A friend lives on a housing estate that is infested with mice, so she bought some traps to catch any that stray into her house. The instructions on the box recommend baiting the traps with peanut butter or chocolate. Not having peanut butter, she decided to raid her daughter’s stash of chocolates accumulated over the Easter holidays.
Naturally, her daughter objected and ‘suggested’ that her mum use her own chocolates! Mum prefers more expensive dark chocolates, so was also reluctant to use them on some mice. She tried to argue that mice, like children, are more likely to prefer the cheaper, sweeter chocolates.
Knowing I work for Rentokil Initial, she asked me to back her case, so I did a little checking. Chocolate is even recommended as bait by the UK Department of Environment, Farming and Rural Affairs in their Technical Advice Note, House Mice — but it doesn’t say which type.
I enlisted the help of the experts in our Global Technical Centre, who, along with many other organisations, have been observing rodent behaviour for many years, to see if they knew whether mice prefer one type of chocolate to another.
Mouse food preferences and eating habits have been investigated extensively, but it appears that little is known about the types of chocolate that mice prefer, so we set up a chocolate trial.
What do mice eat?
The natural food of mice is mainly cereals such as wheat, barley and rice grains, but they are omnivorous and will eat almost anything that is available. They need a balanced diet, just like humans, to obtain carbohydrates, fats, proteins, minerals and vitamins.
Rodent food preferences have been studied by many different organisations over many decades and early food trials showed that mice preferred wheat and maize in the form of pellets and oats in the form of oatmeal over other foods such as pulses.
However, when up to 8% vegetable oil was added to cereal they greatly preferred the cereal with the higher oil content, eating over 10 times more than plain grain, but this tailed off after 11 days or so. This might explain why many people find peanut butter is also an attractive bait to put on traps.
Research around the world has shown that mice and rats are also attracted to sweet foods, but no one can agree on the best level of sweetness to attract them.
The eating habits of mice
Mice eat almost at random when there are multiple sources of food, even preferring to try new sources over existing ones. So their feeding habits are described as inquisitive and unpredictable. This is in contrast to rats, which prefer to visit known sources of food and show an initial fear of new sources of food.
Mice feed mainly at night, visiting 20-30 sources of food per night, if available, and consuming about 20mg of food each visit. However, if there is only one source they will repeatedly visit that.
Mice will make 100-300 visits to food sources per night, consuming in total 3-4gm of food.
The chocolate results
We selected four types of chocolate with different amounts of cocoa (cocoa and cocoa butter) and sugar content:
- milk chocolate: 20% cocoa solids, 56% sugar
- white chocolate (vanilla flavoured): about 30% cocoa butter (no cocoa), 53% sugar
- plain chocolate: 62% cocoa solids, 38% sugar
- dark chocolate: 85% cocoa solids, 15% sugar
The mice had access to the chocolate bars for 24 hours. A video camera recorded the mice’s activity and how they behaved with each type of chocolate. Video analysis software automatically tracked each mouse’s movement and measured how long they spent at any position.
Most visited chocolate
The video analysis created what is called a heat map of the mice’s movements over the period of the trial. The heat map below shows the time spent in a spot using coloured contours, with dark blue showing a short time and light blue, yellow, red and dark red showing increasing time spent.
As you can see in the heat map, the “hottest” spot is in the centre at the bottom where the white chocolate is placed. The next most popular spots are the bottom left-hand corner — where there is no chocolate — and the milk chocolate bar on the left-hand side.
The bottom right-hand corner and right-hand side are also fairly popular, while the mice spent a little time at the two darker chocolate bars.
Mice prefer to move along the edges of a room, which is also shown on the heat map. If we look at the actual figures for time spent and the number of visits to each type of chocolate, some interesting facts emerge. Have a look at the heights of the bars in the chart below.
The mice made 266 visits to the milk chocolate and spent 222 seconds there. They made an equal number of visits to the white chocolate and the 68% cocoa chocolate (both 97 visits) and a few more (114 visits) to the 85% cocoa chocolate.
However, the time spent at the white chocolate was far more than all the others combined, at 735 seconds. They only spent 45 seconds on the 67% cocoa chocolate and 34 seconds at the 85% cocoa chocolate.
The data for the time spent at each place seems to indicate that the higher the cocoa content, the less the mice like the chocolate.
What is more intriguing is the amount of each type of chocolate that they ate. The amounts of chocolate eaten by the mice in one day (or night as they are more active then) are shown in the chart below.
So the mice’s ratings for the four types of chocolate are:
- Milk chocolate: 12
- White chocolate: 6
- 62% cocoa chocolate: 0
- 85% cocoa chocolate: 0
So mice clearly do not like dark chocolates! They didn’t eat any measurable amount despite making around 100 visits to each type.
The mice ate more than twice as much milk chocolate as white chocolate even though they spent three times as long at the white chocolate, as shown earlier. The milk and the white chocolate had similar sugar contents, so the difference could have been either that they like a mild cocoa flavour or the vanilla in the white chocolate was affecting them. For all we know they could just like smelling the vanilla!
Cocoa contains a bitter compound called theobromine, which is similar to caffeine. As the sugar content goes down and the cocoa content goes up in the darker chocolates there is more theobromine, which makes it taste more bitter. This bitter taste could be putting the mice off.
Theobromine is also poisonous to some mammals, especially dogs and cats. Dogs are far more likely to eat any chocolate they can get hold of than cats — cats cannot taste sweetness. So keep chocolate out of reach of dogs and take them to a vet as soon as possible if they do.
Mice, rats and humans are more tolerant of theobromine, but the mice could be protecting themselves by avoiding the chocolate with higher theobromine content.
We hope all you chocoholics enjoy your preferred types of chocolate but don’t let any mice get at them (unless you are using them as bait) as mice (and rats) can spread some nasty diseases!
If you have an infestation of mice or other pests contact us to get rid of them safely and humanely.