Can sheep feel hunger?

Dr. Else Verbeek

Dog behaviour scientist and consultant.

Sheep farming systems extensive in many countries, which means that the amount of land used is relatively large while the input of resources is relatively low. As opposed to the more intensive farming systems (the kind of farming most common in Europe, with limited land, and high input of labour, nutrition and technology). Extensively farmed animals are generally reared outside all year around and receive minimal nutritional supplementation.

This looks like to ideal picture for most people; happy animals in a paddock with plenty of space! Of course it’s great for sheep to be grazed all year around on lush green paddocks. However, what happens when those paddocks are not so lush and green?

For example, during summer droughts or very cold winters?

There was actually a major summer drought in the year I conducted my studies (2008), and many farmers were not prepared for this.

Pasture was insufficient to feed all animals and nutritional supplementation was not readily available and extremely extensive. Unfortunately, this meant poor welfare for both sheep and farmers! However, this was not the first time, and most likely not the last time this happened. Similar scenarios also occur(ed) in Australia! One of the problems with such low nutrition in summer is that animals are thin when they are mated and have to cope with a subsequent pregnancy, possibly without the opportunity to gain sufficient weight later on. Similar problems occur during cold winters, when pregnant sheep may lose a lot of weight and body condition score.

Body condition score (BCS) is a measure of how fat sheep are, and is assessed by manually feeling the fat and muscle cover on the sheep’s back and giving it a score between 1 (extremely thin) and 5 (severely obese). During my research I changed the BCS of pregnant sheep (by changing their diet) to simulate drought conditions in order to get a good indication of ewe welfare.

This sounds pretty mean, but it is very important research because many industry people and policy makers still believe that it is ok for sheep to be extremely skinny because this is “normal and natural”. Although it seems logic that poorly-fed skinny sheep are hungry and have poor welfare, this has not been scientifically proven. Such evidence is really needed in order to change things for the better!

The first problem I ran into is how to know whether sheep are hungry or not. You can’t really tell just by looking at them (although the fact that they’re skinny may give some indication, but again, there is no real scientific evidence that skinny sheep are hungry) and sheep can’t tell you how they feel! However, there are some indirect ways in which we can “ask” the sheep how it feels. So the first study I did was aimed at developing a new methodology to measure hunger in sheep.

For this study, we build 2 identical races (each 3 m wide and 20 m long). Each race had an automated feeding station (equipped with an ultrasound sensor) that delivered about 5 g of a food reward every time the sheep approached it. There was also a moving gate that pushed the sheep to a pre-set distance along the race after the sheep had eaten the reward.

The sheep could eat as many rewards in a day as it liked, as long as it was willing to walk the distance for each reward. We also changed the distance that sheep had to walk for each reward. We call this changing the “cost” or “price” of the reward. It’s kind of like a sheep in a supermarket: if food is cheap they will consume a lot, but the food becomes less attractive when it gets more expensive. Therefore, we expect sheep to “buy” a lot when the price is low, but gradually “buy” less as food becomes more expensive. Just like humans really.

Sheep getting a reward in the race.

The beeps signal that the gate is going to move.

To test whether we can measure hunger in sheep, we divided the sheep into two groups; one group was fed as much food as they wanted before the test, while the other half did not receive any food for 24 h (fasting). We then tested all sheep at 5 different costs.

The hypothesis was that a fasted sheep would be willing to pay a higher price for its food and also consume more food compared to the satiated sheep. Indeed, this is what we found. We successfully developed a methodology that allows sheep to indicate how much they are willing to pay for their food.

In the next study, we wanted to simulate more chronic food restriction conditions similar to what a sheep would experience on a farm. We chose to use pregnant sheep (Coopworth x Texel crosses), as they are likely to be the most vulnerable to undernutiriton.

We divided 22 pregnant sheep with a good BCS (BCS of 3) into three different treatments: low BCS aimed at BCS 2, medium BCS aimed to stay at BCS 3 and high BCS aimed at BCS 4. We adjusted the sheep’s diets to make sure they gained/lost the appropriate amount of weight/BCS for a period of 6 weeks. Note that a BCS of 2 is not an extremely low BCS, the animal is still healthy, alert and energetic. During droughts, farmed sheep may have a BCS of less than 2.

BCS 2 sheep

BCS 4 sheep

After 6 weeks, we tested them at 5 different costs in the races. We found that the low BCS ewes were willing to pay the highest price for their food. When cost was high (50 m per reward), the low BCS ewes were willing to walk 14km on a day just to get their daily requirements! It took them about 22 hours to walk this distance (they took little breaks in between), and they hardly got any sleep. This really shows how hungry they were!

The high BCS ewes, on the contrary, did not want to walk for their food at all. Even when cost was really low (only 1 m per reward) they choose to eat much less then their daily requirements. At the highest cost (50m) they only ate 10% of the daily requirements.

They actually slept the most part of the day, and did not bother to walk for their food! On the days that they were not in the races and food was freely available, however, they ate about 1.6 times the daily requirements.

This is a really interesting result, because it shows that sheep will overeat and get fat if tasty food is easily available. If they have to make a little bit of effort, however, they reduce food intake to very minimal levels!

We also looked at the energy balance of the sheep and at some of the physiological parameters involved in regulating food intake and body weight. As was to be expected, the low BCS ewes were in a negative energy balance and struggled to get enough energy. The high BCS ewes had plenty of energy, leading to significant weight and fat gain! The medium BCS were intermediate, and seemed the most energetically balanced. Interestingly, we also found correlations between glucose (blood sugar), leptin (a hormone secreted from the fat cells that regulates food intake) and hunger; sheep with lower glucose and leptin levels pay a higher price for food.

We have therefore shown that we can measure hunger in sheep! And we have provided evidence that skinny sheep experience hunger! In fact, we can recommend to anyone keeping sheep that a BCS of 3 is optimal. A BCS of 2 is really too skinny and will make sheep feel hungry! There is no more excuse for skinny sheep!

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