Experimental evidence that group size generates divergent benefits of cooperative breeding for male and female ostriches
- Department of Biology, Lund University, Sweden
- Department of Animal Sciences, University of Stellenbosch, South Africa
- Directorate Animal Sciences, Western Cape Department of Agriculture, South Africa
Figures
Figure 1 with 1 supplement
Is there an optimal group size for cooperative breeding ostriches?
Theoretically, accelerating costs of competition and diminishing benefits of cooperation are expected to result in a single optimal group size (A–B, modified from Krause and Ruxton, 2002). (C) Groups in natural populations are, however, highly variable in size: A satellite image of the Karoo National Park with different groups of ostriches plotted. The size of the circles indicates the number of individuals and the blue and yellow segments indicate the proportion of males and females, respectively. To understand natural variation in group size, experiments that manipulate both the number of males and females in groups, and the benefits of cooperation are required. Opportunities for cooperative incubation were manipulated at the experimental study site by collecting and artificially incubating eggs for part of the breeding season (D). Patterns of reproductive success in groups of different size when cooperation was restricted were compared to situations where opportunities for cooperative incubation, such as among these males (E), were allowed by leaving eggs in nests (photo by Julian Melgar).
Figure 1—figure supplement 1
Group size and composition of wild ostrich breeding groups in Karoo National Park in relation to patterns of reproductive success measured under experimental conditions.
(A-D) The number of groups with different numbers of females and males. The reproductive success (mean ± SE of standardised number of chicks) of females (red) and males (blue) in different sized experimental groups is plotted without (C) and with (D) incubation. (E) Observed group sizes and sex rations. The grey rectangle shows the range of group sizes and sex ratios used in the experimental group manipulation. The size of the points shows number of groups observed in the wild. Only groups with both males and females and sexually mature individuals are presented.
Figure 2 with 3 supplements
Group size and opportunities for cooperation over incubation influence male and female reproductive success.
Reproductive success was measured as the number of chicks produced per individual per clutch for each reproductive stage (artificial vs. natural incubation; see Materials and methods for details). (A) The number of chicks males sired decreased with the number of males in the group and increased with the number of females. (B) Opportunities for cooperative incubation reduced the effects of male competition in groups with few females and magnified the effect of the number of females in groups without male competition (Supplementary file 1d). (C) When opportunities for cooperative incubation were removed the number of chicks females produced was independent of the number of males and females in groups. (D) When there were opportunities for cooperative incubation, the number of chicks females produced was dependent on both the number of males and females in groups. Means ± SE are plotted. Full details of sample sizes are presented in Supplementary file 1r.
Figure 2—figure supplement 1
The effect of group size and opportunities for cooperation over incubation on the number of eggs produced per individual.
Standardised number of eggs was measured as the number of eggs produced per individual per clutch for each reproductive stage (no care versus care. See methods for details). (A) The number of eggs males sired decreased with the number of males in the group, but increased with number of females when opportunities for cooperation over incubation were removed. (B) Opportunities for cooperation over incubation had little effect on the number of eggs males sired, apart from in groups with single males where male success increased non-linearly with the number of females as opposed to asymptoted as was the case when eggs were removed. (C) When opportunties for cooperative incubation were restricted, the number of eggs females produced was independent of the number of males and decreased with increasing number of females in groups. (D) When opportunities for cooperative incubation were allowed, there was again a negative effect of the number of females in groups on female egg laying rates. However, there was an additional quadratic effect driven by a slight increase in egg laying rates in groups with six females. Means ± SE are plotted. Full details of sample sizes are presented in Supplementary file 1r.
Figure 2—figure supplement 2
The correspondence between measurements of reproductive success for males and females measured with and without genetic methods.
Reproductive success was measured as number of chick per individual. For measurements using molecular methods, a sample of hatched chicks from each group were genotyped using 96 single nucleotide polymorphism (SNP) markers (see Materials and methods for details). The number of chicks produced by each adult was then calculated as their share of parentage of the genotyped chicks multiplied by the total number of chicks the group produced. The median number of chicks per individual per group is plotted on the x axis (the median was used as the distribution was skewed) against the average number of chicks per individual used in analyses (total number of chicks produced by group / number of individuals) on the y axis. The size of points shows how many chicks were genotyped per group. R^2^ values and regression lines with 95% confidence intervals are presented from GLMs.
Figure 2—figure supplement 3
The total reproductive output of groups in relation to group size and offspring care.
The reproductive success of groups was measured as the numbers of eggs and chicks produced per group per clutch for each reproductive stage (no care versus care. See methods for details). Both in absence (A) and presence (B) of cooperation over incubation, the number of eggs produced by groups was not influenced by the number of males in the group, but increased with number of females. Similarly, the number of chicks produced by groups increased with number of females, but not with the number of males, both in the absence (C) and presence (D) of cooperative incubation. Means ± SE are plotted. Materials and methods for details. Both in absence (A) and presence (B) of cooperation over incubation, the number of eggs produced by groups was not influenced by the number of males in the group, but increased with number of females. Similarly, the number of chicks produced by groups increased with number of females, but not with the number of males, both in the absence (C) and presence (D) of cooperative incubation. Means ± SE are plotted. Full details of sample sizes are presented in Supplementary file 1r.
Figure 3
The benefits of cooperative parental care in relation to group size.
(A) The amount of time nests were incubated was higher in groups with more males and females. (B) Hatching success increased with the amount of time nests were incubated. Regression line from a binomial generalised linear model (GLM) with 95% confidence intervals is shown and the size of the points represents the number of eggs laid by groups. (C) The amount of time females spent incubating decreased with the number of females in groups, although not significantly. (D) Males spent less time incubating in groups with three males compared to when they were on their own. Means ± SE are plotted in A, C, and D. Full details of sample sizes are presented in Supplementary file 1r.
Figure 4
Coordination over reproduction changes with group composition.
(A) The frequency of interruptions per 24 hr period of incubation increased with the numbers of males in groups, especially when there were intermediate numbers of females. Means ± SE are plotted. Full details of sample sizes are presented in Supplementary file 1r. (B) Interruptions to incubations were more frequent in groups with three males when females spent more time incubating than males. (C) More eggs were broken in groups when disparities in the time males and females spent incubating were greater, which decreased hatching success (D). Regression lines from generalised linear models (GLMs) (B=Poisson; C and D=binomial) with 95% confidence intervals are presented for graphical purposes. The size of the points in B, C, and D represents the number of eggs laid by groups.
Additional files
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Supplementary file 1
Supplementary Tables A-S and R session information in html format.
A: The composition of breeding groups observed in the Karoo National Park. B: Group size effects on the number of eggs produced by males. C: Group size effects on the number of eggs produced by females. D: Group size effects on the number of chicks produced by males. E: Group size effects on the number of chicks produced by females. F: Group size effects on the time nests were incubated. G: The effect of nest incubation on hatching success. H: Group size effects on the amount of time males spent incubating. I: Group size effects on the amount of time females spent incubating. J: Group size effects on the interruptions to incubation. K: The effect of the disparity in incubation on the number of interruptions. L: The effect of the disparity in incubation between males and females on % of eggs broken. M: The effect of the % of eggs broken on hatching success. N: Group size effects on the number of eggs produced by groups. O: Group size effects on the number of chicks produced by groups. P: The effect of average within-group relatedness on the number of chicks produced by males. Q: The effect of average within-group relatedness on the number of chicks produced by females. R: Sample size of experiment and summary statistics of reproductive success and incubation. S: Overview of replacements and removals of individuals in experimental groups.
- https://cdn.elifesciences.org/articles/77170/elife-77170-supp1-v1.zip
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Transparent reporting form
- https://cdn.elifesciences.org/articles/77170/elife-77170-transrepform1-v1.pdf
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Source code 1
Contains the R code used for analyses.
- https://cdn.elifesciences.org/articles/77170/elife-77170-code1-v1.zip
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Experimental evidence that group size generates divergent benefits of cooperative breeding for male and female ostriches
eLife 11:e77170.
https://doi.org/10.7554/eLife.77170
