Adaptations to Cave Life

Animals roosting or living in caves must adapt to cope with the unusual environment. Paramount for the cave-roosting vertebrates is the ability to find their way to and from their roosts at the correct time. Not surprisingly, the birds and bats display uncanny skill in memorizing the complex maze to and from their cave roosts. Pack rats use trails of their urine to navigate in and out of caves. Species using the twilight and transition zones can use the daily meteorological cycle for cues to wake and leave the cave. Those roosting in the deep zone may rely on accurate internal clocks to know when it is beneficial to leave their roost.

Organisms that adapt to live permanently underground must make changes in behavior, physiology, and structure in order to thrive in the stressful environment. They need to find food and mates and successfully reproduce in total darkness. Their hallmark is the loss or reduction of conspicuous structures such eyes, bodily color, protective armor, and wings. These structures are worthless in total darkness, but they can be lost quickly when selection is relaxed because they are expensive for the body to make and maintain. How such losses could happen quickly is demonstrated by the cave-adapted planthoppers (Cixiidae). The nymphs of surface species feed on plant roots and have reduced eyes and bodily color whereas their adults have big eyes, bold colors, and functional wings. The cave-adapted descendents maintain the nymphal eyes, color, and other structures into adulthood, a phenomenon known as neoteny.

The high relative humidity and occasional episodes of elevated CO2 concentrations are stressful to cold-blooded organisms. The blood of insects and other invertebrates will absorb water from saturated atmosphere, and the animals literally will drown unless they have adaptations to excrete the excess water. High levels of CO2 force animals to breathe more, which increases water absorption. Cave-adapted insects often have modified spiracles to prevent or cope with their air passages filling with water.

Most lava tube arthropods have specialized elongated claws to walk on glassy wet-rock surfaces. Many have elongated legs to step across cracks rather than having to descend and climb the other side. Jumping or falling might land a hapless animal in a pool or water-filled pit or into the clutches of a predator. Small insects are often too heavy or are unable to climb the meniscus at the edge of rock pools and will eventually drown. However, many cave-adapted insects have unique knobs or hairs near the base of each elongated claw and modified behavioral traits that allow them to climb the meniscus and escape. Some of the latter are predators or scavengers, who wait on pools for victims.

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