Decephalisation in domestication, part 4: more on the horse (Equus caballus)
...continued from https://www.inaturalist.org/posts/67394-decephalisation-in-domestication-part-3-the-horse-equus-caballus#
Cozzi et al. (2013, https://www.researchgate.net/publication/259319958_The_Brain_of_the_Horse_Weight_and_Cephalization_Quotients) published a paper with good original data on brain size in a domestic equid, namely the horse (Equus caballus).
They also analysed the EQ (encephalisation quotient) of the horse, according to the approach of Jerison (https://royalsocietypublishing.org/doi/abs/10.1098/rstb.1985.0007).
Jerison's approach is questionable, because it seems to underestimate EQ. For example, the human species scores only 6.6, while the lion (Panthera leo) scores only 0.6-0.7).
However, the findings of Cozzi et al. (2013) at least allow us to compare the braininess of the horse with those of several other domestic mammals.
At EQ 0.78, the horse is considerably brainier than
- the European bost (Bos taurus) 0.55,
- the domestic pig (Sus scrofa domestica) 0.6, and
- the bactrian camel (Camelus bactrianus) 0.6.
However, the EQ of the horse is similar to those of
- the sheep (Ovis aries) 0.80, and
- the goat (Capra hircus) 0.71-0.93.
The situation is rather confusing, because
- most persons with experience of the horse would agree that it is more intelligent than sheep or goat,
- those who know the domestic pig well insist that it is among the most intelligent of domestic hoofstock, and
- nobody familiar with both the domestic sheep and the horse would regard the former as on a par in intelligence with the latter; so, it is puzzling that both species have EQ about 0.8.
Taking a broad view, we can safely say that the horse does not score above the eutherian mammal average for EQ, which is 1.0 – despite the likelihood that zebras do exceed this standard.
So, how can we explain the obviously greater intelligence of the horse than the ruminant hoofstock such as ‘cattle’?
Well, the answer seems to lie in the degree of convolution of the cerebral cortex. The brain of the horse is more folded than that of the European bost, as discussed in some detail by Cozzi et al. (2013).
I suspect that the wild ancestor of the horse (Equus ferus?) had EQ (perhaps as great as 1.5) more similar to those of zebras, because
- domestication has usually led to decephalisation, and
- this decephalisation has usually been greatest where the ancestral species was brainier than the mammalian average.
The EQ of the horse has been reduced to <1.0, presumably by anthropogenic selection for docility and tractability.
I suspect that the EQ values of the wild ancestors of sheep and goat also exceeded 1.0, however slightly (see the value for Ammotragus lervia in https://www.inaturalist.org/journal/milewski/96127-data-on-braininess-in-mammals-part-5#).
So what I see for the first time is the possibility that an aspect of the brain that has not been reduced by domestication is the degree of folding on the brain.
I infer that the horse is decephalised domestically w.r.t. its brain size, but is not decephalised w.r.t. the folding of its cerebral cortex.
The latter factor, as I see for the first time, may explain why the horse, even in domestication, remains more intelligent than comparable hoofstock belonging to Artiodactyla.
By the way, note that even after the decephalisation of domestication, the horse (EQ 0.78) is no less brainy than its predators the lion (0.6-0.7) and the tiger (Panthera tigris, 0.78). This again contradicts the notion that predators tend to be brainier than their prey (https://www.inaturalist.org/posts/67240-are-carnivores-really-brainier-than-their-prey#).
The original wild ancestor of the horse was not E. caballus, but a congener deserving a different species-epithet. It is extinct, so that we do not know its body mass.
However, my guess is that this was a pony-size species, on the basis that most extant spp. of Equus have body masses of <350 kg.
Contrast this with the body mass of what people assume to be ‘typical’ in the domestic horse, i.e. about 0.5 tonne in thoroughbreds and ‘police horses’. Some breeds of the domestic horse surpass 1 tonne in body mass. There is no extant wild species of Equus that reaches even half this value.
Small breeds of the horse, such as the Shetland pony, may therefore be every bit as valid, as representatives of E. caballus, as large breeds such as thoroughbred or draft horses.
Cozzi et al. (2013) sampled only ‘mongrels’, generally of body mass >500 kg, derived from the larger breeds. They found EQ (relative to mammals generally) of 0.78. This value is unsurprising in one sense, and surprising in another.
It is unsurprising that a domestic species would fall short of the average for wild Equus, because it is well-known that domestic selective breeding results in decephalisation.
On the other hand, it is surprising that the horse has such a small EQ, based on its behaviour relative to mammals generally.
The behaviour of the domestic horse suggests at least average braininess for a mammal. So, based on how intelligent the domestic horse seems to be (compared to all domestic ungulates, including the donkey, Equus asinus), we would expect E. caballus to have EQ > 1 (relative to mammals generally).
I do not know the brain mass of small breeds such as the Shetland pony, but I suspect that they have EQ > 1.1.
Supporting this guess, one of the charts in Cozzi et al. (2013) shows that, in their sample of the horse, there was great variation in EQ depending on body mass.
In individuals of the horse weighing about 800 kg, EQ was only about 0.6. At the other extreme, in individuals weighing not much more than 400 kg, EQ was far greater: about 0.95.
Thus, even within this population of rather large-bodied individuals, we see an increase of EQ from the overall mean of 0.78 to values of 0.95. So, 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: 1.1 or 0.6? Is this not arbitrary, depending on the breeds chosen for study?
There is a controversial perception by 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. (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).
Is the fact that we have given the smaller breeds a different name, i.e. ‘pony’, not in itself an indication 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 has a far inferior EQ to the latter?
I suspect that the wild ancestor (Equus ferus?) had EQ > 1, perhaps as great as 1.3 (again, relative to mammals generally),
- based on zebras, and
- assuming that this wild species was on the small-bodied side (perhaps 300 kg).
So I think that, if one kept body mass constant, and compared the smallish original wild ancestor (now extinct) with small ponies, one would find some decephalisation, but no more than a reduction of EQ from 1.3 to 1.0.
I suggest that 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. This was not because the large breeds are necessarily any more decephalised than the small breeds, but because their body mass has been boosted so much.
Because even thoroughbreds and draft horses are more intelligent than the average mammal:
What may have happened in the production of EQ <0.8 in large breeds of the horse:
It is not so much that there has been decephalisation relative to the wild congeners (which typically seem to have EQ > 1.0), but more that - accompanying a modest amount of decephalisation - there has been a trend for ‘domestic gigantism’ - in which what remains a rather intelligent brain runs an ‘outsize’ body.
So, my criticism of the study of Cozzi et al. (2013) is that they neither studied the ‘right’ breeds, nor even mentioned the need to do so in order to assess the real EQ of the horse.
I suspect that the EQ documented by Cozzi et al. (2013) for the horse underestimates all three of the following:
- the intelligence, relative to the mammalian ‘standard’ or average,
- the EQ of the wild ancestor, and
- the EQ of domestic breeds similar in body size to the wild ancestor.
Also helping to explain why EQ values for the horse might tend to underestimate the intelligence of the species is the greater cerebral folding in equids than in ruminants.
And this raises the question of whether, during domestication,
- Equus has been decephalised both in terms of brain mass and in terms of degree of cerebral folding, or
- the folding has remained the same, somewhat ‘offsetting’ any reduction in the mass of the brain.
Indeed, the whole question of folding of the cerebrum is important to the allometry of braininess in ungulates. However, even if we put this aside initially and just focus on brain mass, I see much that can be improved in our understanding of brain/body allometry in Equus.
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