The purpose of the present study is to explore the patterns of active and passive defense in animal species. In particular, it is presumed that active defense requires more efforts, which has a negative impact on animal’s fitness. In contrast, passive defense mechanisms are less demanding — therefore, they are considered to be used more often. The following project is a correlation study that is based on up-to-date credible academic periodicals. It focuses on the exploration of avoidance of predators’ odors, aggressive invasive behavior, alarm calls, chemical and vibroacoustic signals, alloparenting, caudal autotomy, and toxic weaponry of poisonous and venomous species. The obtained results suggest that animals choose passive defense more often because it is safer in regard to their life. Simultaneously, it is identified that in kinships and social animals, the individual efforts applied in the process of defense are typically higher and accompanied by greater collective benefits. In addition, this study reveals that in most cases, animals establish complex defense systems that consist of several passive, or passive and active defenses.
Key words: active and passive defense, animal species, avoidance of a predator’s odor, alloparenting, autotomy, alarm calls, chemical and vibroacoustic warning, attacks, toxic weaponry.
While discussing the defense mechanisms in animal species, it is important to comprehend that this subject is characterized by a great diversity of anti-predatory mechanisms. Despite the fact that all defense strategies are aimed to ensure survival, many of them require significant efforts. Undoubtedly, such efforts negatively affect animals’ fitness. This research aims to establish both active and passive defense mechanisms and to assess the magnitude of the efforts they require. IT IS HYPOTHESIZED THAT PASSIVE DEFENSE MECHANISMS IN ANIMAL SPECIES RESULT IN A LOWER NUMBER OF HARMFUL EFFORTS THAN THE ACTIVE STRATEGIES REQUIRE, AND THUS, PASSIVE DEFENSES ARE UTILIZED MORE OFTEN COMPARED TO ACTIVE ONES.
Purpose of Study
The goal of this research is to explore credible, up-to-date academic journals that are related to the discussed topic. The design of the study is a correlation research that is based on narrative evidence — qualitative and quantitative data. The purpose of it is to compare and contrast active and passive defense mechanisms of fauna in order to comprehend the efforts they require, as well as their relation to the animals’ fitness. In this regard, it is important to understand that the diversity of species and their habitat encourages the development of a great variety of anti-predatory strategies. Therefore, it is impossible to state that one type of strategy is more appropriate than another. For example, it is natural to assume that active defense requires stronger efforts, but a) it may be more effective than passive mechanisms, and b) it must be utilized when applying less damaging techniques of protection is impossible or irrelevant. That is why the present correlation research does not aim to identity which defense mechanisms are better. Instead, it focuses on investigating the mechanisms and patterns of both kinds of anti-predatory strategies with the aim to increase the understanding of the efforts required and the benefits of this process, as well as to establish the circumstances under which these strategies are implemented.
Results and Discussion
Passive Defense Mechanisms
Avoidance of predators’ odors. Avoiding the predators’ odors is a passive defense mechanism that is typical for many prey species. Hegab with the colleagues (2015) reveal an example of mice avoiding cats’ odors, such as urine, feces, and fur odor. The mechanism of this strategy is the following: when prey species sense the odor of a predator, they limit their activities to apply the defensive approaches. For instance, mice stop foraging and grooming when the odor of a cat is detected. To prey species, the predators’ odor is a sign of proximity of a threat, and thus they do everything to avoid being noticed and caught. Mice, as well as other prey animals, hide or reduce their locomotion activity when they sense the presence of a cat. In addition, they strive to find the place without the predators’ odors (Hegab et al., 2015).
Estimating the costs of this defense, one may rightfully presume that it decreases the changes in food and water consumption. Moreover, hiding for a long time may have an adverse impact on the prey’s health, if the conditions where they are situated are malevolent. Furthermore, in case if an adult mouse stays away from its offspring, it may die without food and warmth provided by a parent. This fact reveals that avoiding the predators’ odors is a passive defense strategy that results in negative outcomes — however, it remains an important way of avoiding the places of predators’ habitat, which reduces the risk of being caught.
Hegab with the colleagues (2015) explain that mammals strive to balance the risks and benefits obtained from avoidance of predators’ odors. To succeed, prey species try to decrease the activity of their organisms in or during high-risk environment or time. In contrast, they attempt to make up for the decreased activity within the safe space and time. This approach lessens the negative outcomes of inability to feed the offspring and other vital activities.
Alarm calls and chemical and vibroacoustic warnings. In contrast to avoidance of the predators’ odor, the alarm strategy is a passive defense in mammals which implies warning the rest of the kin about the proximity of a predator. This mechanism is known as animal altruism (Delattre et al., 2015). The altruism in an animal that produces alarm signals refers to the situation when it puts itself in danger with the purpose of letting other members of the kin run away, hide, or deploy other defense strategies. Producing the alarm signals is possible before the animal is caught, and it undoubtedly increases the risk of being exposed to the predator. Moreover, alarm signals are also produced by the prey when it is already caught. The types of warning vary greatly — in mammals, the warning is mostly produced by releasing noisy sounds.
For instance, scientists studied the communication of forest guenons and detected that social species tend to develop unique sounds for interpersonal communication (Candiotti et al., 2015). This finding depicts a process of social inclusion and integration that occurs by means of sharing the same ‘language’. Under these conditions, the members of the same kin are capable of passing the alarm calls that are understood by their relatives. Furthermore, it is possible to deduce that similar language shared in the kin is a part of a complex passive defense, which resonates through the places where the specific animals are situated. This strategy possesses considerable benefits, whereas the negative outcomes are more prominent at the individual level rather than at the collective one.
Another example is described in the study conducted by Delattre and his colleagues (2015). This research established that termites M. darwiniensis generate chemical alarm signals in order to warn their kin about the approaching danger. Delattre and his colleagues (2015) accentuate that chemical communication is typical for social insects such as bees, termites, ants, and others. In terms of protection of the kin, chemical alarm signals serve to build a composed system of defense. Specifically, releasing chemical warning triggers the processes of protecting the vulnerable members and preparing for active defense (Delattre et al., 2015). The description of this complex defense response in social species of insects implies that these animals combine passive and active defense mechanisms. Moreover, a passive mechanism, such as chemical alarm signal, functions in order to activate an active defense mechanism — a direct attack.
Apart from chemical warning, scientists distinguish vibroacoustic signals (Delattre et al., 2015). This is another approach that allows the species to produce alarm signals, passing them to kin members. Kinships of insects utilize both kinds of signals depending on the situation. Moreover, it is important to stress that not all alarming signals can be attributed to animal altruism. In this regard, mammals that live in kinship and produce alarm signals to attract the predator’s attention are an example of altruism, whereas termites do not attract attention of predators to themselves — their alarm signals have the informative nature aimed to mobilize the activity of the kin for collective defense.
Overall, generating alarm signals in animals is a strong defense mechanism that significantly increases the chances of survival of a kin. In this regard, sacrificing one member to save the entire kin is justified, even though in terms of individual fitness, an animal that produces alarm signals to attract the predator’s attention places itself in a considerable danger that often results in a lethal outcome. On the other hand, in a case when the prey is already caught and its survival is unlikable, altruism behavior is completely justified. The same can be said about the alarm systems of termites, in a case when a member that produces chemical and/or vibroacoustic warning does not risk being caught because of this action.
Finally, it is appropriate to conclude that alarm signals are typically only a part of a complex defense strategy developed by animal species. For instance, after warning, prey species are supposed to deploy other passive defense strategies, such as running, hiding, and expressing unprofitability. Similarly, the alarm calls in predator species function to mobilize the mechanisms of active defense. In other words, alarm calls and chemical and vibroacoustic signals are passive defense strategies that constitute a part of a complex defense in both prey and predatory species.
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Alloparental care. Alloparental care is a term that defines an approach to feed, foster, and protect the youth in a kin that is not directly related to adult members who provide care. Thus, alloparenting in mammals is known to be connected to reciprocal altruism (King et al., 2015). This phenomenon is related to the aforementioned animal altruism in producing the alarm calls. Specifically, it refers to mutualism, when kin members help each other in order to be helped as well in a case of such necessity. Both approaches — warning and alloparenting, are typical for various kins. Discussing the costs and benefits of alloparental care, it is important to consider the required considerable individual efforts and the received collective benefits. Moreover, it can be attributed to passive defense mechanism when not only parents, but also other members of a kin, relatives, tend to protect the youth from predators and other threats.
Undoubtedly, providing alloparenting care increases the chances of the offspring to survive. Typically, in a kin, this task is maintained by adult females, as a part of kin selection (King et al., 2015). However, there are cases when adult males also take part in fostering and protecting the offspring of a kin that is not directly related to them (Stiver, Wolff & Alonzo, 2013; Minhós et al., 2015). In this case, individual efforts are higher than the received benefits, because animals risk their lives to rescue the offspring that does not carry their genes. On the other hand, given that a kin consists of related members, more remote genes are saved, which means that by protecting the youth of the others, an adult member enhances the general fitness of the group.
Revealing the examples of alloparenting care and animal altruism, it is appropriate to refer to a study of reciprocal altruism in eastern grey kangaroos that was conducted by King and the colleagues (2015). This study explores the cases of adoption as a means of reciprocal care and a sign of the animal altruism. Alloparenting in kangaroo can be perceived as a passive defense from predators as well as from illnesses caused by different bacteria. The interaction with the adult members, other than direct relatives, means that a baby-kangaroo advances its immune system, and as a result, it becomes fitter (King et al., 2015). This example illustrates that alloparenting serves as a passive defense not only from predators, but also from viruses and bacteria. The risks of such altruism for adults incorporate the increased exposure to predators and enhanced expenditure of energy. Nevertheless, the benefits of reciprocal altruism are greater than the costs. Therefore, this passive defense continues to be relevant in numerous mammal species.
Furthermore, it is important to emphasize that the term alloparenting more often refers to female members of a kin as the main care-givers — besides, a kin is typically characterized by maternal leadership and corresponding hierarchy. The cases when the animal altruism is registered in male species are referred to as mutualism (Stiver et al., 2013). It is appropriate to stress that all these behaviors — animal altruism, alloparental care, mutualism, may be observed within one kin and manifested through providing reciprocal care, such as feeding and defense. In other words, there are instances where these phenomena intersect — however, they can be perceived as standalone behavioral strategies at the same time.
In this regard, an example of mutualism and male alloparenting as a passive defense can be observed in Tessellated darters (Stiver et al., 2013). This bird species is characterized by males being the main care-givers and guardians of the nests. Stiver and the colleagues (2015) explored the phenomenon of mutualism in these birds and detected that they are prone to protect the offspring of one another. In general, it is proven that mutualism is a means of providing care and defense which reveals the significant reciprocal benefits with less prominent costs.
Autotomy. Autotomy is a specific passive defense that presumes voluntary shedding of a part of body (McElroy & Bergmann, 2013). This phenomenon is also referred to as caudal behavior, which is typical for many lizard species (Cromie & Chapple, 2012; Sanggaard et al., 2012). Most species shed their tails — however, in spider species, it can be a limb or a tail, and a sting in a terrestrial scorpions (Mattoni et al., 2015). The principle of this defense strategy is that a part of a body that has been shed continues to move, attracting attention of a predator, thus allowing the prey to escape (Sanggaard et al., 2012). This anti-predator strategy is greatly effective — however, it requires considerable harmful efforts.
In particular, caudal behavior makes an adverse effect in regard to the locomotion activities of an animal. Considering that lizards and spiders use their tail and limbs for moving, this task becomes more complicated without a part of a body (McElroy & Bergmann, 2013). Besides, lizards accumulate fat in their tails, which means that shrinking away a tail, the lizard loses considerable deposit of energy and nutritious elements (Sanggaard et al., 2012). Consequently, even if an escape is successful, after caudal strategy, the animals need time to recover. This insight implies that autotomy needs to be combined with other passive defense strategies, such as escape, hiding, reduced locomotion, and others.
Considering the aforementioned anti-predatory strategies, it is natural to deduce that caudal behavior makes it impossible to mate for a certain period of time, until the animal recovers (Cromie & Chapple, 2012). On the other hand, scientists detected a significant benefit in shrinking a tail — a new tail is more robust than the old one, which is stipulated by the tougher scar tissue (Cromie & Chapple, 2012). Thus, males with new tails have better chances to win in active defense or while competing for females, since lizards utilize tails for fighting (Cromie & Chapple, 2012). Overall, caudal behavior of animals is an important passive defense which leads to significant negative outcomes, having temporal negative impact on the animal’s fitness. Similarly, it is an effective anti-predatory approach. Hence, to be totally effective, the species that shed away their tails/limbs/sting should combine caudal behavior with other defenses.
Active Defense Mechanisms
To begin with, it is important to stress that the diversity of the active defense mechanisms is much poorer while being compared to the passive ones. This particularity is understandable, considering the costs of fighting/chasing/poisoning are typically higher.
Initiating attack. Aggressive behavior is typical for many species. Scholars believe that initiating assault is a workable approach to conquer the new territories and to defeat the competitors. An example of such aggressive species is the invasive ants (Bertelsmeier et al., 2015). Bertelsmeier and the colleagues (2015) observed behavior of seven such species and concluded that the initiator of the fight is more likely to win. On the other hand, aggressive species that initiate fights are also more likely to be killed than animals who implement passive or passive and active defense mechanisms. In this regard, invasive ants succeed in conquering the territories when they meet species with passive defense mechanisms. At the same time, if faced with more aggressive species, they can co-exist — however, when two invasive species of ants are equally aggressive, they tend to exclude one another (Bertelsmeier et al., 2015). These findings suggest that active defense mechanisms indeed require considerable efforts, which negatively affects the animals’ fitness and may even result in the extinction of the aggressive species.
Venom and poison as defense strategies. Venomous and poisonous animal species are similar in terms of utilizing toxic weaponry as a defense strategy (Harris & Arbuckle, 2016). Nonetheless, it is important to understand that there is a considerable difference between the methods of using this weaponry. Specifically, poisonous species contain toxic fluid within their bodies, and in a case of an attack, when they are bitten, predators are being poisoned with such toxins. This defense process implies that it is a passive defense strategy. In contrast, venomous animals, such as snakes, scorpions, bees, and many others, utilize their venom for both attacks and defense. The ways of infecting the prey or the predator differ. In the first case, animals are infected by absorbing toxins released from a poisoned flesh of the prey, whereas in the second case, venom is transmitted to a body of the prey or a predator by injection — biting or stinging (Harris & Arbuckle, 2016). Furthermore, there are species that are capable of using both kinds of toxic weaponry: they are simultaneously poisonous and venomous. For instance, Harris and Arbuckle (2016) conducted a study of tetrapods, species that are characterized by both active and passive toxic weaponry. These scholars identified that toxic weaponry is utilized in accordance with an animal’s environment — treats, the need to compete, and others (Harris & Arbuckle, 2016). It means that greater diversity within the species contributes to the increased range of toxic weaponry.
In terms of the costs, both approaches are connected with significant losses. For instance, when a bee stings someone, it dies, which means that active defense of venomous species is associated with high level of mortality. Besides, applying the active defense, these species are at risk of killing one another, as it was described in a case of aggressive invasive ants that also use chemical weaponry. Similarly, poisonous species may die or be seriously injured when they are assaulted and bitten. Therefore, it is natural to deduce that the costs of toxic weaponry as a defensive strategy are high in both venomous and poisonous animals.
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The conducted analysis of the recent academic periodical literature that explores active and passive defense mechanisms in animal species proves the working hypothesis to be correct. PASSIVE DEFENSE MECHANISMS REQUIRE LESS FITNESS COSTS THAN ACTIVE DEFENSE STRATEGIES — THEREFORE, THE FORMER IS MORE OFTEN UTILIZED BY ANIMAL SPECIES. At the same time, it is necessary to stress that there is a pattern toward applying the active defense when the passive one has failed. Moreover, this study also reveals that there is greater diversity of passive mechanisms because they possess lesser adverse outcomes at both individual and collective levels. Furthermore, the explored examples depict that in most cases, animals need more than one defense strategy. Typically, prey species utilize several passive defense mechanisms. Among widespread anti-predatory strategies, it is important to distinguish the avoidance of the predators’ odors, alarm calls, chemical and vibroacoustic signals, alloparental care, autotomy, and poisoning. These are some, but not all passive defenses that are often united together as a complex defense mechanism of prey species. In terms of active defenses, it is appropriate to accentuate aggressive behaviors — different forms of attacks, and intoxication of the prey or the competitor. These approaches are effective — however, they are associated with greater risks, increased likelihood of mortality, and even extinction of the entire kinship in case if two equal in power and aggression species choose to compete.