Студопедия

Главная страница Случайная страница

КАТЕГОРИИ:

АвтомобилиАстрономияБиологияГеографияДом и садДругие языкиДругоеИнформатикаИсторияКультураЛитератураЛогикаМатематикаМедицинаМеталлургияМеханикаОбразованиеОхрана трудаПедагогикаПолитикаПравоПсихологияРелигияРиторикаСоциологияСпортСтроительствоТехнологияТуризмФизикаФилософияФинансыХимияЧерчениеЭкологияЭкономикаЭлектроника






Caste division and polyethism in eusocial communities






 

The system of caste division was firstly described for social insects. Wheeler (1928) was the first who proposed a detailed description of caste system in social insects based on anatomy with no fewer than 30 categories. Hö lldobler and Wilson, (1990) define a caste as a group that specializes to some extent on one or more roles. Role means a set of closely linked behavioural acts (for example, queen care). Broadly characterised, a caste is any set of a particular morphological type, age group, or physiological state (such as inseminated versus barren) that performs specialised labour in the colony. A physical caste is distinguished not only by behaviour but also by distinctive anatomical traits. A temporal caste, in contrast, is distinguished by age. The term task is used to denote a particular sequence of acts which serves to accomplish a specific purpose, such as foraging or nest repair. Finally, the division of labour by the allocation of tasks among various castes is often referred to as polyethism, a term apparently first employed by Weir (1958).

A good example of division of labour in eusocial communities based on cast differentiation is existence of soldiers, that is, a specialised cast of workers that defend the colony against intruders (for a review see: Judd, 2000). Termites, social aphids, social thrips, and some ants produce special casts of soldiers. Some species of ants as well as eusocial shrimps and naked mole rats show a distinct polymorphism among workers with larger individuals specialised as guards. In some species of bees and wasps guards differ from other colony members only by their aggressive behaviour but not morphologically (Fig. X-5).

Let us consider several examples of animal social systems based on caste determination and polyethism.

Social insects. Social insects belonging to different orders (Isoptera and Hymenoptera) share essential similarities in the processes of caste regulation. We consider here only a rough schema. There is a great diversity of species: only ants include about 11 000 species and termites about 2 300 species. Different taxa have different numbers of castes, and different degrees of caste specification.

In general, in social insects most members of a community sacrifice their own reproductive potential to provide food and protection for the few reproductive members and their offspring. The so-called primer pheromone causes long - term physiological changes in nestmates within a colony by controlling their endocrine and /or reproductive systems. The primer pheromone is usually dispersed by only one or a few individuals (“queens”) and may regulate sexuality and caste expression. In contrast, chemical signals that cause immediate behavioural changes in conspecifics are defined as releaser pheromones and are produced by numerous nestmates (Wilson, 1971). The social organisation in colonies depends on the control of the proportion of different castes, and on efficient recognition and communication system.

Apis mellifera, the honey bee, has the best studied system of caste differentiation. Differences in caste-specific behaviour are understood for many years (Michener, 1974), but recent molecular studies have shed new light on the mechanisms by which it occurs. In honey bees, the primary determination is between worker bees and gyne (future queens). Gynes are given a special diet that activates queen specific development. Workers assume different roles in the nest as they age, a pattern known as temporal polyethism. Young workers stay in the nest, and as they age they replace foragers, and are replaced by younger workers within the nest. The timing of the progression through the tasks is not fixed. The progression can be delayed, or even reversed, if young workers die. Over the winter, the progression is also delayed, so that there are workers to staff the hive early in the spring

Some ants also have age-correlated division of labour. In ants with multiple worker castes, different morphological types assume different tasks (usually soldiers versus workers), but within each morphological type, work is divided in a temporal fashion.

In termites, in contrast to hymenopterans, the only adults present in colonies are the king and the queen. This one adult caste is initially winged (alate). Termite queens typically become physogastric, due to an enormous growth of the fat bodies and ovaries while the males remain relatively small. Indeed, the termite queen looks awfully fat and large in comparison to workers. For example, in the African termite, Macrotermes subhyalinus, the queen's body becomes so swollen with eggs that she is incapable of movement. When fully engorged, she may be 14 cm long (more than 10 times longer than a worker termite), and capable of producing up to 30, 000 eggs per day.

The second true caste in termites comprises the soldiers. They are always non-reproductive and are more sclerotized and more heavily pigmented than workers. They also have highly sclerotized and powerful mundibles, which make them suitable for colony defence. Soldiers cannot feed themselves and have to be fed by workers. In some species members of the sub-cast “minor soldiers” serve as scouts and leaders for workers being more sensible than workers to trial pheromones. Soldier termites can regulate their own numbers by inhibiting the larval development of other soldiers. Worker termites may be more or less differentiated, depending on the evolutionary status of the species. In primitive species, social tasks are accomplished by unspecialized larvae or nymphs. In the higher developed Termitidae, and some other termites groups, workers constitute a true caste, specialized in morphology and behaviour and permanently excluded from the nymphal development pathway. In theory, each nymph can be developed to an alate and leave the natal colony. In some species the workers are dimorphic having large and small forms, in the Macrotermitinae the larger workers are the males and the smaller workers the females. Workers accomplish different tasks and subtasks in the colony. For example, in the termite Hodotermes mossambicus, one set of workers climbs up grass stems, cuts off pieces of grass, and drops them to the ground below (subtask 1) while the second set of workers transports the material back to the nest (subtask 2). Termites’ lifetime is amazingly long for insects. Sterile workers live for 2-4 years while the primary sexuals live for at least 20 and perhaps 50 years (for details see: Emerson, A. E. 1952; Grassé, 1982 -1986; Kaib et al., 1996; Eggleton, 2000)

Social aphids introduce a whole new direction in the evolution of eusociality and behavioral ecology. Like termites, they are diploid, but in contrast to termites, aphids reproduce both sexually and parthenogenetically, so they have the ability to produce genetically identical individuals. In these clonal stages large colonies are formed comprising of genetically identical individuals. Aphids are the only colonial species that exhibit eusocial behaviour (Alexander et al., 1991). Aoki (1977) was the first who found that the aphid, Colophina clematis, produced instars that defend the colony from intruders. Since then many species of social aphids have been described in the two families Pemphigidae and Hormaphididae (Stern and Foster, 1996). Social aphids produce galls, which are tough pockets artificially induce in a plant by the aphids. All of the alates and reproductive destined instars are normally found inside the galls The individuals on the outside are the soldiers which defend the gall from any predator that would destroy this nest and it's contents. Stern and Foster (1996) describe several types of soldiers based on physical characteristic and behaviour.

Eusocial rodents. Similar to calling termites “white ants” one can call naked mole rats (Heterocephalus glaber) “mammalian termites”. These unique eusocial mammals share many features with termites. They spend virtually their entire lives in the total darkness of underground burrows, they are very small (7 to 8 cm long, and weigh between 25 and 40 g), and, what is the most important, they are eusocial. Besides, like in termites, in mole rats high-cellulose diet is also rather hard to digest, and their stomachs and intestines are inhabited by bacteria, fungi, and protozoa that help break down the vegetable matter. Similarity with insects intensifised with the fact that the naked mole rat is virtually cold-blooded; it cannot regulate its body temperature at all and requires an environment with a specific constant temperature in order to survive. These eusocial rodents cooperate to thermoregulate. By huddling together in large masses, they slow their rate of heat loss. They also behaviorally thermoregulate by basking as needed in their shallow surface tunnels, which are warmed by the sun.

These amazing creatures are neither moles nor rats. Like rats, they are rodents, but they're more closely related to porcupines and chinchillas. Heterocephalus glaber is known since 1842, but only in 1981 Jarvis discovered their eusocial organization system that is believed to be unique among mammals. Since that, this species has been intensively studied (see: Jarvis, 1981; Jarvis et al., 1994; Sherman et al., 1991; Reeve, 1992; Bennett and Faulkes, 2005). The reader can see a fragment of the laboratory colony housed in Sherman’s laboratory on Fig. X- 6.

There are essential ecological reasons for which naked mole-rats have broken many mammalian rules and evolved an oddly insect-like social system. These animals are ensconced in the arid soils of central and eastern Ethiopia, central Somalia, and Kenya, where they must continually dig tunnels with their enlarged front teeth, in search for sporadic food supplies and evade the deadly jaws of snakes.

Naked mole-rats live in well-organized colonies, with up to 300 members in a group (20 to 30 is usual). A dominant female (the queen), who outweighs the others by up to 20 g, leads a colony. The queen is the only female that breeds, and she breeds with one to three males. When a female becomes a queen she actually grows longer, even though she is already an adult, by increasing the distance between the vertebrae in her spine (Fig. X-7, a). These animals are extremely long living; in captivity some mole-rats have lived to 25 years old. One naked mole-rat queen, as the breeding females are called, produced more than 900 pups in her 12-year lifetime at a laboratory colony. The young are born blind and weigh only about 2 g. The queen nurses them for the first month, and then the other members of the colony feed them by faeces (again like termites) until they are old enough to eat solid food.

The breeding female (the queen) suppresses the breeding of all the other females in the colony. She sometimes leaves her nest chamber to check on her workers and to keep them unfertilized by pheromone control as well as by swoops and bites thus demonstrating that they should not “think” about anything but digging tunnels and defending a colony from snakes and newcomers. The worker males are also suppressed, although they do produce some sperm. When the queen dies, several of the larger females fight, sometimes to death, to become a queen. They can regain their fertility quickly.

The majority of workers (both males and females) spend their entire lives working for the colony (Fig. X-7, b). Workers cooperate in burrowing, gathering food, and bringing nest material to the queen and non-workers. They use their teeth to chisel earth and to create piles of soil (Fig. X-7, c). There is a great deal of branching and interconnection of tunnels, with the result that a colony's total tunnel length can add up to 4 km. Tunnels connect nest chambers, toilet areas, and food sources. Burrowing is the only way these animals find food, since they do not travel above ground. Some colony members “farm” succulent tubers that are formed by many of the plant species that grow in arid areas. They generally bore through the tuber, eating mainly the interior flesh while leaving the thin epidermis intact. This behaviour may allow the plant to remain healthy for some time, indeed even to continue growing, thereby providing a long-term food resource for the colony. Judd and Sherman (1996) studied captive colonies in order to determine whether successful foragers recruit colony mates, like many eusocial insects do. It has been revealed that individuals that found a new food source typically give a special vocalisation on their way back to the nest, wave the food around once they got there, and lay odour track for other nestmates to follow.

Whilst most offspring become workers, some continue to grow and become colony defenders. Their main duty is to defend the colony against predators. In particular, rufous-beaked snakes (Rhamphiophis oxyrhynchus rostratus), are attracted to the smell of freshly dug soil and will slither into burrows through mole hills in search of a rodent meal. Soldier mole-rats fight back with their teeth and attempt to block the entrance with dirt. If everything fails, a soldier will directly attack the snake, sometimes sacrificing its own life while others escape.

Should a breeder die, just one of defenders will become reproductive to replace it. They can occasionally disperse to found a new colony with an unrelated member of the opposite sex.

In general, caste differentiation in mole rats bears a strong resemblance (of course, merely superficial) with termite’s one. The sterility in the working females is only temporary, and not genetic. Like in termites, there are castes of fertilised queens and kings and unfertilised workers and soldiers, and workers descend from “nymphs”, that is, under-grown members of the colony. The life span of mole rats is unprecedented among small rodents just like the life span of termites is unprecedented among insects. It is possible that these long living animals will surprise experimenters with their cognitive abilities.

Eusocial shrimps. Tiny marine coral-reef Crustacea offer a new data about the ecology and evolution of eusociality. Colonies of the social snapping shrimp Synalpheus regalis share several features with those of eusocial insects and cooperatively breeding vertebrates (Duffy, 1996). S. regalis inhabits internal canals of tropical sponges, living in colonies of up to a several hundreds of individuals. Colonies consist of close kin groups containing adults of at least two generations which cooperatively defend the host sponge using their large and distinctive snapping claws, and in which invariably only a single female breeds. Irreversible caste differentiation is governed by the queen that typically sheds her large snapping claw and re-grows a second minor-form chela, rendering her morphologically unique among the members of the colony. It is still not completely known how the queen accomplishes social control over sexual maturation of other colony members. Both genetic data and colony structure confirm that many offspring remain in the natal sponge through adulthood. Colonies consist largely of full-sib offspring of a single breeding pair which “regins” for most or all of the colony’s life. In captive colonies researchers have regularly observed a large male in association with the queen behaving aggressively with other large males approach her. The inference of monogamy from genetic data suggests that the queen associates with a single male for a prolonged period. There is a strong competition for suitable nest site and a shrimp attempting to disperse and breed on their own would have low success. Colony members discriminate between nestmates and others in their aggressive behaviour. Laboratory experiments revealed behavioural division of labour within colonies. Large males shoulder the burden of defence, leaving small juveniles free to feed and grow, and the queen free to feed and reproduce (see Fig. X-5). Such size- and age-related polyethism in shrimps has many similarities with poyethism in social insects (Duffy et al., 2002).

Considering intellectual potential of social shrimps, Duffy (2003) refers to Darwin’s (1871) note that “the mental powers of the Crustacea are probably higher than might be expected”. Social shrimps demonstrate coordinated behaviour. For example, they pick up dead colony members and push out of their sponge dwelling. Recent experiments suggested coordinated snapping, during which a sentinel shrimp reacts to danger by recruiting other colony members to snap intruders. The phenomenon of “mass snapping” begins by rhythmic snapping of one individual, following by rapid recruitment of many others. The initial one-to-one confrontation elicited a snap response from the defender. Colony members joined in with a cacophony of snapping thus providing an unequivocal signal that the sponge is already colonized. This distinctive behaviour is the first evidence for coordinated communication in the social shrimp and represents yet another remarkable convergence between social shrimps, insects and vertebrates (Tó th and Duffy, 2005).

Summarising the data on caste division of labour within communities of eusocial organisms we have to admit that the correlation between cognitive and morphological specialisation in these animals is not yet completely described. Even in ants and bees which have been intensively studied for more than hundred years, it remains unclear what effect does the caste determination has on their intelligence. Further we will consider a more gentle system of division of labour in animal societies that perhaps leave more room for cognitive activities.

 


Поделиться с друзьями:

mylektsii.su - Мои Лекции - 2015-2024 год. (0.011 сек.)Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав Пожаловаться на материал