= Solution
\b[Parents share an interest in successful offspring but can differ over mating, fertilization, resource allocation and the costs of care. Resolution usually means a stable outcome under those competing incentives, not disappearance of the underlying conflict.] <sexual conflict> is defined by differences in the outcomes favoured by selection, and can occur without overt aggression. It should be distinguished from <parent-offspring conflict>, which compares parental and offspring interests rather than those of the two parents.
A fundamental source is the unequal return from additional mating or additional investment. <gamete> production, gestation, lactation and care can impose different constraints on the sexes, but those constraints do not dictate one universal sex role. What matters is how changing an action affects lifetime <reproductive success>, including future opportunities. A parent can favour more current investment by its partner while preferring to reserve its own resources for later reproduction. Conversely, both may benefit from increased joint effort when offspring survival requires contributions that neither can supply alone.
The conflict over mating begins before two animals become the parents of a particular brood. The mating rate that maximizes one sex's lifetime return can differ from that favoured by the other. Courtship persistence, resistance and ejaculate effects can then evolve through <interlocus sexual conflict>. In <Drosophila melanogaster>, comparisons involving males deficient in <sex peptide> connected a seminal component to costs of mating for females under the tested mating conditions. <Sex Peptide Causes Mating Costs in Female Drosophila melanogaster by Stuart Wigby and Tracey Chapman>[Current Biology 15, pp. 316–321; sex-peptide deficient males and female lifetime performance]{ref} The result is stronger than a correlation between mating frequency and shortened life, but it does not mean that the component is always harmful: effects depend on exposure, nutrition, remating and the offspring produced. Genetic-background controls and lifetime offspring counts are necessary when separating altered mating behaviour from direct physiological effects.
<antagonistic coevolution> can generate matching manipulative and resistant traits. Water-strider research combined experimental mating outcomes with comparative analyses of male grasping and female anti-grasping structures across <species>. <Antagonistic coevolution between the sexes in a group of insects by Göran Arnqvist and Locke Rowe>[Nature 415, pp. 787–789; comparative and experimental analyses of water-strider mating armaments]{ref} Such evidence supports an arms-race interpretation, while retaining alternatives: correlated structures can reflect shared ancestry, ecology or mechanical compatibility. Demonstrating opposed effects on male and female lifetime fitness is more informative than labelling every elaborate genital structure a weapon. <evolution> can reduce the current effect of a manipulation without removing the possibility of further counteradaptation.
After mating, the identity and number of offspring produced create further disagreements. A male can benefit from fertilizing a greater share of a brood, while a female's favoured allocation can depend on offspring quality, compatibility and future reproduction. Conflict over remating and fertilization should not be equated with conscious knowledge of parentage. <paternity certainty> can influence the return to paternal care only through available information and feasible alternatives; population-level extra-pair paternity is not itself evidence that an individual detects a particular offspring's parentage. A male may still benefit from caring for a mixed brood when withholding care would sacrifice his own offspring as well.
The logic of conflict over <biparental care> can be expressed with female effort $x$ and male effort $y$. Let offspring benefit be $B(x+y)$, with diminishing marginal returns, and let parental costs be $C_f(x)$ and $C_m(y)$. In a simple model, each parent values the same offspring benefit but pays only its own direct cost:
$$
W_f=B(x+y)-C_f(x),\qquad W_m=B(x+y)-C_m(y).
$$
At an interior best response with positive effort, each balances $B'$ against its own marginal cost. The outcome need not maximize the unweighted sum of the two payoffs, because that sum counts the shared offspring benefit twice. This is a transparent model of opposing incentives, not a complete prediction of care in a <species>. Different parentage, offspring dependence, survival and future mating opportunities alter both benefit and cost functions.
When $B''<0$ and $C_f''>0$, the female's local response to an imposed change in male effort satisfies:
$$
\frac{dx}{dy}=\frac{B''}{C_f''-B''},\qquad -1<\frac{dx}{dy}<0.
$$
Thus this particular model predicts <partial compensation in parental care>: reducing male effort increases female effort, but does not fully replace the lost care. Full compensation would give the reducing parent no immediate offspring penalty and could destabilize contributions. The slope is not a universal law, however. It assumes fixed information and benefit functions and does not apply unchanged when a partner's actions provide new information about brood demand.
Experiments on <great tits> used extra begging-call playback during one parent's nest visits to increase that parent's provisioning. The chicks' own begging did not increase, while the other parent also increased its effort. <Negotiation over offspring care?—a positive response to partner-provisioning rate in great tits by Camilla A. Hinde>[Behavioral Ecology 17, pp. 6–12; partner provisioning manipulated independently of chick begging]{ref} Such matching can occur when partner activity reveals that offspring demand or expected returns are higher than previously estimated. It therefore need not refute all conflict over care; it challenges the assumption that each parent treats the partner's effort only as a substitutable input. Experiments should manipulate offspring signals and partner actions separately, measure actual food delivered rather than visit counts alone, and compare immediate responses with later care and reproductive consequences. Handicapping one parent can also change brood condition, so an apparent negotiation response may otherwise combine several causal pathways.
<parental negotiation> can stabilize care through conditional adjustment, coordination and repeated interaction. Desertion opportunities, ecological dependence and availability of replacement partners can constrain unilateral withdrawal. Such constraints can align behaviour without aligning every preferred outcome. The observed result may involve alternating visits, task specialization or unequal contributions; equal time is not a necessary prediction. Conflict can also be reduced through sex-specific expression of a shared trait when its sex-specific optima differ, which concerns <intralocus sexual conflict> rather than manipulation between interacting loci.
Pregnancy extends the problem to allocation through offspring tissues. Under some patterns of relatedness, maternally and paternally inherited copies in offspring can favour different levels of maternal resource extraction. <kinship theory of genomic imprinting> concerns those copy-specific interests, not a claim that every father always favours larger offspring or every mother smaller ones. <Genomic imprinting and the strange case of the insulin-like growth factor II receptor by David Haig and C. Graham>[Cell 64, pp. 1045–1046; original kinship-conflict interpretation of imprinted growth regulation]{ref} Maternal survival, future offspring and mating structure determine the relevant inclusive-fitness effects. An expressed physiological compromise can therefore coexist with genetic conflict over its regulation.
\b[Conflict is resolved through the <evolution> of resistance, constraints, conditional care and negotiated equilibria, often with continuing costs.] Establishing a mechanism requires comparing the parties' reproductive consequences and experimentally separating information, partner capacity and offspring demand; merely observing harmonious care or aggressive mating is insufficient.
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