Cooperative behaviors, such as food sharing, can be studied under a spectrum of conditions from completely natural to very artificial. For example, a careful observer in the field can measure the extent to which male chimpanzees will share meat in the wild after a hunt, or an experimenter in the lab can test the extent to which two chimpanzees in separate cages will pass food to each other by operating a simple mechanical device (like pulling a tray that delivers food to another individual).
There are clear pros and cons to each of these approaches, and each will give you different sorts of information about the animals. For example, field observations of cooperation, albeit natural, won’t allow you to manipulate the situation very much, and often cannot show you what enforces cooperation (i.e. what protects it from freeloading). On the other hand, controlled studies in captive settings won’t necessarily elicit the natural evolved behaviors in which we are all interested. So, for a given helping behavior in a given species, it would be interesting to see what happens when natural forms of cooperation are tested under artificial conditions.
I asked this question in vampire bats by testing if vampire bats would still share food if they had to pass it through a barrier– the metal bars of a cage. This is, of course, a completely unnatural situation. Normally, the bats groom each other awhile before regurgitating food. But if the bats are motivated to feed another individual and not just responding, perhaps they will adapt their food sharing behavior to the novel situation.
After first observing that wild vampire bats will feed unfed individuals inside their roosts, Wilkinson began inducing food sharing by keeping a bat from feeding overnight, first in the wild and then in captivity. For the past couple of years, I have been doing similar trials many times with captive vampire bats at the Bat Zone in Bloomfield Hills, Michigan. This has allowed me to map out the food-sharing network in a group of common vampire bats with varying relatedness but that all know each other equally. One of the things that surprised me is that the majority of the donors approach the hungry bats, not vice versa. This led me to believe that food sharing could not be explained as merely a response to harassment, something which you sometimes see in primates.
In the last couple of weeks, I’ve been placing the hungry bat first inside a small cage in the corner of the larger walk-in colony flight cage, before releasing it into the colony cage. Not only is having to feed a trapped bat across a barrier a novel situation, but the donor and recipient would not be able to groom each other beforehand. This is relevant because Wilkinson speculated that social grooming might allow the bats to detect cheaters by feeling the fullness of a partner’s belly. Also, a trapped hungry bat would not be able to beg or solicit a food donor. Any donating bat would have to willingly fly across the flight cage in order to feed the hungry trapped individual. For all these reasons, I was highly skeptical that this would work, but I thought it was worth testing (my main goal here was to record social calls and get fresh DNA).
Much to my surprise however, when an unfed bat was trapped in a wire cage, other individuals would sometimes come down and regurgitate food to this hungry bat by pressing their face against the metal bars. It was not so surprising that mothers sometimes did this for their grown offspring, but I was more surprised that the adult offspring would also sometimes pass their food across the cage wall to their mothers. Also noteworthy was that the food sharing across the cage wall barrier was much less common than the more natural form of food sharing– hanging belly to belly with copious social grooming before and after. Whereas the natural form occurred for almost every subject, only a few bats regurgitated small amounts of food across the metal bars of a cage.
To me, this demonstrates that even though a given species might often share food in nature, if you make the sharing behavior more difficult or artificial in a lab environment, they might not cooperate as much or even at all. It may also be true that animals that normally would not share food under natural circumstances, might then actually do so under certain lab conditions, for example, if they learned how to obtain a food reward by cooperating during the course of the experiment. This would be simply operant conditioning, not a reflection of whether this species was particularly social or predisposed to cooperation. So caution must be used when interpreting the results of such experiments.