It's Not Easy Being Cool

Last time we talked about the wonderful toucans and hornbills, including the newly-recognised importance of their amazing bills in helping them keep cool. As promised, this time I'm going to talk more about some of the ways in which animals lose heat to avoid overheating, and a bit about the implications for this in a world which is getting warmer all the time. Hopefully it won't be too dry - as usual plenty of pictures of interesting animals! Firstly I must remind us that birds don't have sweat glands - not much point in them under feathers (or indeed fur) - so that's not an option for cooling off.

Before we leave last week's theme of large beaks with networks of surface blood vessels which can be opened to 'dump' surprisingly large amounts of heat quickly, or closed off to conserve heat where necessary, it's worth noting that hornbills and toucans aren't the only ones to practise this strategy. In fact the more that researchers look, the more examples they're finding of it. An unexpected one is among puffins, seabirds of the cold northern oceans. 

Tufted Puffins Fratercula cirrhata were shown to drop the temperature of their bill by
five degrees C within 30 minutes of landing from a foraging flight,
while heat loss from the back was negligible.
Photo per phys.com.

Why would they need to get rid of excess heat in such an environment? The answer seems to lie in observations of a close relation, the Thick-billed Murre Uria lomvia, of the North Pacific and Atlantic. Like puffins they have relatively short stubby wings for diving and chasing after food underwater, so they are not terribly efficient in the air and they must work very hard to fly. In fact the increase in their energy expenditure during flying compared to resting is the highest ever measured in a vertebrate. That, with their efficient insulation, leaves them vulnerable to over-heating, hence the use of the bill to remedy that danger. 

Last time I reported on a South African study which showed that ground hornbills lose a significant amount of heat from the bare skin on the face and neck as well as the bill. It follows that other birds with extensive bare skin on the face, especially in the tropics, are probably doing the same thing.

Blue-and-Yellow Macaw Ara ararauna, Pantanal, south-western Brazil, at nest hollow.
The large white face is bare of feathers except for the artfully arranged lines of black feathers.

It doesn't mean however that these birds are controlling the heat exchange, as the toucans and puffins are. Simply having such expanses of non-insulated skin will allow cooling to take place through blood vessels near the surface. Work on Ospreys showed that heat was lost through the long legs and claws much more than through the beak, but this wasn't managed either.

 

Birds will tuck their beak or a foot under the feathers to stop them from losing heat, but will also do so in hot weather to prevent them taking up any more heat from the air. 

 

Emus and Ostriches do regulate their heat loss, through beaks and bare or lightly feathered necks and legs plus feet and toes. This study, using infrared thermography ('heat pictures'), was done back in 1994, which surprises me.     

Emus near Esperance, Western Australia. The expanses of bare skin on
neck, legs and feet all act as controlled temperature regulators.

 

Somali Ostrich, Shaba Reserve, northern Kenya.
The same comments apply to it as to the Emu, though its barer
neck is presumably a better heat disperser. Both live in hot
arid situations (though Emus also live, or lived, in milder moister regions).

That study also looked at a Southern Wattled Cassowary, a rainforest bird, and concluded that they behaved similarly, but that the legs were less important. (The overall sample size was very small, two ostriches, two emus and the cassowary, all in Brookfield Zoo in Illinois.)  However, they reported that the casque - the big impressive spongy helmet - performed the function instead. Hitherto many suggestions had been made for its purpose (from crashing through dense vegetation to display) none of which were very convincing. Much more recently a 2019 study of 20 captive cassowaries at temperatures from 5 to 36 degrees across eastern Australia confirmed the earlier findings. The bill tip, legs and especially the casque were important in temperature control. At high temperatures they increased the blood flow to the casque, so allowing heat to escape, and when it was cold (they live in mountain rainforests as well as down near the sea) they closed it down. When it was particularly hot they would dip their head in water to increase the heat dump.

Southern Cassowary Casuarius casuarius, in rainforest, Atherton Tableland, north Queensland.

And it seems that many other birds are shedding heat through bills, skin, legs and feet, as this infrared image of an Australian King Parrot Alisterus scapularis shows. Even smaller bills are of value here.

The hottest parts (ie shedding the most heat) show as yellow - the bill,
skin round the eyes and feet. Photo per The Conversation.

The bigger you are, the smaller your surface area is relative to your mass. This is fine if you're in a cool climate, because it means you don't lose heat as easily as a smaller animal. However if it's hot you might be in trouble trying to lose enough heat to stay safe. Accordingly, an elephant's ears represent the biggest 'thermal window' of any animal (along with the bill of the largest toucans).

African Bush Elephant, Queen Elizabeth NP, Uganda.
These magnificent ears are thin and full of fine blood vessels. Even when they are
still they dump heat readily but, if that's not enough, flapping them makes them
even more efficient. (They're pretty good at hearing too!)

Near the other end of the mammal scale the bare tail of many rodents does the job too.

Fawn-footed Melomys Melomys cervinipes, Kingfisher Park, north Queensland.
(No I know you can't see much of the tail - sorry, but you know what it looks like.)

Which brings us to some behavioural tricks for getting rid of some heat (though to at least some extent most of the structures we've looked at need to be operated too).

After my last post I had an interesting but ultimately fruitless discussion with a friend about the behaviour of sea lions and fur seals in the generally cold seas of southern Australia waving their flippers and tails in the air while lolling about in the water. I couldn't see the point in such conditions, although I found it asserted widely on line that it was indeed to cool down, albeit with no evidence that I could find. If anything the reverse seemed more plausible, that it was a strategy to warm up (as long as the air is warmer than the flipper). However the discussion above about puffins suggested something else to me. The 'seals' (no real seals in Australian waters, but that's what we call them for lazy convenience) have superb insulation in the form of a thick layer of blubber and perhaps are in danger of overheating after a busy session chasing fish. Just a thought and if you have another one (or even some actual information!) I'd be very glad to hear from you.

Australian Fur Seal Arctocephalus pusillus, Narooma, south coast NSW in typical loafing pose.

And New Zealand Fur Seals A. forsteri, relaxing and waving en masse at Goolwa, near the
mouth of the Murray River in South Australia.

Many birds in warmer climes have featherless sections of skin along the underwings, so they can lose heat by exposing blood vessels just under the surface by extending their wings.

Cocoi Heron Ardea cocoi, Pantanal, south-west Brazil, exposing bare underwing
patches to lose heat from the blood vessels near the surface.
This big heron is found throughout all of South America except for the Andes.

However this Cocoi is doing something else - its beak is open and it is panting, breathing in and out while fluttering its throat to lose heat by evaporation. This is widespread behaviour among birds in hot situations.  Here are some more examples of panting, though passerines (such as the swallows and bee-eater below) for some reason never learnt the throat-fluttering trick.

Ethiopian Swallows Hirundo aethiopica (above) and
White-throated Bee-eater Merops albicollis (below), Waza, north Cameroon.
Their wings are partly open too to increase air flow over skin.
The temperature was well over 40 degrees, and we felt like panting too!

This lovely bee-eater nests up here near the Sahara (see the long tail streamers which proclaim
its breeding status) and spends winter in the rainforests of west and central Africa.

Wood Storks Mycteria americana, Pantanal.

Galápagos Cormorants Nannopterum harrisi, Isla Fernandina.
This flightless bird is the world's largest cormorant; it nests in the full sun on lava rocks.

Female Australian Darter Anhinga novaehollandiae, Winton, central Queensland.

But what are implications of a relentlessly warming world for all this? Some at least are certainly not good for birds in arid lands in particular. You can only dump heat from a bill (or other exposed body part) if the air temperature is lower than your own temperature. As air temperatures rise that window of opportunity gets smaller, but worse, once they rise above body temperature the once-beneficial bill suddenly becomes a dangerous heat absorber. On the other hand panting is still a viable option - but as droughts become more frequent and more intense a panting desert bird is put under ever-increasing stress from water loss. If you're interested my book Birds in Their Habitats; travels with a naturalist, has more on this (and many other things of course!) - I've just noticed that it's on sale too!

 

Those problems aside for now, we might expect that appendages such as birds' bills and mammalian ears would have increased in size over the last century or so. Can we test that though? Of course we can - in museums around the world are vast treasure troves of specimens waiting to be measured and to contribute to knowledge. An Australian study published in 2015 found that the bills of three common Australian parrots, and a rarer cockatoo, have increased in surface area by up to 10% since 1871.

     

Red-rumped Parrots Psephotus haematonotus....

...Mulga Parrots Psephotellus varius...

...Gang-gang Cockatoos  Callocephalon fimbriatum...

... and Crimson Rosellas Platycercus elegans all have significantly larger bills
than their recent ancestors did.

But there's yet another aspect which I've touched on too. We noted that the elephants need extra big ears to keep cool, as their large bodies have a smaller surface area than smaller bodies. The corollary of course is that smaller bodies have a relatively greater surface area, so can shed heat more readily. Bergmann's Rule accordingly predicts that individuals of a species living in warmer climates will be smaller than members of the same species in cooler places.  But what happens when a place warms over time, as of course is happening all over the world? We might predict that at least some of the animals living there might be smaller than their ancestors - and a team led by Dr Janet Gardner of the Australian National University showed just that. They measured 517 museum skins of eight Australian insect-eating birds, collected over 130 years from 1869 to 2001. Six of the species showed a decrease in size since 1950, four of them being statistically significant. 

 

White-browed Scrubwrens Sericornis frontalis...

...White-browed Babblers Pomatostomus superciliosus...

... Jacky Winters Microeca fascinans...

... and Hooded Robins Melanodryas cucullata living at the latitude of Canberra are
now the size that their relatives at the latitude of Brisbane (nearly 1000km to the north)
were before 1950, just 70 years ago.

We've covered a lot of ground, and if you're still with me - thank you! I think the topic of how animals manage their temperature (especially when it's hot) is an interesting one in itself, but the question of  the impact of a warming world on that is an even bigger and more significant one. We've learned a lot about that in recent years, and I don't for a moment imagine we're close to understanding it all.

Next time, something a bit more scenic perhaps? I'll put my mind to it.

NEXT POSTING THURSDAY 28 October
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