Eat, drink or mate? Here’s how mice decide

Do you know that under the right circumstances, mice might prefer to socialize with their opposite sex than to eat – even when moderately hungry? 

Researchers from the Institute for Systems Physiology at the University of Clinic Cologne in Germany discovered that mice might forgo eating in favor of socializing with the opposite sex. 

The animal model study [1], published in the Cell Metabolism journal, aimed to examine mice’s mechanisms in ranking and sensing nutritional needs over other competing demands. It is long recognized that animals weigh thirst and hunger over other competing requirements, such as mating or socializing with others. 

All about the study

In the study, the researchers treated the male mice with a technique that imitates the effect of a hormone called leptin. Leptin acts on the brain by suppressing hunger or appetite. The researchers observed that treated mice were more likely to approach female mice than their food bowls despite being deprived for almost 24 hours. 

The results were surprising since these revealed that leptin tends to affect the social behavior of male mice. The study offered an essential step into understanding how mice behave when confronted with ongoing needs such as hunger and thirst against different social and behavioral options. 

Findings illuminated how the brain juggles different demands. The researchers, led by Anne Petzold from the University of Cologne in Germany, examined how much decision-making is affected by the hormone leptin. In the study, leptin activated specific cells in the brain that promote a feeling of fullness.

Once the researchers injected the mice with leptin in the study, the mice exhibited suppressed feeding as expected. However, leptin injection also made the mice interact more with female mice. 

Experiment on how mice decide what to eat, drink or mate

The explanation

The lateral hypothalamus is an area of the brain called the hunger center. Leptin activates this hunger center. The authors’ experiment revealed that neurons that can be activated by leptin suppressed hunger when the mice interacted with female members of the study.

Optogenetics artificially activated these neurons, which raised the likelihood that the male mice would approach the female members of the study. The results of this experiment suggested that leptin plays a role in promoting the social behavior of the male mice. 

Interestingly, when mice were stimulated with optogenetics, their leptin-activated neurons tended to favor socializing with the opposite sex than eating food, even when deprived of food for almost 24 hours.

However, in mice deprived of food for four to five days or 96 hours or more, even when their leptin-activated neurons are activated by optogenetics, they tend to choose food over socializing. 

One of the study’s authors explained that when mice were subjected to prolonged hunger, other systems came into play that would make the mice prioritize food over socialization. 

Needs of the mice

Scientists initially thought leptin was essential in suppressing hunger or responding to metabolic signals. Researchers did not expect that leptin may play a role in recognizing social cues or promoting social behavior.

However, these scientists point out that the study’s results made sense. The body initially produces leptin when it is full and the animal’s energy needs have been met. 

Leptin suppresses satiety or appetite once an animal is full. This suppression allows animals to focus on other interests, not only food. One of the interests would appear to be the attraction to the opposite sex. 

While it is impossible to translate the findings of this study to human behavior, it should be noted that the leptin system is present in diverse groups of animals. For example, even flies have receptors for leptin. 

Studying the relationship between leptin and social behaviors in mice might hold clues to why people with eating disorders such as bulimia also exhibit social phobia. 

How thirst might influence social behavior 

The same study also found that thirst might have an opposite effect on behavior compared with leptin. The study’s authors examined a group of neurons that produce neurotensin, a hormone linked to thirst. When these neurons were stimulated, this promoted drinking over social behavior or eating. 

Leptin and current studies 

An article published in the Journal of Medicine and Life outlined the latest studies on leptin and its uses and functions [2]. 

Leptin is derived from fatty or adipose tissue, suggesting that this tissue also acts as an endocrine gland and not only for fat storage, as previously recognized. 

Here are some functions of leptin: 

  • Acts as the brain’s energy regulator by inducing anorexic factors and suppressing appetite factors
  • Leptin reduces intake and increases the expenditure of energy 
  • Low leptin levels can stop energy expenditure and increase food absorption 
  • Increased leptin increases energy consumption and suppresses appetite. 

Leptin and different types of cancer

Current studies on leptin showed that it might play a role in developing breast cancer. As pointed out by the study authors [2], leptin receptors can be used as a maker of breast cancer’s prognosis. For instance, leptin levels were markedly higher in those with advanced tumours than those with early cancer stages. 

Apart from a link with breast cancer, leptin may also be implicated in the development of colorectal cancer. It is also thought to play a role between colorectal cancer and obesity. 

Leptin and colorectal cancer

Leptin and lupus and diabetes

In addition, leptin may also be associated with the development of lupus erythematosus (SLE). Systemic lupus erythematosus is a systemic autoimmune disease characterized by vasculitis and inflammatory arthritis. Patients with SLE exhibit high levels of leptin compared with those without SLE. 

Leptin has been shown to affect type 2 diabetes. Leptin has been shown to do the following: 

  • Decrease plasma glucose and plasma insulin 
  • Increases insulin sensitivity 

Although leptin has been initially thought crucial in regulating appetite and hunger and the increased breakdown of fats, this hormone may also promote social behavior. As the journal article [1] pointed out, leptin might help hungry mice choose mating or sex over food. 

In addition, leptin levels might not change in response to negative emotions and psychological stressors. In a study [3] published in the Physiology and Behavior journal, findings indicated that when both overweight and normal-weight women were exposed to psychological stressors, which generated negative emotions, these women tended to overeat, but their levels of leptin remained unchanged.

The findings showed that there is a relationship between food intake and emotions despite normal levels of leptin in the plasma. 

Since the study was conducted over a short period with the women exposed to psychological stressors, it was difficult to ascertain if chronic exposure to distress or stressors could affect leptin levels in the plasma.

Hence, hormones do not only influence or suppress food intake. Instead, food intake may be influenced by psychosocial factors such as exposure to stressors or being with friends. 

Take home message

Leptin is an important hormone that regulates appetite and helps people feel full following food intake. In animal model studies, leptin may influence social behavior and allow hungry mice to choose mating over food. Although the study showed that mice subjected to prolonged hunger choose food over mating, the findings indicated that leptin might play a role in social behaviors.

Currently, there are still no studies examining the impact of leptin on human social behaviors. Since there are complex factors that influence human social behavior, it would be difficult to point out how leptin can influence humans to choose competing interests when hungry or deprived of food and water for several hours. 


Photograph: wirestockc/Envato
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