the success of environmental enrichment can be measured is by recognizing the behavioral changes that occur from the techniques used to shape desired behaviors of the animal compared to the behaviors of those found in the wild. Other ways that the success of environmental enrichment can be assessed quantitatively by a range of behavioral and physiological indicators of animal welfare. In addition to those listed above, behavioral indicators include the occurrence of abnormal behaviours (e.g. stereotypies), cognitive bias studies, and the effects of frustration. Physiological indicators include heart rate, corticosteroids, immune function, neurobiology, eggshell quality and thermography. It is very difficult for zookeepers to measure the effectiveness of enrichment in terms of the stress due to the fact that animals that are found in zoos are oftentimes on display and presented with very abnormal conditions that can cause uneasiness and stress. Measuring enrichment in terms of reproduction is easier because of our ability to record offspring numbers and fertility. By making necessary environment changes and providing mental stimulation, animals in captivity have been seen to reproduce at a more similar rate to their wild ancestors in comparison to those provided with less behavioral and environmental enrichment. Issues and concerns Habituation Although environmental enrichment can provide sensory and social stimulations, it can also have limited efficacy if not changed frequently. Animals can become habituated to environmental enrichments, showing positive behaviors at onset of exposure and progressively declining with time. Environmental enrichments are effective primarily because it offers novelty stimuli, making the animal's daily routines less predictable, as would be in the wild. Therefore, maintaining novelty is important for the efficacy of the enrichment. Frequently changing the type of environmental enrichment will help prevent habituation. Training Usage of more highly advanced enrichment devices, such as computerized devices, requires training. This can lead to issues as training often consists of food as a reward. While food encourages the animal to participate with the device, the animal could associate the device with food. As a result, the interaction with the enrichment would bring about behaviors that are associated with training instead of the desired playful and voluntary behaviors. Time and resources The process of producing and providing environmental enrichment usually require a large allocation of time and resources. In a survey, "time taken by animal care staff to complete other tasks" was the most significant factor influencing environmental enrichment provisions and scheduling. Therefore, it