This video, from the PBS “Origin of Everything” series, discusses the origins of gender in humans. The concept of gender is distinguished from biological sex, and the homology of sex organs is briefly discussed as it played a role in developing the concept of gender in some societies. Most of the video is spent how ideas of gender have developed through history in multiple cultures. We noted a valuable and rarely made distinction : the video discusses how the concept of “man” has often been closely associated with race and class, not just with gender.
Without sharing food or grooming relationships among all genders, 80% of vampire bats would die from a missed meal instead of 25%. (Gender Showcase, 9-12)
The vampire bat (Demodus rotundus) is a small bat, no bigger than a plum, that “removes a small patch of flesh [from its prey] with its razor-sharp incisors and laps up the blood flowing from the wound. A vampire’s saliva has anticoagulant to keep the blood from clotting. After one bat has drunk its fell, another continues at the same spot.”
Hunting for food is hard and up to one-third of bats miss a meal on any given night.
Bats die after 60 hours without food, after freezing from dropping 25% of its weight.
Life as a vampire is hard. Bats are warm-blooded and, without feathers or fur, lose lots of heat. Their requirements for energy are huge. A vampire bat consumes 50 to 100 percent of its weight in each meal. Yet up to one-third of the bats may not obtain a meal on any given night.
Going without a meal is dangerous. A vampire dies after sixty hours without food because by then its weight has dropped 25 percent, and it can no longer maintain its critical body temperature. To survive, vampire bats have developed an elaborate buddy system for sharing meals. The sharing takes place between mother and pup, as well as between adults.
The dominant male bat fathers half the group’s young by staying and guarding the inner top of the nest.
Subordinate males hang out by the entrance.
Photo credit: Alex Hyde/NPL (https://www.nature.com/articles/d41586-019-03005-5)
One study of vampires on a ranch in Costa Rica focused on a population divided into three groups of a dozen females. The members of a group often stay together for a long time, twelve years in some cases, and get to know one another very well. The group of a dozen adult bats is a family unit from a vampire’s standpoint.
Most of the group consists of females, each of whom usually cares for one pup. A female pup stays in the group as she matures, whereas a male pup leaves. The females in a group span several generations. Group membership is not entirely static, however. A new female joins the group every two years, so at any time the females in the group belong to several lineages, called matrilines.
The bats live in the hollows of trees. Imagine a hollow tree with an opening at its base and a long vertical chamber reaching up into the tree trunk. The females congregate at the top of the chamber. About three males hang out, so to speak, in the tree hollow.
One male assumes a position near the top of the chamber, nearest to the females, and defends this location against aggressive encounters from other males. This dominant male fathers about half of the group’s young. Subordinate males take up stations near the base of the tree by the entrance. Other males are out of luck, roosting alone or in small male-only groups rarely visited by females.
70% of food transfers are mother to pup.
The other 30% are female to someone else’s pup, female to female, and male to pup.
Photo credit (C) Gerry Carter
The food is transferred by one bat regurgitating into the mouth of another. Most (70 percent) of the food transfers are from a mother to her pup. This food-sharing supplements the mother’s lactation [milk production]. The other 30 percent involves adult females feeding young other than their own, adult females feeding other adult females, and on rare occasions, adult males feeding offspring.
Increased cooperation in bats includes same-sex grooming and food sharing.
Some adult females have a “special friendship” with females who are not their kin (males also have same-sex relations; see Roughgarden 141). This bond is brought about in part by social grooming. The bats expend 5 percent of each day grooming and licking one another. Some of this grooming is between special friends, and the remaining among kin. A hungry bat grooms one who has recently fed to invite a donation of food. To solicit food, a hungry bat licks a donor on her wing and then licks her lips. The donor may then offer food.
The mutual assistance is significant. If they didn’t help each other, the annual mortality of vampires would be about 80 percent, based on the chance of missing a meal two nights ina row. Instead, the annual mortality rate is around 25 percent because food-sharing tides bats through their bad nights.
References
G. Wilkonson, 1990, Food sharing in vampire bats, Scientific American (February), 76-82.
Roughgarden uses “cooperation” here in the wider sense to include both helping and not hurting. See R. Trivers, 1984, Social Evolution, Benjamin-Cummings.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 61-2.
The groove-billed ani bird has 4 types of family structures: 2, 4, 6, & 8 adults. The larger the family, the more protection from predators. (Gender Showcase, 9-12)
Larger groups provide more protection against egg predators.
The ani family shows up in groups of 2 adults (1M1F), 4 adults (2M2F), 6 adults, and 8 adults.
The groove-billed ani (Crotophaga sulcirostris) is an insectivorous black bird with a large, deeply grooved bill. It lives in marshes and open pastures in Central America and is related to the cuckoo. Family organizations of the anis may consist of twosomes with one female and one male, foursomes with two females and two males, sixsomes, and even eightsomes.
An ani foursome is two couples cohabiting a one-bedroom flat with one crib. Nests are built in thorny trees or vines. Each male guards one of the females. A female lays an egg every one to two days. The time from egg-laying to fledgling is three weeks.
Females lay fewer eggs each in 4some than in 2some by laying at different times but stop at same time.
Longer ovulator gets 37% of eggs raised, shorter ovulator gets 63% of eggs raised. Longer ovulator lays larger eggs that hatch earlier w/ better chance of survival.
The two females in a foursome wind up with four eggs total. A female in a twosome can also produce four eggs by herself. Thus the number of eggs laid per female is lower in a foursome than in a twosome.
In a foursome, the females start laying eggs at different times. The starter lays bigger eggs and has a longer time between successive eggs than the follower. Both females stop laying eggs at about the same time. Each female “tosses” out of the nest some of the eggs already laid by the other, with the follower winding up mothering an average 63 percent of the eggs and the starter only 37 percent of the eggs. After the tossing is over, four eggs are left in the nest.
Even though the follower mothers more of the eggs, she doesn’t necessarily successfully raise the most offspring. The starter lays larger eggs, which hatch earlier, so these chicks have a better chance of surviving than chicks from the follower.
All four birds in a foursome work together to provide for and protect the eggs.
Males divide day into unequal shifts based on oldest getting riskiest shift but also fathers most young.
Photo credit (C) Dave McCauley.
The males divide the day into unequal shifts. The oldest male incubates at night and most of the daylight hours. As a result, he also incurs the most hazard and the highest death rate. Yet he also fathers the most young.
The benefit from predator protection in a larger nesting group outweighs the disadvantages of a crowded family life.
The starter in a foursome produces the most young, a female in a twosome produces an intermediate number of young, and the follower in a foursome the fewest.
Photo credit (C) Joseph Morlan
By why should a female, who can lay four eggs in a twosome without worrying about a nest mate tossing her eggs out, bother living in a foursome? The answer is that the larger group provides protection against egg predators.
Once loss of eggs to predators is taken into account, the starter in a foursome produces the most young, a female in a twosome produces an intermediate number of young, and the follower in a foursome the fewest. For the anis, the benefit from predator protection of living in extended families of two couples outweighs the disadvantages of a rancorous life at home.
“It’s a cooperative soft goth bird!” —RXS
Citations
S. Vehrencamp, 1978, The adaptive significance of communal nesting in groove-billed Anis, Behav. Ecol. Sociobiol. 4:1-33.
B.S. Bowen, R.R. Koford, and S.L. Vehrencamp, Breeding roles and pairing patterns within communal groups of groove-billed Anis, Anim. Behav. 34:347-66.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 57-8.