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Jarman-Bell Principle

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The Jarman-Bell Principle, coined by P.J Jarman (1968[1]) and R.H.V Bell (1971[2])[3], is a concept in ecology offering a link between a herbivore's diet and their overall size[4][5] by observing the allometric properties of herbivores; namely the non-linear scaling of body size[4][5]. According to the Jarman-Bell Principle, the food quality of a herbivore's intake decreases as the size of the herbivore increases, but the quantity of such food increases to counteract the low quality foods[5][6].

Large herbivores can subsist on low quality food[5]. Their gut size is larger than smaller herbivores[4]. The increased size allows for better digestive efficiency and thus allow viable consumption of low quality food[7]. Small herbivores require more energy per unit of body mass compared to large herbivores[3][7]. A smaller size, thus smaller gut size and lower efficiency, imply that these animals need to select high quality food to function[3]. Their small gut limits the amount of space for food, so they eat low quantities of high quality diet[8]. Some animals practice coprophagy, where they ingest fecal matter to recycle untapped/ undigested nutrients[7].

The Jarman-Bell Principle, however, is not without exception[5]. Small herbivorous members of mammals, birds and reptiles were observed to be inconsistent with the trend of small body mass being linked with high quality food[8].

The implications of larger herbivores ably subsisting on poor quality food compared smaller herbivores mean that the Jarman-Bell principle may contribute evidence for Cope's rule[5][9]. The Jarman-Bell Principle is also important by providing evidence for the ecological framework of "resource partitioning, competition, habitat use and species packing in environments"[5].

The Jarman-Bell Principle expected characteristics in large and small animals. Image by Steven J. C. Gaulin[6].
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Allometry refers to the non-linear scaling of a variable with respect to another. The relationship between such variables is expressed as a power law, where the exponent is a value not equal to 1 (thereby implying a non-linear relationship)[8].

Red line: Straight, imply linear relationship between (x) and (y) axis. Blue and green: Curved, imply non-linear relationship or a logarithmic relationship.

As an organism grows in size (in terms of height and width), their volume grows at a different rate. The organisms body mass can be expressed as such:

[8]

Explanation

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Food intake

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As herbivores, food intake is achieved through three main steps: ingestion (food is taken in), digestion (mechanical and chemical breakdown of food), and absorption (absorb nutrients from breakdown of food) [10].

Plant- based food is hard to digest[11] and is done so with the help of symbiotic microbes in the gut of the herbivore[10][11].

There are different types of stomach plans[12]:

  1. Ruminants: foregut fermenters with 4 chambered stomach.
  2. Pseudoruminants: foregut fermenters with 3 chambered stomach[13]
  3. Monogastric: hindgut fermenters with 1 chambered stomach. Plant- based food is broken down in the enlarged intestine and cecum.

When food is passed through the digestive system (including multiple stomach chambers), it breaks down further through symbiotic microbes[10].

For multi-stomached animals, the rumen is the first stomach and is the site of microbial fermentation[10]. The microbes feed on the food and, through respiration, produce waste products that are nutritious to the host herbivore. Of these waste products, volatile fatty acids make up ~70% of the energy intake of the herbivore[10]. The fermentation site is acidic[10] and the volatile fatty acids cannot be safely absorbed without appropriately moderating the pH levels for safe absorption[14]. Furthermore, fermenting microbes require an acidic condition to function, so increasing pH (hence, decreasing acidity of the rumen) to a moderate level for re-absorption would kill the symbiotic microbes[7]. As such, a site for absorption exists sequential to the rumen[7]. The microbes also take up and store away nutrients for respiration and metabolism. Herbivores tap this source of nutrient (mostly protein/ amino acids) by flushing the microbes, down past the omasum, into the abomasum[10] where the microbes break down into amino acids and are absorbed by the host.- lower quantity bc (to be continued).[15]

Ruminants are arguably better at digesting plant matter than other hindgut fermenters[7].

(to be continued)


Top image (A): stomach of a sheep. Bottom image (B): stomach of a musk-deer | Both herbivores with presence of multiple chambered stomachs, implying the presence of a rumen. (image sourced from wikicommons)
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The Jarman-Bell Principle implies that the food quality a herbivore consumes is inversely proportional to the size of the herbivore, but the quantity of such food is proportional[3]. Although larger animals require more overall energy than smaller animals to function, smaller animals require more energy per unit of body mass than larger animals[3][7].

The length of the digestive tract scales proportionally to the size of the animal[7]. A longer digestive tract allows for more retention time and hence increases the efficiency of digestion and absorption[7].

Poorer quality food selects animals to grow larger in size, and hence develop an increased digestive efficiency compared to smaller animals[3]. Larger sized animals have a larger/longer digestive tract, allowing for more quantities of low quality food to be processed (retention time)[4]. Although herbivores can consume high quality food, the relative abundance of low quality food and other factors such as resource competition and predator presence influence foraging behavior of the animal[16][17] to primarily consume low quality food.

Smaller animals have a limited digestive tract relative to larger animals. As such, they have a shorter retention time of food and cannot digest and absorb food to the same degree as larger animals[4]. To counteract this disadvantage, high quality food is selected, with quantity being limited by the animals gut size. Another method to counteract this is to practice coprophagy, where re-ingestion of fecal matter recycles untapped/ undigested nutrients[7].

However, reports of larger animals, including primates and horses (under controlled, dietary restrictions) have also been observed practicing coprophagy[7].

Through the extra flexibility of subsisting on low quality food, the Jarman-Bell Principle suggests an evolutionary advantage of larger animals and hence provides evidence for Cope's Rule[5].

(may add more things)

Schematic diagram of a monogastric animal (Homo sapiens). The cecum is highlighted in orange. In herbivores, the cecum is enlarged.


Exceptions
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The Jarman-Bell Principle has some notable exceptions[5]. Small herbivorous members of mammals, birds and reptiles are observed to be inconsistent with the trend of small body mass being linked with high quality food[8]. This discrepancy could be due to ecological factors which apply pressure to adapt to the given environment rather than taking on an optimal form of digestive physiology[8]. Small rodents subjected to low quality diet were observed to increase food intake and increase the size of their cecum and intestine, counteracting their low quality diet[18][15] and refuting the link between diet quality and body size.

Applications and examples

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In addition to providing evidence for ecological frameworks such as "resource partitioning, competition, habitat use and species packing in environment" and Cope's rule, the Jarman-Bell Principle has been applied to model primate behaviours and explain sexual segregation in ungulates.

Sexual segregation in ovis whatever has been observed. Male goats are larger than the females. Larger overall size implies larger gut size, and hence digestive efficiency. Males can subsist on lower quality food, leading to resource partitioning of males and females and thus sexual segregation on an intraspecies level[19].

Modelling primate behavior (to be continued)

dinosaurs (to be continued)


  1. ^ Jarman, P. J. (1968). The effect of the creation of Lake Kariba upon the terrestrial ecology of the middle Zambezi valley(Doctoral dissertation, PhD thesis, University of Manchester).
  2. ^ Bell, R.H.V (1971). "A grazing ecosystem in the Serengeti". Scientific American. 225: 86–93. doi:10.1038/scientificamerican0771-86. Retrieved 2019-05-13.
  3. ^ a b c d e f Geist, Valerius (1974-02-01). "On the Relationship of Social Evolution and Ecology in Ungulates". Integrative and Comparative Biology. 14 (1): 205–220. doi:10.1093/icb/14.1.205. ISSN 1540-7063.
  4. ^ a b c d e Hummel, Jürgen; Fritz, Julia; Nunn, Charles Lindsay; Clauss, Marcus (2009). "Evidence for a Tradeoff Between Retention Time and Chewing Efficiency in Large Mammalian Herbivores". Comparative Biochemistry and Physiology - Part A: Molecular & Integrative Physiology. 154 (3): 376–382. doi:10.1016/j.cbpa.2009.07.016. ISSN 1095-6433. PMID 19651229.
  5. ^ a b c d e f g h i McArthur, Clare (2014). "Do we ditch digestive physiology in explaining the classic relationship between herbivore body size diet and diet quality?". Functional Ecology. 28 (5): 1059–1060. doi:10.1111/1365-2435.12301. ISSN 1365-2435.
  6. ^ a b Gaulin, Steven J. C. (1979-03-01). "A Jarman/Bell model of primate feeding niches". Human Ecology. 7 (1): 1–20. doi:10.1007/BF00889349. ISSN 0300-7839. S2CID 85151029.
  7. ^ a b c d e f g h i j k Demment, Montague W.; Van Soest, Peter J. (1985-05-01). "A Nutritional Explanation for Body-Size Patterns of Ruminant and Nonruminant Herbivores" (PDF). The American Naturalist. 125 (5): 641–672. doi:10.1086/284369. ISSN 0003-0147. S2CID 53137013.
  8. ^ a b c d e f Hummel, Jürgen; Codron, Daryl; Müller, Dennis W. H.; Steuer, Patrick; Clauss, Marcus (2013-10-30). "Herbivory and Body Size: Allometries of Diet Quality and Gastrointestinal Physiology, and Implications for Herbivore Ecology and Dinosaur Gigantism". PLOS ONE. 8 (10): e68714. doi:10.1371/journal.pone.0068714. ISSN 1932-6203. PMC 3812987. PMID 24204552.
  9. ^ Hone, David W. E.; Benton, Michael J. (Jan 2005). "The evolution of large size: how does Cope's Rule work?". Trends in Ecology & Evolution. 20 (1): 4–6. doi:10.1016/j.tree.2004.10.012. ISSN 0169-5347. PMID 16701331.
  10. ^ a b c d e f g Moran, John (2005). Tropical Dairy Farming: Feeding Management for Small Holder Dairy Farmers in the Humid Tropics. Csiro Publishing. ISBN 9780643091238.
  11. ^ a b Cabana, Francis; Dierenfeld, Ellen S.; Wirdateti; Donati, Giuseppe; Nekaris, K. a. I. (2018). "Exploiting a readily available but hard to digest resource: A review of exudativorous mammals identified thus far and how they cope in captivity". Integrative Zoology. 13 (1): 94–111. doi:10.1111/1749-4877.12264. ISSN 1749-4877. PMID 29437293.
  12. ^ Dehority, Burk A. (2002-06-01). "Gastrointestinal Tracts of Herbivores, Particularly the Ruminant: Anatomy, Physiology and Microbial Digestion of Plants". Journal of Applied Animal Research. 21 (2): 145–160. doi:10.1080/09712119.2002.9706367. ISSN 0971-2119. S2CID 84022210.
  13. ^ "Pseudoruminant", Wikipedia, 2019-02-19, retrieved 2019-05-17
  14. ^ Navarre, Christine B.; Baird, A.N.; Pugh, D.G (2012). Chapter 5 - Diseases of the Gastrointestinal System. pp. 71–105. doi:10.1016/C2009-0-60474-8. ISBN 978-1-4377-2353-3.
  15. ^ a b Loeb, S. C.; Schwab, R. G.; Demment, M. W. (1991-05-01). "Responses of pocket gophers (Thomomys bottae) to changes in diet quality". Oecologia. 86 (4): 542–551. doi:10.1007/BF00318321. ISSN 1432-1939. PMID 28313336. S2CID 21535229.
  16. ^ Hanley, Thomas.A (March 1982). "The Nutritional Basis for Food Selection by Ungulates". Journal of Range Management. 35 (NO.2): 146–151. doi:10.2307/3898379. hdl:10150/646267. JSTOR 3898379.
  17. ^ McArthur, Clare; Banks, Peter B.; Boonstra, Rudy; Forbey, Jennifer Sorensen (2014-11-01). "The dilemma of foraging herbivores: dealing with food and fear". Oecologia. 176 (3): 677–689. doi:10.1007/s00442-014-3076-6. ISSN 1432-1939. PMID 25270335. S2CID 5678964.
  18. ^ Hammond, Kimberly A.; Wunder, Bruce A. (1991-03-01). "The Role of Diet Quality and Energy Need in the Nutritional Ecology of a Small Herbivore, Microtus ochrogaster". Physiological Zoology. 64 (2): 541–567. doi:10.1086/physzool.64.2.30158190. ISSN 0031-935X. S2CID 86028872.
  19. ^ Pérez-Barbería, F. J.; Pérez-Fernández, E.; Robertson, E.; Alvarez-Enríquez, B. (Aug 2008). "Does the Jarman-Bell principle at intra-specific level explain sexual segregation in polygynous ungulates? Sex differences in forage digestibility in Soay sheep". Oecologia. 157 (1): 21–30. doi:10.1007/s00442-008-1056-4. ISSN 0029-8549. PMID 18481093. S2CID 21262523 – via ResearchGate.
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