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Disclaimer: I did not read Rushton’s seminal book on the topic. I was only first recently able to obtain a complete electronic copy (i.e. not abridged version). I did in fact only skim this literature. I don’t have time to work on this, but I’d like others to do so, hence I put the labor of my initial approach for others to build upon.

A colleague writes to me that (edited):

Last week during skiing holidays I saw a popular scientific program on animal intelligence in the evening, and they presented something like the Lynn-Miller-Rushton cold winter theory! Birds (chickadees) living in Alaska have bigger brains and are more intelligent than birds living in Kansas! This is an independent very valuable support for the cold winter theory on intelligence. I found some of the original studies:

Roth, T. C. & Pravosudov, V. V. (2009). Hippocampal volumes and neuron numbers increase along a gradient of environmental harshness: A large-scale comparison. Proceedings of the Royal Society B: Biological Sciences, 276(1656), 401–405. http://doi.org/10.1098/rspb.2008.1184
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2664346/

Roth, T. C., LaDage, L. D., & Pravosudov, V. V. (2010). Learning capabilities enhanced in harsh environments: A common garden approach. Proceedings of the Royal Society B: Biological Sciences, 277(1697), 3187–3193.
http://rspb.royalsocietypublishing.org/content/early/2010/05/29/rspb.2010.0630

Roth, T. C., Gallagher, C. M., LaDage, L. D., & Pravosudov, V. V. (2012). Variation in brain regions associated with fear and learning in contrasting climates. Brain, Behavior and Evolution, 79(3), 181–190.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3343761/

Pravosudov, V. V., Roth, T. C., LaDage, L. D., & Freas, C. A. (2015). Environmental influences on spatial memory and the hippocampus in food-caching chickadees. Comparative Cognition & Behavior Reviews, 10, 25–43.
http://comparative-cognition-and-behavior-reviews.org/2015/vol10_pravosudov_roth_ladage_freas/

A list of the papers of Timothy C. Roth:
https://www.fandm.edu/timothy-roth

The area is called behavioral ecology. Some years ago I quickly skimmed some of this literature with an eye for comparing it with the human racial evolutionary models. Human populations were not the only ones who were faced with the cold climate of the north. While one can find studies that fit with Rushton etc.’s thinking, one can also find the reverse. Northern does not always mean smarter, larger brain, more behaviorally complex or less aggressive in non-human animals. Among turtles, the evidence shows that the northern ones are towards r, southern ones towards K (measured in egg size and count). Here’s a bunch of other studies I was able to quickly find covering a variety of animals:

• Sol, D., Lefebvre, L., & Rodríguez-Teijeiro, J. D. (2005). Brain size, innovative propensity and migratory behaviour in temperate Palaearctic birds. Proceedings of the Royal Society of London B: Biological Sciences, 272(1571), 1433–1441. https://doi.org/10.1098/rspb.2005.3099
• Barrickman, N. L., Bastian, M. L., Isler, K., & van Schaik, C. P. (2008). Life history costs and benefits of encephalization: a comparative test using data from long-term studies of primates in the wild. Journal of Human Evolution, 54(5), 568–590. https://doi.org/10.1016/j.jhevol.2007.08.012
• Sol, D., Székely, T., Liker, A., & Lefebvre, L. (2007). Big-brained birds survive better in nature. Proceedings of the Royal Society of London B: Biological Sciences, 274(1611), 763–769. https://doi.org/10.1098/rspb.2006.3765
• Sol, D., Garcia, N., Iwaniuk, A., Davis, K., Meade, A., Boyle, W. A., & Székely, T. (2010). Evolutionary Divergence in Brain Size between Migratory and Resident Birds. PLOS ONE, 5(3), e9617. https://doi.org/10.1371/journal.pone.0009617
• Schuck-Paim, C., Alonso, W. J., & Ottoni, E. B. (2008). Cognition in an Ever-Changing World: Climatic Variability Is Associated with Brain Size in Neotropical Parrots. Brain, Behavior and Evolution, 71(3), 200–215. https://doi.org/10.1159/000119710
• Morrison, C., & Hero, J.-M. (2003). Geographic variation in life-history characteristics of amphibians: a review. Journal of Animal Ecology, 72(2), 270–279. https://doi.org/10.1046/j.1365-2656.2003.00696.x
• Jiang, A., Zhong, M. J., Xie, M., Lou, S. L., Jin, L., Robert, J., & Liao, W. B. (2015). Seasonality and Age is Positively Related to Brain Size in Andrew’s Toad (Bufo andrewsi). Evolutionary Biology, 42(3), 339–348. https://doi.org/10.1007/s11692-015-9329-4
• Gillooly, J. F., & McCoy, M. W. (2014). Brain size varies with temperature in vertebrates. PeerJ, 2, e301. https://doi.org/10.7717/peerj.301
• Moore, I. T., Perfito, N., Wada, H., Sperry, T. S., & Wingfield, J. C. (2002). Latitudinal variation in plasma testosterone levels in birds of the genus Zonotrichia. General and comparative endocrinology, 129(1), 13-19.
• Garamszegi, L. Z., Hirschenhauser, K., Bókony, V., Eens, M., Hurtrez-Boussès, S., Møller, A. P., … & Wingfield, J. C. (2008). Latitudinal distribution, migration, and testosterone levels in birds. The American Naturalist, 172(4), 533-546.
• Leggett, W. C., & Carscadden, J. E. (1978). Latitudinal variation in reproductive characteristics of American shad (Alosa sapidissima): evidence for population specific life history strategies in fish. Journal of the Fisheries Board of Canada, 35(11), 1469-1478.
• Iverson, J. B., Balgooyen, C. P., Byrd, K. K., & Lyddan, K. K. (1993). Latitudinal variation in egg and clutch size in turtles. Canadian Journal of Zoology, 71(12), 2448-2461.
• Chalfoun, A. D., & Martin, T. E. (2007). Latitudinal variation in avian incubation attentiveness and a test of the food limitation hypothesis. Animal Behaviour, 73(4), 579-585.
• Heibo, E., Magnhagen, C., & Vøllestad, L. A. (2005). Latitudinal variation in life‐history traits in Eurasian perch. Ecology, 86(12), 3377-3386.

There are many more. To locate them, use search queries like ‘latitude “brain size” bird‘. Or look up the publication lists of the prominent researchers such as Daniel Sol.

Integrating this will require someone to read a lot of varied material and find some overall patterns in the mess. Basically a job for someone biologisty and generalisty, someone like Woodley.

Some comments

With regards to birds, brain size and ecology, there is a problem. Birds living in the high latitudes must either adapt a migrating behavioral pattern or learn how to survive in the winter. Most birds take the first route, but some don’t. However, to fly long distances, it helps to be lean, so there is strong selection against extra weight such as a larger brain. For this reason, bivariate latitude x brain size comparisons might show the opposite pattern than expected. One must account for the solution to the, well, cold winter problem. Some amphibians have an analogous tactic: hibernation. Many insects have yet another analogous solution: they only live in the summer (single year life spans). As far as I understand, fish do not have issues with the water temperature in the winter, so they don’t face the problem. Except for possibly hibernation (which sometimes does require planning ability e.g. in squirrels), these strategies would not seem to select so strongly for intelligence, and so one would not expect the higher latitude species to smarter, less aggressive and so on.

In general, therefore, it seems best to focus on animals that tackle the cold winter problem head-on instead of avoiding it somehow (migrate, hibernate, or single-year lifespans). Among birds, the smartest birds are of the Corvidae family — in particular crows, ravens and magpies — and they generally don’t migrate in the winter. Of the non-Corvidae, I think the smartest birds are some of the parrot species. These also often don’t migrate. (See also bird intelligence.)

So, if I were to look for these relationships/integrate the findings, I would select families/orders (or whatever) of animals that:

1. are widely distributed around the Earth, so that we have natural variation to exploit.
2. that don’t side-step the cold winter problem in some way. If they do, then this must be taken into account.
3. that have a large number of species and sub-species. Small n science is bad science.
4. that haven’t been artificially selected by humans.

Then I would look for reviews and studies of intelligence/cognition, brain size, tool use, latitude, climate harshness, climate variability, (ecological) temperature in these. One can enter some animal family or common term (e.g. ‘bird’) as a search term to avoid the human IQ literature.

It’s a bit tricky to choose suitable families. E.g. hooved animals — cows, horses, pigs — have often been bred by humans, so they don’t work well for testing models (the variation is not natural). They also have too little natural variation, in some cases because we killed most of them already. But deer may work okay.

Carnivores would have worked well, but like the hooved animals, we either enslaved them (wolves -> dogs, cats) or killed them (other wolves, tigers). There some wild medium-sized cat species (Felidae) left — cougars, lynx –, so maybe one could try those. Since they are wild and have claws and teeth, they aren’t so easy to work with.

Would non-human monkeys work? Yeah, maybe. There are in fact already two great studies — Fernandes et al 2014, Navarrete et al 2016 — on primate brain size, cognitive ability tool use etc. Apparently, neither of them actually looked at the ecological correlations?! I smell gold. The dataset for Navarrette is public!

Humans are too slow to migrate effectively, so they are essentially forced to make do in the winter if they are to live in colder regions. So it’s not surprising that we see fairly robust patterns for humans (Beals et al 1984). The Inuit are of course the main exception, as these seem to have IQs in the low 90s despite living far north. Perhaps this is related to small effective population size which slows down evolution for multiple reasons.

Without any integration, what the non-human literature is good for is showing that the proposed model for humans (cold winter theory) is sometimes found for some animal families/orders, and that biologists have no particular trouble with positing these models for non-human animals. As such, there should be no particular scientific resistance to positing them for humans too.