Social Selection, Family Life & Brood Parasitism

Raising a family is challenging. Conflicts of interest arise between parents, who seek to optimize family size each breeding season, and offspring, who seek to maximize the amount of care they receive (to a point). How parents manage their investment in the face of these conflicts has been a focus in life history theory.

American coots (Fulica americana) have a decidedly active — and aggressive — strategy of parental intervention.  My research shows that coots use a mixed strategy for controlling brood size: they use both brood reduction and parental control, but at different points in the parental care stage. After an initial period of high mortality for late-hatching chicks (brood reduction), Parental behaviors such as parental aggression, brood division and favoritism act as mechanisms for parental control that allow adults to optimally invest in their offspring (Shizuka & Lyon 2013). In the process, parents create a “favoritism hierarchy”, that is distinct from the competitive hierarchy created by hatching asynchrony. Changes in parental care strategies across time might be common, but it requires intensive observations throughout the parental care period (both in the nest & outside the nest), which is tractable in subprecocial birds like coots, but may be more difficult in altricial birds.

This work has led me to further ponder the ubiquitous role of learning mechanisms in ecological and evolutionary processes. Equally important is the ecology of information in which organisms evolve, which ultimately influences the evolution of learning. For example, we have now modeled the costs and benefits of parasitic chick recognition under different learning systems (i.e., ‘template acquisition strategies’: Shizuka & Lyon 2020). For example, what is the best way to learn your chicks–to learn all the chicks from your first nest, learn the first-hatched chicks in your first nest, or re-learn the first-hatched chicks each year? This modeling exercise shows us that, while some aspects of the learning mechanisms are neatly explained by the natural history of information (e.g., hatching patterns) other aspects may be constrained by factors such as genetic mechanisms and mating systems. For example, eggs are maternal phenotypes, while chicks contain genes from both mom and dad. Thus, using cues learned from past breeding attempts may be useful for eggs (same mom), while the utility of past information for chicks depends on the mating system (e.g., mate fidelity).

Adult coots are drab gray birds, but coot chicks are born as bright red and orange puff balls. It’s one of the most extreme cases of juvenile ornamentation, i.e., when babies are adorned with conspicuous traits that seem to have some function as signals. The ornamental traits of baby coots–bright red beak and papillae (feathers around their beaks), and the bright red/orange/yellow tips of feathers around their necks–are reminiscent of the bright plumage of many adult (primarily male) birds. But unlike the male birds, juvenile ornaments are not the result of sexual selection because they lose these ornaments well before they reach sexual maturity. Rather, they represent a clear case of non-sexual social selection operating through parental choice. Lyon et al. (1994) showed that, if you trim the brightly colored tips off of the feathers of half the chicks in the brood, the parents will preferentially feed the chicks that retain the bright feathers. In our more recent study (Lyon & Shizuka 2019), we test whether the juvenile ornaments were driven to extremes in American coots because of the coevolutionary dynamics between brood parasites and hosts within species. That is, did the ornaments evolve to extremes because it benefitted parasitic chicks by inducing host parents to favor them? We measured color from nearly 1,500 coot chicks and observed patterns of parental care in many of them to see how this worked.

We actually did not find support for the hypothesis that parasites benefit by being more red. Rather, we find a strong correlation between chick color and the order in which eggs are laid–chicks that come from later eggs are redder. This is a tell-tale sign the mothers are calling the shots by making their youngest babies redder. And we also show that redder chicks are more likely to be favored when they are the youngest chicks. So it seems that chick color has more to do with the internal dynamics of families than coevolution of brood parasites and hosts–at least currently. The big question still stands though: why are coot babies SO red? Many animals seem to manage the allocation of food within families with less gaudy cues (e.g., begging), so why are coots so flashy?