Analysis of Chromosomal Variation and Reproductive Fitness as a Conservation Management Tool

Cytogenetic research was first proposed as an essential management tool for zoo populations in 1969. Since that time, a large number of mammalian taxa have been karyotyped, leading to a variety of surprising findings. It was discovered that many of the animals commonly bred in zoological gardens possessed unexpected chromosome complements. Inadvertent hybridization of species with such chromosome variation often results in sterile offspring. One early finding was a chromosome difference that distinguishes Bornean from Sumatran orangutans. These two species were often misidentified based on physical appearance, especially when the animals are young. Routine chromosome analysis provided a reliable method for differentiating the two subspecies. This observation has been of great importance in the captive management of orangutans by preventing unwanted hybridization.

A similar situation was encountered with the two species of dik-dik commonly held in zoological gardens. The Genetics Division at CRES found significant chromosomal differences between Kirk’s and Guenther’s dik-diks and employed karyotype analysis to identify hybrid animals in 300 zoological collections. The breeding success of these two species was in rapid decline until management decisions based on the chromosomal analyses prevented further hybridization, allowing the captive dik-dik populations to rebound.

The Soemmerring’s gazelle is another species for which chromosome analysis has been an important conservation management tool. Captive propagation of this vulnerable African gazelle is notoriously difficult. A chromosome study undertaken by the Genetics Division revealed widely varying chromosome numbers among individuals of this species that is now considered to be the primary cause for the difficulty in establishing self-sustaining captive populations. Zoological Society of San Diego curators and the Antelope Taxon Advisory Group (TAG) have requested ongoing chromosome analyses on all living captive Soemmerring’s gazelles to be used to assess reproductive fitness for management purposes.

These examples demonstrate how analysis of chromosomal variation and reproductive fitness has become an essential conservation management tool. Similar findings have been documented in other species including owl monkeys, waterbucks, genets, and spider monkeys. Other types of chromosome errors known to cause reduced fertility or sterility have been found in rhinoceroses, Siberian tigers, gelada baboons, and gorillas. Planned investigations of this type are needed on as many species as possible to prevent an array of reproductive difficulties such as hybridization, reduced fertility, and sterility caused by chromosome variation.