The evolutionary pros and cons of group living vary for males and females, according to a newly published study of ostriches
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A group of wild ostriches (Struthio camelus) in the savannah. Masai Mara National Park, southwest … [+]
Raising children is a cooperative effort in humans — and in ostrich, Struthio camelus. Ostriches form groups that lay their eggs in a large communal nest and individual members of the group take turns incubating them. It seems very idyllic and cooperative until you realize that, like everything, cooperative breeding has its costs. For example, competition can be fierce over mating opportunities and whose eggs are incubated. The counteracting forces of cooperation and competition are predicted to select for an optimal group size, so knowing this, you might think that ostrich have precisely worked out the most beneficial group size and the best male-female balance within them. If so, and if real life scenarios are anything to go by, you’d be wrong.
“The competing forces of competition and cooperation are expected to result in there being an optimal group size in nature”, said lead author, ecologist Julian Melgar, a postdoctoral researcher at Lund University. “But in the wild, groups are highly variable in size and it is not clear why.”
Which raises the question: Why do groups of wild ostrich vary so much both in size and in their ratios of males to females? A common explanation for why group size varies is that fluctuating ecological conditions shift the optimal size of groups over time and space. Although ecological conditions clearly have important effects on group living, they do not explain why the composition of social groups is still highly variable under similar ecological conditions (Figure 1).
F I G U R E 1 : Is there an optimal group size for cooperative breeding ostriches? Theoretically, … [+]
To better understand why group sizes and gender numbers can vary, Dr Melgar and an international team of scientists studied the group dynamics of captive breeding ostriches and compared them to the distribution and composition of group sizes seen in wild ostrich.
“We set out to study the costs and benefits of group size under consistent ecological conditions, to separate out the effect of individual differences from group attributes on reproductive success and disentangle how competition and cooperation change with group size”, Dr Melgar explained.
To do this, Dr Melgar and his collaborators set up 96 ostrich groups over eight years in large enclosures in Klein Karoo South Africa. They placed different numbers of males (1 or 3) and females (1, 3, 4, or 6) into these enclosures and manipulated their opportunities for cooperation during the incubation period. These captive group sizes are similar to those seen in wild ostrich.
Cooperative breeding behavior was hindered by temporarily removing eggs from the nest during part of the breeding season. The resulting impacts upon both males and females in these groups was monitored, and cooperation during incubation was measured as it impacted reproductive success — the number of chicks hatched.
Dr Melgar and his collaborators found that females benefitted from cooperative chick care, and overall, they did better in larger groups. In contrast, males have only one optimal group size (one male with four or more females) because of the high costs of competing with other males for mates compared to the negligible benefits of cooperative chick care. Forming an intermediate-sized group as a potential compromise was not optimal for either females or males due to sexual conflicts over the timing of mating. Basically, intermediate-sized groups saw the eggs either broken or left exposed to the elements when males mated with incubating females.
These results reveal that variation in cooperative breeding groups is independent of ecological conditions, breeder quality or relatedness — all of which are common explanations for variation in cooperative breeding groups for other animals. Instead, the different priorities of males and females explain why there is no single optimal group size for ostriches and generally, it can help explain how animal groups balance these differing (and often conflicting) priorities in nature. This information can also inform animal breeding efforts and conservation work by underscoring how different social pressures influence reproductive success.
Source:
Julian Melgar, Mads F Schou, Maud Bonato, Zanell Brand, Anel Engelbrecht, Schalk WP Cloete, and Charlie K Cornwallis (2022). Experimental evidence that group size generates divergent benefits of cooperative breeding for male and female ostriches, eLife 11:e77170 | doi:10.7554/eLife.77170
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