Sex & behaviour: sexual investment CfE Advanced Higher Biology Unit 2: Organisms and Evolution

SQA mandatory key areas Comparison of investment in sperm and egg production – number and energy store; greater investment by females. Problems and solutions of sex for sessile organisms. Parental investment, optimal reproduction and reproductive strategies in terms of the number and quality of current offspring versus potential future offspring. Classification of parental investment into discrete r-selected and K-selected organisms does not reflect continuous range of life history strategies.

Key concepts Parental investment is costly but increases the probability of production and survival of young. Simplistic various reproductive strategies have evolved ranging from polygamy to monogamy.

Darwin’s Puzzle: Why are males and females different? Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. 1st ed., Murray, London.

Parental investment and sexual selection Trivers 1972

Assumption Assumption: every organism has adaptations that function to facilitate reproduction Members of a population/species live in the same environment, so why do some animals have different adaptations than others? Morphs: age, sex, others SEX: male and female adaptations are different WHY?

Parental investment “Any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring” (Trivers 1972)

Sperm vs. egg In sexually-reproducing species, the relative size of gametes define who is male and who is female.

Sexual dimorphism Amongst vertebrates, the clearest dimorphism is between gamete (sex cell) size. This single physical difference explains why behavioural sex differences exist. Females gametes: large, nutrient-filled, expensive to produce, limited in number, and produced infrequently. If fertilised this will lead to high costs to the female. Male gametes: small, have no nutrients, cheap to produce, constantly made throughout life. Reproductive Capability: females are thus classed as the ‘slow sex’ and males the ‘fast sex’.

Nurturant females In most animals, and almost all mammals, females provide far more parental investment than just the egg Internal fertilization protects, but at a cost Cod vs. gorillas Humans (mammals): Prolonged internal gestation (pregnancy) Placentation Lactation

Female reproductive strategy Females have much to lose if they mate with the wrong male, they are thus selective about who they mate with. They look for certain criteria: Physical Features: size and strength which confer dominance and so preferential access to resources. Behavioural Features: may indicate willingness to invest or good parenting skills. Females will compete with other females for the right to choose the most desirable (alpha) males. They gain little from multiple matings and seek quality not quantity. Almost every reproductively capable female will be able to find a mate of some sort.

Male reproductive strategy Males are far less choosy as they as they little to lose and everything to gain if they can have as many mating opportunities as possible. Males are not tied to rearing offspring and so seek quantity. While they would prefer a superior female, they are less choosy. If presented with a sexual opportunity they will take it. Males compete vigorously with other males for access to fertile females. Male reproductive success is however very variable, a small number of males will achieve many matings, while many males may never mate.

Competitive males Males are fighting with each other to mate with as many females as possible More females = more offspring (sharp contrast to females)

Sexual selection and parental investment theory For members of the sex that invests more in offspring, reproductive success is limited by the amount of resources an individual can secure for itself and its offspring. For members of the sex that invests less in offspring, reproductive success is limited by the number of mates one can acquire.

Bateman’s Gradient Bateman (1948) observed that the number of offspring fathered by male fruit flies increased in proportion to the number of females with which the male had mated. Female reproductive success did not increase as her number of partners increased. This is 'Bateman's gradient' - the steeper the gradient the stronger is sexual selection. males No. of offspring females No. of mates From Anderson & Iwasa (1996) p 54.

Sexual selection and parental investment theory What of it? Selection acted on males differently than it acted on females Specifically, differences in parenting strategies cause differences in adaptations Sex that invests more: adaptations to survive and get resources for offspring Sex that invests less: adaptations to help them get as many mates as possible It explains why, in many species, males look and behave differently than females

Sexual selection and parental investment theory Explains primary sex differences (uteruses vs. testes) Explains secondary sex differences Differences in weaponry (intrasexual selection) Differences in ornaments (intersexual selection) When the sexes have different adaptations, they are “sexually dimorphic”

r & K Selection r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus upon either increased quantity of offspring at the expense of individual parental investment, or reduced quantity of offspring with a corresponding increased parental investment, varies widely, seemingly to promote success in particular environments.

How many, and how often? r Selection (aka. Quick-and-many) K selection (aka. Slower and fewer) Age of maturation Young – usually before the next breeding season Older – usually many seasons after birth Number of offspring Many Few Frequency of breeding Usually frequently (many times a season) – high fecundity = many eggs produced per breeding season Generally once a season. Low fecundity Size of offspring Usually small Generally larger Mortality rates High – many offspring do not live to sexual maturity Low – offspring generally survive Examples of species Mice, rabbits, most insects, cane toads, octopus, mass spawning organisms Humpback whales, elephants, humans, some birds

Eggs or liveborn young? Oviparity Viviparity Literally means Ovum = egg, parus = bearing Vivus = living, parus = bearing Description Eggs released by mother, embryos develop outside mother’s body, nourished by egg yolk Embryo develops in mother, born as young. Mode of nutrition varies Benefits Reduced energy use in care of young Yolk provides good nutrient source More likely for offspring to survive to birth Drawbacks Eggs may need to be incubated Less chance of survival to birth due to eg. Eggs desiccating, predators, poor environment Energy expenditure for female carrying offspring Examples Birds, sharks, reptiles, monotremes Humans, some snake species, most mammals

Oviparity Bony fish and frogs Birds and reptiles Known as Shell - Amniote eggs Shell None, or leathery membrane Usually a hard, calcerious shell Benefits Wedge into safe crevices Better protected from desiccation – do not have to reproduce in water Dangers Desiccation Damage Cannot be hidden in crevices Examples Port Jackson shark, amphibians Hens, monotremes, crocodiles

Viviparity Egg yolk viviparity Placental viviparity Types of viviparity are recognised by the nutrient source for the developing embryo Egg yolk viviparity Placental viviparity Other source of nutrient More notes Cool habitat – kept warmer within body Largish eggs Any – nutrient sent via blood stream to embryo Very small eggs Feed them unfertilised eggs Feed them “uterine milk” – secretion from uterus Examples Some sharks and snakes. Sea snakes – so that they do not have to return to land to breed Mammals except monotremes, hammerhead shark Porbeagle shark (feeds with eggs), Bat rays (feed with “milk”)

Parental care or not? No parental care Care of laid eggs Care of young What is it? No contact with offspring after eggs are laid Guarding and/or incubating eggs to hatching Care of young after hatching/birth Benefits Free to mate more No energy expenditure Eggs have protection from predators/ harsh conditions High chance of offspring survival Drawbacks High levels of mortality Energy expenditure Some mortality after hatching Very high levels of energy expenditure – may not be able to mate for many years after offspring birth Examples Reef fish, frogs, turtles Seahorse, diamond python, cephalopods (eg. Octopus, squid), spiders Humans, primates. Mammals (milk), emperor penguins, emus

Assessment task Choose two organisms to compare reproductive strategies (one r and one K selection) including: Comparison of investment in sperm and egg production – number, size, energy store. Parental Investment – number of offspring produced and by which method (oviparity or vivaparity), degree of parental care. Explain each organisms chance of survival in light of this information.