Over time, mutualism may become codependence, or it may develop into an even more intimate relation: union. Endosymbiosis is the intracellular capture of former symbionts.
Biological merger has repeatedly occurred, in seemingly creative ways. The mitochondrion, chloroplast, and nucleus of eukaryotic cells have double membranes: an apparent vestige of evolutionary union of an eocyte and bacteria.
Eocytes are anaerobic thermophiles that thrive in a sulfur-rich marine environ. Eocytes are archaea, with an evolutionary lineage distinct from bacteria.
These prokaryotic cousins – archaea and bacteria – have repeatedly teamed up. Purple sulfur bacteria and eocytes are naturally complementary endosymbiotic partners, as the metabolic needs of one are met by the metabolic waste products the other. An endosymbiotic relationship has flourished between a purple bacterial host and an eocyte symbiont, based on hydrogen and sulfur recycling, with CO2 taken in.
The selective integration of cellular life was merely a matter of time given coherence in evolution. Much later plants and animals coevolved symbiotically. The waste product of plants – oxygen – became a ready fuel of animal respiration, as well as plants themselves being nourishment.
The coevolution of flowering plants and pollinating animals is well-known. To attract pollinators and seed dispersal agents, plants evolved advertisements of various sorts: distinctive visual patterns and colors that contrast with their background, unique scents, and morphological features. Plants place their flowers and fruits on long peduncles (stalks), facilitating detection and approach. Even leaves may act as billboards.
Over time, a relationship between a plant and pollinator may even become exclusive: the animal and plant coevolving to a private mutualism.
Darwin noted the Malagasy star orchid, with a nectary at the base of a corolla tube 25 cm deep. While he knew of no pollinator, Darwin predicted one existed, one with a tongue long enough to reach the nectar. Years later, a local giant hawk moth was discovered, named Morgan’s sphinx moth, with a 30-cm tongue, longer than its body. In a pattern of coevolution, star orchids and hawk moths have developed such that certain hawk moths are suited to pollinate specific star orchids.
In the cloud forests of the Ecuadorian Andes mountains live nectar bats. Many varieties of these bats have tongues that reach 3.9 cm. One, Anoura fistulata, has a tongue that can extend to 8.5 cm: 150% of its body length, comparable only in tongue length to a chameleon. When not in use, the bat stores its tongue in a special tube in its thorax. With it, in an exclusive relationship, the nectar bat can dine on the flowering Centropogon nigricans, which stores its nectar at a depth only the tongue of A. fistulata can reach.
The Cuban tropical vine Marcgravia evenia has a unique disk-shaped leaf. For nectar bats foraging by echolocation, the leaf produces an echo signature which can be clearly differentiated from the echoes produced by background foliage. This reduces bat search time by half in finding the vine they were looking for.
The Cuban tropical vine’s echolocation trick is especially impressive when considering that different bat species have different echolocation techniques. Somehow the Cuban tropical vine knew exactly who its customers were and catered to them.
Cave-dwelling bats, such as fruit bats, make quick echolocation clicks by popping the tongue from the floor of the mouth to the roof. Bats that feed upon flying insects have a different clicking action: one that lasts 5 times that of the quick pop of cave bats. The longer signal is better for detecting flying prey than for navigating obstacles.