The use of a tool by a non-human primate -- for instance, using a rock as a tool to crack nuts -- is not a matter of necessity but rather one of simple opportunity, researchers suggest.

Necessity is not the mother of invention, at least for the chimpanzee that avails himself of the rock, the researchers argue; it comes down to ecological opportunity, when the local environment provides both the tool and a reason to use it.

"By ecological opportunity, we mean the likelihood of encountering tool materials and resources whose exploitation requires the use of tools," says Kathelijne Koops of the University of Cambridge in England.

The researchers looked at behaviors of three primate species known to use tools: chimpanzees, capuchin monkeys and orangutans.

Chimpanzees have been observed using a number of tools, from stones for cracking nuts to sticks wielded to harvest army ants. Orangutans also utilize sticks as tools to gather insects as well as for prying seeds from fruits. Capuchin monkeys likewise use stones to crack open nuts and will dig into the ground for tubers using sticks.

It had long been assumed such primates began using such tools during periods when food was scarce, to maximize what they could obtain from the environment.

The researchers argue just the opposite, that an abundance of desirable food sources -- that "ecological opportunity" -- was the original driving force behind the development of their tool use behaviors.

"When chimps, orangutans and bearded capuchin monkeys alike encounter calorie-rich but hard-to-reach foodstuffs, such as nuts and honey, it seems to act as an incentive for an ingenious use of materials," Koop says.

Such foodstuffs are among the most nutritious in the primate's habitats, so using tools to extract them is beneficial at all times, not just during periods of food scarcity, she says.

Making and using tools, and the transmission of such skills from one individual to another, is seen as a significant step in the development of culture, the researchers point out.

"Given our close genetic links to our primate cousins, their tool use may provide valuable insights into how humans developed their extraordinary material culture and technology," Koop suggests.

Local environments can exert a significant influence and should be considered in any attempt to understand the occurrence and distribution of such material cultures, the researchers argue.

Koop and her fellow researchers published their findings on primates, ecological opportunity and tool use in the journal Biology Letters.