Fisher assumption violated:“no competitive or cooperative interactions between relatives”

Local Resource Competition (LRC) & Enhancement (LRE)

In essence:Bias offspring sex ratio to the sex that cooperates (LRE) and away from the sex that compete (LRC)

For example:Diploid species, generations NOT overlapping, siblings interact

Mum’s fitness = +

# * rep value # * rep value

Reproductive value of sons and daughters is influenced by their interactions mums fitness depends on relative number of sons & daughters (SEX RATIO)

e.g. if daughters compete with each other for resources then rep value of daughters has -ve correlation with # daughters so a male biased offspring sex ratio decreases this competition and maximises the rep value of daughters

LRC

e.g. if daughters cooperate then should produce a female biased SR[~ daughters ‘’repaying parental investment” so cheaper to produce]

LRE

Split sex ratiosCan evolve under LRE & LRCWhen interactions occur within 1 sex

e.g. if daughters out-compete sons then its best to produce sons in male only broods (LRC)

[Diff parents produce diff sexes / facultative allocation?]Predicted more often than observed and not obvious why this isPopn sex ratio least informative

e.g. if daughters cooperate only with each other, rep value of daughters increases with production of daughters. But the rep value of sons is not influenced by their brood SR. So to get the most out of daughters its best to produce them in female only broods (LRE)

…Seems simple but its very difficult to make testable predictions…

When are LRE / LRC likely to be important?LRC predicted / observed when 1 sex competes for resources (such as reproductive opportunities, space, food).

LRE predicted / observed when 1 sex stays and helps the family.

How much should sex allocation be biased?Until the returns from each sex are equal

i.e. where the fitness returns from biasing towards one sex arebalanced against the Fisherian freq dependent advantage for the rarer sex

Bias sex allocation to cooperative sex and away from competing sexThis can sometimes be achieved by unequal investment in the dispersing sex where non-dispersing sex is cooperative (LRE) or ends up in competition (LRC)

Making testable predictions for LRE:

Under LRE:helping sex = 1 + 2*bh

…But in reality…Value of helpers can vary within and between families: e.g. diminishing returns from more helpers, helpers costly in poor territories

Predicting ES sex ratios requires:Identifying & quantifying the effects of cooperation and / or competitionRelative importance of simultaneous LRE and LRC effects

And is further complicated by:Facultative sex allocation (i.e. parents able to vary sex allocation)Cross generational cooperation & competition Whether helping effort varies with relatedness to recipientsSex determination mechanisms skewing relatedness to sons & daughtersWhich parent controls offspring sex ratioOther influences on sex allocation

non-helping sex

Do cooperatively breeding vertebrates produce more helpers when useful?

Support prediction with helpful males & helpful females:

1 - 4: Helpers are not particularly helpful. e.g. Harris hawk, acorn woodpeckers*, sf wrens

5 - 11: include taxa above, 1 sex beneficial

Clearest support for LRE as unambiguous predictionSR bias to helpful sex is mediated by how helpful they are (& if LRC occurs)

* Sons & daughters possibly equally helpful

When both LRE and LRC occur (or at least more forces than just LRE)

Daughters help family & sons disperse Benefit of helper linked to territory quality80% sons on low quality [cannot support helpers, LRC]90% daughters on high quality [can support helpers, LRE] - pairs moved from low to high quality switch to 85% daughters (control high-> high, no change) - but more than 2 helpers would lead to LRC - switch to 93% sons when have 2 helpers

Also:Benefit of sons = daughters on low quality territories (low importance for LRC)Daughters help lots on high quality territories (LRE)Fitness of daughters increased by being reared on high quality territory

More generally:Expt. Manips. required to test predictions & info about mechanismCase where popn SR not useful as territory circumstances are importantFitness implications of SR variation quantified (rarely done)Detailed fitness data showing that LRC may not be so important

LRE in invertebrates - allodapine bees

1 generation / year1-9 sisters & nieces cooperate to rear offspring-likely that sisters found together r=~0.5-females found singly if can’t find a sister

Sisters help each otherDaughters help mums Females help brothers/sons/nephews

Popn SR female biased (after adjusting for cost of sons)

Sex ratios (proportion of males) in relation to brood size in two species of allodapine bee: Exoneura robusta (circles = newly found nests, squares = overwintering nests; 10-15 and 16+; data from Schwarz, 1988) and Exoneura angophorae (triangles = all nests; 10-14 and 15+; data from Cronin and Schwarz, 1997).

Factors to consider:

Shape of fitness benefits from daughtersShape of fitness benefits from sons [Fisherian?] - do sons help too [nest construction role?]

Timing issues where males and females more/less useful throughout year ?

Insurance benefits of early daughters if mum dies

Feeding and defence role of daughters over winter- 2 daughters is minimum to forage and defend

Nest founding role of daughters and optimal ‘founding unit size’Suboptimal fitness when daughters are not multiples of optimal unit size

[Why not split sex ratios?]

…case where specific and detailed theory neededand more data on enhancement effects of males, integer problem…

LRC in primates - bushbabies (galagos)

Male biased SR (popn level)Males disperse and females (sisters and mothers) compete [food?], esp in breeding season as they can’t move very far with young

Supporting data:Male biased birth SRs in species when

rep success decreases with group sizefemales disperse lessranges of females overlap

…but…Data not from wild popns (problem if facultative adjustment)Facultative adjustment leads to rank dependent SR adjustment (high rank can make it hard to have daughter if low rank)

SR adjustment in response to rank for other reasons toodaughters inherit rank / matrilineal dominancesons benefit more from high rank / condition mothers

Controversial

Popn SR data do not show any trends which isn’t a big suprise

Testing LRC through interaction with other forces - more primates

Strong competition for resources -> LRC-high ranking should produce daughters -also predicted by advantaged daughter (inherit rank)Weak competition for resources -> TW -high ranking should produce sons as in good enough condition

[pattern predicted by LRC & advantaged daughter? No rank-SR effect?][LRE pattern? When LRC high, best to be with relatives?]

* Effect hinges on 1 popn…

Low popn growth = high LRCEffect size = adjustment in relation to rank

High rank females switch from daughtersto sons less LRC (high popn growth)

Repeatable case of LRC - brush tailed possums

Popns of possums really LRC?

T&W: males are bigger so mothers in better condition should have sons[- how do #dens & condition correlate?]Timing effect?-if young, have sons to avoid LRC, but respond to condition if older

Litters contain >1 offspring so could also be SR-#offspring effects-bad for daughters to be in female biased litters, doesn’t matter for males-diff mech for LRC?

Males disperseDaughters stay -compete for den sites-not condition, popn effects

Questions about LRE & LRC

Is there SR adjustment or just noise?-comparative approach good when hypotheses predict adjustment in same direction. Though hard to identify causal force.

Data on what the enhancement / competitive effects are & magnitude

Fitness benefits to parents & offspring of SR bias?-assumptions of explanations tested?-LRE, LRC, env effects & T&W may all be present in some cases-how, when & why do LRE and LRC interact with env effects and T&W

What can be learned from brood and popn level SRs-comparative approach / facultative adjustment / temporal data

Case-specific theory needed, even to predict direction of effect

Less pseudoreplication

Is absence of split SRs due to constraints or biology we’ve not looked at