Transportation in the Japanese Army also relied heavily upon horses and other pack animals, largely due to Japan's rather limited industrial capacity. As Edward Drea notes, the Japanese Empire in 1940 possessed 220,000 automotive vehicles (most of them imported) and could produce 30,000 trucks annually, whereas the United States in 1940 produced 3.7 million passenger cars and over 750,000 trucks. The poor state of infrastructure in places like China also played a role, however.

Throughout the Second Sino-Japanese War and Pacific War, mobilized infantry divisions often had roughly one horse for every three or four persons. The organization and strength tables for the 3rd and 18th Divisions in 1937 report 8,197 horses for 25,375 personnel and 5,493 horses for 20,087 personnel, respectively. The numbers for the 18th Division look essentially the same in December 1941 even after the addition of 50 vehicles (with the majority of these going to its mountain artillery regiment). Similarly, the 201st Division raised in 1945 had 5,202 horses for 19,946 personnel.

Various exceptions existed, of course. For instance, the triangularized 16th Division in December 1941 needed only about 2,470 horses for its 14,640 personnel, owing to the addition of nearly 500 motor vehicles (89 of which went to the supply regiment). The coastal defense divisions created very late in the war also had an authorized strength of 1,609 horses for 17,469 personnel, though by this point the Army was scraping the barrel, so to speak.

The Nationalist Chinese Army in the same period faced even greater logistical challenges, possessing only 3,000 vehicles in the 1937 while simultaneously suffering from a shortage of pack animals. To quote Chang Jui-te: "Twenty Nationalist divisions needed 10,647 horses and 20,688 mules. Yet at the end of 1935, they had only 6,206 horses and 4,351 mules. According to one statistic, in 1936, the mortality rate of the Nationalist military's horses was a high 5 percent. The number even reached 10 percent for some units." Unsurprisingly, perhaps, the Chinese addressed the deficits by drawing upon manpower, and porters often ended up carrying much of the food, ammunition, communications equipment, etc.---anything smaller than a mountain gun, it seems.

The paper-strength of a National Revolutionary Army division in 1933 allocated large numbers of porters to the artillery and transportation battalions. In 1945, porters still comprised the majority of transportation battalions while infantry regiments, engineer battalions, and artillery battalions now received support staff for working with pack animals, including veterinarians, horseshoers, and saddlers. In fact, the Army never had a chance to implement these changes, due to the sudden end of the Second World War, the renewal of the Civil War, and the attempts to downsize the military.

I'll look around some more in Chinese and Japanese sources and let you know if I gain any further insights, though I really should get back to working on my ancient history dissertation. :)

Sources:

Chang Jui-te,"The Nationalist Army on the Eve of the War," in The Battle for China: Essays on the Military History of the Sino-Japanse War of 1937-1945, ed. Mark Peattie et al. (Stanford: Stanford University Press, 2011), 83-104.

Harry H. Collier, Organizational Changes in the Chinese Army, 1895-1950 (Taipei: Office of the Military Historian, 1968).

Edward Drea, "The Imperial Japanese Army (1868–1945): Origins, Evolution, Legacy," in War in the Modern World since 1815, ed. Jeremy Black (London: Routledge, 2003), 75-115.

Kuwata Etsu and Maebara Tōru, eds., Nihon no sensō: zukai to dēta [Wars of Japan: Illustrations and Data] (Tokyo: Harashobō, 1982).

Morimatsu Toshio, Zusetsu rikugun shi [Illustrated History of the Army] (Tokyo: Kenpakusha, 1991), 174-176.

Nihon rikugun hohei rentai [Japanese Army Infantry Regiments] (Tokyo: Shin Jinbutsu Ōraisha, 1991).

Taiheiyō sensō shidan senshi [Pacific War Division Histories] (Tokyo: Shin Jinbutsu Ōraisha, 1996).

Toyama Misao and Morimatsu Toshio, eds., Teikoku rikugun hensei soran [Overview of the Organization of the Imperial Army], 3 vols. (Tokyo: Fuyō Shobō, 1987).