Decephalisation in domestication, part 3: the horse (Equus caballus)

@matthewinabinett @elliotgreiner @simontonge @variani18

My reference is COZZI ET AL. (2013, 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).

The overall value for the encephalisation quotient (EQ) of the horse (Equus caballus), given by these authors, is 0.78.

However, this may be misleading.

TECHNICAL PROBLEMS

The sample included sexually immature individuals. These are likely to boost the EQ falsely, because the brain grows faster than the body, in an ontogenetic context.

Another reason why the EQ calculated by Cozzi et al. (2013) is somewhat exaggerated is that their mean body mass for males was less than that for females. This is odd, because in the horse mature males are somewhat more massive than mature females.

It seems that, in the particular abbatoir used by these authors, for some reason males tended to be smaller-bodied than females. This quirk resulted in a somewhat overestimated EQ for males.

Although Cozzi et al. (2013) 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. And 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 the European bost (Bos taurus) obtained by the same authors in their study of the latter species.

I also question the use, by Cozzi et al. (2013), of encephalisation quotient 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/population 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 of the horse, as these authors do, seems invalid.

Cozzi et al. (2013) show remarkable individual variation in the brain mass of the domestic horse. Some individuals have massive brains compared to the mean for the sampled population. However, this does not mean that these individuals had a boosted EQ – because EQ is an average, applicable only to the collective.

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.

Thus, part of the individual variation in brain mass relative to body mass may be 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.

Because their analyses 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 the finding by Cozzi et al. (2013) 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 value of about 500 kg), and
• one of their charts 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 horse have remarkably heavy brains.

Here, I list all the individuals with brain mass > 700 g (compared with overall mean of 606 g):

• 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; 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 such a large brain that, 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; 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, with an unusually massive brain (relative to the adult male mean of 590 g), but not a particularly massive body.

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. The result is that some individuals of the horse have disproportionately massive brains.

Whether these individuals are particularly intelligent is unknown.

However, it is noteworthy that the European bost shows relative homogeneity, being consistent in 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 horse, adult female body mass has mean 567 kg, with brain mass having a mean of 613 g (Cozzi et al. 2013).

The EQ works out as 0.74, according to the Jerison formula.

In the European bost, adult female body mass has mean 600 kg, with brain mass having a mean of 486 g (Ballarin et al. 2016, https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0154580). 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 slightly heavier than the equid.

At such great body mass, we expect some natural decephalisation, based on wild species such as the African savanna buffalo (Syncerus caffer, https://www.inaturalist.org/taxa/42405-Syncerus-caffer).

It is hard to know how to interpret the above EQ values relative to the wild ancestors, which are now extinct in both cases.

However, what these results show clearly is that the equid is brainier than the bovine. The EQ values are clearly different, at more than 0.7 for the horse, but less than 0.6 for the European bost.

This difference

• is statistically significant, and
• makes sense given the obviously more intelligent behaviour in equids than in bovines.

However, a problem remains in interpreting these values. EQ reflects brain mass and body mass, both of which 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 – if such a species ever existed.

It is also hard to say whether the European bost exceeds the horse in the degree to which EQ 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.

DISCUSSION

Imagine a woman riding a fine horse, putting it through various 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 EQ 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 has EQ about 6.5 times greater than the mammalian average, while the horse has EQ about 1.5 times less than the mammalian average.

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 horse.

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 is considerably inferior in braininess 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 horse, such as the Shetland pony (https://en.wikipedia.org/wiki/Shetland_pony), are as valid, as representatives of the species, as large breeds such as thoroughbred or draft horses.

Cozzi et al. (2013) 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 informative to study, for the first time, the smallest breeds or mongrels (body mass about 200 kg) of the horse, by methods similar to those of Cozzi et al. (2013).

I suspect that the EQ would turn out to be boosted, to about 1. This is because brain mass relative to body mass would be much greater than in the large-bodied breeds and mongrels - despite the latter animals having brains of about 400 g, far less than the ca 600 g documented by Cozzi et al. (2013),
 
The ancestral wild horse was probably rather small-bodied, perhaps 300 kg. I suspect, based on zebras, that it had EQ > 1.
 
So, I suspect that, keeping body mass constant, and compared the smallish original wild ancestral equid (now extinct) with small ponies, one would find some decephalisation. However, this may have been modest: possibly a reduction of EQ from 1.2 to 0.9.

I suspect that

• it is only with an increase in body mass, via the breeding of large-bodied breeds of the horse, that the EQ was reduced to a value far below the mammalian average,
• this is not because the large-bodied breeds are necessarily any more decephalised than the small-bodied breeds per se, but because their body mass has been boosted so much,
• the EQ documented by Cozzi et al. (2013) underestimates the braininess of breeds similar in body size to the wild ancestor, and
• all EQ values (=all values for braininess) for the horse tend to underestimate the intelligence of the species owing to the greater cerebral folding in equids than in ruminants.

Which are more ‘typical’ of the species, Equus caballus: small-bodied ponies or large-bodied horses? I.e. which EQ is more ‘typical’ of the species, Equus caballus: 1.0 or 0.75?

Do readers see how arbitrary/capricious this is, depending on the breeds available/chosen for study?

https://www.inaturalist.org/journal/milewski/67396-decephalisation-in-domestication-part-4-more-on-the-horse-equus-caballus#...

Posted on June 19, 2022 01:47 AM by milewski