the burden. The dominance hierarchy also comes into play, as the offspring of the more dominant group members get preferential treatment. The gestation time is about 163–165 days, and births are typically of a single young. The births usually happen at the beginning of the rainy season, when sufficient food is available. The young are weaned at about six months of age and are fully mature in four to five years. The life expectancy of the green monkeys is 11–13 years in captivity, and about 10–12 years in the wild. Human interaction In the Caribbean islands, interactions between humans and monkeys are sometimes problematic. On the island of Barbados, farmers complain about the monkeys damaging their crops, and many try to find ways to keep them at bay. On Halloween 2006, a monkey was suspected of causing an island-wide, eight-hour blackout. The monkey apparently climbed a light pole and tripped an 11,000- and 24,000-volt powerline. In some African countries, many monkeys are killed by power lines, dogs, predatory animals e.g. wild cats, vehicles, shooting, poisoning, and hunting for sport. Added to this, an increase in desertification, and loss of habitat due to agriculture and urbanisation has occurred. As a result, the population numbers in troops are declining in urban areas to an average of between 15 and 25 individuals, with many troops disappearing altogether. Use in scientific research and vaccine production The African green monkey has been the focus of much scientific research since the 1950s, and cell lines derived from its tissues are still used today to produce vaccines for polio and smallpox. Chlorocebus species are also important in studying high blood pressure and AIDS. Unlike most other nonhuman primates, they naturally develop high blood pressure. In Africa, the monkeys are massively infected with simian immunodeficiency virus (SIV), related to the ancestor of human immunodeficiency virus (HIV), both of which are widespread throughout populations. Chlorocebus monkeys are a natural host of SIV and do not succumb to immunodeficiency upon infection; therefore they are an important model in AIDS studies to understand protective mechanisms against AIDS. The monkeys infected with SIV and humans infected with HIV differ in microbial responses to infection. Vero cells are a continuous cell line derived from epithelial cells of the African green monkey kidney, and are widely used for research in immunology and infectious disease. Similar cell lines include buffalo green monkey kidney and BS-C-1.