Biology · Behaviour, Communication & Social Life
Mate choice, sexual selection and reproductive behaviour
Suppose one animal attracts several partners while another has fewer partners but more attributed offspring. A mating count, a preference and reproductive success describe different parts of the story. None is automatically a measure of strength or proof of a genetic advantage.
Nervous Systems, Senses & Movement supplies the sensory and motor context. Evolutionary Processes & Adaptation introduces heritable variation, selection and fitness. Begin with the first lesson and the ordinary numerical examples; return to those chapters when a mechanism or evolutionary comparison needs more detail. Animal Reproduction, Development & Life Histories is an optional return path for reproductive timing and the distinction between resource expenditure and lifetime outcomes.
Back to the Biology mapBy the end, you can
Relate mate choice and reproductive competition to specified outcomes while distinguishing observed associations, parentage and evolutionary change.
Selection concerns reproductive consequences
Sexual selection concerns differences in reproductive success associated with competition over mating or fertilisation. Mate choice and competition among potential partners can interact. Competition can occur through access, timing or post-mating processes rather than only through visible fighting.
A preference is a tendency to respond differently to available alternatives under a specified comparison. It does not by itself show that a preferred trait is heritable, increases survival or produces more offspring. Those links need their own evidence.
Roles differ among species and settings. Either sex can compete or exercise choice, and the same individual can do both. Selected research on two-spotted gobies reports male preference involving female coloration. That bounded example challenges a fixed rule that females always choose while males only compete; it does not provide a rule for all fish.
Count the outcome that the claim needs
The model follows five focal individuals in a stipulated sexual reproductive setting. Their distinct mate counts are zero, one, one, three and five. Attributed offspring counts are zero, one, three, two and four. Counterpart partners are outside this focal set, and no closed mating network is implied.
Both totals are ten, yet the individual records differ. One focal individual with one mate has three offspring, while another with three mates has two. Observed access and realised reproductive output are related quantities without a one-to-one conversion.
Parentage, observation completeness, offspring survival and the chosen interval can affect an actual record. At a stated male–female association scale, monogamy describes one partner per individual, polygyny one male with several females, and polyandry one female with several males; multi-partner systems can involve several partners in both roles. A social pair bond need not identify genetic parentage. These labels describe associations rather than care duties, motives or moral rankings.
Equal totals can hide different individual outcomes
Read the diagram in words
Mate counts 0/1/1/3/5 and offspring counts 0/1/3/2/4 both sum to ten and average two. Their squared-deviation totals are sixteen and ten; dividing by five gives variances 3.2 and 2. The focal individual with one mate and three offspring differs from the one with three mates and two offspring.
| Tracked focal identity | Distinct mates in the interval | Attributed offspring in the interval |
|---|---|---|
| Focal individual one | 0 | 0 |
| Focal individual two | 1 | 1 |
| Focal individual three | 1 | 3 |
| Focal individual four | 3 | 2 |
| Focal individual five | 5 | 4 |
| Distinct mates in the interval | 10/5 = 2 |
|---|---|
| Attributed offspring in the interval | 10/5 = 2 |
| Distinct mates in the interval | 16/5 = 3.2 |
|---|---|
| Attributed offspring in the interval | 10/5 = 2 |
Σ(x − mean)² / N
No sample correction or causal selection estimate
Original complete focal set in an assigned sexual setting and one observation interval; counterpart mates outside the focal set. No source data, closed mating network, causal trait, sampling inference or hereditary effect.
Twenty observations need not mean twenty independent choosers
A second original record supplies ten chooser identities, each with two observations involving alternatives X and Y. Eight identities choose X twice; two choose X once and Y once. The eighteen X choices out of twenty describe the observation record.
The animal identity remains visible because repeated choices by one individual can be related. Treating twenty observations as twenty independent animals would discard that structure. A large observed fraction also does not establish a causal effect of the alternatives' traits or a population-wide preference.
A comparison needs to address opportunity and context as well as who was chosen. Availability, prior experience and other correlated features can influence behaviour. The model supplies labels X and Y only, without a colour, genotype, sex-role assumption, experimental treatment or offspring consequence.
Twenty choices belong to ten identities
Read the diagram in words
Eight identities choose X twice. Two identities choose X once and Y once. Totals are twenty observations, eighteen X and two Y; the X observation fraction is 0.9, while eight of ten identities choose X in both observations.
| Chooser identity | Observations from this chooser | Choices of X | Choices of Y |
|---|---|---|---|
| Chooser 1 | 2 | 2 | 0 |
| Chooser 2 | 2 | 2 | 0 |
| Chooser 3 | 2 | 2 | 0 |
| Chooser 4 | 2 | 2 | 0 |
| Chooser 5 | 2 | 2 | 0 |
| Chooser 6 | 2 | 2 | 0 |
| Chooser 7 | 2 | 2 | 0 |
| Chooser 8 | 2 | 2 | 0 |
| Chooser 9 | 2 | 1 | 1 |
| Chooser 10 | 2 | 1 | 1 |
| Choices of X | 18 |
|---|---|
| Choices of Y | 2 |
| X fraction among all observations | 18/20 = 0.9 |
| Identities choosing X in both observations | 8/10 = 0.8 |
Repeated observations need not be independent
Original two-alternative choice labels and ten identities with two observations each. No actual animals, trait manipulation, causal effect, independence assumption or offspring outcome.
Variation is the start of an explanation
Unequal reproductive output provides variation on which a selection question can be based. Demonstrating a particular cause requires more than finding unequal numbers. The relevant trait, its association with outcomes and the surrounding alternatives need to be specified.
Evolutionary change additionally depends on inheritance and population context. A preferred display can carry costs in some settings, and a current advantage need not persist when resources, competitors or frequencies change. Reproductive success is not a universal rank of health, dominance or strength.
Behavioural descriptions remain biological comparisons with defined settings. They do not prescribe what human relationships should be or turn one species' mating system into a rule for another. The useful question is which mechanism and consequence the evidence actually connects.
Worked example
Equal means can hide different distributions
Each focal-record mean is ten divided by five, or two. For mate counts, deviations from two are −2, −1, −1, 1 and 3. Their squared deviations sum to 4 + 1 + 1 + 1 + 9 = 16; dividing by all five supplied records gives a population variance of 3.2.
For attributed offspring, deviations are −2, −1, 1, 0 and 2. Squared deviations sum to 4 + 1 + 1 + 0 + 4 = 10, giving population variance 2. These are descriptive variances of the complete assigned set, not estimates with a sampling correction.
Eighteen of twenty choice observations select X, or 0.9. There are still ten chooser identities, with eight always choosing X in the two observations and two showing one choice of each. Neither the variance comparison nor the choice fraction establishes a causal selection mechanism or a prediction for a new population.
Check your understanding
Does more distinct mates guarantee more attributed offspring in the focal record?
Show reasoning
No. The individual with three mates has two attributed offspring, whereas one with one mate has three. These outcome counts need separate evidence and cannot be converted by a fixed rule.
Does twenty choice observations from ten identities justify calling the sample twenty independent animals?
Show reasoning
No. Each identity contributes two observations, which can be related. The observed X fraction is 18/20 = 0.9, but there are ten distinct choosers.
Does higher variance in mating counts prove which trait caused sexual selection?
Show reasoning
No. Descriptive variation does not identify a causal trait, its inheritance or its fitness effect. It defines a pattern needing further mechanistic and evolutionary evidence.
What to carry forward
Preference, mating opportunity, parentage and evolutionary change are connected through evidence rather than automatic conversions.
References
Resources supporting this lesson’s explanations and examples.
- Sexual selection and mating systems (PNAS 106 Suppl1:10009–10016, 2009; doi: 10.1073/pnas.0901132106)Stephen M. ShusterScientific review
- Male mate choice selects for female coloration in a fish (PNAS 98(23):13155–13160, 2001; doi: 10.1073/pnas.211439298)Trond Amundsen and Elisabet ForsgrenPrimary research article