To learn, we need to remember, and therefore, memory is essential. Evolutionary processes had to remember this axiom, equipping us (both ourselves and animals) with different types of memory that, modulated by varying degrees of motivation, enabled easier adaptation to environmental and social situations. The stakes were so high, however, that these processes also selected “innate predispositions to learning” to make everything easier and more immediate, and above all, more certain.
In social species, where there is parental and family continuity between generations, learning and adaptation are guided by the group. But consider those species in which there is no overlap between parents and children. To whom has the adaptive process entrusted the continuity of species existence in terms of behaviour and lifestyle, if not to genes and therefore instinct? In this way, even complex behaviors are identically transmitted across generations.
The European beewolf (Philanthus triangulum) and the Little Sand Wasp (Ammophila campestris) are two wasp-like insects (Hymenoptera Vespoidea Sphecidae) commonly found on the Dutch coast of the North Sea. These species provided Niko Tinbergen and Gerard Baerends with the opportunity to explore the capabilities of their memory.
Female Philanthus excavate nests in the ground to lay their eggs. A single burrow leads to a cluster of cells, each containing an egg. The females’ job is to provide nourishment for the larvae that will hatch. Unfortunately, this nourishment consists of bees, which they are expert hunters at ambushing while the bees are busy collecting nectar or pollen from flowers. Grabbing one with their legs, both fall to the ground, but as they roll down, the Philanthus expertly strikes its prey with its sting, paralyzing it (fig. 1). It then flies, holding the bee tightly, toward the nest, placing 3-4 paralyzed bees in each cell, which will feed the developing larvae. Left there, the larvae will complete their development, leaving the nest the following summer to “continue their mother’s work,” as K. von Frisch describes it.
A solitary Tinbergen, walking among the dunes of one of the Frisian Islands, wondered how the stingrays managed to find their nests amid the uniform dune landscape. He watched them arrive in flight and, after a few twists, descend decisively over the entrance to their burrows; but on those beaches, there were so many stingrays and so many nests, all reached without hesitation. Sight? Smell? Senses that were certainly not foreign to them? But what, among the sparse tufts of halophilous grasses and driftwood, constituted their reference points, and what were the scent trails that the winds never tired of confusing?
The first step was to clear a few square meters around some nests of every possible creature or plant. The result was enlightening: the filanto, which had detached itself from the nest with a few spirals before cleaning, appeared disoriented upon returning with the prey. It fluttered, landed, and flew again with the prey tightly grasped until it released it to begin searching nearby by pawing at the sand. In some cases, it was lucky enough to find the hole in the tunnel; in others, it gave up, exhausted.
Let’s now look at image 2. Among the washed-up debris were many pine cones, and Tinbergen used them to make a crown around a nest while the female was inside placing a bee in a cell. When she emerged again, she made a few exploratory circles around the area and then went off to hunt for new prey. He immediately moved the crown of pine cones a short distance away, and when the female returned, she headed resolutely for a spot inside the crown, where she obviously found no entrance to the nest.
Image from The Foundations of Ethology, by Irenaus Eibl-Eibesfeldt, Adelphi, 1995.
The mystery was solved with a simple experiment, which brought with it an important corollary: the emerging philanthus was attentive to the details surrounding the nest, which it memorized to then orient itself toward it, easily finding it again. What it was doing was a process of spatial localization based on details of the landscape: a small insect of a couple of centimeters was capable of using the same orientation strategy as organisms of far greater structural complexity, such as birds, reptiles, or ourselves. A strategy we today call “piloting,” based on a process of learning and subsequent memorization.
Tinbergen passed on to his student Baerends his passion for predatory solitary wasps, introducing him to the study of sand wasps, which, although very similar to Phyllanthus, have a much more complex breeding behavior.
This time, its favorite prey is butterfly caterpillars, which it captures and paralyzes by dragging them to the nest dug in the ground, sometimes walking long distances (Image 3). It also uses a navigation strategy to find the nest, relying on details of the surrounding landscape. Once it reaches the nest, it introduces the caterpillar, lays an egg on it, then emerges and seals it with pebbles (Image 4). It typically digs two or three more nests, repeating the same process as the first. After some time, it alternates removing the pebbles to enter the burrows and monitor the development of the eggs, now larvae, and, if necessary, supplies them with more caterpillars, which they then devour. Morning visits to the nests continue as the larvae develop, each regularly supplied with the right number of caterpillars it needs. This isn’t a routine, blind operation, but the food (expensive for the mother to catch!) is calibrated to each individual’s actual needs. If the caterpillars haven’t been consumed yet, she closes the burrow and ignores them for the day; otherwise, she adds more. This process continues until the larvae begin to spin cocoons and pupate, ceasing to feed. The mother will never visit those nests again or see her daughters again. Their genes contain all the information needed to continue their typical offspring-rearing behavior the following year.
Baerends wondered how the wasp managed to deliver the right amount of food to each nest without making errors. He observed this with great patience and skill, personally removing or replenishing caterpillars in the various nests. As he verified, every morning the wasp visited the nests before hunting for caterpillars, checking whether and how many were needed for each nest. Then it set off on the hunt. This was a remarkable memory exercise, but if Baerends modified the nests’ caterpillar content before the visit, the mother wasp correctly took this into account. However, if he modified the contents after the morning visit, the mother wasp was unaware of the change and did not adjust her behavior: she would stuff a nest she had found empty but had already been filled with larvae placed by Baerends.
Image from The Foundations of Ethology, Irenaus Eibl-Eibesfeldt, Adelphi, 1995.
His research highlighted several important facts at a time when ethological disciplines were growing. First, female Ammophila wasps had an instinctive organization of their nest-building and offspring-rearing behavior. The individual actions they performed were organized according to a rigid and repetitive hierarchy: digging the nest – capturing prey – transporting it – opening the nest – dragging the caterpillar back – placing it in the cell – laying the egg – exiting and closing it. This indicated the existence of an internal program of action that was fixed and immutable, generation after generation, and genetically controlled.
Even caring for the larvae followed a fixed pattern of actions, with the added task of inspecting and memorizing the contents of the caterpillars. However, since external interventions were not anticipated by the adaptive reality, there was no need for rechecking; it had already been done, no more energy could be wasted, and no one could open the nests and shuffle the cards! The behavior of the Ammophila, thus generated by adaptation, was sufficient to perpetuate its species. Baerends’ experimental interventions verified the wasp’s behavioral patterns, particularly its remarkable memory capacity, which was sufficient to guide its predation on the caterpillars: a memory imprint lasting up to 15 hours, as Baerends discovered, equivalent to the hours of a summer day, their hunting season. Evolutionary miracles…
Simple experiments, observations in nature, and the exploration of phyletic relationships: this was how ethological sciences made a brilliant start in the decade preceding the Second World War. When I arrived in the Netherlands in 1971 as an “apprentice ethologist,” Tinbergen was a Nobel Prize nominee, and Baerends, director of the Institute of Zoology at the University of Groningen, welcomed me to his institute to share with me a lifelong connection to animal behavior.
Credits
Author: N. Emilio Baldaccini, Former Professor of Ethology and Conservation of Zoocenotic Resources at the University of Pisa. He has published over 300 scientific papers in national and international journals. Actively engaged in scientific education, he is also a co-author of academic textbooks on Ethology, General and Systematic Zoology, and Comparative Anatomy.
Translated by Maria Antonietta Sessa
