Intensive human contact correlates with smaller brains: differential brain size reduction in cattle types

Abstract

Cattle are one of the most intensively bred domestic animals, providing humans with a multitude of products and uses. Using data from the fossil record, we test if their domestication, as for other taxa, has resulted in a reduction of their brain size. We not only conclude that Bos taurus (domestic cattle) have smaller brains than their wild ancestor, Bos primigenius (aurochs), but that brain size varies significantly by breed, with some having much smaller brains than others. Differences in husbandry practices between several breed categories align with a range of human engagement, which also aligns with the degree of selection for docility. Sampling 317 domestics from 71 breeds, we investigate if differences in brain size correlate with the intensity of human contact. A clear pattern emerges whereby a brain reduction gradient parallels a gradient in behavioural selection. Bullfighting cattle, which are bred for fighting and aggressive temperament, have much larger brains than dairy breeds, which are intensively selected for docility. Our results add to a fundamental aspect of animal domestication theory: the interplay between basic features of the domestic environment-selection for docility, absence of predators and human provision of resources-seems to explain differences in brain size.

Keywords: allometry; aurochs; behaviour; domestication; encephalization; phylogeny.

Figure 1.

(a) Relative brain size difference between wild and domestic cattle. Regression of EV estimate versus MZW; table 1 for statistics. (b) Skull measurements used for EV estimation in current study (bold) and in Finarelli [41]: BCL = braincase length and BCW = braincase width. BCH = braincase length was omitted from analyses as it is a poor estimator of EV in bovids (Finarelli [41]). MZW = muzzle width, was used for allometric size correction. Silhouettes credited to: DFoidl (modified by T. Michael Keesey) (aurochs), https://creativecommons.org/licenses/by-sa/3.0/; and Steven Traver (dairy). (Online version in colour.)

Figure 2.

(a) Brain size variation between breeds; table 2 in electronic supplementary material, data D6 for statistics. (b) MZW variation between breeds, measured against overall size (PTL = posterior tooth row length); table 3 in electronic supplementary material, data D6 for statistics. Silhouettes credited to: DFoidl (modified by T. Michael Keesey) (aurochs), https://creativecommons.org/licenses/by-sa/3.0/; Steven Traver (dairy); Tracy Heath (beef); and Mariana Ruiz Villarreal (bullfighting). (Online version in colour.)