coef = −0.06 ± 0.09 Coef = 0.39 ± 0.18 F1,78.1 = 20.21, P = 0.0000 F1,81.7 = 15.70, P = 0.0002 F1,80.6 = 19.77, P = 0.0000 Environmental predictability Coef = 0.19 ± 0.08 Coef = 0.12 ± 0.08 F1,81.9 = 5.26, P = 0.024* F1,80.7 = 2.02, P = 0.16 Environmental quality Coef = 0.32 ± 0.09 Coef = 0.32 ± 0.09 F2,183.8 = 6.93, P = 0.0013** F2,184.3 = 6.86, P = 0.0013** Brood size Coef = −0.08 ± 0.02 Coef = −0.07 ± 0.03 F1,82.2 = 11.87, P = 0.0009*** F1,85.1 = 8.2, P = 0.005** No. future broods possible Coef = −0.03 ± 0.01 Coef = −0.03 ± 0.01 F1,83.5 = 7.22, P = 0.0087** F1,85.5 = 5.13, P = 0.026* Model 5 Intercept Coef = 0.64 ± 0.18 Coef = −0.06 ± 0.09 Coef = 0.46 ± 0.20 49 species, 96 studies, 314 effect sizes F1,76.5 = 8.58, P = 0.0045 F1,80.7 = 14.47, P = 0.0003 F1,76.1 = 8.42, P = 0.0049 Environmental predictability Coef = 0.17 ± 0.08 Coef = 0.07 ± 0.09 F1,81.0 = 4.44, P = 0.038* F1,79.1 = 0.63, P = 0.43 Environmental quality Coef = 0.32 ± 0.09 Coef = 0.32 ± 0.09 F2,184.5 = 6.94, P = 0.0012** F2,185.3 = 6.79, P = 0.0014** Siblings present y/n Coef = −0.11 ± 0.14 Coef = −0.07 ± 0.15 F1,84.2 = 0.42, P = 0.42 F1,83.7 = 0.19, P = 0.67 No. future broods possible Coef = −0.05 ± 0.01 Coef = −0.04 ± 0.02 F1,95.4 = 9.39, P = 0.0028** F1,96.4 = 7.87, P = 0.0061** Full vs. half siblings likelihood Coef = −0.39 ± 0.15 Coef = −0.39 ± 0.15 F1,112.5 = 6.94, P = 0.0096** F1,111.5 = 6.76, P = 0.011* Model 6 Intercept Coef = 0.83 ± 0.16 Coef = −0.06 ± 0.09 Coef = 0.62 ± 0.20 F1,76.5 = 21.95, P = 0.0000 F1,80.7 = 14.47, P = 0.0003 F1,77.7 = 21.23, P = 0.0000 Environmental predictability Coef = 0.17 ± 0.08 Coef = 0.06 ± 0.08 F1,81.0 = 4.44, P = 0.038* F1,78.9 = 0.62, P = 0.43 Environmental quality Coef = 0.32 ± 0.09 Coef = 0.32 ± 0.09 F2,184.5 = 6.94, P = 0.0012** F2,186.8 = 6.78, P = 0.0014** Brood size Coef = −0.05 ± 0.03 Coef = -0.04 ± 0.03 F1,87.1 = 3.07, P = 0.083. F1,87.1 = 2.51, P = 0.12 No. future broods possible Coef = −0.05 ± 0.01 Coef = −0.05 ± 0.01 F1,86.4 = 13.09, P = 0.00050*** F1,89.7 = 10.15, P = 0.0020** Full vs. half siblings likelihood Coef = −0.33 ± 0.14 Coef = −0.31 ± 0.14 F1,89.6 = 5.98, P = 0.016* F1,90.3 = 5.14, P = 0.026* Model 7 Intercept Coef = 0.65 ± 0.23 Coef = 0.08 ± 0.11 Coef = 0.56 ± 0.25 31 species, 68 studies, 230 effect sizes F1,55.6 = 4.93, P = 0.030 F1,2.7 = 9.24, P = 0.0000 F1,54.8 = 4.32, P = 0.045 Environmental predictability Coef = 0.13 ± 0.10 Coef = 0.07 ± 0.11 F1,33.1 = 1.43, P = 0.24 F1,58.1 = 4.73, P = 0.036* Environmental quality Coef = 0.22 ± 0.09 Coef = 0.22 ± 0.09 F2,128.2 = 2.95, P = 0.056. F2,127.4 = 3.09, P = 0.049* Siblings present y/n Coef = 0.00 ± 0.22 Coef = 0.06 ± 0.24 F1,60.5 = 0, P = 1 F1,60.2 = 1.11, P = 0.30 No. future broods possible Coef = -0.03 ± 0.02 Coef = −0.03 ± 0.02 F1,58.4 = 3.10, P = 0.083. F1,60.0 = 0.08, P = 0.79 Full vs. half siblings likelihood Coef = -0.43 ± 0.22 Coef = −0.44 ± 0.22 F1,76.0 = 3.77, P = 0.056. F1,74.9 = 1.07, P = 0.31 Promiscuity (% EPPbr) Coef = 0.00 ± 0.01 Coef = 0.00 ± 0.01 F1,58.5 = 0.04, P = 0.84 F1,57.9 = 1.22, P = 0.28 Model 8 Intercept Coef = 0.81 ± 0.19 Coef = 0.08 ± 0.11 Coef = 0.68 ± 0.23 F1,55.9 = 17.26, P = 0.0001 F1,2.7 = 9.24, P = 0.0000 F1,55.4 = 18.95, P = 0.0001 Environmental predictability Coef = 0.13 ± 0.10 Coef = 0.05 ± 0.10 F1,33.1 = 1.43, P = 0.24 F1,57.5 = 4.73, P = 0.035* Environmental quality Coef = 0.22 ± 0.09 Coef = 0.22 ± 0.09 F2,128.2 = 2.95, P = 0.056. F2,128.0 = 2.94, P = 0.057. Brood size Coef −0.07 ± 0.03 Coef= −0.06 ± 0.03 F1,66.2 = 3.87, P = 0.053. F1,64.6 = 3.29, P = 0.076. No. future broods possible Coef = −0.04 ± 0.02 Coef = −0.04 ± 0.02 F1,58.3 = 4.94, P = 0.038* F1,60.5 = 0.03, P = 0.87 Full vs. half siblings likelihood Coef = -0.19 ± 0.18 Coef = −0.18 ± 0.19 F1,67.1 = 1.00, P = 0.32 F1,67.8 = 2.45, P = 0.12 Promiscuity (% EPPbr) Coef = 0.00 ± 0.01 Coef = 0.00 ± 0.01 F1,58.5 = 0.27, P = 0.61 F1,57.5 = 1.24, P = 0.27 Coefficient, Wald statistic, and P value for fixed effects for three sets of models: (i) a model with life history variables only (models correspond to Table 1); (ii) a model with environmental variables only; and (iii) a model with all fixed effects included. Results are the mean from 500 ASReml linear mixed models. Models controlled for phylogeny, repeated measures, and were weighted by study sample size (the number of broods used to calculate the original test statistic). Fixed effects in bold are significant at the P < 0.05 level, and in italics at the P < 0.10 level.
Model no. Log Lik Fixed effects df Wald statistic P value N species N study N observations Notes 9 107.5 Intercept 1, 16.8 4.23 0.056 18 33 140 Full dataset Brood size 1, 18.8 1.06 0.32 10 107.0 Intercept 1, 13.9 2.67 0.12 Brood size 1, 17.0 0.18 0.68 Predictability 1, 11.3 0.69 0.42 Quality 2, 133.0 7.61 0.00075 Predictability * Quality 2, 133.0 3.74 0.026 11 84.0 Intercept 1, 12.4 2.24 0.16 16 30 115 Excluding 2 species missing data on adult mortality or the broods possible each breeding season Brood size 1, 14.7 0.88 0.36 No. of future broods possible 1, 10.1 0.11 0.75 12 86.3 Intercept 1, 10.5 1.83 0.20 Brood size 1, 12.1 0.38 0.55 No. of future broods possible 1, 11.4 0.21 0.66 Predictability 1, 9.6 0.03 0.88 Quality 2, 107.0 10.18 0.00009 13 75.0 Intercept 1, 5.2 0.78 0.42 10 20 80 Excluding 8 species missing data on adult mortality, the broods possible each breeding season, or mating system Brood size 1, 5.7 0.00 0.97 No. of future broods possible 1, 4.3 0.24 0.65 Full vs. half siblings likelihood 1, 4.3 0.46 0.53 14 73.6 Intercept 1, 4.0 0.99 0.38 Brood size 1, 4.3 0.10 0.77 No. of future broods possible 1, 3.7 0.13 0.74 Full vs. half siblings likelihood 1, 3.4 0.06 0.82 Predictability 1, 3.6 0.16 0.72 Quality 1, 73.0 4.83 0.011 Mean results of 500 ASReml linear mixed models. Models controlled for phylogeny, repeated measures on studies, and species, and were weighted by study sample size (the number of broods used to calculate the original test statistic). Fixed effects in bold are significant at the P < 0.05 level and in italics at the P < 0.10 level. Models are grouped by the dataset used for analysis, and the notes column describes the loss in sample size due to missing life history data. Models did not converge when promiscuity was included as a fixed effect.
No. Log likelihood Total I2 Random effects Component SE N species N study N observations Notes Null (a) 110.4 5.87% Phylogeny 0.0143 0.0329 18 33 140 Full dataset Species 0.0197 0.0259 Study 0.0118 0.0105 Residual 0.0166 0.0068 9 107.5 5.79% Phylogeny 0.0149 0.0267 Species 0.0147 0.0215 Study 0.0146 0.0116 Residual 0.0162 0.0067 Null (b) 90.2 13.1% Phylogeny 0.0980 0.0354 16 30 115 Excluding 2 species missing data on adult mortality or the broods possible each breeding season Species 0.0222 0.0285 Study 0.0106 0.0108 Residual 0.0197 0.0078 11 84.0 6.04% Phylogeny 0.0110 0.0320 Species 0.0213 0.0259 Study 0.0128 0.0118 Residual 0.0192 0.0077 Null (c) 82.2 4.74% Phylogeny 0.0046 0.1095 10 20 80 Excluding 8 species missing data on adult mortality, the broods possible each breeding season, or mating system Species 0.0450 0.0460 Study 0.0002 0.0035 Residual 0.0000 NA 13 75.0 9.07% Phylogeny 0.0988 0.0769 Species 0.0000 NA Study 0.0009 0.0044 Residual 0.0000 NA Mean results of 500 ASReml linear mixed models. Models were weighted by study sample size (the number of broods used to calculate the original test statistic). Models are grouped by the dataset used for analysis, and the notes column describes the loss in sample size due to missing life history data. Sample error variance was constrained to 1. Total heterogeneity (I2) is a measure of the proportion of observed variance due to true differences in effect sizes in the null model (34). I2 values of 25%, 50%, and 75% indicate low, moderate, and high levels ratios of signal to noise (34). This measure does not take effect size dispersion into account (34). Model numbers correspond to Table 1. NA indicates that models could not estimate the standard error of this variance component.
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Sibling rivalry and the evolution of dishonesty
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