When Dale DeNardo saw a Gila monster wandering around the Sonoran Desert with its cloaca protruding from its body, the Arizona State University physiologist speculated that the animal might be shedding heat as well as waste from its cloaca. DeNardo went on to show that evaporative heat loss from a Gila monster's cloaca can cool the animal by 3°C. Ty Hoffman, working with DeNardo and Glenn Walsberg, immediately wondered if birds can also use their cloacae to cool off when the temperature soars(p. 741).
`For the last 50 years or so, physiologists have assumed that any evaporation that doesn't happen through a bird's mouth must happen through the skin,' Hoffman explains. He points out that the cloaca is a large, moist surface that's perfect for evaporative heat loss, yet nobody had measured how much heat birds lose through their cloacae. Hoffman decided to examine cloacal evaporation in Inca doves, which are known to have high rates of skin evaporation.
To determine how much water doves lose through their mouth, skin and cloacae, Hoffman placed each dove in a glass chamber separated into two compartments by a latex sheet with a hole for the bird's head to poke through,so that the top compartment captured water lost through the bird's mouth and the bottom captured water lost through the skin and cloaca. To measure water loss through the birds' skin only, Hoffman sealed the birds' cloacae with glue and measured the water content of air samples from the bottom compartment using a hygrometer. To measure water loss through the skin plus the cloaca, he removed the cloacal seals and repeated the water content measurement.`Unsealed' birds had higher rates of water loss than `sealed' birds,suggesting that cloacal evaporation was playing a large role in evaporative heat loss.
But can doves regulate how much heat they lose through their cloacae? To find out, Hoffman prevented evaporative heat loss from the birds' mouths by pumping humid air into the chamber's head compartment, and then cranked the temperature up. At 30°C, 35°C and 40°C, doves hardly relied on their cloacae. But at 42°C, cloacal evaporation accounted for a quarter of evaporative heat loss. `This mirrors what we saw in the Gila monsters,'Hoffman says. Because cloacal evaporation only kicks in at extreme temperatures, it may be an emergency thermoregulatory tactic.
To show that this is not simply a `by-product' of having a cloaca, Hoffman repeated the measurements for Eurasian quail, which have a large cloacal opening and may therefore be expected to use cloacal evaporation. Unfortunately, Hoffman could only test quail up to 32°C, because the birds became heat stressed above this temperature. Yet, despite the fact that the birds were showing obvious signs of heat stress, the evaporative heat loss from quail cloaca was negligible. This supports Hoffman's conclusion that doves actively regulate the contribution of their cloacae to heat loss.
Hoffman speculates that this ability could increase doves' fitness in hot places, as it may allow the animals to be active when predators aren't around,or increase the time they spend foraging at the hottest time of day. He would now like to establish whether other birds can also use their cloacae to cool off. `It would be nice to show that I haven't simply stumbled across the only species to do this!' he laughs.