Investigating the effects of incremental conditioning and supplemental dietary tryptophan on the voluntary activity and behaviour of mid-distance training sled dogs

PLoS One. 2020 Aug 13;15(8):e0232643. doi: 10.1371/journal.pone.0232643. eCollection 2020.


Serotonin is a neurotransmitter synthesized by the amino acid tryptophan, that has the potential to impact the behaviour and activity of dogs. The objective of this study was to assess the effects of supplemental tryptophan and a 12-week incremental training regimen on the voluntary activity and behaviour of client-owned Siberian Huskies. Sixteen dogs were blocked for age, BW and sex and then randomly allocated to either the control or treatment group. Both groups were fed the same dry extruded diet; however, the treatment group were supplemented with tryptophan to achieve a tryptophan: large neutral amino acid ratio of 0.075:1. Once a week, a 5-minute video recording was taken immediately pre- and post- exercise to evaluate dogs' behaviours. Activity monitors were used to record voluntary activity on both training and rest days. Linear regression analysis was used to assess the relationship between training week and time spent performing each behaviour. Additionally, a repeated measure mixed model was used to test differences between diet groups and training week for both behavioural and activity count data. The time spent performing agonistic behaviours prior to exercise was negatively associated with week for treatment dogs (β = -0.32, 95% CI [-0.55, -0.10], P < 0.05) and no change was observed for control dogs (β = -0.13, 95% CI [-0.41, 0.15], P > 0.10). Treatment did not have any effect on activity levels (P > 0.10). For all dogs, locomotive behaviours decreased prior to exercise as weeks progressed (P < 0.05), while run day voluntary activity depended on the distance run that day (P < 0.05). These data suggest that sled dogs experience an exercise-induced reduction in voluntary locomotion in response to both single bouts and repetitive bouts of exercise. Additionally, tryptophan supplementation may decrease agonistic behaviours, without having any effect on voluntary activity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Behavior, Animal / physiology
  • Dietary Supplements*
  • Dogs / physiology*
  • Dogs / psychology
  • Endurance Training / methods
  • Endurance Training / veterinary
  • Female
  • Humans
  • Linear Models
  • Male
  • Motor Activity / physiology
  • Physical Conditioning, Animal / methods*
  • Physical Conditioning, Animal / physiology
  • Running / physiology
  • Serotonin / biosynthesis
  • Serotonin / physiology
  • Snow Sports
  • Time Factors
  • Tryptophan / administration & dosage*
  • Tryptophan / metabolism


  • Serotonin
  • Tryptophan

Grants and funding

This study was funded by the Mitacs Accelerate Program ( (Grant 460735) with matching funds from Champion Pet Foods ( The funders provided support in the form of salaries for authors (ER and JRT). The funders did not have any additional role in the study design, data collection and analyses, decision to publish, or preparation of the manuscript.