This Week in Evolution

The beehive was an early Mormon icon, symbolizing hard work and cooperation. To an evolutionary biologist, however, a beehive could symbolize reproductive skew, a situation where some individuals reproduce much more than others. Extreme reproductive skew is one of the defining characteristics of eusocial species, of which honey bees are a prime example. Reproductive skew can differ between the sexes. In honey bees, the queen lays most of the eggs, and most females don’t reproduce at all. Polygamous species and groups show the opposite pattern: males vary much more in reproductive success than females do. Maybe an inverted beehive would have been a better symbol. Note that the cells in our bodies behave somewhat like a eusocial bee colony; any children we have are directly descended from a few sex cells, while brain cells and skin cells play the supporting role of worker bees.

This week’s paper, “Ancestral monogamy shows kin selection is key to the evolution of eusociality” was published in Science by William Hughes and others. Like humans, some bees are monogamous, meaning that the queen mates with only one male, so her daughters (the workers) are all sisters. In other bee species, the queen mates with several males, so her daughters are half-sisters. Relatedness generally favors cooperation, although there are some possible complications, discussed below.

This week’s paper asks how mating behavior affects the evolution of eusociality. They reasoned that, if mating system doesn’t matter, then today’s eusocial species could be descended from either monogamous, polygamous, polyandrous (each female has multiple mates), or promiscuous ancestors. Alternatively, eusociality may evolve more easily with one of these mating systems than with the others.

Continue reading "Traditional values in bees" »

Posted by R. Ford Denison at 09:47 PM | Permalink | Comments (1)

The “Coolidge effect” – I would have named it for a different American president – is a tendency of some males to be more interested in a new sex partner than one they have mated with in the past. Males that don’t help care for young may have more descendants this way than if they put all their eggs (so to speak) in one basket. But to avoid remating with the same partner, one first needs to remember them all.

Continue reading "Sex! Identity theft! Burying beetles!" »

Posted by R. Ford Denison at 11:24 PM | Permalink | Comments (1)

Three recent papers in Proceedings of the Royal Society discuss differences between males and females or, in one case, among males.

“The costs of risky male behaviour: sex differences in seasonal survival in a small sexually monomorphic primate” by Cornelia Kraus and others, is based on a 10-year study of differences between male and female behavior in grey mouse lemurs. During the breeding season, males had lower survival than females, despite any possible risks associated with pregnancy or raising young. The higher risk for males apparently resulted from their tendency to travel more, looking for females.

The sexes also differ in winter behavior: females hibernate, while males remain active. Is there something about female physiology that makes hibernation healthier for them than it would be for males? Maybe, but there was no difference in winter survival between the sexes, which don’t differ much in size in this lemur species. The authors suggest that hibernation might have longer-term benefits in females, such as increased lifespan, whereas males need to stay active to bulk up in preparation for the breeding season.

This paper reminded me of an earlier paper on albatrosses, in which "in each pair, the male spent the winter just north of the pack ice in Antarctic waters whereas the female stayed south of Madagascar." It’s not hard to understand why males and females might differ in various ways (size, color, etc.) but differences in behavior outside of the breeding season are more interesting.

The second paper addresses an old argument between Charles Darwin and Alfred Russel Wallace, who developed similar explanations of evolution by natural selection at about the same time.

Continue reading "Separate vacations and other sexual differences" »

Posted by R. Ford Denison at 11:02 PM | Permalink | Comments (2)

My wife and I have been watching the Planet Earth series. Week after week, mother polar bears and mother snow leopards care for their young, while fathers are either absent or dangerous. But then we got to Emperor penguins. What's the difference? This week's paper Parental conflict in birds: comparative analyses of offspring development, ecology and mating opportunities tries to answer this question.

Every baby bird -- except some turkeys -- has two parents. How much care does each parent provide for their chicks, and why? If animal behavior were ordained by a god, as a guide to human behavior, then we might expect all wild species to exhibit the same exemplary behavior. Or maybe those species that have more opportunities to interact with and influence humans -- ducks, say -- would exhibit divinely inspired behavior, while those remote from human settlements -- Emperor penguins, for example -- are left to the whims of natural selection?

The authors of this week's paper didn't waste time testing such nature-as-morality-lesson hypotheses, which are left as an exercise for creationists. Instead, they explored how parental care behaviors have evolved in response to various factors. These factors include how chicks of different species depend on parental care, and also whether a bird that leaves its mate alone to care for their chicks has additional opportunities to reproduce.

Continue reading "Conflict over parental care" »

Posted by R. Ford Denison at 07:44 PM | Permalink | Comments (0)

This week's paper is Facultative Mate Choice Drives Adaptive Hybridization by Karin Pfennig of the University of North Carolina, Chapel Hill (where I went to kindergarten), published in Science.

Spadefoot Toad (Wikipedia)

In contrast with the “hybrid vigor” sometimes seen with crosses between different genotypes within a species -- corn, for example -- hybrids resulting from mating between related species are usually less likely to survive and reproduce. For example, when two different species of spadefoot toad mate, their daughters usually produce fewer eggs. But apparently there are situations where genes from the other species are beneficial enough to outweigh problems due to genetic incompatibilities.

When Spea bombifrons females mate with Spea multiplicata males, their tadpoles mature faster. This is beneficial in shallow ponds that don’t last long, but not in deeper ponds, which will last long enough for nonhybrid tadpoles to mature. Therefore, the smart thing for S. bombifrons females to do would be to mate with their own species in deeper ponds, but with S. multiplicata in shallower ponds. But how smart is the average toad?

Continue reading "When hybrids are best" »

Posted by R. Ford Denison at 08:07 PM | Permalink | Comments (5)