Population size, one of the basic biological parameters is particularly difficult to estimate for nocturnal animals with cryptic life style and little individual distinctiveness like Eurasian otters (Lutra lutra). Because telemetric methods often fail and also expose the animals to a high risk of injuries and even mortality, we analysed DNA and hormones of spraints to obtain data on population density and structure of free-living otters in a Nature Park in north-eastern Germany. We were able to assign 53 different individual profiles from faecal samples and obtained six more profiles from animals found dead inside the park. The total population estimate (n ¼ 59) consisted of at least 32 males and 27 females; 33 animals were adult, 23 younger than 2 years (three of unknown age). Marking points were frequented by up to 12 individuals. Estimated density was one animal per 4.7 km of shoreline. The genotypically estimated total population size was more than 2.5 times as high as estimated in the past census. The method was also suited to compare otter population densities in different areas or at different times in the same area.

Water consumption by 12 female Przewalski horses was automatically measured in a semireserve for 17 months. Average daily consumption on a yearly basis was between 2.4 and 8.3 liters, but significant individual variation in water consumption and drinking frequency was recorded throughout the observation period. Individual differences were more clearly detectable during periods of hot and dry weather. Individuals with high water demands initiated herd movements toward watering places independent of their social rank. Discussed as possible reasons for individual differences in water demand are: acquired preferences, distinct food selection, and genetically determined physiological differences. Watering behavior should not be regarded as an important factor only in re‐introduction projects, but also with respect to animal welfare in general. Zoo Biol 17:181–192, 1998. © 1998 Wiley‐Liss, Inc.

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