Carrots and Sticks
Last revised April 20, 2017
Carrot on a stick.1
Horsemanship — both natural and traditional — emphasizes training and motivation. If you can get your horse to do what you want, then you have succeeded.
Not surprisingly, much has been learned about learning since we were in school. I’ll share some of this here.
In horse psychology, a reinforcer is anything that occurs after a behavior which makes that behavior more likely. A punishment is anything that occurs after a behavior which makes that behavior less likely.
The formal language of the psychology of learning uses these terms:
- Positive reinforcement (PR): receiving a pleasant stimulus. Your horse gets a treat or praise. In everyday language, this is usually what we call “reward”.
- Positive punishment: receiving an unpleasant or aversive stimulus. Your horse receives a yank on the bit. In everyday language, this is usually what we call “punishment”.
- Negative reinforcement (NR): taking away an unpleasant stimulus. You relax the reins, releasing pressure on the bit.
- Negative punishment: taking away a pleasant stimulus. Your cell phone rings, and you stop grooming to answer the call.
In these terms, positive means that you add the consequence to his life, and negative means you remove the consequence. Reinforcement is something that increases the probability of a behavior that it follows; punishment is something that decreases the probability of a behavior that it follows.
A positive reinforcer is rewarding. It is something we want. Negative reinforcement (NR), as defined above, is OK, but we don’t feel any improvement in our well-being if we sit on a tack then jump up. I like positive reinforcement (PR). So does your horse. And nobody likes punishment, whether positive or negative.
|Pleasant Stimulus||Aversive (Unpleasant) Stimulus|
|Adding/Presenting||Positive Reinforcement (PR)||Positive Punishment|
|Removing/Taking Away||Negative Punishment||Negative Reinforcement (NR)|
|When a traditional cowboy sets out to break a horse, he makes things unpleasant for the horse — positive punishment — such as tying its feet and pinning it to the ground and whipping it. When the horse’s spirit is broken, and it seems willing to yield, the cowboy may stop whipping and allow the horse to stand — NR. In the cowboy movies, he may simply get on the horse and ride it until it gives up, demonstrating his heroism and skill, and convincing the horse that efforts to escape are pointless. In real-life, once a horse is broken, a real cowboy is likely to make an effort to be kind and understanding, and try to see the world through the horse’s eyes.2
When a modern horse trainer sets out to teach a trick, she often uses a clicker, praise and treats. The horse that gives the correct response to some cue gets a click, a treat, praise, and other reward — PR.
When a rider uses spurs to make their horse move faster, tapping until the desired speed is reached, they are using positive punishment — adding an unpleasant stimulus. When the rider stops spurring, they are using NR — removing an unpleasant stimulus.
When a trainer snaps a lunge whip in a round pen to move a horse around it, they are using positive punishment. Once the horse begins moving satisfactorily, the trainer may lower the lunge whip, offering NR.
Punishment and NR are entirely different.
Natural horsemanship favors the use of PR where possible, but regularly strays. Shaking a horse’s lead rope to make it back up is positive punishment at the horse’s mouth. Raising a lunge whip or using spurs is positive punishment. A bit is positive punishment until it is removed from his mouth. The issue is not about the degree of unpleasantness of our stimulus. It is whether our stimulus is pleasant or unpleasant.
Comparing Reward and Punishment
|Natural horsemanship favors the use of PR — reward — in daily interactions with your horse. Traditional horsemanship is more prone to using aversive influences, such as bits, spurs, and whips, but tries to disguise them as “aids”, “moving away from pressure”, etc. Is one approach better than the other?|
The answer to this question might hinge on the training techniques used, not the language we use. Your horse would choose to live in a world in which PR was the norm. So would you. But would he learn better with PR than with the other options? Positive and negative feedback have somewhat different roles. Both improve performance, but positive feedback is important to maintain a desired behavior, while negative feedback is most useful in adjusting current behavior.
Psychologists at Washington University used a simple test that either rewarded or punished students for giving correct or incorrect answers. Punishment caused students to change their behavior at a rate that was up to three times greater than if they had been offered a reward. With reward, behavior change depended on the size of the reward, whereas the size of the punishment did not seem to matter. The authors conclude that “the asymmetry in the effects of the reward magnitude and the punishment magnitude was so striking that it is difficult to conceive that one factor is just a weighted or transformed form of the other factor. Instead, the data suggest that rewards and penalties are fundamentally distinct factors in governing behavior.3” This makes the case for very mild punishment in shaping behavior, and for using “jackpots” when rewarding a desired behavior. A mild stick might work better than the carrot when shaping behavior, and two carrots work better than one for maintaining it once learned.
A similar result was found by British researchers: negative feedback, whether graded or binary, accelerated learning. Positive feedback did not speed up learning, but it increased retention of the motor memory when performance feedback was withdrawn.4 Here the researchers clearly distinguished between learning and retention effects. Negative feedback shaped learning best, while positive feedback best helped with retention.
Of course, there are many studies that help us realize that the best answer is “it depends”. In one interesting study, compared with adults, 8- to 9-year-old children performed disproportionately more inaccurately after receiving negative feedback relative to positive feedback. The dorsolateral prefrontal cortex and superior parietal cortex were more active after negative feedback for adults, but after positive feedback for children. It appears that when we are young, we do better with positive feedback, but as we grow, our brains change and we learn faster from negative feedback than we did as children.5 “Eight-year-olds learn primarily from positive feedback (‘Well done!’), whereas negative feedback (‘Got it wrong this time’) scarcely causes any alarm bells to ring. Twelve-year-olds are better able to process negative feedback, and use it to learn from their mistakes. Adults do the same, but more efficiently.6” If we were to extrapolate to horses, we might conclude that because horses have mental ages that are in some ways comparable to those of young children, they might do better with positive feedback. And maybe little soccer players should all get a trophy.
Advocates of PR are stuck with the problem of adjusting the horse’s behavior when they are in the saddle and in an awkward position to use a clicker or hand out carrots. Some of this problem is addressed by our use of language. In the round pen, we pretend we are speaking the horse’s language when we extend an arm with open fingers, and drive it forward. Under saddle, we claim that horses like to “move away from pressure”.
Motivating a horse to learn using PR has better outcomes beyond training. One study found that “the association of a reward with a learning task induced positive reactions towards humans during training. It also increased contact and interest, not only just after training, but also several months later, despite no further interaction with humans. In addition, this ‘positive memory’ of humans extended to novel persons. Overall, PR enhanced learning and memorization of the task itself.7”
Trick Training: A Swiss study
A Swiss study compares the effects of clicker training on mood and learning with the effects of NR. Horses in the study needed to learn to perform 10 tasks:
- Stand still next to the trainer, with the head forward.
- Walk next to the trainer.
- Stop next to the trainer.
- Move backward.
- Flex the neck alternatively on the right and on the left.
- Lower the head down towards the ground.
- Walk on a tarp (1.5 m × 2.5 m) that was covered by plastic bottles.
- Climb on a wooden podium (70 cm large × 70 cm long × 50 cm high).
- Push a blue balloon (diameter 1 m) with the nose or leg.
- Lift one of the front legs upward.
The PR condition used a clicker and a food pellet to reward behavior, and used a target (stick with tennis ball) to guide them. Horses in the NR condition received a variety of unpleasant stimuli: gentle pressure on the halter, gently shaking or pulling the lead or/and a gentle pressure on the horse’s flank using a stick, depending on the exercise. The intensity of the stimulus was gradually increased if the horse was not responding to it or not responding correctly. As soon as the expected response was shown, the experimenter stopped the stimulus immediately and the horse could rest for a short break (5 s) before continuing the training. Horses in each condition were given the same amount of training time per exercise.
A number of behavioral measures were used: body tension, amount of contact with trainer, ear position, and head level.
As suggested earlier, mares in the NR condition progressed slightly faster than those with PR. Three PR horses reached exercise 8, and five NR horses reached it; two NR horses actually reached exercise 9. So we might conclude that NR is slightly more efficient at teaching new behaviors than is PR.
The researchers also found that PR mares showed “a more motivated body tension, a more positive attitude towards the trainer, a lower head position and more forward ears than NR mares.”
This study was apparently the first to study judgment bias in horses, and some of the findings demand further study. But the general conclusion should be the obvious one: horses given rewards during training will be in better moods; horses trained with mild NR might learn a bit faster.
A Danish Study of Trailer Loading
But not all studies reach this conclusion. Consider the issue of trailer loading, a very stressful job for some horses (and owners)! Most trainers use NR, tapping the horse on the butt with a whip, tugging on a lead rope. Heart rates rise, and this method regularly fails for any horse initially reluctant to enter the trailer. After all, this task is not about training at all. A horse knows how to board a trailer. It simply does not want to. The problem is motivation.
A study in Denmark8 compared traditional NR (pulling on a lead rope, tapping with a dressage whip) with PR (using clicker training and target training first, then using the target to lead the horse on, clicking and giving a food reward for good performance.) None of the horses in the study could be loaded into trailers by their owner at the start of the study. Horses were assigned to one of two groups, and both groups were first trained in one of these methods, and tested by using the methods to get them to walk across a tarp. Horse heart rates and behavior were carefully monitored and recorded throughout the trials.
Training was completed when the horse could enter the trailer upon a signal, or was terminated after a maximum of 15 sessions. Of the 12 horses, 10 reached the criterion within the 15 sessions. The findings certainly make the case for the value of PR, clicker training, and target training:
- Horses trained with PR spent less time per session to complete the training criterion. PR got the job done in 80% of the time that it took for a horse to load with NR.
- Horses trained with PR displayed significantly less discomfort behavior (widening of the eyes, widening of the nostrils, tail whipping and avoidance) per training session than horses trained with NR. PR produced only 25% of the discomfort and 30% of the avoidance measured in the NR condition.
This Danish study shows that for getting a horse to load, PR gets a horse to load faster, with less discomfort, than NR.
It might be that stress is a factor in learning: NR adds stress, and when the situation is already stressful, it may be less effective than PR; when we are just learning tricks, it may be more effective.
Training efficiency must always be a consideration when choosing a training technique with your horse. Most of us would always choose the most efficient technique, to get to our goal fastest. But there is more to life with your horse than efficiency. What effect does your chosen technique have on your relationship with your horse? If we are going for fear, your NR technique can certainly be chosen for this goal. If we are going for affection, you may need a different technique.
Researchers in France9 were interested in learning how NR and PR affected a pony’s attitude toward the trainer. Their training goal was to teach a pony to back up on a verbal signal: ‘‘reculé!’’ (French for ‘‘go back!’’).
When training with NR, the trainer stood in front of the pony, gave the verbal signal, then shook a riding stick in front of its head. If the pony took a step back, the agitation of the stick immediately ceased and he was led back to his home stall or pasture.
When training other ponies with PR, the trainer took a step forward towards the pony after giving the vocal order to go back. If the pony took a step back in the allotted time (just 10 seconds), the trainer gave the pony a commercial carrot pellet, and he was led back to his home stall or pasture.
The five training sessions lasted 1-3 minutes.
At the end of these training sessions, the differences in training methods were clear:
- Ponies with PR responded more quickly to the voice command for “go back” than those given NR. All of these PR ponies responded correctly on the second trial, while only half of the NR ponies did so. On the third trail, all the PR ponies again backed up as requested, and one fifth of the NR ponies still did not understand what they were to do.
- Pony mood during training also differed significantly as revealed by ear position (ears forward vs back), neck position (round vs flat or hollow), number of side steps, head lifting, and horizontal head movements.
- Consistent with what we will learn in the section on Vision, the NR ponies more often turned their heads to the right, so that they could view the stick with their left eye, than vice versa. PR ponies rarely turned their heads.
- Heart rate, a good indicator of stress, also varied between the two groups. Those receiving PR maintained lower heart rates. Heart rates did not jump when the trainer stepped forward or when she gave pellets for reward. But heart rates did jump with the stick was agitated in the pony’s face in the NR condition.
- Before training, ponies from both groups behaved similarly toward the trainer. But after training, the PR ponies were much faster to initiate contact with the trainer, and spent more time standing near her. After training, only 2 of the 11 NR ponies approached the trainer at all, whereas all of the PR ponies did.
- The differing attitude towards trainer generalized to an unknown motionless human. All PR ponies approached an unknown person after the training, but only 2 of the NR ponies did so. These attitudes lasted: five months later, 10 of the 11 PR ponies approached an unknown person, while only 2 of the 11 NR ponies did. The authors conclude “the positive reinforcement training experience created a deep, long-lasting trace in the ponies’ memories despite many other interactions with diverse humans in the mean time.10”
Other studies cited by Freymond et al (2014)11 support such findings. One study12 observed more forward ears, less head lifting, lower neck position and faster contact initiation with trainers in PR compared to NR horses trained to walk backward. Another13 found less discomfort behavior (eye and nostril widening, tail whipping) and less avoidance towards the training in PR than in NR horses trained to enter a trailer. Another study14 showed that PR horses made more active contact with the trainer than NR horses. And another15 found that horses trained with both PR and NR to halt, shook their heads vertically less than horses trained only with NR. All these results strongly suggest that PR could be associated with more positive emotions, more positive contact with the trainer, and thus better horse welfare than NR.
Aversive stimuli regularly have undesirable, unwanted, unintended effects. For example, they can result in aggressive responses16 and decreased learning.17
Along the same lines, research has shown that fear can result18 if NR is not perfectly delivered to the horse. Compare this with the dangers of giving your horse a carrot at any time he has not asked for one: you make him happy.
What Positive Reinforcement?
You have decided that today (Tuesday on the east coast) you will teach your horse a trick. You pack your clicker and head to the barn. Today Mr. Horse will learn to play your prized guitar. The two of you will improve your relationship during your time together. What should you use as his reward as he heads to his first successful strum?
I know riders who think that all their horse needs is a pat. I find this odd, because while I’ve seen plenty of horses playing the guitar, I’ve never seen a horse pat another horse. I suspect patting doesn’t do anything for Mr. Horse.
So how about a loving rub? Would that work during the guitar lessons? There is plenty of evidence that grooming reduces the groomee’s heart rate.19 You have personal evidence that your horse likes grooming and scratching in some places — withers, neck, chest, and chin perhaps. Or would a chunk of carrot be a better reward? There is plenty of evidence that horses love carrots and other food treats.20
To answer the question, five researchers in France and Poland set out to ask horses this question. Young horses were trained to stand still for a minute, using six sessions of 5 minutes each. As the horses stood as instructed, one group was rewarded with a small piece of carrot hand delivered by the instructor. Individuals in the other training group were given three vigorous scratches on his withers under the same performance condition.
Maximum duration of immobility when trained with food reward (FR) and grooming reward (GR). Slices of carrot resulted in quicker learning than did scratches on the withers.21
Performance — as measured in seconds the horse held still — was measured for each horse on each training day.
Turns out carrots work better than scratches as a reward in training. On the last day of training, almost all horses trained with the food reward had successfully reached the last step and managed to maintain immobility for 1 min, only 4 of the 10 in the grooming reward group did. The researchers also used a “motionless human test”, in which the experimenter remained motionless and the time elapsed before the horse approached and the time spent near the experimenter was recorded.
Latency to approach the experimenter and time spent near her (distance <0.5 m) in the “motionless person test” by food-rewarded and grooming-rewarded horses, before and after training.22 Horses approached the experimenter more quickly and lingered longer when trained with carrots.
Horses seem to learn faster when given rewards they crave, and they seem to crave carrots over scratches. It also seems that the best way to your horse’s heart is through his stomach, rather than his withers. So use carrots in your guitar lessons. Even if he doesn’t learn to play, he might learn to love.
Treats are handy when training tricks. You drop your hat. Your horse reaches down, and picks it up, and hands it to you. You hand him a slice of carrot. He has acted on his world to get this reward he wanted. Trick training relies heavily on positive reinforcement. It is reinforcement because it makes the desired action more likely. It is positive because your horse loves carrots.
Treats like carrots can be good for a horse, but most succeed because they contain more sugar than his hay. In some horses, a steady supply of carrots can raise blood sugar, and make your horse temporarily hyperactive. The same can be said for peppermints or anything mixed with molasses. If you find that your horse is more rambunctious at the end of a training session in which he scored lots of sugar, consider some sugar-free candy has his treats. Once you have found some that he loves, you can then buy it in bulk.23
Horses are hedonists, and under the right circumstances love to be stroked. When I first met my horse, my first touch was to scratch his chest. He stretched his neck and head out, closed his eyes a bit, his lower lip went loose and trembled, and he groaned. I then scratched his neck, and he did the same. I knew he was the right horse for me, because he was able to tell me that my efforts felt good, and when I saw that, I felt good. Horses are very articulate about what feels good. If you rub your horse just right, you’ll know.
How do horses groom each other? Allogrooming is most commonly done by rubbing or scratching the withers, and this has been shown to lower the heart rate24 — and presumably leads to relaxation. My horse also likes me to rub his face with a towel, particularly when it is wet with sweat or a bath. He taught me this by putting his head into my armpit, and rubbing up and down. Now, when I hold both ends of a towel in front of him, he puts his head into it. He bobs his head up and down, and I rub side to side. Your horse will have specific areas that he likes you to scratch or rub. He’ll tell you when you’ve found the spot, and are doing it right.
Try this experiment. Stand next to your horse, and slowly reach for his throat. As he accepts your touch, begin to scratch it gently. Watch his lower lip, his eye, his ears, his neck position. If he appreciates your effort, you’ll see his neck lower, his eye partly close, his lower lip tremble and his chin sag. He may yawn once or twice. And he may turn his head so you can get some additional spots that need love. His front end sends many different signals that he is enjoying what you are doing. Use any of them as clue to his mood when you are massaging him, grooming him, or rubbing him. Practice reading your horse’s mood by monitoring these markers.
Humans love to pat their horses on the neck, but this has no comparable effect within the herd — horses don’t pat each other. So if you ask me, a pat either has no meaning, or a meaning that must be learned. It is unlikely to serve as a primary reinforcer, unlikely to “feel good” to Mr. Horse. Grooming, done right, could work like a treat as a positive reinforcer. Rubbing — not patting — his neck on the trail will make you both feel better.
I started my grooming career with the conventional brushes, but I’ve moved to more intimate contact: I rub him with a small towel. Once I’ve gotten most of the dust off, I alternately spray a bit of the towel with Show Sheen, then rub. Toweling seems to get him cleaner than brushing, even if the towel has been used many times. And he seems to enjoy it more.
Horses love to play. While horse play can be rougher than playing with Bowser, horses enjoy it just as much as dogs do. Horsing around just comes naturally. Humans typically are offended by horse play when the horse tries playing with them, especially when it leaves tooth marks. But there is no reason you should not play carefully with your horse. Play is a handy reward because it is cheaper than treats and can last longer. It is less work on your part than grooming. It absolutely works with dogs as a reward — consider the story of Chaser, the dog who learned over a thousand words, using only play as a reward (in the introduction to the Tricks section).
Some of the games my horse enjoys include mumbly peg (I fiddle with his lips, and he tries half-heartedly to bite a finger. He never gets one), soak ‘em (in the wash stall on a hot day, I spray a fine mist above his head, letting it rain down on his face. He closes his eyes a little and tries to catch the falling water); drink from the hose (also done in the wash stall, on a hot day, with low water pressure); roll the ball (a giant ball, that he gets to push around with his nose in the round pen); and more.
Play takes too much time to serve as a positive reinforcement for trick training, and doesn’t work when you are mounted. But it is a good way to end a session with your horse, in part because it will leave him with pleasant memories of your time together — memories that will be stronger than if they were embedded within the session, thanks to the power of a recency effect. (A serial position effect is the tendency of a person or horse to recall the first and last items in a series best, the middle items worst. How you start your session with your horse matters just as much as how you end it because of a primacy effect. When your horse is back in the pasture, or dreaming in his stall, you want him to be thinking of the best parts of being with you.)
Other Horse Rewards
Your horse enjoys a much longer of activities than just reducing pressure, treats, grooming, and play. For instance, he loves to while away the hours grazing with his friends, or standing in the shade with them. He loves to roll in a dust bath. And so on. But if we are to amuse our horse while he interacts with us, and reward him for some behavior, we must come up with rewards that are quick and easy to deliver. We can’t offer him a roll in a dust bath whenever he lifts his foot for the farrier or walks into the trailer. Keep your eyes open for motivators that you can deliver efficiently, to use as rewards for behavior. Does he like official horse cookies more than peppermints, or peppermints more than carrot slices? If you’ve got two pockets and two hands, I bet you can find out.
In summary, reward is important in maintaining a desired behavior, while negative feedback is most useful in adjusting current behavior. Sometimes PR is more efficient in teaching something new, sometimes NR is, and this might depend on whether we are actually trying to motivate a horse to overcome a fear, or simply teach a new trick. Effective training will wisely intermix both carrots and gentle sticks.
Larger rewards are more effective than small ones, but small punishments may be as effective as big ones for changing behavior.
Both reward and punishment are used in both natural horsemanship and traditional horsemanship, regardless of the claims of natural horsemanship advocates.
Rewards are certain to leave our horses with better memories of their time with us, regardless of how much they learn.
From what we know about learning, neither natural horsemanship nor traditional horsemanship has it right. The right approach is the one that works best in learning, in retention, and in residual feeling about the learning experience and the trainer. This may involve a mix of PR and the gentle application of NR during training, and the robust application of PR when we are working to solidify retention and build lasting good feelings. And certainly, if you care more about your horse learning to like people than learning to play the xylophone, stick with PR for your early days.
1 Image source: Dye, David. “The Problem with Motivating People” May 2016 trailblazeinc.com https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSV6KTdLKK1zRUXYU18fnCYoOTJYazny7cuFlkRYzZOtyCIL_NH
2 Miller, Robert M., and Rick Lamb. The revolution in horsemanship and what it means to mankind. The Lyons Press, 2005.
3 Kubanek, Jan, Lawrence H. Snyder, and Richard A. Abrams. “Reward and punishment act as distinct factors in guiding behavior.” Cognition 139 (2015): 154-167.
4 Galea, Joseph M., Elizabeth Mallia, John Rothwell, and Jörn Diedrichsen. “The dissociable effects of punishment and reward on motor learning.” Nature neuroscience 18, no. 4 (2015): 597-602.
5 Van Duijvenvoorde, Anna CK, Kiki Zanolie, Serge ARB Rombouts, Maartje EJ Raijmakers, and Eveline A. Crone. “Evaluating the negative or valuing the positive? Neural mechanisms supporting feedback-based learning across development.” Journal of Neuroscience 28, no. 38 (2008): 9495-9503.
6 “From 12 years onward you learn differently.” (e) Science News September 25, 2008.
7 Sankey, Carol, Marie-Annick Richard-Yris, Helene Leroy, Severine Henry, and Martine Hausberger. “Positive interactions lead to lasting positive memories in horses, Equus caballus.” Animal Behaviour 79, no. 4 (2010): 869-875,
8 Hendriksen, P., Elmgreen, K., Ladewig, J., 2011. Trailer-loading of horses: is there a difference between positive and negative reinforcement concerning effectiveness and stress-related signs? J. Vet. Behav. 6, 261–266.
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11 Freymond, Sabrina Briefer, Elodie F. Briefer, Anja Zollinger, Yveline Gindrat-von Allmen, Christa Wyss, and Iris Bachmann. “Behaviour of horses in a judgment bias test associated with positive or negative reinforcement.” Applied Animal Behaviour Science 158 (2014): 34-45.
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Hausberger, M., 2010a. Reinforcement as a mediator of the perception
of humans by horses (Equus caballus). Anim. Cogn. 13,
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is there a difference between positive and negative reinforcement
concerning effectiveness and stress-related signs? J. Vet. Behav. 6,
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17 Haverbeke, A., Messaoudi, F., Depiereux, E., Stevens, M., Giffroy, J.-M., Diederich, C., 2010. Efficiency of working dogs undergoing a new Human Familiarization and Training Program. Journal of Veterinary Behavior-Clinical Applications and Research 5, 112–119.; Kratzer, D.D., Netherland, W.M., Pulse, R.E., Baker, J.P., 1977. Maze-learning in quarter horses. Journal of Animal Science 45, 896–902.
18 Waran, N., McGreevy, P.D., Casey, R.A., 2002. Training methods and horse welfare. In: The Welfare of Horses. Kluwer Academic Publishers, The Netherlands, pp. 151–180.
19 Feh C, deMazières J (1993) Grooming at a preferred site reduces heart rate in horses. Anim Behav 46: 1191–1194.;Normando S, Haverbeke A, Meers L, Ödberg F, Ibanez Talegon M, et al. (2003) Effect of manual imitation of grooming on riding horses’ heart rate in different environmental situations. Vet Res Com 27: suppl615–617.
20 Hausberger M, Henry S, Larose C, Richard-Yris M-A (2007) First Suckling: A Crucial Event for Mother–Young Attachment? An Experimental Study in Horses (Equus caballus). J Comp Psy 121(1): 109–112.; Jansen J, deWeerth C, Riksen-Walraven M (2008) Breastfeeding and the mother-infant relationship – A review. Dev Rev 28: 503–521.; Henry S, Briefer S, Richard-Yris M-A, Hausberger M (2007) Are 6-month-old foals sensitive to dam’s influence? Dev Psychobiol 49: 514–521.; Emery N, Seed A, von Bayern A, Clayton N (2007) Cognitive adaptations of social bonding in birds. Phil Trans R Soc B29 vol.362 no.1480: 489–505.; deKort S, Emery N, Clayton N (2006) Food sharing in jackdaws, Corvus monedula: what, why and with whom? Anim Behav 72(2): 297–304.
21 Image source: Sankey C, Henry S, Górecka-Bruzda A, Richard-Yris M-A, Hausberger M (2010) The Way to a Man’s Heart Is through His Stomach: What about Horses? PLoS ONE 5(11): e15446. doi:10.1371/journal.pone.0015446
22 Image source: Sankey C, Henry S, Górecka-Bruzda A, Richard-Yris M-A, Hausberger M (2010) The Way to a Man’s Heart Is through His Stomach: What about Horses? PLoS ONE 5(11): e15446. doi:10.1371/journal.pone.0015446
23 An example is LiOR Sugar-Free Mint-Flavored Candy Mint in a 3.3-lb Bag, available here: https://www.amazon.com/LiOR-Sugar-Free-Candy-Mint-3-3-lb/dp/B004JR4QMI/ref=sr_1_18_a_it?ie=UTF8&qid=1470935030&sr=8-18&keywords=sugar+free+bulk+candy
24 (Feh and de Mazières, 1993; McBride et al., 2004).