Care and violence in animal families.
Text: Christoph Dieffenbacher
Parents looking after their young is a common feature of family life in many animal species. The goal of parental care is to ensure offspring survival. Yet, this often involves trickery and conflict, and sometimes even naked aggression.
The ground nest is in turmoil: a mother earwig of the European species Forficula auricularia is feeding her young. Only a few of them can survive, so the earwig larvae have developed a ploy to boost their chances: a wax-like substance forms on their skin, making them smell like well-fed specimens. This prompts their mother to give them a bigger share of the food. After all, it makes sense for her to invest in off spring with higher chances of survival. Conversely, the young earwigs can also smell when their mother is sick, meaning that provisions are likely to be scarce. In short: the earwigs communicate with each other.
Likewise, the caterpillars of gossamer-winged butterflies produce chemical substances that make them smell familiar to worker ants, tricking the ants into adopting and raising them. Larvae of the burying beetle use their legs to tickle their parents’ heads, causing them to regurgitate their food and give it to their offspring, while juvenile treehoppers vibrate the branch they are on to signal danger and request maternal assistance.
Cost-benefit analysis
Ingenious young insects employ countless tricks and survival strategies of this sort. Are they unique in this regard? “Other species exhibit a broad range of intra-family care arrangements, too – as well as rudimentary forms of communication, and conflicts,” says zoologist Dr. Mathias Kölliker, formerly an SNSF professor at the University of Basel and now a curator at Basel’s Natural History Museum and a family man himself. Kölliker used to explore the evolution of family life in animals by studying earwigs. Today, he deals with fundamental questions in ecology and evolution, publishing papers and curating exhibitions.
Parental care increases the chances of an animal’s genes living on in its offspring. For this to happen, protection against harmful germs and predators is a must for all living beings. Many species, Kölliker says, simply leave their offspring to their own devices – such as frogs that lay their spawn in water. In these cases, survival is left to chance. In the course of evolution, under certain conditions species that developed care strategies – which, to begin with, consist in staying with their offspring and protecting them – prevailed.
It is simply a matter of weighing the costs against the benefits, Kölliker says: “Care involves expending energy and time that progenitors could otherwise use to reproduce.” Later in the evolutionary process, parents began to supply their newborns with food and teach them basic skills to help them manage life on their own and compete successfully.
Over time, this has led to a growing dependence of young animals on their parents that can no longer be reversed: for birds and mammals, brood care has become so important as to be indispensable. “Without parental care and nurturing, these young animals wouldn’t stand a chance.”
Keep it down!
Hungry off spring have come up with a variety of ways to draw attention to themselves: birds cheep, for instance, while insects produce pheromones – smells undetectable to the human nose. Adult birds, for their part, use warning cries to keep their offspring from begging for food too loudly whenever a bird of prey is circling overhead.
“Once brood care emerges in the evolution of certain species, it is followed by communication and social contact, and consequently a form of family life, a first step toward more sophisticated forms of co-existence,” Kölliker observes, adding that this is even the case when animals are manipulated into caring for the offspring of another species, as in the case of cuckoos or gossamer-winged butterflies.
He reports that for many insects and mammals, it is primarily the females that care for their young, while in birds the burden is almost invariably shared between male and female. However, “parents often disagree about which of them should perform a given task, which can also lead to conflicts.”
In the case of certain insects and most species of fish, it is the males that look after offspring, while females defend their territory. A particularly extreme example of parental care described by Kölliker relates to certain female spiders: After laying their eggs, they sacrifice themselves by predigesting their insides and lying on top of the spiderlings, which consume their mother’s body until she eventually dies.
In species where the male parent is not involved in brood care, offspring are sometimes killed by males. This can occur in species as diverse as earwigs, lions or polar bears, requiring females to protect their offspring against the often highly aggressive males. There are also species in which care duties are shared by individuals other than the parents. These “helpers” can be relatives, offspring from a previous brood, or outsiders that are tolerated in exchange for their childcare duties, a form of cooperation known as “pay to stay” among researchers.
The origins of milk
Comparative studies have shown that brood care as a strategy for efficient reproduction has evolved independently in a number of diff erent groups of animals, the zoologist recounts. But is parental care something that can be compared across all species, from single-celled organisms to humans? “While it can take very different external forms, the fundamental challenges faced by parents are not so different.” Kölliker has a keen eye for mechanisms occurring within his own family, and finds it “comforting to observe that human beings are a part of nature, and do not operate on an entirely different level to the other creatures on the planet.”
He concludes with a remarkable story about how milk is likely to have evolved: certain female insects, like earwigs for example, treat their eggs with antibiotic substances to protect them against bacteria and fungi. The very first mammals, around 200 million years ago, still laid eggs, and similarly coated them with a bodily secretion for protection. This substance is thought to be a precursor to mammalian milk. Rather than providing nutrition, its original function was to offer protection against harmful organisms – an antibiotic effect that milk still retains today.
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