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In the sandy deserts on the coast of Brazil, a lizard and a cactus have
evolved an unusual arrangement that benefits both: the cactus provides the
lizard with food and water, and has its seeds dispersed in return. In fact,
they cannot germinate unless they have passed through the lizard.

Joao Vasconcellos-Neto and his colleagues at the State University of
Campinas observed the lizard, Tropidurus torquatus, in the Linhares Nature
Preserve to the north of Rio de Janeiro, where the temperature can top 50
°C by midday on sunny days. The lizard feeds on ants, while keeping
a watchful eye on Melocactus violaceus, a cactus that grows in open, sandy
areas.

Each day this cactus produces between one and four small, pink fruits. They
mature within the cactus’s spine-covered corona, and are pushed up and out
at the top. ‘The exposure of the fruit is synchronised with the lizard’s
period of greatest activity in the morning,’ says Vasconcellos-Neto. ‘As the
temperature goes up, the velocity with which the fruits rise increases, and
they are left most accessible to the lizards.’

When a lizard sees a fruit, it climbs up the side of a cactus and grabs it.
Although the fruits contain little sugar, they have a high water content,
and Vasconcellos-Neto believes this is why they are important to T.
torquatus.

As for the cactus, it provides the fruit during the day, when few other
herbivores apart from these lizards are active. In return, the seeds are
dispersed to other sandy areas when the lizard defecates. Each fruit
contains 22 seeds on average.

‘The seeds we collected from lizard faeces germinate in our lab,’ says
Vasconcellos-Neto, ‘while seeds taken directly from ripe fruit will not
sprout. This suggests that passing through the lizard’s digestive tract
breaks the seeds’ dormancy through some chemical changes.’

Vasconcellos-Neto believes that the high temperatures and the spines of M.
violaceus – which don’t inconvenience the lizard – prevent the small
mammals of the area from eating the fruit, ‘leaving Tropidurus torquatus as
apparently the exclusive disperser of its seeds’ (Ciencia Hoje, vol 15, No
89).