New scientific discovery! Why does the rampant growth of rats have obvious seasonal patterns?

Produced by: Science Popularization China

Author: Bingbingbang (Institute of Zoology, Chinese Academy of Sciences)

Producer: China Science Expo

The four seasons change a lot. Spring is warm, summer is hot, autumn is cool, and winter is freezing. The reason why we can experience the difference of the four seasons is that the rotation and revolution of the earth cause the annual changes of light, precipitation, temperature, etc., resulting in these distinct four seasons.

(Photo source: Veer Gallery)

"Spring is here, everything is coming back to life, and it's the mating season for animals again..."

This is a sentence that often appears in "Animal World". As the weather gradually warms up, everything seems to be coming back to life. Why do animals' activities vary in different seasons?

In order to adapt to the changes in the environment in different seasons, mammals living in temperate and frigid zones have gradually developed a series of seasonal physiological characteristics that are synchronized with changes in the external environment during the long process of evolution. They can perceive and predict environmental changes in a timely manner and respond appropriately to them, thus catering to the cyclical alternation of abundance and scarcity of survival resources.

Among them, reproduction behavior, as a high-energy-consuming physiological behavior, is naturally limited to a specific period of each year, which leads to the typical seasonal reproduction phenomenon in small animals.

The photoperiod is the most stable environmental factor among all environmental factors with periodic changes, so it has become the most important basis for animals to predict environmental changes. In other words, animals can perceive seasonal changes by sensing the length of light and darkness in the outside world, and thus adjust their body state to a state suitable for reproduction in the appropriate season.

(Photo source: Veer Gallery)

Voles that breed in spring and summer

I don’t know if you have noticed that some small animals always like to go out in warm seasons. In fact, the relatively warm period may happen to be their breeding season.

Take Brandt's voles distributed in the grasslands of Inner Mongolia as an example. As a typical wild mouse species, its breeding season starts in the spring and summer (March to September) when the day length gradually increases, and ends in the autumn and winter. This characteristic of only breeding in a specific season of the year is called seasonal reproduction. So how is this seasonal reproduction regulated by the photoperiod?

A team led by researcher Zhang Zhibin from the Institute of Zoology, Chinese Academy of Sciences, recently discovered that intestinal microorganisms play a vital role in the seasonal reproduction of Brandt's voles.

An article entitled "Gut microbiota is associated with the effect of photoperiod on seasonal breeding in male Brandt's voles (Lasiopodomys brandtii)" published in the microbiology journal Microbiome shows that the photoperiod can regulate the seasonal reproduction of Brandt's voles by affecting the types of microbial flora in the vole's intestines (gut microorganisms refer to the large number of microorganisms that exist in the intestines of animals. This group of microorganisms lives in the intestines of animals and helps the host animals complete a variety of physiological and biochemical functions) and acting on the HPG axis-melatonin-kisspeptin/GPR54 system.

(Photo source: Veer Gallery)

The team artificially manipulated the length of the light cycle indoors to measure the physical condition characteristics of Brandt's voles under different light cycles.

The results showed that Brandt's voles exposed to short light cycles not only had a slower weight gain, but also a significant decrease in the weight of their gonads (testicles and epididymis), the levels of reproduction-related hormones (such as follicle-stimulating hormone, luteinizing hormone and testosterone) and the expression levels of related genes (such as Dio2, Rfrp-3, Kiss-1 and GPR54), which can be seen as a manifestation of decreased reproductive capacity.

These data indicate that Brandt's voles have better reproductive capacity under long photoperiods than voles living under short photoperiods, which also explains why voles have the most reproductive behavior during the longer photoperiod period from March to September.

Effects of different seasonal photoperiods

From the above research, we know that the photoperiod will affect the microbial community in the body. So how are the specific microbial communities affected?

The research team measured the composition and structure of the intestinal microorganisms of voles under different light periods, and found that the light period does determine the microbial groups in the intestines of voles.

Researchers found that voles living under short light periods and voles living under long light periods showed significant differences in intestinal microbial composition. After transplanting feces from Brandt's voles living in long and short light periods, they found that the levels of three reproduction-related hormones (melatonin, follicle-stimulating hormone and luteinizing hormone) and three reproduction-related genes (hypothalamic Kiss-1, testicular Dio3 and Dio2/Dio3 ratio) in the voles that originally lived under short light periods became consistent with those of voles living under long light periods after receiving feces transplants from voles living under long light periods.

This proves that the photoperiod does affect the composition of intestinal microorganisms, and the composition of intestinal microorganisms further affects the reproductive capacity of animals. Changes in intestinal microorganisms affect more than just the reproductive capacity of animals. In addition, the above-mentioned transplantation of intestinal microorganisms also changed the activity rhythm of Brandt's voles. After receiving the microbial transplantation of voles living under a long photoperiod, the activity rhythm of voles that originally lived under a short photoperiod became consistent with that of voles living under a long photoperiod. This shows that in addition to affecting the reproductive performance of animals, intestinal microorganisms also affect the activity rhythm of animals.

(Photo source: Veer Gallery)

Through the above experiments, researchers can almost confirm that the light cycle in the external environment can significantly affect the intestinal microbial community of animals, and this change in the intestinal microbial community determines the reproductive performance of animals by acting on the HPG axis-melatonin-kisspeptin/GPR54 system in the hypothalamus, ultimately forming a phenomenon that reproductive performance is closely related to the light cycle.

Enlightenment from the study of seasonal changes

Studying the changes in photoperiods in different seasons not only explains why rodent reproduction has obvious seasonal characteristics, but also provides important theoretical guidance in pest control, livestock breeding and human health management.

For example, the intestinal microbiota after exposure to a specific short light period can make the host fall asleep more easily, which can be of great benefit in improving sleep problems of some patients with difficulty falling asleep or jet lag problems of people who travel frequently on business trips.

In addition, the forestry or agricultural sectors can use microbial communities exposed to specific short light periods to slow down the reproduction of harmful rodents and avoid excessive populations of pests and rodents, thereby achieving the goal of controlling rodent infestations.

In addition, the reproductive performance of livestock in many farms is also greatly affected by the photoperiod (for example, horses and donkeys prefer to reproduce under long photoperiods, while sheep and camels prefer to reproduce under short photoperiods). Therefore, farmers can also use the microbiota after exposure to a specific photoperiod to regulate the reproduction of livestock and reap greater economic benefits.

Editor: Guo Yaxin