Walking in tropical rainforests, my eyes constantly scan the ground for cryptic patterns and symmetries like the white streaks of an Antirrhea butterfly or the broken hourglasses of a fer-de-lance. Usually, animals tend to be inconspicuous to reduce detection by predators, with exception of those that harbor chemical defenses, most famously the poison frogs, or bear a powerful sting such as a spotted velvet ant. For these reasons, it’s surprising to encounter something that sticks out so noticeably yet poses no harm. Perhaps some of the largest that fall into this category are the turquoise eggs of the great tinamou (Tinamus major). While hiking along the foothills of the Cordillera Talamanca, I saw several of these eggs in the crux of a buttress, openly exposed on top of the drab leaf litter. And to my surprise, something even brighter lay right above. With striking yellow marks and matte black scales, it was a bird snake (Phrynonax [Pseustes] poecilonotus)— caught in the process of swallowing two of the eggs.
For a predator without any limbs, the round and slick surface of eggs are challenging to handle. Snakes often arch their necks and push down into the substrate for sufficient traction. Their lower jaw uncouples as the egg fills the mouth completely, and a tubular external opening that connects to the trachea protrudes to allow for breathing throughout the endeavor. By the time I had arrived, the snake had just finished swallowing one egg, and it moved its snout to the left and right to squeeze the egg further down its body. I thought it was so cool that as the snake’s body wall stretched to accommodate the egg, the egg’s bright colors were still visible from inside, even under such dim ambient light conditions. In a beautiful demonstration of the power of its musculature, the snake flexed its body rapidly like a rope being pulled from two directions, crushing the egg in place. Just a few centimeters down further, I could pick out evidence of yet another recently crushed egg. In many egg-eating snakes, once all the nutritious goop is digested, they will regurgitate the fragments of calcified eggshell (but be sure to check out the unique feeding behavior of kukri snakes!). Unfortunately, the bird snake was perturbed by my presence and halted any further egg consumption or shell regurgitation, so after a period of time I left in hopes that it would finish the entirety of the clutch in peace.
Tinamous are well known for their unusual egg colors, varying widely across species, including blue, green, purple, pink, and dark brown. In the great tinamou (Tinamus major), the egg’s calcium carbonate surface is smooth and glossy, even weakly iridescent as it fluctuates from blue to green (489–514 nm) depending on the observer’s viewing angle. Why conspicuous egg coloration has evolved in this group is still uncertain, though looking to the life history and ecology of tinamous provides some clues. Female tinamous lay a single egg every couple of days, depositing it in an existing nest where eggs have previously been laid by other females. These communal nests are beneficial in that a female’s eggs will reside within a larger clutch, increasing the chance of individual egg survival in the event of predation. Male tinamous are solely responsible for attending and incubating the eggs, sitting atop to regulate their temperature and cloaking the bright eggs by his cryptic gray-brown plumage. Because the eggs are so conspicuous, it’s possible that their vulnerability to predators necessitates intensive parental care for survival, hastening the eggs’ incubation time and overall protection— in what has been called the ‘blackmail’ hypothesis. A more straightforward explanation is that egg color functions as a cue to females that are searching for communal nests. Egg color fades as development progresses, so females may also detect subtle differences in appearance to preferentially lay their eggs together with fresh clutches. Coordinating the timing of hatching may be advantageous because a male’s chance of clutch abandonment remains exceedingly low when no chicks yet departed.