A new study by Australian researchers shows that well-endowed guys are more attractive to women — but only to a point.

Seriously, look at the size of the thing sticking out of that beetle's head, and that's not even the maximum size those things grow to. All male rhino beetles have these giant horns growing out of their heads as a way of showing off their sexual prowess to any female rhino beetles that might be around. Crazy, unwieldy…

Some traits get passed down because they help animals stay alive. Some traits get passed down because they enhance fertility. And some traits get passed down because they just make creatures more sexually attractive.

Some species share the exact same territories, rely on the exact same resources, and are sufficiently closely related that they can easily interbreed. So why don't these just merge into a single population? Because they simply don't want to interbreed.

Brain size can vary tremendously between species. For instance, the average human head holds around three pounds of cerebral matter; your typical chimp, on the other hand, packs about a third of that.

Female chickens are among the most promiscuous members of the animal kingdom, with wild and domestic fowl alike tending to mate with way more males than necessary to fertilize their eggs.

To compete for mates, male beetles often grow fierce mandibles that they use to fight other males. Unfortunately, there's a tradeoff. Researchers in Japan discovered that beetles with longer mandible weapons have smaller testicles and less ejaculate than their brethren.

Among the tiny insects known as water striders, males who aggressively attempt to mate with females don't wind up with as many offspring as their more gentlemanly counterparts. How can aggressive mating ever be a losing strategy?

Accepted scientific wisdom holds that new species arise because of geographic separation - the same bird evolves differently on two different islands. But a new study overturns this idea, challenging the importance of environment as a driver of evolution.

Seed beetles are polyandrous – females mate with multiple males, and choose which sperm will fertilize their eggs afterward. Scientists long believed they did this to get the best sperm. But a new study shows the fittest males always lose.