Examples of Melipotis cellaris
Why do individuals of some animal species look practically identical while others are so variable as to fool taxonomists into thinking they aren't that closely related? As someone who identifies insects and spiders on a regular basis, I can't help but notice that certain spiders, moths or grasshoppers come in a wide range of colors and even sizes. In fact, within certain groups, variability is the rule and that factors into my recognition process when I encounter an unfamiliar species.
Many factors contribute to the evolutionary pressures that shape any particular type of animal. One of the most familiar and well understood cases involves the Domestic Dog (Canis familiaris). Because our canine friends are so familiar, we learn at an early age that, in spite of their amazingly different appearance, Yorkshire Terriers, Greyhounds and Old English Sheepdogs belong to the same species. We take it for granted, especially because the reason for the diversity is so well documented: humans selectively bred the animals. Many of our domesticated pets and livestock have been pushed to their biological limits of variability as we, rather than nature, became the force that decided which ones reproduced. Whether it is pigeons and goldfish for the sheer delight of the exotic, or horses and cattle for practical concerns having to do with labor and food, we have been messing with genetic engineering for an awfully long time.
But humans are not the only thing that can push or lead a species to be variable. Although we might think that our social pressures are exceptional, those of certain wasps can make adolescent angst seem rather overrated.
Paper wasps in the genus Polistes live in small to medium-sized colonies and have either one or more foundresses, the queens that have mated and produce the eggs that become workers. Some Polistes species have only one queen per colony, while others almost always have several. Then there are those species which are flexible and can have either one or more. This last group includes a few species in which quick recognition is important to reducing aggression between potentially competing individuals who might attempt to move up in the hierarchy, say from simply feeding the queen's young to producing eggs of their own. Due to the plasticity of status, competition is more common between closely ranked individuals while very high and very low wasps don't bother to test their limits with each other. In order to avoid unnecessary aggression, the wasps need a quick way to know who is who in the nest. Enter variable markings that help with facial recognition. Yes, some of these wasps know their sisters by sight, as each wasp has distinctive markings on its head. A curious result of this capacity is that the wasps guarding a nest quickly learn to recognize different humans as well; an observer who keeps their distance evokes no reaction after just a few encounters while a stranger puts the whole colony on alert. Less amusing is that identifying certain Polistes species is a real pain in the behind because they all look different, sometimes even resembling other species.
While the wasp example is interesting, it is not the most common reason for variability among individuals in the insect world. It has been said that insects and, in fact, many wild animals, almost never die of old age. There are too many dangers that can lead to an early demise and one of the most pervasive is predation. As humans we rarely have to worry about becoming a meal for another organism. Arthropods, on the other hand, are not so carefree. A great deal of evolutionary pressure on small organisms results from the need to stay alive long enough to reproduce and it is definitely a bug-eat-bug world out there.
Among the many defenses that potential prey species exhibit, camouflage ranks pretty high. It takes less energy to sit in one place than to run away. If, by staying still, an insect can avoid detection, then it definitely behooves it to do just that. Blending in with the background, though, is sometimes trickier than we think. An animal cannot remain in one ideal spot for its whole life. Of course it needs to move about while foraging, finding water or a better temperature, or locating a mate. So any single individual will end up in a range of situations from optimum to disastrous. If they happen to be in the wrong place at the wrong time and a predator sees them, they are toast.
There are ways, though, that the odds can be nudged in the direction of survival. Most moths have short adult life spans. Two weeks is a decent average. If many individuals emerge at the same time, they then all need to hide during the day when their diurnal hunters will be seeking them. A common technique among predators such as birds and wasps is to learn what a specific prey species looks like and then use that as a search image. The hunter usually gets pretty good at this because they are putting in a lot of time doing one thing: finding food. For some delectable prey species, the chance of survival is greater if there is a wide variation in appearance because if one is detected, the next, which doesn't look the same, might not be.
Variability can aid a species' survival in one more way that works well for gregarious caterpillars. While a few kinds of caterpillars can eat almost any sort of plant, the majority of lepidoptera species are limited to only one type. This is known as their host and various species of caterpillars have evolved incredible forms of camouflage to match their plant while they feed. An alternative strategy, though, is to be so variable that a predator would miss at least some of a group because they look so different that they aren't noticed as part of the target search image. There are a couple of sphinx moths in our area that illustrate this, especially the Banded Sphinx (Eumorpha fasciatus). The larvae feed on primrose and present a delightful array of colors, all of which tend to blend in with the red stems and green or yellowed leaves.
In a complex habitat, the chances of an active insect finding a place where it blends in is greater if each member of the species does not need the exact same location. Variable moths are often quite capable of assessing the suitability of their surroundings. A moth flushed out in a field and then watched as it moves to new resting spots displays behavior that looks unmistakably like it is searching for the perfect location. The habits of grasshoppers tend to lead them to seek shelter where they best blend in. Pursued spiders scoot around and reposition themselves as if they are very aware of trying to find the best hiding place as quickly as possible. Some spiders can even gradually change their color to match their surroundings, which helps not only in evading their own predators but in remaining invisible to their prey.
 Normal and outbreak examples of the Red Katydid |
Since the diversity of life on our planet is so ubiquitous, and there is just so much of it, variation within a particular species may not even be noticed by a casual observer. But, as with many things that require a bit of effort, curiosity and attentiveness yield rewards. One of these is a better understanding of just how the world that surrounds us actually operates. Another is the delight we can experience when learning something new.