In the fertile grasslands of central Nigeria the roar of a motorcycle is enough to instill fear in the Christian cattle herders stalked by an increasingly bloody conflict.

The rev of an engine is the first warning sign that gangs of kidnappers have emerged from the forest for their latest sortie in a battle over diminishing farmland that appears to be drawn along sectarian lines.

Across Africa’s most populous country, an undeclared war, triggered in part by climate change and fought over cattle, has turned Muslims and Christians against each other in a confrontation so bitter it threatens to tear Nigeria apart.

Warring over cattle is almost as old as human history in parts of Africa. But across a swath of the continent, cattle-related violence is unleashing more bloodshed than at any time in living memory.

Fights over cattle have claimed thousands of lives in South Sudan and the Central African Republic, worsening the humanitarian crises in two states devastated by civil war. Militias raised by armed cattle herders have brought anarchy to parts of northern Kenya, killing farmers white and black.