Indirect Benefits:

Mate Choice and “Good Genes”

Why do females sometimes show preferences for males that offer nothing

but sperm?

What indirect benefits do such males offer?

“Good Genes” Hypothesis

Females choose mates whose genes improve their offspring’s fitness.

Prediction:

Females allowed to choose males with ‘good genes’ should produce more fit offspring than females not allowed to choose.

gray treefrog

house mouse barn swallow

“Good Genes” in House Mice

Each female is given a choice between two males. Males are designated as preferred (P) or nonpreferred (NP).

A female is permitted to mate with either a P male or a NP male.

Offspring production and viability then monitored for both P mated and NP mated treatment groups.

femalefemale

Some Results:

1. P-mated females produced more litters than did NP-mated females.

2. At 30 days survival was greater for P-sired offspring.

3. P-sired male offspring had larger home ranges.

(from Drickamer et al. 2000)

Evidently, females are choosing males with ‘good genes’.

But there is still a problem:

How can females determine if a male has good genes?

Hypothesis:

Males indicate their quality in some way.

They sing, they dance, they display ornaments.

Hey, look at me!!

Case Study: Call duration in Hyla versicolor

Eastern gray treefroggreen form

Some males have long calls:

Some males have short calls:

In choice tests, females prefer long calls over short calls by 3:1.

gray form

Call duration in Hyla versicolor and offspring viability

Fitness Measure High Food Low Food

Larval Growth Long Callers > Short Callers

Long Callers > Short Callers

Larval Survival No Difference No Difference

Welch et al., 1996 data

Evidently, females sometimes choose mates with traits that signal high genetic quality.

But… there are two more questions:

How can mate choice for ‘good genes’ lead to such exaggerated male traits?

What prevents a male from deceiving a female as to his genetic quality?

Hmm… if I look C-G-A-G-T, will she

know I’m really G-T-T-G-A?

Zahavi’s Handicap HypothesisZahavi’s Handicap Hypothesis

Exaggerated male traits are essentially handicaps.

Female prefer males with these handicaps…… … because such males have demonstrated an because such males have demonstrated an ability to survive despite their handicaps!!ability to survive despite their handicaps!!

Hypothetical Example

Peahens prefer peacocks with absurdly long tails because such males have demonstrated an ability to survive despite such tails.

Wow, how attractive

am I!!

At first glance, Zahavi’s idea seemed ridiculous!

But Zahavi’s handicap model can work if…

… the signals used are costly, such that only the highest quality male can pay the cost of the very longest tail.

Darn! And these

styrofoam antlers

seemed like such a good

idea…

“Bright Birds and Parasites”

Birds are susceptible to parasites, both external and internal.

Natural selection should favor female choice for parasite-resistant males.

How to choose?

Parasitized birds have dull plumage.

Therefore, natural selection should favor female choice for males with brighter-than-average plumage.

“Bright Birds and Parasites” cont’d

But why do males evolve to be so extremely But why do males evolve to be so extremely bright?bright?

Because parasites evolve ways to defeat resistance.

Once a new parasite form arises, natural selection again favors males who can resist the parasite.

In other words, females who choose males with still brighter plumage. Thus, coevolution between host and parasite Thus, coevolution between host and parasite

leads to brighter and brighter males.leads to brighter and brighter males.

Tail Length in Barn Swallows and Parasite Resistance

Males tails are longer than female tails.

1. Females prefer to pair up with males with long tails.

2. Females that paired with short-tailed males were more likely to engage in extra-pair copulations.

3. Long-tailed males were more likely to engage successfully in extra-pair copulations.

So what’s so important about long tails?

Females prefer long-tailed males

Long Tails as an Indicator of Resistance to Parasitic

Mites

No. m

ites

on o

ffsp

ring

Mail tail length

P < 0.001

Tail length of an offspring’s biological father is inversely correlated with mite load.

What we hear:

Bud- weis-

-I got great DNA!

What a female Hyla versicolor hears: