The relationship between partners, filled with care, signs of attention and empathy, is called love by poets, but biologists call it an inter-gender relationship aimed at survival and procreation. Some species prefer to take in quantity - to breed with as many partners as possible in order to increase their offspring, thereby increasing the chances of survival of the entire species. Others create monogamous couples, which can cease to exist only after the death of one of the partners. For many years, scientists believed that the first option is much more profitable, but this is not entirely true. Monogamous couples, as a rule, raise offspring together, i.e. protect him from predators, get food and teach certain skills, while in polygamous relationships all this most often falls on the fragile shoulders of females. Of course, there are exceptions, but today is not about them. Biologists have long been interested in another amusing moment - males continue to show signs of attention to females, even when their pair has already been formed and has existed for several years. What is the reason for such behavior, what is the benefit from it, and what evolutionary aspects are associated with it? We will find answers to these questions in the report of the research group. Go.
Research basis
Given the subject of the study, we will not focus on polygamous species of birds, but will focus on feathered romantics who fall in love once and for all.
Speaking of monogamy, it is worth noting that there are several types of monogamy depending on the duration: one season, several years, and for life.
Among birds, seasonal monogamy is the most common. Wild geese are a prime example. The females nest and incubate the eggs, while the male guards the territory. On the second day after hatching, the family goes to the nearest reservoir, where the goslings learn to look for food. In case of danger on the water, the female fiercely protects the offspring, but the male, apparently remembering important matters, most often flees. Not the most ideal relationship, anyway.
Family of wild geese.
If we talk about relationships, the basis of which is constancy, then storks are the best in this matter. They create a monogamous couple for life and do not even change their place of residence without special need. One nest of storks, which can weigh up to 250 kg and reach 1.5 m in diameter, serves them for many years if natural disasters or human intervention do not destroy it. In the Czech Republic there is a nest that was created back in 1864.
The construction skills of storks do not need to be assessed when you see such structures.
Unlike wild geese, storks have equal responsibilities: both partners incubate eggs, look for food, teach offspring to fly and protect them from danger. An important role in the relationship of storks is played by various kinds of rituals: singing, dancing, etc. The most curious thing is that these rituals are carried out not only during the formation of a pair (on the first date), but throughout their life together (even when replacing the female during incubation, the male performs a small dance). To us, this looks very sweet, romantic and completely illogical, since from a biological point of view there is no benefit to such behavior. It is so? And here you can smoothly begin to consider the study itself, which was supposed to answer this question.
Ethologists* believe that the constant manifestation of their feelings by males is associated with the preservation of the reproductive state in females.
Ethology* - a science that studies genetically determined behavior, i.e. instincts.
At the same time, it remains unclear why such behavior lasts not only during the period of primary mating, but throughout life, because it would be more logical for males to invest more strength and energy in offspring, and not in demonstrating feelings for a female. To date, many researchers have believed that the intensity of the expression of affection for the female directly affects the quality of mating and, consequently, offspring (ie, the number of eggs laid).
A male bird of paradise dances in front of a female. As we can see, the male looks much brighter than the female.
This theory is supported by observations. The female, whose partner is an unwritten handsome man and the first flyer in the village, puts more effort into the offspring than if the male is neither fish nor meat. It sounds funny and funny, but the rituals that males perform in front of females are aimed at demonstrating not only beauty, but also strength. It just so happened that bright plumage, beautiful singing and other manifestations of attention from males are just cognitive signals for females, which she decodes into information about the male.
Scientists from the Universities of North Carolina and Chicago, whose work we are reviewing today, believe that such behavior of males is aimed at optimizing the behavior of females in relation to the breeding process.
The model proposed by scientists is based on numerous experiments that have shown that the increase in these signals from males increases the contribution of females to the process of procreation. There is an assumption that the source of such stimulatory effects are perceptual responses arising from the properties of the environment, signals and the nervous system as such. At the moment, about 100 examples of such "deviations" from the usual sensory systems (hearing, sight and smell) are known.
When a male once again demonstrates his advantages over other males, this can positively affect the male himself (the female will definitely choose him). But for the female, this can be a disadvantage, as it will reduce future reproductive realization. In other words, we have a "better than expected" situation. The male, who is much better than other males and constantly shows signs of interest in the female, will get what he wants - mating and procreation, or rather his kind. A female, who expects similar behavior from other males, but has not received it, may find herself in a deplorable situation. Such a case is referred to by scientists as an inter-gender conflict: the demonstration by males of themselves beautiful increases among the population, and resistance to this tactic grows among females.
This conflict was modeled using a computational approach (neural networks). In the models obtained, the signaler (signal source - male) uses the perceptual perception of the receiver (signal receiver - female), which stimulates the signals themselves to the detriment of perception. At a certain moment, there is a change in the perception of signals in the female population (a kind of mutation), from which the signal strength of the source (male) will greatly decrease. The gradual increase in such changes will lead to the fact that one or another type of signal will be completely ineffective. As such changes proceed, some signals disappear, having lost their strength, but new ones arise, and the process begins anew.
This highly convoluted system is quite simple in practice. Imagine that a male with a bright feather (one single) has appeared, he stands out from the others, and the females prefer him. Then a male appears with two bright feathers, then with three, and so on. But the strength of such a signal, due to its growth and propagation, begins to decrease proportionally. And then suddenly a male appears, who knows how to sing beautifully and build nests. As a result, beautiful plumage as a signal ceases to be effective and begins to degenerate.
However, there is always an exception to the rule - some inter-gender conflicts can develop into full-fledged and very effective inter-gender cooperation.
Scheme of the emergence of inter-gender conflict and inter-gender cooperation.
The bottom line is that the male with a more pronounced signal forces the female to lay not three eggs, but four. For the male, this is good - he will have more offspring with his gene pool. For the female - not very much, because she will have to spend more energy to ensure that all the offspring survive and reach an independent age. Consequently, females begin to develop in parallel with males in order to be more resistant to their signals. As a result, there can be two paths: conflict or cooperation.
In the case of cooperation, the females evolve to lay 3 eggs, as before the appearance of a stronger signal from the males, but continue to respond to these signals. So much for women's tricks in the natural world. Thus, not just a pair is formed, but a pair that supports each other at the optimal level for procreation in terms of signal-response interaction.
Males cannot evolve back, roughly speaking. Their increased signals to females result in a clutch of three eggs, i.e. not as expected. However, reducing the signal to the previous level will also be ineffective, as it will lead to a decrease in the number of eggs in the clutch to two. It turns out a vicious circle - males cannot reduce the signal strength and cannot increase it, since females in the first case will give fewer offspring, and in the second they will not respond.
Naturally, neither males nor females have any malicious intent or desire to enslave each other. This whole process takes place at the genetic level and is aimed solely for the benefit of the offspring of a single pair and the well-being of the species as a whole.
Results of the study
Using mathematical modeling, the scientists assessed the conditions under which intergender cooperation might occur. Quantitative characteristic with average value zf describes the main contribution of the female to her offspring. Initially, the mean is allowed to evolve to its optimal value. sopt, which depends on two variables: the benefit from the contribution (the number of surviving offspring) and the value of the contribution for females (cf). The last variable is estimated after breeding, implying that a proportion of females survive and may reproduce again the next year, resulting in an increase in the number of generations.
In this study, several terms will often be used, which are worth a little clarification:
- signals - manifestation of attention from males to female partners (singing, dancing and other rituals) that take place in formed pairs;
- contribution / investments - the response of females to these signals, manifested in the form of a larger number of eggs in the clutch, more time to care for future offspring, etc.;
- respondent - a female responding to the male's signals;
- costs β the value of the contribution of females to offspring (time in the nest, time to search for food, health status due to more / fewer eggs in the clutch, etc.).
New male signals and female responses to them were modeled by freely recombining diallelic single-locus modifiers, thus combining quantitative and population genetic approaches. IN locus*, which controls the response of the female (A), initially there is a high frequency of allele-responder* (A2), corresponding to pre-existing perceptual perception
Locus* - the location of a particular gene on the genetic map of the chromosome.
Alleles* - different forms of the same gene, located in the same loci of homologous chromosomes. Alleles determine the path of development of a particular trait.
Responder gene* (Rsp) is a gene functionally associated with the segregation disorder factor (SD gene), the active allele of which (Rsp+) is capable of suppressing SD expression.
The signal locus (B) is initially fixed for the allele without signal (B1). Then the B2 allele is introduced, which causes the appearance of signals from the males.
The manifestation of signals for males also has its price (sm), but increases the contribution of the female partner (A2) by Ξ±. For example, Ξ± can be expressed as an extra egg in the clutch. At the same time, an increase in the contribution of the female can also manifest itself in the form of the positive effects that she has on her offspring.
Therefore, a pair in which the male carries the signaler allele and the female carries the responder allele (i.e., the A2B2 pair) has an additional contribution from the female and hence higher fecundity than the other 3 combinations.
Variants of combinations of males and females according to the ratio of signals and responses to them.
The number of offspring surviving to breed in the following year is affected by density dependence* inside the brood and the dependence on the density of the brood after fledging.
Density dependence* Density-dependent processes occur when the growth rate of a population is controlled by the density of that population.
Another group of variables is associated with the mortality of females and males after the birth of offspring. These variables are determined by the contribution to the brood (cm - the contribution of males, cf is the contribution of females), the cost of signals for males (sm) and nonselective mortality (dm - males and df - females).
Widows, widowers, juveniles, and any previously single individuals unite to form new pairs, and the annual cycle is completed. In the model under study, the emphasis is on genetic monogamy; therefore, all types of sexual selection (i.e., competition between individuals for a partner) are excluded from the calculations.
Relationship between the evolution of signals, respondents and contributions.
Modeling has shown that a stable balance is achieved when males give signals and females respond to them. In a state of equilibrium, the entire contribution to the offspring is restored to the level that it was before the appearance of additional male signals.
On the chart Π The above shows an example of evolutionary dynamics where the contribution of females to the offspring returns to the optimal level, which is the result of the evolution of a quantitative sign of contribution (the dotted green line is the real contribution, and the solid green line is the contribution that was not realized due to the lack of response of females to additional male signals). On the chart Π an alternative example is shown, when an inter-gender conflict leads to the loss of a respondent.
And on the chart Π‘ two parameters are identified that affect this result: the increase in the contribution caused by additional signals (Ξ±), and the costs of females for this contribution (cf). In the red area on the chart, the signals never increase, as their cost will exceed the benefit. In the yellow and black areas, the frequency of signals increases, which leads to an increase in costly investment by females. In the yellow region, the response to this occurs by reducing the quantitative investment trait, which leads to permanent fixation of alleles of both signals and respondents. In the black region, where responding females have more induced investment, the responding allele is rapidly lost, followed by signals, as in traditional patterns of sex conflict (graph Π).
The vertical border between the red and yellow regions represents the point at which males make an additional contribution to the offspring due to the fact that females balance the costs of their signaling. The horizontal border that separates the yellow and black regions from the red region occurs similarly, but for a less obvious reason. When the costs of females for the contribution (cf) are low, then the optimal value of the contribution (sopt) will be relatively high, and hence the contribution of females will be much larger in the initial conditions. The consequence of this is that the signals give the male a proportionately smaller benefit from the contribution he causes, which is again offset by his costs.
The parameter space in which signals and responses are fixed (yellow) varies depending on the strength of selection and the genetic dispersion of the respondent's allele. For example, when the initial allele frequency of a responder is 0.9 instead of the 0.99 shown in image #2, the introduction of signals results in more efficient selection in relation to the responders (the original genetic variance is higher), and the black area expands to the left.
Male signals may occur even if they involve costs that reduce the male's contribution to the current brood (parametrized sfec), thereby directly influencing fitness* both male and female rather than reducing the likelihood of the male surviving.
Fitness* - the ability to reproduce individuals with a certain genotype.
The ratio of fertility costs to signals (left) and the ratio of viability costs to signals.
In terms of fertility, when male signals are fixed (yellow area), all males invest less in offspring than before signaling. In this case, the contribution of females will be greater than it was before the manifestation of male signals.
The greater contribution of females, when male costs are governed by fecundity (rather than viability), increases the average number of offspring per pair, but this does not fully compensate. Over time, more female input leads to an increase in the average number of offspring that reach fledging, but reduces the average female viability. This leads to the formation of a new balance between these two forces, where the average number of offspring is lower than in the case of normal viability or in the initial conditions (before the appearance of signals).
From a mathematical point of view, it looks like this: if male signals increase fertility by 1% (but do not increase viability), then the cost of females per offspring increases by 1.3%, but their mortality also increases by 0.5%, and the number of offspring per pair decreases by 0.16%.
If the average value of the contribution of females is initially below the optimal level (for example, due to the influence of the environment), then a balanced system arises when the signals stimulating the growth of costs appear, i.e. intersexual cooperation. In such a situation, male signals not only increase the contribution of females to offspring, but also their fitness.
Such behavior of males and females most often occurs due to external changes (climate, habitat, amount of food available, etc.). In view of this, scientists suggest that the formation of monogamy in some modern species, while their ancestors were polygamous, is due to migration and, accordingly, a change in the environment.
For a more detailed acquaintance with the nuances of the study, I recommend looking at ΠΈ to him.
Finale
This study demonstrated the relationship between polygamy and monogamy in terms of evolution. In the bird kingdom, males have always tried to outdo each other in order to get the attention of a female: with bright plumage, beautiful dance, or even a demonstration of their building abilities. This behavior is due to competition among males, which is inherent in polygamous species, most often. From the point of view of females, all these signals make it possible to assess the qualities of the male that their common offspring will inherit. However, over time, males began to evolve in such a way that their signals were brighter than those of competitors. Females, in turn, have evolved to counteract these signals. After all, there must always be a balance. If the costs of females for offspring are disproportionate to the benefits, then there is no point in increasing costs. It is better to lay a clutch of 3 eggs and survive the process of incubating and raising offspring than to lay five and die trying to protect them.
Such an intersexual conflict of interests could lead to a catastrophic decline in the population, but evolution took a more sensible path - the path of cooperation. In monogamous pairs, males continue to show themselves in all their glory, and females respond to this with an optimal contribution to the offspring.
It is curious that the world of wild animals is not burdened with moral principles, laws and norms, and all actions are due to evolution, genetics and the thirst for procreation.
Perhaps for romantics, such a scientific explanation of winged love will seem too prosaic, but scientists think otherwise. After all, what could be more beautiful than to evolve in such a way that between the female and the male there is a balance and a true partnership that takes into account the interests of both parties and is aimed at the benefit of future generations.
Friday off-top:
Despite the fact that the name of these birds is not the most beautiful (βgrebesβ), their reunion dance is simply beautiful.
Off-top 2.0:
Birds of paradise are (literally) a prime example of the variety of signals males send to females during the nesting season (BBC Earth, voice-over - David Attenborough).
Thanks for watching, stay curious and have a great weekend everyone! π
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Source: habr.com