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PeerJ2019; 7; e7678; doi: 10.7717/peerj.7678

Why the long face? Comparative shape analysis of miniature, pony, and other horse skulls reveals changes in ontogenetic growth.

Abstract: Much of the shape variation found in animals is based on allometry and heterochrony. Horses represent an excellent model to investigate patterns of size-shape variation among breeds that were intentionally bred for extreme small and large sizes. Methods: We tested whether ponies (wither height < 148 cm) have a diverging size-shape relationship in skull shape as compared to regular-sized horse breeds (wither height > 148 cm, here-after called horses) during ontogenetic growth. We used a dataset of 194 specimens from 25 horse and 13 pony breeds, two of which are miniature breeds (wither height < 96.5 cm)-Falabella, Shetland. We applied three-dimensional geometric morphometrics, linear measurements, and multivariate analyses (Procrustes ANOVAs) to quantitatively examine and compare the ontogenetic trajectories between pony and horse breeds with an emphasis on the miniature breeds as an extreme case of artificial selection on size. Additionally, we tested for juvenile characteristics in adult horse and miniature breeds that could resemble "paedomorphosis"-retention of juvenile characteristics in adult stage; e.g. large eyes, large braincase-to-face-relationship, and large head-to-body relationship. Results: Allometric regression of size on shape revealed that 42% of shape variation could be explained by variation in size in all breeds. The ontogenetic trajectories of ponies and horses vary in slope and therefore in rate of change per unit size, and length. The differences in trajectory lengths and slopes result in ponies having a similar skull shape in an older age stage than horses of the same size in a younger age stage. This pattern could cause the generally perceived "paedomorphic" appearance of ponies. Miniature breeds have larger heads in relation to wither height compared to horses, a non-paedomorphic feature in horses specifically. Also, rostra (faces) are longer in adult individuals than in juveniles across all kinds of breeds. This pattern can be explained by the long-face hypothesis for grazing ungulates and could possibly be caused by the mismatch of selection by humans for shorter rostra and the dentition of ruminants. Conclusions: Miniature breed specimens do not exhibit any of the classical mammalian "paedomorphic" features (large orbits, large heads), except for the adult Falabella that has enlarged orbits, possibly because they are herbivorous ungulates that are affected by functional and metabolic constraints related to low nutrient-food consumption. Instead ponies, including miniature breeds, have faster and shorter ontogenetic growth compared to horses, resulting in adult pony skulls looking in part like juvenile horse skulls.
©2019 Heck et al.
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Publication Date: 2019-09-16 PubMed ID: 31576240PubMed Central: PMC6752190DOI: 10.7717/peerj.7678Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research investigates the variations in skull shape among different breeds of horses- particularly regular-sized, pony, and miniature breeds- and the effects of size and shape on their growth process. The study sheds light on some of the characteristics found in these horse breeds that could be attributed to “paedomorphosis” – the retention of juvenile traits in the adult stage.

Methods Used in the Research

  • The researchers used a dataset of 194 specimens which included 25 horse breeds and 13 pony breeds, including two miniature breeds – Falabella and Shetland.
  • They employed three-dimensional geometric morphometrics, linear measurements, and multivariate analyses (Procrustes ANOVAs) to quantitatively assess and compare the growth trajectories of pony and horse breeds, with a special emphasis on miniature breeds.
  • The team also sought to identify any juvenile features present in adult horses and miniature breeds that could be an indication of “paedomorphosis”.

Key Findings of the Research

  • It was discovered that 42% of shape variation within all breeds could be accounted for by variations in size.
  • The pace of change per unit size, or rate, and the ontogenetic trajectories’ lengths varied between horses and ponies. As a result of these variations, older ponies had similar skull shapes to younger horses of the same size.
  • The pattern mentioned above could be a potential explanation for the paedomorphic appearances often seen in ponies.
  • Miniature breeds were found to possess larger heads in relation to wither height, a trait that is absent in regular-sized horses. Additionally, across all breeds, adults had longer faces than juveniles – a pattern possibly explained by the long-face hypothesis for grazing ungulates.

Conclusions of the Research

  • The study concluded that miniature breeds do not display most traditional mammalian “paedomorphic” traits (apart from adult Falabellas having larger eye sockets).
  • It was proposed that this lack of paedomorphic traits could be due to the metabolic and functional constraints faced by herbivorous ungulates who consume low nutrient-foods.
  • The researchers noted that ponies, including miniature breeds, experience faster and shorter growth compared to regular horses, making adult pony skulls appear similar to juvenile horse skulls in some aspects.

Cite This Article

APA
Heck L, Sanchez-Villagra MR, Stange M. (2019). Why the long face? Comparative shape analysis of miniature, pony, and other horse skulls reveals changes in ontogenetic growth. PeerJ, 7, e7678. https://doi.org/10.7717/peerj.7678

Publication

PeerJ
ISSN: 2167-8359
NlmUniqueID: 101603425
Country: United States
Language: English
Volume: 7
Pages: e7678

Researcher Affiliations

Heck, Laura
  • Palaeontologisches Institut und Museum, University of Zürich, Zürich, Switzerland.
Sanchez-Villagra, Marcelo R
  • Palaeontologisches Institut und Museum, University of Zürich, Zürich, Switzerland.
Stange, Madlen
  • Department of Biology & Redpath Museum, McGill University, Montréal, Q, Canada.

Conflict of Interest Statement

The authors declare there are no competing interests.

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