Parental care and sexual conflict René van Dijk Email: R.E.van.Dijk@bath.ac.uk
Papers for 15 November Team 1 Royle,, N. J., I. R. Hartley & G. A. Parker. 2002. Sexual conflict reduces offspring fitness in zebra finches. Nature 416: 733-736 736 Team 2 Osorio Beristain & Drummond. 2001. Male Boobies expel eggs when paternity is in doubt. Behavioral Ecology, 12: 16 21
Course Outline 1. Sexual selection, speciation and extinction * 2. Sexual conflict over mating * 3. Sexual conflict over parental care 4. Sex allocation 5. Sexual size dimorphism
Lecture 5 Sexual conflict over parental care Why is a conflict? Sexual conflict over parental care Theories of parental care Desertion games Parental effort games Tests of care models Summary
Intra-familial conflicts
What is sexual conflict? Sexual conflict occurs if the evolutionary interests of males and females do not coincide Types of sexual conflict Conflict over mating (pre-zygotic) Mate choice, sperm competition Conflict over care (post-zygotic) Parental effort, offspring desertion
Why is a conflict over care? The common interest of parents is to raise the young to independence, and thus spread their own genes If parents care together, then their young grow & survive better in many insects, fish, birds and mammals e.g. chicks survive better if both parents feed the young in junco (a small N Am songbird) e.g.
So biparental care looks like a good idea in many vertebrates Cichlid fish 182 genera Crocodiles 21 species Birds About 9600 species Primates 203 species % biparental care % female-only care % male-only care 40 38 90-95 95 32 60 62 5-10 68 <1 0 <1 0 % no care 0 0 0 0
.. and in some insects Terrestrial arthropods (no. of orders) Female- only care Male-only care Biparental care Egg care 20 3 3 Caring of young 17 1 5 Provisioning young 7 0 3 For instance, in burying beetles the male and female cooperate to prepare carcass & feed the young
Why is a conflict? cont BUT care is costly Metabolic costs of laying eggs, and caring for the offspring Brood-rearing rearing Incubation These metabolic costs may have long- term consequences* cost of reproduction e.g. several Cladocera species (water fleas) The caring parent is vulnerable to predators The chances of finding a new mate is limited if the parent cares
The cost of reproduction Reproduction is costly in terms of survival or reproduction later in life ( cost( of reproduction ) Thus reproduction should decrease survival and/or reproduction in future years These predictions were tested in collared flycatcher by experimentally manipulating their clutch size, and then looking at survival and clutch size of parents next year
Why is a conflict? cont... thus each parent prefers the other to do the hard work of raising young tug-of of-war between male and female over care: evolutionary increases in male duration of care were associated with decreases in female duration of care in shorebirds and vice versa
Sexual conflict over care is manifested by (i) offspring desertion one (or both) parent(s) abandon the offspring the parents decide for each brood whether they care or desert (discrete decision) (ii) reduced parental effort the parents decide how much effort they spend on raising their young (continuous decision) In both situations the behaviour of one parent may depend on the behaviour of its mate require game-theoretic analysis
(i) Desertion game (Maynard Smith 1977) G = care; D = desert P n = survival of brood cared for by n parent p = probability of remating W, w = number of eggs laid by deserting or caring females Above diagonal: payoff for the female Below diagonal: payoff for the male
Desertion game, cont.. Solving the game: We are looking for the Evolutionarily Stable Strategy (ESS), ie the strategy that can not be invaded by any mutant For instance, biparental care is an ESS, if wp 2 > WP 1 and wp 2 > wp 1 (1 + p)
Desertion game, cont.. The conditions of 4 ESSs can be deduced for (i) (ii) (iii) (iv) (i) biparental care (L)RS must be greater than if than if or deserts (ii) female-only care If deserting has a good chance of mating again (iii) male-only care No difference in survival of young relative to presence (iv) no care No or does better (in terms of surviving young) by providing care
There are more sophisticated game-theoretic models of C/D We may want to model the behaviour of all members of a population (males and females) - because in reality, the animals interact with each other We may allow the parents to produce several broods in a breeding season We may allow the behaviour of parents depend on their state, e.g. body reserves ( state-dependent game )
More sophisticated desertion games, cont Reserves (arbitrary units) 25 20 15 10 5 0 0 20 40 60 80 Days from start of breeding season The body reserves of male ( )( ) and female (---( ---)) parents over a breeding season and the predicted pattern of care ( ( female-only care, biparental care, male-only care) Early in the season the body reserves of females are high and thus the males can desert them, whereas in the middle of the season the females f are in poor condition, and thus the males decide to care. The females can then exploit the males decision and desert
(ii) Parental effort game Each parent makes a decision on a continuous scale: how much effort should they spend on raising the young What is the evolutionarily stable parental effort for the male and the female? Houston & Davies 1985:
Parental effort game, cont (Houston & Davies 1985) cont The ESS depends on the slopes and intercepts of the functions If males can work harder than females, then males will end up with doing all care (middle( middle) If parents are willing to compensate, but only partially, then biparental care is the ESS (left( left) If parents are overcompensating, then either of them may end up with doing all care (right( right)
Testing the Parental effort game Prediction: in biparental species, the parents should NOT compensate fully their handicapped mate Experimental test in starlings by handicapping the male or the female parent during brood-rearing: rearing:
Testing the Desertion game The Desertion game is very simplistic, and thus may not be suitable for direct testing Bearing this caveat in mind, we can derive predictions from this model Prediction 1: 1 Biparental care occurs if both parents provide much better care than a single parent Prediction 2: 2 Males desert if they are likely to find new mate
Prediction 1: Biparental care occurs if both parents provide much better care than a single parent Male removal experiments in birds brood survival growth Removal reduced 11 13 Removal did not influence 5 3
Prediction 2: Parents desert if the probabilities of finding a new mate are high Observational test Female rock sparrows desert more often their brood when unmated males are in the population
Prediction 2: The probabilities of finding a mate should influence caring/deserting decisions, cont cont. Experimental test St Peter s s fish desert their brood more often when the population sex ratio is biased: biased sex ratio deserts biased sex ratio deserts
State dependent desertion game Females may strategically reduce their reserves as to force males to provide more care
Parental manipulations Parents may manipulate the behaviour of their mate Forced baby-sitting : male social voles drag females back to nest to force the females to care Parents may withhold care e.g. in magnificent frigatebird - males always desert the chick - male desertion time varies among broods - after male desertion, the female fully compensates
Summary Sexual conflict occurs if the evolutionary interests of males and females do not coincide Care is costly, so each parent prefers the other to do the hard work To solve the conflicting interests of male and female parent researchers use game theory: (i) Desertion games (ii) Parental effort games Experimental and observational studies are partially consistent with the predictions of the game theoretic models There is growing evidence that parents DO manipulate their mate s behaviour