Among animal welfare professionals, those who work at zoos might have the toughest jobs. Keepers and curators at zoo must alternately serve as biologists, psychologists, trainers, chefs, janitors, and educators. Often, those hardworking individuals take on multiple roles at once. Another important job that keepers and curators perform at the zoo is that of gerontologist. Gerontology, or the study of aging, is a field that has only been formally defined for forty years, and is becoming a more important consideration for the welfare of captive animals.

With the exception of animals raised in a specific breeding program who are destined for reintroduction, animals that are born in zoos will typically live out their lives, and ultimately die, in zoos. Zoos need to therefore adequately prepare to deliver proper care - both physical and psychological - for their aging residents. Providing that sort of proper veterinary care might involve making adjustments to an animal's environment, routine, or social groupings. Those changes, while made in the service of an animal's welfare, could nonetheless result in psychological distress.

Like any health care provider, a zoo's animal care staff has to balance the medical health requirements of their charges with their psychological well-being. Human doctors can simply ask their patients how they feel; veterinarians do not have this option. Instead, zoo researchers conduct detailed observations of their animals to determine what consequences might follow any major change in management procedures.

Tigers are typically thought of as solitary creatures. In the wild, according to common knowledge, if you see two or more tigers together (and it isn't mating season), you can bet its a mother and her cubs. However, the social systems of big cats may be more malleable than once thought. There is evidence, for example, that wild tigers gather in temporary aggregations to feast on a recent kill. And other large cats like cheetahs and snow leopards, despite being thought of as solitary in the wild, have been well-suited to social housing in zoos.

So, what happens when you take a species that is traditionally thought of as solitary, house them socially their entire lives, but then need to separate them on occasion as they become elderly? Disney's Animal Kingdom is home to six adult female tigers (Panthera tigris). They were originally born into three different litters in the spring of 1997 and were then transferred together to Disney in the Fall of 1998, where they've been socially housed ever since. But the tigers are aging. In captivity, tigers live on average twenty years (and fifteen years in the wild), meaning that the sixteen-year-old Disney tigers are in their golden years, and will increasingly require specialized veterinary care.

The Animal Kingdom's keepers and curators have found that providing personalized medical care or enrichment, or allowing slow eaters more time to eat their food, requires that they separate the tigers overnight. Therefore, while the tigers sleep most nights in groups of three, they sleep individually two nights each week. As they continue to age and become more susceptible to illness, the frequency of nights spent alone may increase. The researchers at Disney's zoo wondered if the social separation was a stressful experience for their tigers.

Over the course of four three-month periods spread out over two years, the night staff conducted observations of each tiger on four nights per week: two nights in which the tigers were kept apart (though their enclosures still allowed them to see, hear, and smell each other), and two nights when they remained in groups. With two observation periods per night, the researchers had 369 total observations.

Most importantly, they found that the tigers slept the same amount of time - seventy-five percent of the time - whether they were housed together or alone. Sleeping behavior is an important marker of stress, so this was a good sign. Brief periods of social isolation did not seem particularly stressful for the cats. In other words, even if being alone was slightly stressful, it was not severe enough to impact their sleep. And there were no differences in sleeping position, either.

Other indicators of stress such as pacing, a common behavior seen in stressed big cats, or door pounding occurred extremely rarely, and did not occur more often while the tigers slept solo.

When it came to other questions about the tigers' behavior across conditions, the differences that did emerge were either expected or unimportant.

For example, whether in groups or by themselves they spent the least amount of the time they were awake in a sitting position. But they took up this position more often while individually housed, 1.68% of the time, than while socially housed, when they sat for only 0.79% of the time. While this is a statistically significant difference, the researchers don't think that it "reflects a significant impact to welfare." In other words, it doesn't really matter.

The tigers chuffed more often while together than while alone. This makes sense and would be expected, as chuffing is a social vocalization. They were more likely to groom themselves while alone than while together, but self-grooming is typically thought of as an indicator of positive welfare in big cats. Further, the night keepers did not observe any signs of over-grooming, which, like pacing, would have suggested a negative reaction to their isolation.

Overall, the tigers adjusted quite well to their occasional nighttime separation. As they continue to age and will probably require more nights spent alone, the keepers can rest a bit easier knowing that providing the tigers with better, more personalized veterinary care will not come at the cost of their psychological well-being.

Miller A., Leighty K.A. & Bettinger T.L. (2013). Behavioral Analysis of Tiger Night Housing Practices, Zoo Biology, 1-6. DOI:

Header photo: a tiger yawns at Disney's Animal Kingdom via Flickr/trenchfoot; Second photo: Malayan Tiger (Panthera tigris malayensis) at the San Diego Zoo copyright the author.