Digestive physiology and feeding behaviour of equids – a comparative approach

Take home messages

• Like other members of their order – the perissodactyla or odd-toed ungulates – equids had a much richer species diversity in fossil times than they have today.
• The main reason for their decline is seen in the comparative success of artiodactyls – even-toed ungulates – and amongst these, especially of ruminants.
• Possible reasons for the comparative success of ruminants in evolutionary time include locomotion (where a single hoof precludes use of swampy or rocky habitats), higher water dependency due to lack of a ‘selective brain cooling mechanism’, a slower overall reproductive rate due to slower in utero maturation, and digestive physiology.
• The digestive physiology of equids, compared to ruminants, is summarized as lower digestive efficiency compensated by higher intake levels (which also necessitates more time spent foraging).
• Historically, this has been linked to differences in fermentation site (foregut vs hindgut fermentation) and digesta retention times.
• Historically, it has been postulated that equids are at an advantage during times of low forage quality, because they are supposed to be able to increase intake in such a situation. The graphical depiction of this theory has been widely reproduced, in spite of a lack of support for it by empirical data.
• Empirical data so far suggests no fundamental difference in the reaction of equids and ruminants to decreasing forage quality (increasing fibre levels): a reduction of ad libitum intake. • Preliminary data suggests that horses may be more susceptible to food scarcity (constrained food intake) than ruminants.
• In contrast to ruminants, the digestive tract of horses does not selectively discriminate by particle size.
• A major difference between equids (and other nonruminants) and ruminants is chewing efficiency.
• Among nonruminants, equids achieve extremely high chewing efficiency, most likely due to their cheek tooth surface anatomy (elaborate enamel folds).
• Ruminants achieve an even higher chewing efficiency, not because of dental anatomy, but due to the sorting mechanism in their forestomach.
• Because particle size-reducing chewing is postponed to rumination in ruminants, their ingestive behaviour and ingestive chewing pattern differs from that of equids. Equids chew more thoroughly during ingestion, with a chewing pattern similar to that used by ruminants during rumination.
• It is an interesting question how extant equid species survived in their respective habitats together with ruminant presence.

References

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