Nature's Nap Time

Cold winter temperatures cause many animals to rely on different forms of hibernation.

As fall turns to winter, many animals begin to alter their patterns as the days become shorter, temperatures turn colder, and food becomes scarce. Depending on the species and their environment, there are several strategies that different animals can use to overwinter. Current thinking is that these strategies are not necessarily distinctly separate, but rather lie along a single spectrum. 

True hibernation or dormancy is a state of nearly complete reduced activity defined by a lower body temperature and metabolic rate. A loud noise or movement won’t wake an animal in this state.  However, not all animals undergo a true hibernation, especially in warmer climates areas like central Texas.  Native bumblebees are an example of true hibernators in our area, as they seek shelter in a hole in the soil, under leaf litter, or in hollow plant stems.

Native bumblebees are true winter hibernators in Central Texas.

Brumation is another hibernation-like state exhibited by several cold-blooded reptiles and amphibians such as snakes, lizards, turtles, and fish.  This state can last for months and is triggered by colder temperatures and shorter days. These animals will den in rock crevices or dirt burrows, sometimes alone, but often in groups as in the case of garter snakes. While their body temperature, heart rate, and respiratory rates are greatly reduced, some animals in this state may awaken periodically to drink water.

In winter, Black-necked Garter Snakes often den in groups.

This Green Anole came out of brumation on a warmer winter day.

Not as deep as hibernation, topor is a state that can last for several days or weeks. This light hibernation lasts for short periods of time, allowing animals to wake up on warmer winter days. Such is the case for ground squirrel species such as the Rock Squirrel. Some bird species, such as hummingbirds, go into torpor nearly every night to conserve energy, and can even enter a torpid state during colder days.

Rock Squirrels enter a state of torpor during the colder winter months.

While there are different types of hibernation strategies, hibernation states can also be categorized by the factors that induce that state.  Facultative hibernation in animals is caused by environmental conditions such as lack of food, short daylight hours, cold temperatures, or a combination of these stressors. This can induce short or longer periods of topor throughout the colder season. Obligate hibernation applies to animals that hibernate spontaneously every year for long or short periods, regardless of temperature or food supply. 

Most animals build up fat reserves in the fall to survive the colder winter periods in which they are dormant or less active. However, climate change may disrupt these patterns, as drier summers, warmer autumns, and long-standing droughts can lead to lower food production.  This may cause animals to enter hibernation states without the stored calories they need, and emerge early from hibernation hungry for food that is not yet available.  These types of timing mismatches will become more prevalent as our planet warms, disrupting the rhythms of nature’s nap time.

Observing Orbweavers

Drops of morning dew sparkle on a spider's web.

The family of spiders known for building flat, spiraled, wheel-shaped webs are called Araneidae, or the orb-weavers.  They are the most common group of spiders often founds in yards, gardens, fields, and forests.  In our area, those include the Black & Yellow Garden Spider (Argiope aurantia), Lichenmarked Orbweaver (Araneus bicentenarius), and Spinybacked Orbweaver (Gasteracantha cancriformis).  While all of these spiders produce venom, that venom is harmless to humans but helps them to immobilize their insect prey.

Also called the Yellow Garden Spider, the Black & Yellow Garden Spider is a large spider that has an abdomen with a series of pale, yellow spots on a black background, and legs that are orange at the base, black near the tips, and often held in pairs. Females are much larger than males, with up to an inch in body length. While both appear in late summer, females can be seen through December in our area.

A Female Black & Yellow Garden Spider.

While not quite as common, the Lichenmarked Orbweaver has an abdomen with a pattern of light green and reddish-brown markings and two small humps on front.  Females can grow to one inch in body length. Its legs have alternating bands of orange and brownish-black, and the coloration on its abdomen acts as a natural camouflage against the lichen-covered branches of trees where it often rests.

Lichenmarked Orbweaver, showing its characteristic green abdomen with two humps.

Often called crab spiders, one of our most distinctive spiders is the Spinybacked Orbweaver, a small, half-inch wide spider with a hard exoskeleton that has six spines around the edge.  There are many color forms, but most are yellow or white with black or dark red spines. Males are tiny and seldom seen, but females can be seen year-round. They build their webs in the morning in shrubs or low tree branches.

Spinybacked Orbweaver, yellow form.

Spinybacked Orbweaver, white form, showing relative size.

Orb-weaving spiders are one of nature’s engineers, and their process to build a web is intriguing.  They start by floating a line of silk on the wind to another surface. Once that line is secured, they drop another line from the center, forming a Y.  The rest of the web is a series of non-sticky spirals ending with a final spiral made of sticky capture silk.  The non-sticky webbing is nearly invisible, but the more visible sticky webbing is what normally traps the spider’s prey.  As such, there is always a tradeoff between the visibility and stickiness of the web.

The webs built by some orb-weavers include a vertical zigzag band of highly visible silk through the center of the web.  This is called the stabilimentum, and is most often found in the web of a Black & Yellow Garden Spider. While scientists are unsure of the exact purpose of this structure, theories include it posing as a lure to bring prey to the web, a warning for birds not to fly through the web, or as a way to camouflage the spider as it sits on the web.

A male Black & Yellow Garden Spider
rests on the web's stabilimentum.

Many orb-weavers build a new web on a daily basis.  They often hide during the day and become more active in the evening, when they consume the old web, rest for a bit, then begin spinning a new web in the same general location. Consider all that hard work the next time you think about destroying a web!

 

Migration Isn't Just For The Birds

 

Wandering Glider dragonflies do more than wander...

Most people who are interested in nature know that each spring many species of birds migrate north, and each fall they migrate south.  But birds aren’t the only animals that participate in this amazing natural phenomenon, as even some insects (other than the well-known Monarch butterfly) participate in long-distance migrations as well.  While insects are the most species-rich and abundant group of macroscopic organisms on the planet, understanding many aspects of their annual cycles and behavior drastically trails behind the ability to identify and classify them. 

Migration is defined as the cyclical travel of an animal as it returns to its place of departure.  It can be accomplished in one or multiple generations.  Some animals don’t migrate at all, and others move only short distances south or north.  While the guiding factors for navigation are still being discovered, migration is typically triggered by local climate, food availability, seasonality, or reproduction.

Adult Common Green Darner

Interestingly, about nine dragonfly species are known to migrate, including the Common Green Darner (Anax junius), Wandering Glider (Pantala flavescens), as well as some species in the Skimmer family such as meadowhawks and saddlebags. These dragonflies are known to travel up to 1500 miles over the full cycle of their migration, taking three generations.

Adult Wandering Glider

The secrets of dragonfly migration were discovered and confirmed by biologists measuring the chemical makeup of their wings.  In their immature or nymph stage, dragonflies grow their wings while still developing in local water.  Because the stable hydrogen isotope concentration varies in concentration by latitude, biologists can estimate the geographic region where adult dragonflies developed as nymphs.  

The female Common Green Darner (bottom) is held by
the male (top) after mating, when ovipositing in water.

From February to August the first generation emerges in the south and migrates north.  From June to October the second generation emerges in the north and migrates south.  Both of these generations have the ability to undergo rapid development from egg to adult.  But from November to March the third generation emerges in the south and is non-migratory, focusing mainly on producing another first generation for the cycle to start again.  It is this generation that employs a different strategy, developing slowly in late fall, entering diapause or actively induced dormancy over the winter, and emerging in early spring.

Dragonfly migration was not well understood until very recently.  Research published in 2018 detailed the journeys of Common Green Darners by analyzing 852 wing samples from eight different countries and utilizing 21 years of citizen scientist observations to link each specimen to its place of origin and establish its migration history. However, it is important to note that the timing of migration and the development of nymphs are both highly temperature dependent, so continued climate change could lead to fundamental changes in the migration cycles for dragonflies and other migrating insect species. So as you can see, migration isn’t just for the birds!