Explaining some Definitions
A significant interest was dedicated to the topic of longevity in mammals in recent years; a lot of results of scientific research have been published with a special focus on zoo animals. First of all this short summary wishes to explain some of the main definitions related to this subject, like the term maximum longevity, mean life expectancy, relative life expectancy, survivorship and mortality. The commonly used parameters used to determine population management success in zoo animals are the following, modified by CLUBB et al. 2008. and CLAUSS et al. 2010.
Maximum longevity is the published age record, this data is valid for a single animal and not representative for the whole population (unit of measurement is year).
Mean (or median) life expectancy is the number of years an individual is expected to live, can be determined for different age classes and makes comparison possible between populations of the same species (unit of measurement is year).
Relative life expectancy is the life expectancy of a population as a proportion of the longevity record of the species and makes comparison possible between populations of different species (unit of measurement is percentage).
Survivorship is the proportion of a cohort that is alive at a defined point in time and makes comparison possible within species (unit of measurement is percentage).
Mortality is the proportion of a cohort that has died at a defined point in time and makes comparison possible within species (unit of measurement is percentage).
Animal Well-being and Longevity
An above-average life span is usually considered a sign of appropriate husbandry protocol of zoo mammals. Zoological gardens mostly provide an „easy life” for their animals which is free of natural predators and the veterinarian team can help with several health problems, so consequently captive mammals live longer than their free-living conspecifics. This „hypothesis” is supported by the fact that longevity records are often held by captive animals. For concrete and detailed information it is worth having a look at the undermentioned literature like AnAge: The Animal Ageing and Longevity Database (www.genomics.senescence.info/species/), or WEIGL, R. 2005. Longevity of mammals in captivity; from the Living Collections of the world. A list of mammalian longevity in captivity. Zoo professionals have the opportunity to browse on ZIMS (Zoological Information Management System).
Many zoological gardens still delineate the maximum longevity data which is not a really appropriate method. These data are mostly exceptions and the bulk of the animals will not reach that age. Therefore the visiting public may have the impression that the given institution does something wrong in the husbandry and management of the species.
...and some Exceptions
Contrary to general opinion there are several species that have shorter life expectancies in captivity than in the wild. These taxa include sea mammals such as the Killer Whale (Orcinus orca) and the Asian and African Elephants (Elephas maximus and Loxodonta africana), and furthermore many ruminants, for example the Moose (Alces alces). Some interesting studies have found that several ruminants, especially the browsing species, find it difficult to accommodate to captive husbandry methods. A significant positive correlation was found between the relative life expectancy of a species and the percentage of grass in their natural diet, suggesting that there are more health problems, mostly dental ones, in the captive care of browsing species than of grazing ones. The mating system also has an influence on life expectancy: the males of monogamous species reach higher ages than those of polygamous ruminants.
However, it is worth mentioning that high longevities can be a sign of good and appropriate husbandry methods, but at the same time this is in connection with the problem of aged zoo animals and the disfiguring of the desired age-structure in zoological gardens.
CLAUSS, M., MÜLLER, D. W. H., STEINMETZ, H. W. & HATT, J.-M. 2010. The more the merrier or happy when alone? Hypothesis on stress susceptibility in captive individuals of solitary species. Proceedings of the International Conference on Diseases of Zoo and Wild Animals, Madrid. Pp 92-95.
CLUBB, R., ROWCLIFFE, M., LEE, P., MAR, K.U., MOSS, C. & MASON, G. J. 2008. Compromised survivorship in zoo elephants. Science. 322: 1649.
HATT, J.-M., MÜLLER, D. W. H., LACKEY, L. B., CLAUSS, M. 2011. Life expectancy in zoo mammals: What a zoo veterinarian should know. 2011, Proceedings AAZV Conference, Pp. 181-183.
JURADO, O. M., CLAUSS, M., STREICH, W. J., HATT, J.-M. 2008. Irregular tooth wear and longevity in captive wild ruminants: a pilot survey of necropsy reports. J. Zoo Wildl. Med. 2008. 39 (1): 69-75.
MÜLLER, D. W. H., LACKEY, L.. B., STREICH, W. J., FICKEL, J., HATT, J.-M., CLAUSS, M. 2011. Mating system, feeding type and ex situ conservation effort determine life expectancy in captive ruminants. Proc. Biol. Sci. 2011. 278 (1714 ): 2076-80.
MÜLLER, D. W. H., LACKEY, L.. B., STREICH, W. J., HATT, J.-M., CLAUSS, M. 2010. Relevance of management and feeding regimens on life expectancy in captive deer. Am. J. Vet. Res. 2010. 71 (3): 275-80.
WEIGL, R. 2005. Longevity of mammals in captivity; from the Living Collections of the world. A list of mammalian longevity in captivity. Kleine Senckenberg-Reihe, Band 48. Frankfurt am Main.