What causes the variation in inherited traits

Genes and Environment: How Do They Affect Behavior and Physiology in Songbirds?

Research report 2013 - Max Planck Institute for Ornithology

Behavioral Neurobiology Department
Environmental factors can strongly influence behavior and physiology. For example, Canary Islands girls breed prematurely if green plants are offered to them despite the short length of the day. In the zebra finch, song and the underlying brain structures show little heredity and react strongly to changing environmental conditions, while the size of the brain depends on the interaction between genes and the environment. In this way, a high level of genetic variability can be maintained. These results show the great importance of environmental factors for behavior and neural development in songbirds.

Influence of genes and the environment on behavior

The question of the proportions to which behavior is genetically determined or shaped by environmental influences has long been a central issue in behavioral research. Today we know that our personality and our behavior are determined far less strongly by genes than was previously assumed. Especially during youth development, environmental factors can have a decisive influence on the formation of the brain and behavior through so-called epigenetic effects. Epigenetic mechanisms regulate the extent to which the genes are activated by modifying the genes or chromatin. Whether the influence of the environment is strong or weak can, however, depend crucially on genetic predisposition. Hereditary and environmental factors are therefore still difficult to tell apart.

Influence of environmental factors on breeding activity in Canary Islands girls

Environmental factors can fundamentally influence defined behavior. For example, the seasonal change in the length of the day, also known as the photoperiod, controls the annual rhythm of animals in the temperate zones and enables them to reproduce in times of favorable environmental conditions. This is important for the rearing of the offspring and so crucial for biological fitness [1]. A very well-studied phenomenon is the timing of the start of breeding in birds: The increasing day length in spring causes an increase in the concentration of sex hormones in the blood. We speak of the photoperiod as a proximate factor, as this alone is sufficient to trigger fundamental changes in behavior and physiology. Seasonal breeding bird species then use additional factors such as food supply, rainfall and social interactions to fine-tune their breeding activity. However, under favorable conditions, these additional factors can also trigger breeding activity on their own. For example, Stefan Leitner and his team at Kanarengirlitzen (Serinus canaria, Fig. 1) show experimentally that the presence of growing green plants is sufficient to trigger breeding activity during the shortest days of the year - up to two months before the actual start of breeding! Normally, the birds do not breed on such short days (so-called short days). In order to maintain this effect, however, the animals must be guaranteed full access to the growing green plants under short-day conditions (Fig. 2). If the animals are only offered individual components of a plant, such as the smell of plants as an olfactory stimulus or artificial plants as a visual stimulus, there is no premature breeding activity [2]. However, not all observed animals start breeding at the same time, so they react differently to environmental stimuli. This reflects a variation in the characteristic expression (phenotypic variation) in behavior and in physiology.

Causes of Phenotypic Variation

If one wants to explain how genetic variation is maintained in a trait even though the trait is subject to selection forces, one has to investigate the causes of the phenotypic variation of a behavioral trait [3]. In order to determine how the complex chants of songbirds evolved, we need to measure the genetic variation and the nature of the environmental factors responsible for the expression of the chants, as well as any interrelationships between the two. The determination of such genotype-environment interactions (GxE) is important because they show the range of characteristic expression that a genotype can show under changing environmental conditions and determine whether a characteristic such as birdsong can signal the hereditary characteristics of an individual (see [4] for an overview). Thus, songbirds are an important model for studying behavioral and brain correlations. At the zebra finch (Taeniopygia guttata) For example, essential singing parameters such as the number of syllables or the length of the singing phrases are less heritable and are more dependent on changing environmental conditions [5]. Brain structures that are responsible for controlling singing, on the other hand, are more hereditary [6].

These results from earlier studies suggest that the song structure and the so-called song nuclei on which it is based in the brain depend on a combination of changing environmental conditions and the genetic makeup of an animal. However, there is a possibility that certain genotypes react more strongly than others to unfavorable environmental conditions. developmental stress) was shown. This hypothesis states that the developing organism draws resources from organs such as the developing brain under unfavorable environmental conditions during youth development, such as food shortages, and is more likely to be involved in the development of vital body functions [7]. The singing thus represents an "honest signal" for the recipient to indicate that the sender is in good physical condition. To be honest, a signal like birdsong in the individual must be supported by appropriate physiological foundations. For example, a weakened animal would not be able to produce an elaborate song. In general, such signals are expected to be costly to the broadcaster in order to keep them honest. So although the importance of genetic and environmental influences on song learning in songbirds has been investigated several times over the past forty years, there has been no study to date that has examined the influence of both factors on the neural control of song.

Investigation of the interaction of genetics and environmental factors in zebra finches

In a long-term experiment on zebra finches, the genotype-environment interactions were investigated together with colleagues from England and Australia. In this experiment, two important environmental factors were changed: the availability of food during adolescence development and the acoustic environment, which is important for young males to learn to sing. In order to determine the respective contribution of the genotype and the environmental conditions that are responsible for possible differences in brain and song development, a so-called partial cross-fostering Approach chosen. Half of a clutch is exchanged and pushed under other parents so that half of the eggs in each nest consist of “cuckoo eggs”, so to speak. Thus half of the hatched young birds grew up with their own parents, while the other half was raised by stepparents (Fig. 3). In half of the couples, the amount of grain feed available was restricted by the controlled addition of worthless husks (so-called feed stress). In this way, the parent animals have to spend longer searching for food and have less time to look after and feed the young. In addition, the males have less time to sing, which could affect the vocal development of young males. A food restriction of the parent animals was necessary in order to produce a greater and, above all, controlled variation in the environmental conditions. Apart from the acoustic environment, i.e. the assignment of the young animals to the singing of their own or the stepfather, the remaining, rather unspecific environmental factors such as feeding rates, nest temperature or actual singing activity of the fathers cannot be changed in a controlled manner in the experiment. In fact, the food shortage had an impact on the development of the young animals [8; 9]: Females who were exposed to feed stress during the rearing phase were less active in partner selection trials than females who grew up normally. However, these females preferred the songs of male zebra finches in the same way as normally reared females.

Effects on vocal and brain structure

In the long-term experiment, a comparison of the blood plasma concentrations of the stress hormone corticosterone initially showed slightly increased values ​​in male and female offspring who grew up under feeding stress conditions. Figure 4 shows the proportions that the genetic origin, the rearing conditions and the interactions between genotype and feed stress contribute to the development of phenotypic variation. These proportions were calculated both by comparing two brothers who grew up in different nests and by comparing fathers and sons.