Decephalisation in domestication, part 3: Equus caballus
(writing in progress)
I wonder if the fact that people have given the smaller breeds of horse a different name, i.e. ‘pony’, indicates that there is a difference in the kind of intelligence recognised by humans, within the species Equus caballus.
In which other domestic mammal do we call the species a different name depending on its body size? Is it too far-fetched to suggest that part of the reason why horses and ponies are named as if they are different species is that the former has a considerably inferior EQ to the latter?
Please see https://www.thespruce.com/the-difference-between-horses-and-ponies-1886998 and https://www.quora.com/Is-there-a-behavioral-difference-between-ponies-and-horses.
Small breeds of the domestic horse, such as the Shetland pony, are every bit as valid, as representatives of the species, as large breeds such as thoroughbred or draft horses.
Cozzi et al. (2014) sampled only ‘mongrels’ derived from the larger breeds, generally of body mass >500 kg. They found EQ of about 0.75, which is a puzzlingly small value notwithstanding the likely decephalisation resulting from domestication. The puzzle arises because the behaviour of the domestic horse indicates at least average braininess for a mammal, which would correspond to EQ = 1.
It would be interesting to study the smallest breeds or mongrels of the horse, by methods similar to those of Cozzi et al. (2014), in which body mass was more like 200 kg. Nobody has done this. However, I suspect that the EQ would turn out to be far greater, perhaps about 1. This is because, although these animals would certainly have brains lighter than the ca 600 g documented by Cozzi et al. (2014), and might be more like 400 g, the brain mass relative to body mass would still be much greater than in the large breeds and mongrels.
Now, the ancestral wild horse was probably on the small side, perhaps 300 kg. I suspect, based on zebras, that it had EQ > 1.
So I think that, if one kept body mass constant, and compared the smallish original wild ancestral horse (now extinct) with small ponies, one would find some decephalisation, but not much: possibly a reduction of EQ from 1.2 to 0.9. It is only with the increase in body mass, via the breeding of large breeds of the horse, that the EQ was reduced to a value far below the mammalian average, not because the large breeds are necessarily any more decephalised than the small breeds per se, but because their body mass has been boosted so much.
So, the trouble with the study of Cozzi et al. (2014) is that they neither studied the ‘right’ breeds from my viewpoint, nor even mentioned the need to do so in order to get a fairer overall view of EQ in the domestic horse.
So I suspect that the EQ documented by Cozzi et al. (2014) underestimates the braininess of breeds similar in body size to the wild ancestor. And I also think that all EQ values for the domestic horse, i.e. all values for braininess for this species, tend to underestimate the intelligence of the species owing to the greater cerebral folding in equids than in ruminants.
Supporting my speculations above is one of the charts in Cozzi et al. (2014), which shows that, in their sample, there was great variation in EQ depending on body mass. In individuals weighing about 800 kg in their sample, EQ was only about 0.6. At the other extreme, in individuals weighing not much more than 400 kg in their sample, EQ was far greater, about 0.95. If even within Cozzi et al’s (2014) population of rather large individuals we see an elevation of EQ from the overall mean of about 0.75 to values of 0.95, then it seems fair to predict that, if ponies of only about 200 kg were studied, the EQ would easily exceed 1.
Which are more ‘typical’ of the species, Equus caballus: small ponies or large horses? I.e. which EQ is more ‘typical’ of the species, Equus caballus: 1.0 or 0.75? Do you see how arbitrary this is, depending on the breeds available for study?
There seems to be a perception by some equestrians that ponies are more intelligent than horses, i.e. that the smaller breeds of E. caballus are the smarter, more wily, more cunning, more mentally resourceful, etc. Please see the excerpts below, although this website was not unanimous on this question.
(writing in progress)
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Cozzi et al. (2014, https://www.researchgate.net/profile/Michele_Povinelli/publication/259319958_The_Brain_of_the_Horse_Weight_and_Cephalization_Quotients/links/59d48ef34585150177fc4da4/The-Brain-of-the-Horse-Weight-and-Cephalization-Quotients.pdf) show remarkable individual variation in the brain mass of the domestic horse.
Because the analyses presented by Cozzi et al. (2014) on this point are rather confusing, I had a look for myself in their supplemental data, in which each individual is listed with its sex, age, brain mass, and body mass.
The problem with Cozzi et al.’s (2014) statement that the horse has remarkable individual variation in brain mass is that their sample also varied greatly in body mass (the heaviest individuals exceeding 800 kg compared with the normal 500 kg or thereabouts), and a chart presented by them shows that much of the variation in ‘individual EQ’ is accounted for by individual variation in body mass.
However, my own scrutiny of the raw data confirms that at least a few individuals of the domestic horse have remarkably heavy brains.
Here I list all the individuals with brain mass > 700 g (compared with overall mean of 606 g) as follows:
female 7 y old, brain 759 g at body mass 813 kg. This individual is ambivalent because it was so big that it is hardly surprising that its brain was so big.
female 6 y old, brain 708 g at body mass 830 kg, same interpretation as that above: no real puzzle because this was such a massive individual.
female 9 y old, brain 735 g at body mass 567 kg. This individual clearly shows the anomaly, because it had exactly average body mass and yet possessed a far more massive brain than the average of 613 g.
female 7 y old, brain 946 g at body mass 433 kg. Wow! This is a remarkably massive brain compared to the mean of 613 g. And the body mass is far less than average. This is a small individual with a big brain, and so if all members of the population were like this the EQ for this population would be similar to those for primates such as certain monkeys, and certainly greater than the likely EQ of the original wild ancestor of the horse (which I assume to have been about 1.1).
male 8 y old, brain 853 g at body mass 403 kg. Again, wow! This is another anomalous individual.
male 9 y old, brain 722 g at body mass 518 kg. This individual also shows the anomaly to some degree. It has an unusually big brain (relative to the adult male mean of 590 g) while its body is not particularly massive.
The pattern remains rather unclear. However, I infer that, although domestication has reduced the mass of the brain relative to body mass, it has not removed the individual variation in brain mass relative to body mass.
So, some individuals of the domestic horse have disproportionately massive brains. Whether these individuals are particularly intelligent is anyone’s guess. However, it is interesting that the European bost (Bos taurus) seems to show a more homogeneous pattern, i.e. a more consistent brain mass relative to body mass, at least in adult females.
COMPARISON OF BOS TAURUS AND EQUUS CABALLUS
I limit the following summary to adult females.
In the Equus caballus, adult female body mass has mean 567 kg, with brain mass having a mean of 613 g (Cozzi et al. 2014). The EQ works out as 0.74, according to the Jerison formula.
In Bos taurus, adult female body mass has mean 600 kg, with brain mass having a mean of 486 g (Ballarin et al. 2016). The EQ works out as 0.58, according to the Jerison formula.
Note that, in both cases, adult female body mass is about 0.6 tonne, with the bovine a bit heavier than the equid. At such great body mass, we expect some natural decephalisation, based on other species sampled in the wild, such as the African savanna buffalo (Syncerus caffer, https://www.inaturalist.org/taxa/42405-Syncerus-caffer). So, it is hard to know how to interpret the above EQ values relative to the wild ancestors, which are now extinct in both cases.
What this does show clearly is that the equid is brainier than the bovine. The EQ values are clearly different, at more than 0.7 for E. caballus but only less than 0.6 for B. taurus. There is no doubt that this difference is statistically significant, and it also makes sense given the obviously more intelligent behaviour of equid than bovine.
However, there is still a problem interpreting these values. This is because EQ reflects both brain mass and body mass, and both of these seem to have been changed by the artificial selection of domestication. Both species are more massive than their wild ancestors. At the same time, selective breeding is also likely to have led to a reduction in brain mass at any given body mass. So it is hard to know how much of the reduction in EQ in the domestic vs wild species is owing to a decrease in brain mass with domestication, and how much is owing to a boosting of body mass with domestication. In the case of Equus in particular, it remains difficult to know what the EQ would have been in any wild species naturally exceeding 0.5 tonne in body mass – is such a species ever existed.
It is also hard to say whether the EQ of B. taurus has been reduced by domestication more than the EQ of E. caballus has been reduced by domestication.
What is sure is that the equid is brainier than the bovine, and this conclusion can be stated with particular confidence because the two species are so similar in their body masses of adult females.
Imagine a woman riding a fine horse, putting it through all sorts of paces and feats such as steeplechasing. Imagine the bond between horse and ‘owner’, and the trust required for the horse, with a visual system so different from ours, to jump over obstacles more out of faith in its rider than out of an ability precisely to see what it is doing. Imagine that this particular individual horse is every bit as much of a ‘personality’ as its ‘owner’ is, perhaps kind, perhaps jealous, perhaps mischievous, perhaps phlegmatic, perhaps highly-strung, perhaps quick to temper, perhaps vindictive, perhaps playful. Imagine the fine cooperation that occurs between these two beings and the great amount of training and practise that has gone into their performances. And then remember how different they are in braininess: the woman with > 6-fold the braininess of the average mammal, while the horse does not even meet the mammalian average. The human is about 8-fold brainier than the horse, because the human is about 6.5 times brainier than the mammalian average while the horse is about 1.5 times less brainy than the mammalian average.
The overall EQ value given by Cozzi et al. (2014) for the domestic horse in their study is 0.78. However, but this is misleading because they included sexually immature individuals, which boost the EQ falsely because their brains have grown faster than their bodies. Another reason why their EQ is somewhat exaggerated is that the mean body mass for males was less than that for females in their study. This is odd, because everyone knows that in the domestic horse mature males are somewhat more massive than mature females. It seems that, in the particular abbatoir used by Cozzi et al. (2014), for some reason males tended to be smaller than females. This quirk resulted in a somewhat exaggerated EQ for males.
Although these authors present, in their summary, the overall EQ value for the domestic horse as being 0.78, their value for females - the more standard sex - was only 0.76. This, on closer examination, turns out to be exaggerated because of the inclusion of juveniles. The real value for adult females is 0.74, and this is the value we should compare to the value for Bos taurus obtained by the same authors in their study of the latter species.
I can also criticise the sloppy use, by Cozzi et al. (2014), of the term EQ as applicable to individuals as well as the whole population/taxon. Implicit in the true definition of EQ is that it is a property of the taxon as a whole. To assign different EQ values to males and females, one would have to define EQ differently. And to use EQ as applicable to a given individual horse, as these authors do, discredits their study. It is obviously true that some individual horses have massive brains compared to the mean for the sampled population, but this does not mean that these individuals had a great EQ – because EQ is a concept valid only for the collective.
Anyway, the individual variation in brain size in the domestic horse is noteworthy. Individuals of about 700 kg body mass had ‘EQ’ of about 0.6, whereas individuals of about 400 kg body mass had ‘EQ’ of about 0.9. In other words, part of the individual variation in brain mass relative to body mass is explained by the great variation in body mass in this sample. This implies that brain mass tends to be consistent despite variations in body mass among individuals. I.e. that the main reason why certain individuals seem so ‘brainy’ is that they happen to have small bodies.
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