BEACON Researchers at Work: Tadpole sibling rivalry

This week’s BEACON Researchers at Work blog post is by MSU graduate student Sara Garnett. 

If you’ve grown up with siblings, you’re probably familiar with the potential for conflict. Deep down, you love each other and want the other to succeed, but that doesn’t stop you from competing for attention and other parental resources. It’s not just you; there’s an evolutionary basis to this balance between competition and cooperation among siblings. The pressure to compete for the resources needed to survive and produce offspring – the component we think of as direct fitness – is pretty simple to understand. If an individual is going to pass its genes on to the next generation, it needs to compete against others to get whatever food, habitat and mates necessary. If you’re only taking this direct fitness into account, you might expect that individuals will compete as intensely as possible to control these resources.

This isn’t always what we see, however, especially where relatives are concerned. Competition to pass on genes is a significant component of evolution, but a given individual isn’t the only one who has those genes. If it isn’t a novel mutation, at least one parent also has a copy, which means that it may also have been passed on to other siblings. Genes may also be shared as a result of common descent in half-siblings, cousins, grandparents, and other relatives, depending on how far back the gene is present.

What this means is that behaving in ways that help relatives survive and reproduce can also be an effective strategy when it comes to passing on genes. This is illustrated by Hamilton’s rule, which states that behavior benefiting a relative can spread in a population when the fitness cost to an individual who helps a relative is less than the fitness benefit that relative gains, adjusted for how closely related they are (that is, how likely they are to share genes). This adjusted fitness benefit for a relative can be considered to be the indirect part of the first individual’s fitness. When taken together with the genes passed on by producing offspring, both components make up an individual’s inclusive fitness. Thinking about ways to maximize this quantity, rather than focusing solely on direct fitness, can shed light on a variety of kin-directed behavior.

Common examples of Hamilton’s rule feature individuals performing some behavior that increases risk to themselves, such as ground squirrels that are more likely to perform alarm calls (and attract the attention of predators) if their kin are nearby, but similar motivations can also explain individuals refraining from behaving in a certain way. Restraint is often more subtle and difficult to detect than active helping, but it can be just as important. Amphibian species whose young develop as cannibal types or morphs are less likely to associate with siblings and, given the choice, prefer to consume unrelated individuals. In several species of birds, nestlings will beg more intensely in nests where average relatedness is lower, suggesting that they refrain from competing as much as possible with more closely related nestmates. Some plants even alter allocation to structures used to compete for limiting resources based on whether strangers or siblings are growing nearby. Investigating examples of such kin-discriminating behavior can provide insight into the strategies organisms use to maintain this balance between competition and cooperation, allowing them to maximize inclusive fitness.

I have been pursuing these types of questions as a Ph.D. student in Tom Getty’s lab, using American toad tadpoles as a study organism. Many pairs of adult toads may each lay thousands of eggs in a particular pond. While lacking any particularly complex social structure, the tadpoles (like some other species) form groups that are more likely to contain siblings than non-siblings. Because they cannot leave their birth pond until metamorphosis, any strategy to minimize direct competition with siblings cannot be based on avoiding siblings.  In this case, perhaps restraint is involved, with kin deferring to one another based on who benefits the most from a given resource in a particular set of circumstances.

A variety of population responses have been seen when tadpoles interact at high densities, but one common trend is for larger tadpoles to inhibit the growth of smaller conspecifics when competing for limited resources, probably through a chemical signal in the water. As there is evidence that chemical cues are used to recognize kin in some species, this may also be a way to communicate growth information to siblings. Although this could simply be a way for larger tadpoles to suppress the growth of their competitors, in some cases smaller tadpoles have been observed to perform better when grouped with kin versus non-kin. Rather than uniformly suppressing growth, larger individuals aggregating with kin may produce a signal that smaller siblings respond to by sacrificing resources to larger kin, who see a greater increase in marginal fitness benefits with additional resources, until the former have attained an adequate size. At this point smaller individuals, who now receive a greater fitness benefit from additional food, respond by using more food and increasing their growth rate. In this way, inclusive fitness and individual fitness are maximized for all siblings. For this to occur, the kinship and size structure of groups of tadpoles in nature must be predictable or detectable by some mechanism. On the other hand, unrelated tadpoles have no indirect fitness component to worry about and should always compete as intensely as possible, in order to maximize direct fitness.

My first step in addressing this question has been to investigate whether the conditions perceived by individual tadpoles affect their growth and development. To do this, I exposed tadpoles in individual enclosures to water from tanks containing either siblings or non-siblings receiving one of two food levels, then monitored several measures of growth and development until metamorphosis. By examining whether patterns differ across treatments and what form these differences take, I can gain further insight into what strategies may prove beneficial under various circumstances and how these strategies may have evolved.

For more information about Sara’s work, you can contact her at garnett3 at msu dot edu.

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