Plants use many methods to distribute their seeds and succulent fruits such as lemons are not necessarily designed to be eaten. Many, such as blackberries and plums, are bitter until the seeds are ‘ready, while others, apples and many tropical fruits are designed to encourage pecking by birds, which scatters the seeds. Another group including figs and senna pods encourage animals to eat the fruits without digesting the seeds, allowing the seeds to pass undamaged through the animals, be it a mouse and elephant.
Yet other fruits remain unappetizing to animals until they drop to the ground, where they are eaten or scattered when fully rips or rotten.
Lemons come into this category, though it is possible that monkey, baboons or other animals may be fascinated by the bitterness and attack them earlier, giving themselves an unexpected dose of vitamin C and, of greater benefit to the tree, subsequently spitting out the pipe.
Many citrus trees that are natives of and regions have sour fruits to discourage animals from eating it. The flesh of a lemon is there for three main reasons: to add weight so that it will roll a long way after it falls from the tree. To dissuade foraging animals from eating the seeds before they can develop and to supply water and nutrients as the flesh rots around the germinating seeds. The main aim of any seed is to propagate the species, not to feed the local animals. Animals benefit only as a side effect of plants wanting to use them as a form of transport for their seeds.
The trouble with citrus fruits is that they have been cultivated for so long that nobody knows what their original seeds dispensers actually were. In cultivation, however, they do seem to be eaten by monkeys. May be monkeys like acid tastes more that more than people do. Many tropical fruits are dispersed by becoming over ripe, falling to the ground and being eaten by animals. May be the acid in citrus fruits was meant to act as a deterrent to these foraging animals so that the fruits and the seeds the contained were left to grow where fell.