and altruism

Francesco Cavraro

In many vertebrate species offspring stay with their parents for a certain period of time, even when parental care has ceased. The presence of helpers (i.e. immature individuals of a parental couple) helping parents in breeding subsequent broods has been described in dozens of bird species and, to a lesser extent, also in mammals. In fish this phenomenon, which belongs to the sphere of the so-called altruistic behaviours, is much less widespread. Anyway, a particularly interesting example is that offered by some species of cichlids of Lake Tanganyika, belonging to the genera Lamprologus, Julidochromis and Telmatochromis.

One species in particular, Lamprologus birchardi, has been studied in depth. In this species, the helpers are young individual fish of previous broods that remain in their parents’ territory and participate in the parental care, which in this species consists in the maintenance (removing sand and debris from the place chosen for laying) and in the defence (from the intrusions of both conspecifics and potential competitors and predators belonging to other species) of the territory within which a pair of adult fish reproduces. Helpers contribute also to the care of the offspring (helpers assist their parents by cleaning and oxygenating newly hatched eggs and larvae with fin movements).

Helpers are young individual fish of previous broods that remain in their parents’ territory and participate in the parental care

Three factors have proved to be the most important in the cost-benefit analysis of the phenomenon of helping when compared to leaving the parental territory and forming a new family. For the energy spent on helping the reproductive pair, helpers grow less than non-helper individual fish. The main reason could be the hierarchy that is established in the territory: helpers are tolerated but, still, the two parents are in charge. Since only larger individual fish are able to establish a territory and reproduce, a slow growth is clearly a disadvantage. However, this cost is offset by a lower mortality rate, as helpers have access to a well-defended territory. In addition, at the species level, the presence of helpers determines a positive contribution to the future reproductive success of parents: females aided by helpers, in fact, lay more eggs and, as a result, have a larger number of offspring. In the presence of helpers, the two parents spend less time caring for and defending their territory and offspring, and therefore have more time devoted to feeding. Thus, the energies saved can be dedicated to reproduction.

By increasing the reproductive success of parents, helpers favour the spread of a part of their genes, according to the process known as kin selection, through which an individual ‘accepts’ bearing a cost to favour another conspecific with which it is closely related. In the scientific community there is no unanimity on the issue, but it seems that kin selection, even if not being the main reason that drives an individual to remain in their parents’ territory and not reproduce, seems to be the reason that determines their assumption of the role of helper. In the Florida scrub jay Aphelocoma coerulescens, or in the pied kingfisher Ceryle rudis, helpers with various degrees of kinship with respect to the offspring are observed. Well, in these species the energies spent by helpers are directly proportional to the degree of kinship.

In various species of birds, suitable habitat saturation and a localised distribution of resources are the main factors that determine the permanence of young individuals within the territory of parents, giving rise to extended families. This also seems to be valid for L. birchardi, since the habitats of Lake Tanganyika suitable to this species are densely populated, both by conspecifics and individual fish belonging to other species with similar ecological needs. L. birchardi fry remain in their native territory also because the protection offered by the family is fundamental in the first months of life. Experiments in the aquarium have in fact shown that helpers remain linked to their family even when having the opportunity to abandon them and reproduce on their own.

[1] Taborsky M. (1984) Broodcare helpers in the cichlid Lamprologus brichardi: their costs and benefits. Animal Behaviour 32(4), 1236-1252.

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