(Photograph = Melocactus violaceus, Andrew Nicolle)
Introduction
Melocactus violaceus fruits develop internally in the stem of the mother plant. They are completely protected in the cephalium, but they are driven out to the surface by internal pressure. They are pink and full of water when mature.
The Details
The fruits are driven to the exterior of the plant by pressure during the hottest part of the day where they are easily noticed and consumed by a lizard, Tropidurus torquatus. Apparently, this specific lizard is the only consumer of the fruits. The fruits are just the right size for the lizard, and they deliver a useful dose of water in the mid-day heat. Presumably the lizard consumes the fruit to gain water and as well as food.
The seeds pass through the lizard and are deposited in various places. Under laboratory conditions, lizard-passed seeds germinated quickly and easily. In contrast, seeds that came directly from the fruits (not passed through the lizard) did not germinate easily under laboratory conditions.
Thus, the plant makes fruits that entice the lizards during the hottest part of the day when the animals could use water. The lizards deposit the seeds in some places far from the mother plant and help distribute the cactus species. It is a win-win situation.
Additional Reading: Pollination and Seed Dispersal by Lizards
Great photographs! The one on the bottom on the right hand side looks like a Melocactus matanzanus. How can we distinguish one from the other?
No photos of the lizard?! Interestingly the subspecies Melocactus violaceus subs. margaritaceus has white fruits that would not seem to attract lizards.
Mike,
Thanks for your note on my blog. I did not know about the white-fruited cactus. And, sadly, the lizard photos were unavailable.
Thanks again for writing.
Joe Shaw