Decephalisation in domestication, part 2: Bos taurus
(writing in progress)
Ballarin et al. (2016) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4851379/ document the brain mass and body mass of adult female Bos taurus of the breeds Holstein/Friesian/Charolai/Limousine. The same team has, in other papers, presented similar data for the domestic pig (Sus scrofa) and the domestic horse (Equus caballus).
Ballarin et al. (2016) sampled more than 100 individuals of adult female Bos taurus of the breeds mentioned above. These breeds reach adulthood after three years old, so all the individuals I report here were 3-16 years old. I exclude the individuals < 3 years old reported in the same paper.
The mean body mass was 600 kg and the mean brain mass was 486 g. I take these figures as reliable, allowing us to establish the EQ of Bos taurus.
The EQ calculated from the above values, using the Jerison allometric equation, is 0.58. Compare this with the usual value of about 1.0 for smaller members of the same family, Bovidae, i.e. antelopes.
Although Bos is closely related to antelopes, the main difference being the far greater body size of Bos, the EQ values differ to a surprising degree: antelopes tend to have EQ of about 1.0, which is about average for mammals, whereas adult female Bos taurus has EQ of only about 0.6. And bear in mind that the EQ would probably be even smaller in mature males, which were not sampled here.
There is a pattern in which the largest ungulates, other than elephants, tend to have small values for EQ compared with the medium-size and small ungulates. So the value of 0.6 or less for the EQ of Bos taurus is not surprising in view of the great body mass of these breeds. However, another factor involved, which cannot be quantified because the wild ancestor is extinct, is the degree to which Bos taurus has been decephalised by selective breeding in domestication.
The first surprising fact is that in adult female Bos taurus, the brain keeps growing past four years old and perhaps past 9 years old. I.e. the brain, although smaller than predicted based on extrapolation from smaller ruminants (antelopes and deer), continues to grow for years after sexual maturity is reached. This seems somewhat paradoxical, because I would have expected that a decephalised species would show this ontogenetically, by ceasing to grow its brain at a relatively early age, such as that of sexual maturity (about 3 years in this case).
The second surprising fact is that the individual variation in brain mass in adult female Bos taurus is far less than in adults of the domestic horse.
The horse can be assumed to have been decephalised by domestication, relative to its wild ancestor, and the EQ of the domestic horse is only about 0.8 (which is considerably less than that of zebras at > 1.0). Nevertheless, the surprising fact, found by these authors in their study of the horse, is that different individuals of the horse vary greatly in brain size. Some individuals have surprisingly large brains, which if typical of the species would give the domestic horse a far greater value for EQ.
So, in the domestic horse the paradox is not that the brain continues to grow for longer than expected; instead it is that the domestic decephalisation of the horse on average has not obliterated a great individual variation in brain size, such that certain individuals of the domestic horse seem to be every bit as brainy as their extinct wild ancestors were on average.
I see this fact as important for an understanding of the human relationship with the domestic horse. Not only do individual horses vary in the usual ways on an individual basis, with the usual personality differences we’re used to in all mammals, but individual horses differ in more than personality: some individuals are far more intelligent than others.
This individual variation in intelligence does not seem to hold for Bos taurus, which presents greater homogeneity: all individuals are equally dumb, as it were.
Is it possible that there is some systematic difference between ruminants and equids in this way, which domestication has not been able to eradicate despite the human success in dumbing down the horse in the sense of a decline in the average EQ of the species from perhaps 1.0 to the current 0.8 or less?
(writing in progress)
Comments
Brain mass is surprisingly variable from individual to individual within the species.
This is evident in the domestic horse, based on a paper by Cozzi et al. It was not merely a matter of ponies being relatively large-brained owing to their small body size. At any given body size, brain size varies greatly from individual to individual in the domestic horse.
It is evident in the domestic dog, based on a graph presented by Kruska (2005). The point about breed and body size, which I have just made about the horse above, applies here too.
It is evident in the mule deer (Odocoileus hemionus), from data collected by Anderson (1974). This was not just a matter of body size varying greatly between summer and winter, which it does in deer. Even within a given sex and season, brain size varies individually in the mule deer.
My commentary:
It would be remarkable enough to find such individual in variation in brain size in any wild species of large mammal. However, to find that even domestic species retain such variation is truly food for thought. This is particularly because, when domestic species go feral, all this variation seems to count for naught in the ‘reversion’ to the original braininess of the wild ancestor – which has never been observed to occur in any feral population of large mammals.
We must assume that the dingo has as much individual variation in brain size as the domestic dog and wolves. Yet, over thousands of years of living as a wild mammal in Australia, the dingo has failed to regain the braininess of wolves. Instead it lives wild in the domestically-decephalised form.
I do not know if there is any correlation between individual brain size and individual intelligence. It is one thing to establish that brain size varies so much individually. It is another to assume that the large-brained individuals in the population are the most intelligent ones is quite another thing.
INDIVIDUAL VARIATION IN BRAIN SIZE IN THE MULE DEER (Odocoileus hemionus):
My reference is Anderson A E et al. (1974). I corresponded with the author in 1989, when I was in Chicago, and he also sent me all his raw data for brain mass of both males and females.
The brain of males of the mule deer grows rapidly until an age of about 1.5 years, then continues growing more slowly until full maturity is reached about the age of about 5 years. Anderson’s sample includes a total of 21 male individuals older than 5 years. From his graph (page 59) I see a surprising range of variation in individual brain masses, from 144 g to 243 g. The average for all individuals older than 5 years is about 200 g.
Turning to the raw data Andersen sent me for confirmation of this variation, I see that the individuals of extreme brain mass had the following body masses (which are bled body mass, i.e. the intact carcase minus only the blood, which makes only a kg or two of difference I presume). An individual with brain mass 243 g has body mass 85 kg; one with brain mass 242 g had body mass 64 kg; and one with brain mass 144 g had body mass 88 kg. The seven individuals which had about average brain mass (about 200 g) had various body masses of 70 kg, 112 kg, 79 kg, 61 kg, 52 kg, 100 kg, and 75 kg, which means a range of 52-112 kg, which is a large range (possibly partly owing to the fact that the animals were shot in winter as well as in summer).
My conclusion is that males of the mule deer, about 5-5.5 years old, vary in brain mass from 144 g to 243 g. This confirms that wild deer can vary remarkably in their brain sizes on an individual basis. Although body mass changes greatly from autumn to spring, the brain mass would hardly change seasonally.
Does the variation in brain size correlate with variation in individual intelligence? My guess is that the correlation would be weak, i.e. that the largest-brained individuals were not necessarily much smarter than the smallest-brained individuals.
I turn now to the adult females sampled by Anderson et al. (1974).
The total number of individuals of adult females was 33. Bled body masses ranged from 48 kg 75 kg. Again, I think that part of the reason for so much variation in adult body mass was that animals were shot in both winter and summer, i.e. at both the lean and the fat times of year.
Brain masses vary, in this set of 33 individuals of female mule deer, from 160 g to 242 g, which is a big range. It is as if the brain itself ‘gets fat in summer and lean in winter’ but of course this is not so, regardless of how much the body mass as a whole follows a seasonal cycle. I doubt that the brain varies seasonally by more than a gram or two..
The adult female mean for brain mass was 192 g, compared with the mature male value of 199 g. This is surprisingly similar between the sexes, considering that the mean body masses differed greatly: up to about 110 kg in mature males vs up to about 75 g in adult females. The sexual difference in brain mass is so limited that mean brain mass in mature males is only 3.5% greater than that in adult females. As he matures, the male puts on brawn and antler mass, but hardly puts on extra brain mass. This is a less extreme version of the pattern I myself found in the Maasai giraffe.
The important point is that even wild ungulates can show remarkable variation in brain size on an individual basis within a single category of age and sex.
On 17 Nov. 1989, while working at Field Museum of Natural History in Chicago, I phoned Allen E Anderson about the development of males in the mule deer (Odocoileus hemionus). I mentioned a pattern I had noticed around this time, which was the great individual variation in brain size in ungulates.
Anderson told me he did not know why brain size was so individually variable.
However, he assured me that there is no seasonal effect on brain mass. I.e. even if an individual of the mule deer becomes emaciated in winter, its brain does not shrink.
So his guess was that it is simply part of the biology of brains that individuals differ greatly in brain size. This, of course, means that if we want to document the mean braininess of a species, we have to sample many individuals.
As far as I can make out from my notes from the time, Anderson told me that brain mass in the male mule deer is complete before other aspects of his masculine development are complete. The brain is fully grown at adulthood, which I assume to be about 2-3 years old. Testicular parameters peak far later than this, after 7 years old. Then, later again, antler development peaks with the attainment of maximum body mass (perhaps by 10 years old?). No sooner has the male mule deer reached maximum body mass and antler size, than he starts to senesce, losing reproductive competitiveness year by year. I infer that his brain is precocial relative to his secondary sexual features, and particularly relative to his antlers. This sounds to me like a pattern similar to what I myself found in the giraffe (Giraffa tippelskirchii), albeit less extreme.
What I infer from these notes is that if one samples a fair number of male individuals of the mule deer, one will find considerable individual variation in brain mass. However, this variation cannot be attributed to either season or age beyond adulthood, which I infer may be about 4-5 years old for the brain. The body condition of the animals does vary greatly, but this leaves brain mass unaffected. The body and antler size continues to increase post-adulthood, until at least 7 years old, but this too leaves brain mass unaffected.
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