The idea that animal brain mass and body mass are correlated makes intuitive sense: larger animals, after all, tend to have larger brains than their smaller cousins.
However, it has proven difficult to find a definitive model for how these two figures relate to each other.
A new study published in Nature Ecology and Evolution this month suggests that this problem arises from a fundamental error in a long-held assumption about the mathematical relationship between the two measures.
The study proposes an alternative model, one that fits the data better and promises explanations for some other long-standing questions about cerebral evolution.
As the study explains, the general assumption for decades has been that the relationship between body mass and brain follows a relatively simple power law. However, this proposed relationship has been hotly debated as it does not appear to apply to all groups of species.
Until now, researchers have not been able to find a configuration of this equation that works across the board. In particular, the difference between different taxonomic levels has been of such concern that it has been given its own name: the taxon-level problem.
While the value and source of a critical part of the relationship referred to as the allometric component have been the subject of much debate, the new paper addresses a more fundamental question: whether we are correct in assuming that the relationship between brain mass and body mass follows some sort of linear relationship. logarithmic.
To do this, the authors took an extensive database that included 1,504 mammalian brain/body mass values and looked at what kind of model fit the data best.
They found that instead of being log-linear, the relationship was log-curvilinear: when plotted on a logarithmic scale, the graph itself had to be curved to accommodate it.
The authors tested several different equations to construct this curve and found that the one that fit best was a second-order polynomial—the kind of quadratic equation one tends to encounter in high school.
The key feature of the curve is that “as mammalian mass increases, the rate at which brain mass increases with body mass decreases.” It thus predicts that very large animals have smaller brains than predicted by the linear model – which is exactly what the data set shows.
Having a single curve to which the brain/body relationship can be fitted allows comparisons between groups that had different allometric coefficients in the previous model.
The study points to one advantage of the new model: “[it] makes it possible to study evolutionary trends in the evolution of traits (or relative traits) over time.” To this end, he examines the rate at which brain mass increases—or, in other words, the rate at which brains evolve larger Finds important differences between species.
Surprisingly, primates evolved large brains extremely quickly, but so did rodents and carnivores. Only three groups of animals showed an association of increased brain mass with body mass over time.
The study focuses on mammals, but the authors also examined a dataset of body/brain mass pairs for birds and found that the curvilinear relationship also fits these data well. But perhaps the most intriguing mystery that remains to be solved is why brain and body mass are correlated in this way.
This question remains very open, and its answer may have a lot to say about how and why animals evolved the way they did.
As the study says in its conclusion, “the search for the theoretical and empirical basis for curvilinear relationships between species will probably lead to major contributions to all of biology.”
This research was published in Nature Ecology and Evolution.