Natural history of a mother
For many a newborn in the animal kingdom, survival to adulthood rests crucially on the care they receive from their parents, and in particular from their mother. Maternal care comprises many different types of behaviours that, perhaps most prominently, include diverse ways to feed their young. From the trophic eggs that some amphibians and invertebrates lay beside their fertilized eggs to the nutritive epidermal mucus segregated by some fish, nature’s mothers showcase an astounding array of ways to feed their brood. Queen above all, or so we like to think, stands mammalian milk. Produced by the mammary glands that name our taxonomic group, this chalky liquid constitutes the only source of nourishment for the common mammal newborn, and is exquisitely designed to tackle the particular challenges faced by each species in the wild. In the gelid waters in which the hooded seal breeds, floating amidst an unstable world of ice that only allows for four days of nursing, fat is critical for survival. With a 60% content in fat, this seal’s milk contains between 10 and 20 times as much fat as human milk. More than many ice-creams. In our own species, breastmilk contains certain substances that cannot be assimilated by the baby, but whose aim is rather to nourish beneficial bacteria in our gut. These are but a few examples that support the instinctive notion that mammal mothers are nature’s utmost, most loving, parents.
«Maternal care comprises many different types of behaviours that, perhaps most prominently, include diverse ways to feed their young»
Toxeus magnus females behave like the typical mammal mother. After giving birth, they nourish their young with milk rich in proteins, fat and sugars. This is the only source of food the young will know until they are capable of hunting by themselves. Beyond that point, mothers will continue to supplement their teenage offspring with milk, increasing their survival and, ultimately, their reproductive success. During this time in which they share the nest until their young grow into adults, these dedicated mothers will also regularly invest time in cleaning the family home to keep it free of parasites. Even more, when the time comes for their male offspring to disperse, they will still shelter their female offspring for a while. Such staunch care and steadfast determination are not in vain. Quite the contrary, mothers of this species secure the survival of around 75% of their young, a feat well above what is common in nature even among birds and mammals. The natural history of these mothers is admirable, but surely not that extraordinary. After all, it’s the mammalian way. So much so that we can easily picture these mothers grazing among the top grass with a calf clumsily stumbling along, or maybe patiently stalking a prey in the African Savannah, while her young play in the background within sight of the pack.
Yet, these particular mothers live in no pack or flock, they shed no hair or feather, they have no breasts. These mums milk their brood using modified glands in their abdomen, clean their nest scuttling on eight legs, and hunt with the aid of sharp chelicerae fangs and a body that closely resembles their favourite prey, ants. Through their four pairs of eyes, these peculiar mothers see the world in a way until recently unknown to science, but perhaps far more common than we suspect. Behold the “spider-sense”1.
1 The original scientific paper on which this piece is based was recently published in Science (Chen et al. 2018. Prolonged mil provisioning in a jumping spider. Science. 362, 1052-1055). It is the first description in an invertebrate of milk of strikingly similar characteristics to mammal milk along with parental care that extends beyond sexual maturity. (Go back)