Author: Joba without antlers
Edit: Yuki
When it comes to breastfeeding, the first image that comes to mind is probably this:
Image source: Figureworm Creative
Or something like that.
Yes, it is assumed that mammalian behavior is a characteristic and specialty of mammals – after all, they are originally named for the ability to produce milk through the breast glands to feed their offspring.[1] Although this feature has always been "imitated" by other animals, it has never been surpassed.
For example, some birds (pigeons, penguins, flamingos, etc.) have behavior similar to "breastfeeding", but they do not have mammary glands, and can only produce nutrient-rich "milk" from the throat sac to feed the young birds.[2]
Flamingos are feeding their young birds with "milk". Image credit: robin müller| wikimedia commont
Another example is the diploptera punctata, a fetal cockroach discovered by scientists a few years ago, which can also produce a high-protein "crystalline milk" to provide nutrition for offspring [3], but they do not feed offspring for a long time.
"Crystalline milk" in Pacific cockroach embryos. Source: http://journals.iucr.org
Recently, a small spider less than 1 centimeter long broke the monopoly that has been monopolized by mammals and became the first invertebrate other than mammals that can "lactate" and raise offspring for a long time.
The intriguing new discovery, led by a team of Quan Ruichang and Chen Zhan of Xishuangbanna Botanical Garden in China, just appeared in the November 30 issue of Science magazine.
Dr. Chen Zhanqi and his big ant spider. Photo: Wang Xiaoliang
This overnight popular "mammal" spider is called the big ant spider (toxeus magnus), which lives in tropical and subtropical areas such as Singapore and Guangdong, China. Although the large ant spider looks like an ant at first glance, a closer look reveals that it is actually a jumping spider that "pretends to be an ant".
Toxeus magnus ant-like spider (toxeus magnus) Source: sarefo| independent
Researchers have found that large ant spiders can not only produce milk, but also take care of young spiders step by step until adulthood. This phenomenon is very rare in invertebrates, and it has not been found in other spiders at present, which can be said to subvert everyone's existing cognition.
How did scientists discover the secret of the giant ant spider's breastfeeding?
Nest of the wild giant ant spider. Image source: References[2]
In their natural state, the nests of large ant spiders are closed and secretive (as shown above), and it is not easy to observe the inside of the nest, while in the laboratory, the transparent boxes used to cultivate them provide convenience for scientists to observe their behavior.
A box used in the laboratory to keep large ant spiders. Image source: References[2]
In the process of studying the large ant spider, the researchers inadvertently found a strange phenomenon - the larvae of the large ant spider never left their nest in the first 20 days of life, and the mother did not bring food back to the nest from outside during this period, but the size of the larvae became nearly four times larger in 20 days (from 0.9 mm at birth to 3.5 mm on the 20th day).
This made the researchers curious: How did the female giant ant spider feed her offspring?
In the process of raising large ant spiders, researchers inadvertently found that many juvenile spiders will surround the mother's abdomen for a long time, just like the behavior of young people in mammals who actively ask their mothers for milk.
The baby spider crawls to the mother's abdomen in search of "milk". Source: sciencemag.org
After more careful observation, the researchers were pleasantly surprised to find that the female large ant spider had a reproductive groove in its abdomen and secreted a milk-like droplet that the juvenile spider grew up by sucking on these droplets.
Enlarged figure of the female giant ant spider's abdomen (a) and female giant ant spider's reproductive groove and milk (b). Image source: References[2]
Through further analysis, the researchers found that the main components in the milk of the spider are proteins, fats and sugars. Compared with milk, the fat and sugar content will be slightly lower, but the protein content is four times that of milk, so it seems that spider milk is still a well-deserved high-nutrition dairy product[2]!
During the first 7 days of hatching, the mother of the large ant spider will secrete milk throughout the nest for the baby to eat. After 7 days, the female spider no longer actively secretes milk, and the baby spider will climb to the mother's reproductive groove to find the milk on her own.
When scientists seal the female spider's breast with a graffiti solution, the young spiders will still flock to the past in search of milk. Source: sciencemag.org
Starting 20 days after birth, these young spiders begin to learn to leave the nest to find food, but at the same time, they are not completely "weaned", just as babies begin to eat "complementary foods".
It is not until 40 days after the birth of the juvenile spiders, which is close to the stage of adulthood, that they begin to rely entirely on going out to prey to fill their stomachs, and at this time the juvenile spiders are close to 4/5 of the mother's.
These baby spiders are very "loving family", and they do not forget to "go home to see" after going out to eat every day: even if they are completely weaned, in the next 20 days, the baby spiders will leave the nest during the day to find food, and return to the nest at night for the night.
It is not until after 60 days of growth, when the larvae are fully adult, that they no longer return to their nests and roam the world on their own.
So, what is the significance of the behavior of "feeding" for the growth of juvenile spiders?
The answer is essential! If the baby spider does not drink "breast milk" at all, it will all die within ten days or so after hatching. In the 20-40-day-old "complementary food" period, although it is possible to not be completely dependent on breast milk, breastfed larvae are more likely to survive than larvae without breastfeeding [2].
In addition to providing "breast milk", the meticulous care of the mother of the big ant spider for the baby spider can also allow more babies to survive. For example, in the case of the same lack of breast milk, juveniles with female spiders in the home after 20 days of incubation have a higher survival rate than juveniles without females.
On the one hand, because female spiders can better protect juvenile spiders from predators, on the other hand, female spiders can regularly remove parasites from their nests, providing a better environment for the growth of juvenile spiders.
What is more interesting is that the large ant spider family is more "female over male" - after the adult baby, the female spider will expel the adult male individuals from the nest, so that the proportion of daughters in the nest is significantly higher than that of the sons.
Researchers speculate that this phenomenon is probably due to the fact that more females are beneficial to produce more offspring and more beneficial to growing the family .[2]
After the baby spider is born, the body size changes over time. The abscissa coordinate is the number of days after hatching, and the ordinate coordinate is the change in juvenile body length (mm). Within 0-20 days of incubation (a + b stage), the larvae have not yet left the nest and survive entirely by eating breast milk; after 20 days of incubation, the larvae begin to leave the nest to prey on other foods and survive by preying on other foods + breast milk (stage c); 40-60 days after hatching, the young spiders begin to gradually stop eating breast milk and rely entirely on predation, but will still return to the original nest (stage d); fully adulthood, began to live independently (stage e). Image source: References[2]
Previously, it was generally assumed that these behaviors existed only in groups of higher social mammals with longer lifespans. It can be said that the nursing behavior and long-term care of offspring displayed by the giant ant spider have refreshed our traditional impression of invertebrates.
This new discovery by scientists in the giant ant spider has forced us to rethink the evolutionary origins, evolutionary history and significance of the phenomenon of breastfeeding. Perhaps, the giant ant spider is not alone, and more invertebrate "mammals" will be found in the future!
Author business card
Typography: Xiao Shuang
Source: Photo: Wang Xiaoliang
Reference Links:
[1]https://zh.wikipedia.org/wiki/%e5%93%ba%e4%b9%b3%e5%8a%a8%e7%89%a9
[2]z. chen et al., “prolonged milk provisioning in a jumping spider,” science, doi:10.1126/science.aat3692, 2018.
[3]https://www.sciencealert.com/scientists-think-we-should-start-drinking-cockroach-milk-superfood