Bizarre Facts in Biology

Home > How Organisms Evolve > Bizarre Facts in Biology

Bizarre Facts in Biology

No Sibling Rivalry Here
by Stephen Kilpatrick

The evolution of eusocial systems in the animal kingdom was once a mystery. Meaning "truly social," eusociality is characterized by a reproductive division of labor, overlapping generations, and having a class of workers (usually sisters of the offspring) cooperate in care of the young . Only one or a few females (the "queens") actually reproduce with only a few of the males. There are "sterile castes" of males and females that do not reproduce, but perform the necessary tasks of the colony, including helping to raise the offspring of those that do reproduce. At first glance, it seems that this system is doomed from an evolutionary perspective. Why would the majority of the colony sacrifice their own reproductive fitness to benefit those that do reproduce? Assuming this altruistic behavior is genetically based, it should be quickly eliminated by natural selection, since those altruistic genes would not be directly passed on to future generations.

For some eusocial species, the key to the mystery is in the genetic relatedness of the individuals. Most eusocial species are insects belonging to the orders Hymenoptera (which includes bees, wasps, and ants) and Isoptera (termites). Hymenopterans are haplodiploid, meaning that males mature from unfertilized eggs and are haploid, while females mature from fertilized eggs and are diploid. Fathers cannot have sons, only daughters. Fathers pass along all of their genes to their daughters, and sisters share 100% of the genes from their father and 50% from their mother, for an average of 75% relatedness. (Contrast this with more common diploid animals, in which full siblings have 50% relatedness to each other and to each of their parents.) Since females in haplodiploid systems are 50% related to their own daughters and 75% related to their sisters, they actually ensure survival of more of their own genes by helping their sisters to survive rather than having their own offspring. The males, on the other hand, are 100% related to both their mothers and their daughters, so it benefits them just as much to aid their mothers as to reproduce. In other words, the inclusive fitness of both males and females is higher than their individual fitnesses, and the altruistic behavior is favored.

Strangely enough, there are eusocial mammals, specifically the African naked mole rat, which lives in underground colonies much like ants. Unlike other mammals, they are ectotherms; they warm themselves by basking in the shallow surface tunnels during the day. One queen reproduces with a few males, while the rest of the males and females are either "workers" maintaining the colony or "soldiers" defending the colony. Researchers believe that behavioral interactions with the queen cause the nonbreeders to suppress their own hormone production, rendering them infertile. Unlike eusocial insects, naked mole rats are not haplodiploid, so why would such a system be favored? It has been suggested that because environmental conditions in the naked mole rats' habitat are so harsh, it is nearly impossible for one or a few individuals to dig a burrow and find enough food to survive. Working cooperatively, the colony survives. The inclusive fitness of nonbreeders is high, because at least some of their genes are shared by the breeders, and there is little hope of reproducing independently.

1. What are the three major criteria for eusociality?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. Assuming that the evolutionary goal of reproduction is to have as many of your own genes in the next generation as possible, what is the potential parent-offspring conflict in haplodiploid eusocial insects? (Think about the perspectives of the mothers and their sons and daughters. Would it be better for each of them if the mothers were producing more sons or more daughters?)
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. Naked mole rats differ from other mammals in one important physiological mechanism. What is this?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

4. Explain how the eusocial system of naked mole rats may have evolved by selection.
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.




1.	What are the three major criteria for eusociality? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	Assuming that the evolutionary goal of reproduction is to have as many of your own genes in the next generation as possible, what is the potential parent-offspring conflict in haplodiploid eusocial insects? (Think about the perspectives of the mothers and their sons and daughters. Would it be better for each of them if the mothers were producing more sons or more daughters?) To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	Naked mole rats differ from other mammals in one important physiological mechanism. What is this? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	Explain how the eusocial system of naked mole rats may have evolved by selection. To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 15: How Organisms Evolve

Bizarre Facts in Biology