Harare – Zimbabwe's ex-president Robert Mugabe has reportedly not said a word since the death of his toughest rival Morgan Tsvangirai. The former trade union stalwart, who posed the most formidable challenge to Mugabe’s nearly four-decade hold on power, died on Wednesday last week in a hospital in South Africa where he was being treated for colon cancer.Tsvangirai, 65, founded the MDC in 1999. He was among the most prominent critics of Mugabe, the long-time authoritarian leader who was ousted from power in November. According to New Zimbabwe.com, Mugabe has kept a "conspicuous silence" over his long time rival's death. Tsvangirai once served under Mugabe for five years during the national government of national unity formed in 2009 – 2013.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook.