The lemurs swallow seeds of such size because they are covered in a fleshy reward that the primates are unable to separate from the seed. To get it, they have to swallow.
"The seeds are normally shaped so that they are easy to swallow—similar to the shape of capsules or tablets given by the doctor—and are also slippery, so that once they are taken into the mouth, they are swallowed almost without noticing," Birkinshaw said.
As the lemurs digest the fleshy fruit, the seed is passed through their system, normally away from the parent tree. Once on the ground, the seeds take root.
Special Adaptations?
According to Birkinshaw, some fruits in Madagascar are especially adapted to lemurs. For example, he said their fruits tend to be dull in color, owing to lemurs' poor color vision, but have a smell the primates find attractive.
Birkinshaw said that while he is unable to distinguish the smell of the fruits, lemurs have a much greater sense of smell and "clearly can" smell a particularly desirable fruit from a great distance.
"Sometimes groups move a long way, say 300 meters [1,000 feet] out of their normal territory to exploit a particularly desirable fruit source that they had not previously been near," he said. "I can only think that they knew it was fruiting because of the scent of the ripe fruits."
The other alternative, he said, is that the lemurs are able to predict when particular fruit sources are available in different parts of the forest based on previous experience. "[That] would be even more remarkable," he said.
Ganzhorn, the German primatologist, said he has a difficult time finding convincing evidence that fruits are adapted to lemurs, which he says will eat anything that is ripe.
"There are definitely fruits that are only eaten by lemurs and that situation is in all forests of Madagascar," he said. "But except for size, there is no convincing correlation between characteristics."
Nevertheless, lemurs are the only animals left on the island nation large enough to disperse many of these seeds. And they have become a long-term conservation priority, Ganzhorn said.
"If these animals are gone, then the forest has a harder time regenerating. For long-term conservation, it is important [to have lemurs]," he said. "The problem is that in the short interval and time scale, these forests will be gone, because people simply cut them."
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