Climate Change’s Profound Effects on Eggs

When you think of an egg, what do you see in your mind’s eye? A chicken egg, hard-boiled? A mermaid’s purse, the egg of a shark or ray, entangled in seaweed thrown onto shore? Perhaps you see a human egg cell, prepared on a microscope slide telegraphed onto a TV screen in a laboratory? Or the majestic turquoise mottled eggs of the blue jay?

Each egg is unique, and that is one of the finest things about them. Each egg on Earth has its own charisma, allure, and evolutionary backstory, easily (I have learned from years of zoological research) as diverse and interesting as the animals that hatch from out of them. Every egg there has ever been, is an emblem of survival; a product whittled, chiselled, and crafted by the unthinking forces of natural selection for the purpose of passing genetic lineages forwards in time—days, weeks, months, sometimes years.

In the last 500 million years, eggs (like animals) have come a long way. As geological periods came and went, as climates changed, ecological disasters raged and animal fortunes waxed and waned, the egg has changed. In some animal groups, including birds, the egg cemented itself in a crystalline shell to harden itself to planet’s changing atmosphere. In others, most notably mammals and large sharks, the egg resided for longer and longer inside the body to stay warm. In some groups, like spiders, eggs were wrapped in silk; in others, such as insects, the egg was endowed with a breathable suit-of-armour that helped insects quietly pioneer even the driest and most inhospitable parts of Earth’s continents.

Eggs, I have learned, are survival machines unthinkingly sculpted by what the planet throws their way. And so, in the coming decades and centuries, what will this (our) impending period of climate change do to these astonishingly adaptive life-vessels?

There is some evidence that changes to eggs are already occurring.

Today, because of our changing climate, the eggs of insects, predictably, are adapting most quickly. In the UK, for instance, butterflies and moths hatch up to six days earlier than they did just ten years ago. In the U.S., wild bee activity begins 10 days earlier than it did 130 years ago. Many aphids, considered a pest upon trees and other plants, now hatch from their eggs a month earlier than they did half a century ago. Fast generation times and variation between insect populations is seeing natural selection work quickly in these species. Insect distributions are also altering because of human-induced climate change. Traditionally, cold winters (which kill off insect eggs) were a natural barrier to the movement of invasive species, but milder winters have seen their spread across countries and continents continue apace. This is why, entomologists argue, mosquitoes have brought new diseases to the European continent in the last decade, including dengue fever to France and Croatia, chikungunya in Italy and malaria in Greece.

Read More: How Climate Change Is Leading to an ‘Ecological Recession’

Mild winters are affecting other problematic invertebrates, too. In the U.S., cases of Lyme disease, spread through blood-sucking ticks whose eggs are no longer killed off in the winter months to the same degree, have increased three-fold in 25 years. There are similar concerns over populations of tree-munching ash borers and the invasive Asian giant hornet, a potential threat to honeybee populations across Europe and North America. As they have always done, invertebrate eggs are adapting quickly, sometimes unthinkingly profiting from the climate crisis.

Although their life history plays out at a slower pace, the life-history patterns of land vertebrates have also begun to shift in recent decades in response to shifts in climate. Information on birds, which have a long history of amateur study, best show the changes. Most notably it is very clear that, like insects, many bird species are adjusting the moment at which their eggs are laid and at which point they hatch, potentially to align themselves better with the seasonal availability of food. In North America, roughly a third of birds lay their eggs earlier, by about twenty-five days, than they did a century ago. In the UK, between 1971 and 1995, 63% of bird species nested earlier, by an average of 9 days.

Perhaps a more pernicious danger to vertebrate eggs today is that posed by extreme weather events. In sea turtles, for instance, we may point to warmer temperatures and thus more of their population turning female (a strange quirk of turtle embryos is that their sex is determined by temperature), but unpredictable storms and the flooding of shoreline nests is perhaps the far greater risk. Hurricane Irma in 2017, for instance, saw 56% of green turtle nests and 24% of loggerhead turtle nests on the coast of Florida destroyed. In 2019, Hurricane Floyd killed up to 100,000 turtle hatchlings in one fell swoop. The truth is that, for many species, there may not be time for the egg to adapt to environmental changes so sharp and jagged as this.

There are likely to be both many losers and perhaps a few winners in these turbulent times. The wolf spider, Pardosa glacialis, may become one of those that benefit from climate change, for example. In 1996, this tundra-living spider used to lay one clutch of eggs each year, but now it takes advantage of a longer spring and summer and regularly lays a second clutch of eggs later in the season, often with more eggs than the first. Because the spider has more months in which it can hunt, the size of adult spiders at the end of their season has also increased. The significance of just this one change to an ecosystem could be far reaching: a single square kilometer of tundra can be home to 1 million of these ground predators—spider-eating birds, perhaps, have a lot to gain. Finding a way to study these populations, to record ecosystem shifts happening due to climate change, will prove crucial to the human response.

For decades, museums have been one of the best tools we have to measure the impact that the climate crisis is having on eggs and the organisms that hatch from out of them. Globally, there are specimens, records, and data from 5 million bird eggs collected from as far back as 250 years. Eggs are different to other museum specimens. Carefully stored, they do not rot; they require very little by way of preservative fluids and, within each specimen is a chemical record, through fragments of DNA, of breeding biology and diet. Nowadays eggs can be scanned with electron microscopes and spectrophotometers; they can be genetically sequenced or scraped of their isotopes for carbon dating; they can be scanned and stored and shared in a digital format. There may be no more perfect kind of specimen in the world from which to gather data about our changing planet.

It was museum eggs, of course, that first helped scientists to prove the link between heavy DDT (dichlorodiphenyltrichloroethane) use and declining bird populations, a story detailed so eloquently in Rachel Carson’s landmark Silent Spring. By comparing eggshells from the late 1940s to the 1960s, when DDT use was high, with museum-curated eggshells before this time, scientists were able to show that DDT was pooling in birds, particularly those higher in the food chain such as birds of prey. This caused them to lay easily broken eggs with thin shells, causing wild populations to decline. The result was the agricultural use of DDT being banned in most developed countries. Over the years, other studies have seen environmental changes linked to eggshell thinning. Most notable are those that firmed up the causal link between ‘acid rain’ and eggshell malformation in birds. Studies have shown a long and drawn-out reduction in eggshell thickness in blackbirds, song thrushes and mistle thrushes, for instance, all linked to acidification of their lowland environments, mostly through pollutants such as sulphur dioxide and nitrogen oxide. The main problem appears to be that acidification, which removes calcium carbonate from the soil, leads to a reduction in the number of snails, whose shells provide calcium for nesting birds readying themselves to lay. In each case, museum specimens act as the vital comparison group for the data that scientists collect from the field.

Eggs may, in part, help us better understand environmental changes, but the clock is clearly ticking. The agreements arising from global climate conferences, useful as they are, have so far failed to budge carbon dioxide emissions from their upward trajectory. Eggs cannot arrange summits or conferences and they make lousy politicians, but, I suspect, if we observe them more closely, take them more seriously, celebrate them where we can, they would have plenty more to tell us about how and why we might limit the impacts of the raging climate change we are now inflicting upon our world.

That an extinction crisis is approaching is obvious. The question is becoming, at this rate and trajectory of change, what kind of world do we want on the other side, in the next chapter? What level of impoverishment will we tolerate? How much suffering are we comfortable with?

The mud and silts laid down today will record the trials and evolutionary experiments of new eggs, as they always have. Baked and dried, hidden for millennia, the fossil strata will become the pages upon which the story of eggs is written in future chapters. As long as our nearest star shines, I have no doubt that there will be eggs on Earth. Their journey will never be finished.

I hope we will be around as long as possible to see it continue.

Adapted from Infinite Life: The Revolutionary Story of Eggs, Evolution, and Life on Earth by Jules Howard. Published by Pegasus Books, September 3rd 2024.