Individual variation in brain size in the Rocky Mountain mule deer (Odocoileus hemionus hemionus)

https://assets-eu.researchsquare.com/files/rs-3143852/v1_covered_700fff66-ae6d-4c30-be28-d6a9613531cf.pdf?c=1705331304

INTRODUCTION

There is individual variation in most features of mammals.

However, on an intuitive basis I would have predicted that the size of the brain would be one of the least variable features, within a given species.

This is because the central nervous system is so basic to the functioning of the whole animal.

However, in reality the size of the brain can vary to a remarkable degree among individuals.

I first noticed this phenomenon during my fieldwork on ungulates in Kenya, in the late 'eighties.

Remarkable individual variation in brain size has subsequently been shown in the horse (Equus caballus, https://www.inaturalist.org/journal/milewski/67394-decephalisation-in-domestication-part-3-the-horse-equus-caballus).

This Post is about another ungulate in which such variation has been well-documented, viz. the Rocky Mountain mule deer (Odocoileus hemionus hemionus, https://www.inaturalist.org/observations?taxon_id=143865). This form of cervid is additionally a good example of sexual dimorphism in the size of the brain, relative to the size of the body.

SOURCE OF DATA FOR THE MULE DEER

My reference is Anderson A E et al. (1974, https://www.jstor.org/stable/3830385).

I corresponded with the author, Allen Anderson, in 1989, with the result that he sent me all his raw data for brain mass of both males and females.

RESULTS

The brain of males grows rapidly until an age of about 1.5 years. It then continues growing more slowly, until full maturity is reached at about 5 years old.

The sample in Anderson et al. (1974) includes a total of 21 male individuals older than 5 years. The average for all individuals older than 5 years is about 200 g.

From their graph (on page 59), I see a surprising range of variation - from 144 g to 243 g - in individual brain masses.

The raw data sent to me by Allen Anderson confirm this variation.

The individuals of extreme brain mass had the following body masses (which refer to the intact carcase minus only the blood, which makes only a kg or two of difference):

• one individual with brain mass 243 g has body mass 85 kg;
• one individual with brain mass 242 g had body mass 64 kg; and
• one individual 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 (in decreasing order) 112 kg, 100 kg, 79 kg, 75 kg, 70 kg, 61 kg, and 52 kg,

This means a range of 52-112 kg, which is a surprisingly 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 Rocky Mountain 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.

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 sample of 33 individuals of female mule deer, from 160 g to 242 g, which is a surprisingly wide range.

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 it 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, Giraffa tippelskirchi (task: Cite recent ref.).

On 17 Nov. 1989, Allen Anderson (by phone) acknowledged that brain size was remarkably variable among individuals, but had no explanation for this.

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, Allen Anderson told me that brain mass in males of the Rocky Mountain mule deer is complete before other aspects of masculine development are complete.

The following sequence applies:

• 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 the brain in males is precocial relative to the secondary sexual features, and particularly relative to the antlers.

This sounds to me like a pattern similar to what I myself found in the Maasai giraffe, albeit less extreme.

DISCUSSION

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.

What Anderson's data also show is that the variation in brain mass 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.

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.

It would be naive to think that the brain itself ‘gets fat in summer and lean in winter’. 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.

Publicado el junio 27, 2024 03:39 TARDE por milewski