As an endurance athlete and naturalist, I tend to think a lot about what endurance actually is, particularly on those lonely 3+ hour Sunday long runs (here’s my running log; note: I haven’t used it much since June 2021). Working with my coach, Sam Davis, I’ve been honing the various physical and mental characteristics that I’ll need to get myself in peak endurance shape for my 100 miler. In season 5 of the podcast we explore these ideas in much more detail. Here will look at the definition and then some examples in the animal world.

A chickadee removing debris from a potential nest cavity (Centennial Woods, Burlington)

So what is endurance?

As with anything in biology, definitions are slippery little things that get harder and harder to pin down the more specific you try to be. I take endurance to have something to do with being able to experience extreme physiological stress and persist through a combination of physical and mental traits.

A formula might look something like this:

Endurance required = (Amount of Pain ^ coefficient of ability to suffer) * Time

As impressive as it might be for a 1,000 year old eastern hemlock to weather every climatological torment Vermont’s weather can throw at it, the tree still has no cognitive capacity to recognize pain and therefore suffer. Lacking sentience, hemlocks, and by extension plants, prokaryotes, fungi, and most animals, cannot summon courage, alter their behavior in the moment to overcome obstacles and press forward, and therefore cannot “endure.”

A house wren peaking out of its nest box (Centennial Woods, Burlington)
Endurance in animals

So just because an animal can suffer doesn’t mean it’s an endurance athlete. The ability to experience pain/hardship through time is key. There seem to be three different arenas in which animals most clearly manifest an ability to “endure”:

  1. reproduction (primarily in courtship and rearing young, not in sex itself),
  2. migration
  3. hunting

In a recent study, biologists revealed that natural selection can actually work to reverse the fickle hand of sexual selection (source). This seems to imply – as might already be obvious – that sexual selection can pull animals in all sorts of wild directions that make it harder and harder for the animal to survive. The stronger the force of sexual selection and the necessary energy investments in reproduction, the less energy the organism has to meet its basic survival needs. Sounds like a great place to look for endurance! And so, in honor of the nesting season (plus Father’s Day is coming up), I’ll highlight some examples of endurance in reproduction in this and the next newsletter.

A male common yellow throat between singing to defend territory and attract a mate (Centennial Woods, Burlington)

During the breeding season male birds sing for two primary reasons: to attract mates and to defend/define their territory. The song broadcasts to conspecifics (other individuals of the same species) a particular message: I’m here, I’m fit, and I’m ready to breed. And indeed a song may be a sign of an individuals fitness. In song sparrows, less inbred males have greater variability in their repertoire, which females use to select mates (source). In other birds, a bird who sings the local dialect may be favored as they may be more likely to know the landscape and sources of food and therefore be more fit (source).

But this broadcast is also heard by other species and is dangerous business. Male birds may sing more in their last year of life as they balance the risk between being heard by a predator and missing out on one last chance to breed (source). Even knowing potential threats are nearby doesn’t seem to reduce the frequency and duration of songs (source). Sex overrides our better judgement.

Despite the hazards, there’s an obvious tradeoff to singing, and males spend a lot of time and energy every day singing. For example, the metabolic rate in Carolina wrens increases 4 fold when it’s singing as compared to when it’s at rest (source). Across all songbirds, singing accounts from 10-25% of a male’s energy budget (source). Interestingly, in urban areas, where background noise may be higher, birds, like the zebra finch, have to expend more energy to sing louder (source).

Red-eyed vireos sing their tune up to 22,000x per day!! (Centennial Woods, Burlington)
When and how much to sing

The dawn chorus partly solves the singing problem. Birds can get all that sexy courtship stuff out of the way in the wee hours when predators might not be able to see as well. It also constrains the amount of time a bird can sing. But for some of the daytime singers, it can be an incredible investment in time. Many birds sing well over 1,000 times per day. But the real endurance athlete here is the red-eyed vireo. Males will sing there tune – “Where are you? Here I am?” – up to 22,000 times in just 10 hours (source)! And time singing is time not eating or resting.

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