Some form monogamous pairs. There is a 'polygyny threshold', a threshold level of wealth inequality, at which a female is marginally better off as the second wife of a rich male than she would be as the only wife of a poor male. In species with the possibility of extreme wealth-inequality, very large harems are built up by very few males. Monogamous mating systems typify species whose economic goods are by their nature difficult to hog. Even the richest male is not so different from the poorest as to tempt a female to become his second wife.
Indirect clues suggest a leaning towards the harem.
First, men tend to be larger than women, a pattern unsurprisingly typical of polygynous mammals. The Darwinian anthropologist Napoleon Chagnon has made a lifelong study of the Yanomamo of the Amazon jungle, one of the few tribes that retain our ancestral hunter-gatherer way of life. The Yanomamo call themselves 'the fierce people', and violent and lethal fighting is rife. The founder of one bloc of villages had forty-five children by eight wives. The Darwinian historian Laura Betzig has shown that, both in the Roman Empire and in medieval Christendom, though marriage was monogamous, mating was often polygynous.