Sizing up saquaros
White doves, foraging ants, hungry tortoises - they all help in the prickly business of dispersing the seeds of the saguaro cactus. Which is just as well, since the chances of a seed growing into a fully-grown 10-metre-tall cactus are many, many millions to one.
Mature saquaro cacti are perfectly adapted to survive the intense heat of the Sonoran Desert. But their seeds are delicate and must find shade to avoid being scorched to death before they germinate.
The seeds are covered with sweet flesh that is irresistible to all sorts of desert creatures. White-winged doves are the first to arrive and eat the seeds before depositing them many mjles away in their droppings. Fallen fruit provides a bonanza for creatures on the ground and the greater the range the seeds can attract, the more likely they are to succeed.
Ants carry the seeds in the shady underground, and tortoise head for shade where they're sure to leave undigested seeds in their dung. The seeds then await the rains. Of the 40 million or so seeds a cactus produces in its lifetime, only one is likely to develop into a plant that outlives its parent. If it's lucky enough to find shade it'll take 10 years to become a 5cm tall cactus. To reach the size of its parent may take at least a hundred years.