Cannon ball fruits in Mexico need horses, old or new

Shading the car park in George Brown Darwin Botanic Gardens is a tree with fruits that only a horse could break open. Or perhaps an animal now extinct in Mexico and Central America, the current homeland of the Mexican Calabash, Crescentia alata.

Yep, a trip to the tropics is fertile ground for a blogging botanist, although presumably the same applies to a tropical botanist slumming it in the temperate region for a few days. Anyway, I'm close to the end of my northern Australian obsession but I couldn't resist another exotic tropical species with a cute back story.

In a learned paper on 'the fruits the gomphotheres [now extinct elephant relatives] ate', Daniel Janzen and Paul Martin hypothesise that horses and cattle may now provide a decent substitute for extinct 'megafauna'. Not just these elephant relatives but perhaps extinct horse ancestors.

The Mexican Calabash, or Jicaro, is the only member of the plant family Bignoniaceae in Janzen and Martin's list of Costa Rican lowland plants with seeds 'probably dispersed by extinct megafauna'. There are quite a few members of the pea family, figs, palms and others.

Bignoniaceae includes well known temperate trees such as the Indian Bean (Catalpa) and Jacaranda, bearing flowers with the male bits (stamens) attached to the petals, and typically the whole flower looking a bit like a trumpet or vase.

The Calabash has a shy and crooked, bell-shaped flower, followed by a hard and heavy ('cannon-ball-like') fruit. Both arise directly from the trunk or stems, making this a cauliflorous plant.

Today introduced horses crack open the ripe fruit of the Calabash in their mouth, eating and then distributing the 200-800 seeds held within a slippery pulp inside. Tests showed that 97% of seeds extracted from the horse dung and washed would germinate.

In the absence of horses, the fruit rots on the ground during the wet season and the fermenting pulp kills the seeds. The fruit drops from the tree while still green then, over a month or so, browns and ripens, with the innards becoming black and slimy. Although the pulp is sweet, it was a 'fetid' odour.

It seems likely the tree is more common in Cost Rica today than it was historically, thanks to the free-ranging horses. One consequence of the plant going through a bit of a bottle-neck when the local horses died out might have been a reduction in some pollinating animals such as bats.

The flowers of the Calabash are visited by four different kinds of bat today, and their numbers and diversity may have suffered as the horses, and then the Calabash, declined. This in turn could have led to changes to other plants species that may have depended on these bats for pollination. Such a tangled web!