However, even under the best of conditions, repellents frequently fail to live up to user expectations. The reason for this is twofold. First, many repellents simply don't work. For example, peer-reviewed publications have consistently shown that ultrasonic devices do not drive unwanted animals away. Second, even when the repellent has been shown to work, animals in dire need of food will "hold their nose" and eat anyway because the alternative is essentially death by starvation. Repellents are most successful (referring to products actually demonstrated by peer-reviewed research to be effective) when animals have access to alternative food sources in a different location.