As with all behavioral neuroscience animal model experiments, food deprivation is used as a way to increase motivational response. This is also true for using Drosophila as an animal model to study the mechanisms of behavior.
The effects of food deprivation on the behavior of flies has been studied for a long time. One of the first books I ever read on fly behavior was 'The Hungry Fly' by Vincent Dethier (1976 - pictured here). But, you might notice, that there is very little information on how food deprivation is done in the methods sections of behavior papers. Maybe this is because food deprivation is a pretty simple protocol, but in my lab, we have spent way too many hours figuring out the appropriate length of time and method to food deprive flies to optimize motivational response. So in my opinion, it would be nice if people included more details on this in their methods.
Notes about terminology: Lots of people term the process of removing food in order to increase motivational response as ‘starvation’ as opposed to ‘food deprivation’. I think either is accepted, I just prefer food deprivation because it’s more consistent with the other animal models and sounds less extreme.
How is it done? Flies are typically placed in a vial with plain agar or a moist kimwipe or moist filter paper for a certain amount of time. That amount of time depends on the response they want to see and the genetic background of the fly. Note that in older protocols, people sometimes placed flies in a plain vial so they were both food and water deprived. Newer protocols usually provide a wet surface so flies are not water deprived. Sometimes the methods don’t differentiate so you have to read the results carefully.
What does food deprivation do to fly behavior? Well, obviously, it makes them hungry. But what else does that do, how hungry does it make them and how long does it take to make them hungry? There is a LOT of info available on there so my recommendation is to spend a few days being lost in pubmed if you are interested in this. I’m probably missing a bunch of studies, but here are a few behavior studies that are relevant if you are food-depriving your flies
Hunger increases foraging behavior (including increasing sensitivity to food-associated cues like odors). There are a lot of great studies in flies on this and Mahishi et al, 2021 have a great first paragraph on this in their Panopticon paper that I’m quoting below (including a handful of extra references that I added in):
“Flies show hunger-motivated ranging or foraging walks to find food; they also show explorative walks (or local searching behaviour) after food encounter (Dethier, 1957; Bell et al., 1985; Bell, 1990; Turelli & Hoffman, 1988; Burns et al., 2012; Corrales-Carvajal et al., 2016; Kim and Dickinson, 2017; Murata et al., 2017; Hughson et al., 2018; Mahishi and Huetteroth, 2019), and much has been achieved in identifying the circuits and dynamics involved in this behaviour (Zhang et al., 2015; Corfas et al., 2019; Lin et al., 2019; Moreira et al., 2019; Sayin et al., 2019; Seidenbecher et al., 2020; Behbahani et al., 2021, Landayan et al., 2018). Most of these studies either used hunger-motivated behaviour to focus on the underlying navigational strategy of the flies, or they focussed on exploration–exploitation trade-offs under different motivational settings.”
Hunger reduces male courtship behavior and (Cherriyamkunnel et al, 2021) and affects female sexual receptivity (Sun et al, 2023).
Hunger enhances acquisition and retrieval for odor-sucrose memory (Krashes et al 2009; Chouhan et al, 2017), memory consolidation during sleep (Chouhan et al, 2021), and can reduce odor-shock memory (Placais & Preat, 2013).
Hunger influences naive and learned choice behavior in larvae (Brunner et al, 2020).
How long should you food-deprive your flies? If you are trying to reproduce a behavior, food-deprive flies for as long as the protocol you are using recommends. Hopefully that will work! Sometimes it doesn’t. If it doesn’t work, my recommendation is to check to see if your genetic background is different. Different backgrounds respond to food deprivation in different ways (Nelson et al, 2016; Hughson et al, 2018).
What if you aren’t sure how long to food-deprive your flies? I recommend that you dive into the literature. A number of labs have looked at how food deprivation affects various behavioral measures (for example Weaver et al, 2023). There’s a good chance that the food deprivation protocol you need will be out there.
If that doesn’t work or you can’t find anything, consider the ‘hitting it with a hammer’ approach. By this, I mean set up a few agar vials of your flies in the density you plan to store/test them plus a few extra flies. Start recording the time at which they start dying (this can be anywhere between 16-48 hours in our experience). From your measurements, figure out the time point at which 5-10% of your flies are dead. Then try reducing that amount of time a little bit and perform your behavior experiment. Consider this the ‘maximal food-deprived motivation’ time.
When possible, I'd also recommend doing a careful study of different lengths and methods of food deprivation to be able to see how that impacts the behavior you are studying. In my experience, this can be a great undergraduate or rotation project. Make sure to take careful notes when you do these experiments because this can be very helpful information to have on hand, and in my opinion should be added in detail in the methods and the data plotted for a supplemental figure when possible.