The enigma of female COMPETITIVE traits
Bright colors, lethal weaponry, bizarre ornaments, complex
songs, intense aggression. These sorts of traits are gaudy and costly, and seem to scream, “Here I am, come eat me”.
Consequently, they have been a source of fascination for
biologist for decades. But after decades of research, we have a good understanding for why they exist. Females are limited in the number of eggs they can produce, while males are limited by the number of eggs they can access. This can exert strong pressure on males to out compete one another for access to females. Any and all traits that improve competitive ability (i.e. competitive traits) are favoured, to the point they are more costly than helpful.
But we don't see these traits just in males. There are many species with females that have weapons, fight, display bizarre ornaments, and sing complex songs. This is puzzling for biologists, females are supposed to be limited by egg production and offspring care, and ornaments and competitive behaviours traits take energy away from those things. So why are they so common?
There are multiple ways to tackle this problem. We can ask WHY females have these traits, and to answer that we’d need to look at whether these traits are helpful or costly, an if the answer is different in different contexts. We can also ask, HOW are females expressing these traits? In other words, what aspects of the female affects the traits, is it controlled by genetics? Is it affected by early life experiences? Can the female change the trait if she finds herself in a new situation?
My research program integrates these two perspectives in order to develop a better understanding of how and why females express competitive phenotypes.
Why are females bright & aggressive?
Some press discussing my work in female aggression in the dark-eyed junco.
Recent article on my research on female song in the superb fairy-wren
Though females in most species are not limited by the number of males they can mate with, that doesn't mean that they don't have problems of their own. What is important to a female varies. For example,in baboons, dominance rank is hugely important for female and her offspring's survival; in tree swallows, females only get a chance to breed if they can win a nesting cavity; and in dung beetles, females lay their eggs in big balls of dung. In all of these examples, females aren't held back by the number of mates they have, but they still might struggle if they can't get other important resources.
Biologist have known about these limitations for a long time, but only recently have we started to put it all together. Based on what we now know, it looks like being bright or aggressive might be related to an individual female's ability to acquire these resources, just like being bright or aggressive is often related to the number of mates in males.
But, being aggressive or brightly colored isn't cheap. Colorful feathers and loud songs can attract predators, and fighting can lead to injury or even death. Plus, the increased time and energy spent fighting might reduce the amount of care a female provides to her young.
Understanding the costs and benefits females experience due differences in competitive trait expression is a central goal of my research program.
What I've learned so far from Juncos:
What I've learned so far from Fairy-wrens.
How are females expressing these traits?
A big part of understanding aggression, ornaments, weapons and songs in females is understand what factors influence these traits. For a long time biologist thought females have these traits simply because they are good in males. I've been exploring this possibility by looking at how a female's genetics, physiology and ecology interact to shape how she looks and acts.
For animals like birds and mammals (including humans) the steroid hormones are a major player affecting how individuals look and act. In particular, testosterone is important for regulating sex-differences and individual differences in traits expression. Testosterone is also influential in how animal their energy, whether they invest more in being a good parent, living a long time, or fighting with others.
We know a lot about these relationships in males, but although we often here testosterone referred to as the "male hormone", females produce testosterone too. Unfortunately, we don't know much about what testosterone does in females. I've been working on this problem in Juncos, and have a new project soon to start in Crimson finches.
So far I've learned that
The genes that make a good male may be different from the genes that make a good female.
I'm using a long-term data-set to build a pedigree for Gouldian finches. I'll use this to investigate the importance of genes and early stress in shaping colour and size. I'm also looking at how these traits change with age in both sexes. I've just started on this project so stayed tuned for what we learn.
The costs of Competitive traits
the evolution of song learning
Most animals communicate with innate sounds, but a select group of animals can learn new sounds. This special group includes humans, bats, elephants, some marine mammals (whales and dolphins), and three types of birds (parrots, songbirds, and hummingbirds). Birds are excellent models for human language research due to their accessibility and obvious behaviours. Future progress in this field requires a clear understanding of the evolutionary origins of vocal learning. However, recent research has drastically reordered the bird family tree. The new family tree has challenged many past assumptions, and lead to a hotly contested debate about when and how vocal learning evolved. The New Zealand wrens (Acanthisitti) are a critical missing link in this debate due to their unique position in the new bird family tree. However, very little is known about the vocal abilities of this group, and no one has tested them for vocal learning. Here, we combine our expertise in behavioural analyses with recently developed acoustic tools and techniques to determine whether this group of special New Zealand birds, are vocal learners. By studying the tītipounamu or rifleman, we aim to resolve the crucial question of when and how vocal learning evolved in birds.
Until quite recently, it was assumed that song and song learning evolved in males and in the Northern hemisphere. But recent research has shown that song evolved in females and males together, and that this complex behaviour originated in Gondwanaland (specifically in Australia/NZ). We're collecting data on the structure, function and ontogeny of female song in NZ birds. We know that in many species both sexes sing, but we don't know how often each sex sings, whether they sing different songs, whether they learn differently or from different tutors, and whether those songs change as the birds shift from winter into the breeding season. We also don’t know much about the function of song in this species: is it to advertise to potential mates; to defend a territory; to tell others that your mate is taken; to tell your neighbours that your fertile?