Cooperative breeding systems, in which the offspring of a species is raised and nurtured by not only the parental individuals, but also by alloparents, are widespread among social animals. In birds, around 9 % of all species engage in cooperative breeding. The question why an individual engages in cooperative breeding instead of breeding independently has been a continuous point for researchers. The fitness benefits that an individual gains from cooperative breeding differ from inclusive fitness in the Florida Scrub Jay to a rise of available food sources and group benefits for the Azure-Winged Magpie and Brown-headed Nuthatch. Since the graphic distribution of cooperative breeding in birds is highly variable, it has been suggested that ecological conditions must play a part in what drives cooperative breeding. The ‘Hard-Life Hypothesis’ states that the more barren the environment in which a species has to raise their offspring, the more likely it is that the individuals will participate in cooperative breeding. The ‘Ecological-Constraint Hypothesis’ states that, if an individual can not find an own habitat due to saturation of the surrounding territories, it will stay and act as an alloparent for its relatives instead. Other, more recent theories take the life history into account as well, stating that the survival rates of not only the offspring but all group members of the system rise.
Cooperative breeding is defined as a system in which the offspring of a species is not only minded by the direct parental individuals but also by non-breeding individuals, the so called alloparents. (Lukas & Clutton-Brock. 2017). While cooperative breeding systems are found in non-vertebrates, for example in ants (Bourke & Heinz. 1994), they can be witnessed in several mammals, fishes and birds as well. In the subphylum of vertebrate, the highest frequency in cooperative breeding species is found in birds, a total of 9 % of all species (Koenig. 2017).
When is a species considered to be a cooperative breeder system? One definition is that this is the case when at least 10 percent of nests in a population is attended by the parents and at least one additional alloparent (Cockburn 2006, cited by Hatchwell 2009). In most cases, cooperative breeding directly relates to kin-selection with cooperative breeding groups consisting of family members with former offspring (Riehl 2013) or, in rare cases, grandparents (Polygottus, 2010). The reason why the former offspring acts as an alloparent instead of breeding independently is discussed by ecologists but is often explained by the Hamilton rule.
Hamilton, an evolutionary biologist, argued that there are two metrics for evolutionary success in a species, personal and inclusive fitness. While personal fitness is described as the number of offspring that an individual procreates, the inclusive fitness is the number of offspring equivalents which an individual supports. This is also known as kin-selection (Hamilton. 1964)
Hamilton’s rule is a mathematic description whether an individual decides to become an alloparent. It is stated as C, the reproductive cost of the alloparent, such as not gaining personal fitness, has to be outweighed by r, the genetic relatedness of the alloparent and the offspring in combination with b, the reproductive benefit to the recipient of the altruistic behaviour, most often the increasing chance of survival for the minded offspring (Hamilton. 1963).
Kin-selection in combination with cooperative breeding takes place in several bird species, for example in the Florida Scrub-Jay (Aphelocoma coerulescens) (Erickson. 2009). In Florida Scrub-Jays, the cooperative breeding society is a nuclear one, which means that the centre of the society is a mated pair, which is monogamous and polytocous (Riehl. 2013). A pair can have up to 6 alloparents, usually older offspring, which help to mind the nestlings and the brooding parent and defend them from predators. While the numbers of helper do not influence how many eggs are produced, the number of surviving hatchlings increase significantly with the increasing number of helpers. Some of the alloparents stay their whole life with their parents instead of mating and breeding independently (Erickson. 2009). There are several theories which try to explain the reasons of individuals in related and non-related groups to become alloparents instead of breeding independently: The earliest theory is the ‘Ecological Constraint Hypothesis’, which states that the offspring participates in cooperative breeding since the chance of finding a mate on their own and a habitat to breed in are too low due to the area being ‘saturated’ (Emlen, 1982). This circumstance leads, following Hamilton’s rule, to a decrease in personal fitness and therefore to a rising chance in taking care of the next offspring of their parents to increase the inclusive fitness. Staying in the group also offers them the benefit of a ‘safe haven’, such as access to food resources and defence benefits (Kokko & Ekman. 2002).
Another theory, the ‘Hard-Life Hypothesis’ explains that barren and unpredictable environments with low food sources in combination with a density of predators, decreases the chance of offspring survival without helpers (Hing, Klanten et. Al. 2017).
A third theory, the ‘Life-History Hypothesis’ claims that the low annual mortality that is offered by the increasing of survival for not only the offspring but for all members in a cooperative breeding system predisposes bird species to breed cooperatively (Russel. 1989).
A combination of all three hypothesis and one of the newest is the ‘Broad-Constraints Hypothesis’. The life history predisposes since the chance of survival in a group and the survival of the species rises with the cooperation, but the ecological constraints facilitate the cooperative breeding behaviour. The ecological constraints can be an oversaturated area and therefore a lack of territory for the offspring to disperse to, but also the variability of environmental productivity in the habitat (Hatchwell & Komdeur. 2000).
In Florida Scrub-Jays, a combination of the hard-life hypothesis and the life-history hypothesis makes more sense than the ecological constraint hypothesis. The territories are in most cases, not overly saturated, the main cause of death in this species is predation and the food resources are spare in many territories (Erickson. 2009). The more helper a breeding pair has, the easier it is to share the work of food gathering and staying in vigilance. This increases the chance of survival not only for the nestlings, but also for all other members of the group.
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Figure 1: Global patterns of richness in avian cooperatively breeding species
While kin-selection certainly helps and promotes cooperative breeding in many species, it is not the only important factor for it, as implied by the ‘Hard-Life Hypothesis’. The distribution of cooperative breeders is higher in environments that have a high variability in environmental productivity (Rhiel. 2013), as shown in figure one, provided by Feeney, Medina et al. in their essay ‘Brood Parasitism and the Evolution of Cooperative Breeding. For the purpose of this essay, we only look at the A part of this figure.
Cooperative breeding seems to be heavily influenced by the environment, since the areas in which the richness in cooperative breeders is the highest are known for being harsh and barren with a high variability in rain fall, especially the areas in Africa and Australia with their deserts and savannas.
A good poster species for non-related cooperative breeding that is influenced by the ecological factors of their habitat is the Azure-Winged Magpie (Cyanopica cyanus), a species that is common in South-East Asia. The individuals live in small colonies up to 70 birds and are not monogamous (Xian, Luo et al. 2018). The alloparents help the brooding pair, to which they are in most cases not related, by assisting in the building and restoring of the nest, minding the nestlings and females and defending them against predators. That the variability of their environment influences their cooperative breeding system was visible in a year in which the frequency and mass of rain increased around 30 %. The number of alloparents increased as well (Polygottus. 2010). This has to do with one of their main food sources, insects. The insects tend to hide during rainfall which increases the difficulty to find them and therefore decreases the available food source.
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Figure 2: Average Are of Types of Habit in Territories of Non-Cooperative and Cooperative Breeding (Adapted from Kesler & Haig 2007)
Not all cooperative breeding is correlated to kin-selection or multi-altruistic behaviour. In Brown-headed Nuthatches (Sitta pusilla), the role of a helper is only taken on for personal benefits. The lack of altruism in helping becomes visible when looking at the following results of a survey regarding the frequency of cooperative breeding and the survival rate of the nestlings: While in the studied population about 20 % of adults engaged in cooperative breeding, the average number of nestlings raised by cooperatively breeding and single-pair breeding families remained the same: Around 4 chicks would be raised. It is notable that the dominant breeding pair will still tolerate the alloparent, even if no help is provided (Wong et al. 20). 12This leads to personal benefits of the helper, which are an increase in its own survival, since the participation in a group will secure its security and make food sources more available. The latter is caused by the higher area in territory that is held by cooperative breeding groups (compare figure 2). The motivations for helping are either personal nest failure of inability to successfully find a mate to breed with (Polygottus 2010, 5-8). This behaviour is also known as redirected helping (Sheng-Feng et al. 2017. 709).
The benefits of living in a cooperative breeding system are therefore not only limited to inclusive and personal fitness, since, in the case of the Brown-headed Nuthatches neither is achieved. The Brown-headed Nuthatch that becomes an alloparent, does neither produce own offspring due to the nest failure, therefore not gaining personal fitness, and also does not gain any inclusive fitness since the survival rate of the nestlings does not rise with the number of helpers. Instead, they gain a number of indirect and direct benefit from living in a group. These benefits can be divided in resource defence benefits and collective action benefits. Resource defence benefits are best described as the defence of vital resources such as food resources and are indirectly caused by the larger shared territory (compare Figure 2) (Sheng-Feng et al. 2017). Collective action benefits are more direct and caused by the coordination within the group. This coordination increases the efficiency of the group in vital activities such as foraging, defending the young, vigilance and homeostasis. The importance of collective action benefits increases intensively when the conditions are harsh (Koening & Mumme 1987), underlying the claims of the ‘hard-life hypothesis’.
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FIgure 3: Classification of group composition and social mating system for all 213 species of cooperatively breeding birds for which data are available
These indirect and direct benefits have to be taken in consideration in the same way as the personal and inclusive fitness are. In a study which researched 213 completely cooperatively breeding bird species, around 15 % nestled with only unrelated group members (compare Figure 3) (Riehl. 2013). The absence of monogamy in these species is an important factor as well, since the alloparents of one year are potential breeders in the next year while in a nuclear cooperative breeding group with a dominant breeding pair, the alloparents stay as non-breeding helpers until either one or both members of the dominant pair die, or they leave the group. An important and well studies species that lives in a non-related cooperative breeding system is the pied kingfisher. In this species, the alloparents solely consist of unrelated males that join the group and act as helpers until breeding themselves (Reyer. 1984). This includes a prospect of future personal fitness gain to the variety of group benefits.
The establishing of cooperative breeding systems not only in birds but also in other vertebrata and invertebrate can be seen as a result of group forming due to the benefits of groups in general and adapting to the circumstances of the environmental challenges. The examples presented in this essay show that different species have different reasons to establish a cooperative breeding system. These reasons circle most often around the survival of the whole group and the benefits gained for the helping individual and are the cause of challenging environmental obstacles. The participation of the helper in the group always leads to benefits that outweigh the costs, no matter if the helper will be able to breed in the future or not. The research about how and why exactly these systems form and are part of some species living in social groups and not in others is ongoing and, in the case of non-related cooperative breeding groups still in need of further investment.
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- National University of Ireland, Maynooth
- Cooperative Breeding Life History Enviroment Environmental Influences Hard-Life Hypothesis Ecological-Constraint Hypothesis Hamilton rule