Table of Contents
Overview: Why Reproduction and Care Are Behavioral Issues
Reproduction is the only way genes are passed on to the next generation. From the viewpoint of behavioral biology, mating behavior and parental care are not random; they are shaped by natural and sexual selection to maximize reproductive success (number of surviving, reproducing offspring).
This chapter focuses on:
- How animals find and choose mates (reproductive behavior)
- How and why they invest in offspring (parental care)
- The trade‑offs and conflicts that arise between parents, between sexes, and between parents and offspring
General ideas such as “adaptiveness,” “fitness,” and “evolutionary strategies” are treated in the parent chapter; here, they are applied specifically to reproduction and care.
Reproductive Behavior: From Finding to Securing a Mate
Mate Search and Attraction
In many species, simply encountering a potential mate is nontrivial. Behavior has evolved to increase encounter rates and ensure recognition of conspecifics (members of the same species).
Typical mechanisms:
- Signals to attract mates
- Acoustic: birdsong, frog calls, cricket chirping
- Visual: bright plumage, courtship dances, bioluminescence (e.g., fireflies)
- Chemical: sex pheromones in insects and mammals
- Tactile: courtship touches or vibrations (e.g., spiders drumming on webs)
- Aggregation behavior
- Leks: communal display sites where (usually) males gather and females visit to choose mates (e.g., many grouse species)
- Breeding swarms: synchronized emergence in insects or frogs at breeding sites
Species recognition is a key function: courtship signals and rituals help prevent wasted reproductive effort with the wrong species.
Mate Choice (Intersexual Selection)
Often, one sex (commonly females) chooses among potential partners of the other sex. This is intersexual selection.
Criteria for Mate Choice
Commonly selected traits:
- Direct benefits: resources or protection directly affecting the chooser
- Access to good territories (nest sites, food)
- Nuptial gifts (prey items, nutrient‑rich secretions)
- Protection from predators or other conspecifics
- Indirect (genetic) benefits: traits thought to signal “good genes”
- Ornamentation (long tails, bright colors, large antlers)
- Complex courtship displays (songs, dances)
- Indicators of health (symmetry, parasite‑free appearance)
These traits can be costly (e.g., large tail reduces flight performance), which makes them more reliable as “honest signals” of quality: only high‑quality individuals can afford them.
Sexual Conflict in Choice
The sex investing more in gametes or care tends to be choosier; the other sex has an advantage if it can appear more attractive than it truly is (e.g., deceptive displays). This creates an evolutionary “arms race” between choosiness and manipulation.
Competition for Mates (Intrasexual Selection)
When mates are limited, members of one sex (often males) compete among themselves.
Forms of Competition
- Physical combat:
- Establishing dominance hierarchies, territories
- Fighting with specialized weapons (horns, antlers, enlarged claws or mandibles)
- Display competition:
- Rival songs, display postures, or dances
- Assessment without direct physical damage
- Sperm competition (after mating):
- Producing more sperm
- Removing or displacing rival sperm (e.g., specialized genital structures)
- Mating plugs or chemical substances that reduce female receptivity
Competitive behaviors and traits evolve if they increase access to mates, even at a cost to survival.
Mating Systems and Their Behavioral Basis
Mating systems describe the pattern of mating relationships in a population.
Monogamy
- One male and one female form a pair bond for at least one breeding season, sometimes for life.
- Often associated with substantial biparental care (both parents care), where both are needed to raise offspring successfully (e.g., many songbirds).
Behavioral components:
- Pair‑bond formation and maintenance (duets, mutual grooming, coordinated defense)
- Mate guarding to prevent extra‑pair copulations
Polygyny
- One male mates with multiple females; females usually care for offspring alone or with limited male help.
- Common when:
- Resources can be defended in patches (territories), attracting multiple females (resource‑defense polygyny)
- Females are clumped and can be defended as a group (female‑defense polygyny)
- Males aggregate in leks and provide mainly genetic material (lek polygyny)
Behavior emphasizes male competition and display, female choice of best male or territory.
Polyandry
- One female mates with multiple males; males often provide most of the care.
- More likely when:
- Males can independently raise young (e.g., some shorebirds)
- Females can increase total reproductive success by laying multiple clutches cared for by different males
Behavior involves aggressive or dominant females, role reversal in mate choice and competition.
Promiscuity and Polygynandry
- Promiscuity: No lasting bonds; both sexes mate with multiple partners.
- Polygynandry: Several males and females form social groups with multiple mating relationships; care may be shared among group members.
Behavioral consequences include complex social structures, kin recognition, and often sperm competition.
Parental Care: To Invest or Not to Invest?
The Cost–Benefit Framework of Parental Investment
Parental investment is any effort by a parent that increases offspring survival and future reproduction at the cost of the parent’s ability to invest in other offspring.
Trade‑offs:
- More care → higher survival of current offspring, but fewer offspring overall or reduced parental survival
- Less care → more offspring possible, but each with lower survival probability
Selection favors the level and form of care that maximizes lifetime reproductive success, not survival alone.
Forms and Levels of Parental Care
Parental care ranges widely, from none at all to intense care lasting years.
No Parental Care
- Many fish, amphibians, invertebrates, and plants produce large numbers of gametes or offspring, then invest nothing after fertilization or egg‑laying.
- Strategy: “many, cheap offspring” relying on numbers and favorable conditions.
Behavior: brief mating, egg‑laying, often synchronized with environment (tides, seasons).
Egg and Nest Care
Common in insects, fish, amphibians, reptiles, and birds:
- Choosing or building a safe nest site (burrows, nests, foam nests, etc.)
- Guarding eggs from predators and conspecifics
- Cleaning or aerating eggs (fanning by fish, rotating by birds)
- Thermoregulation of eggs (incubation, shading)
Care is generally limited in time but critical for early survival.
Brooding and Transport of Young
Parents may physically carry eggs or offspring:
- Eggs carried on body (e.g., some frogs, seahorses, some aquatic insects)
- Young riding on parent’s back or in pouches
- Mouthbrooding (fish hold eggs or fry in mouth)
Behavioral implications:
- Restricted feeding and movement of parents
- Increased vulnerability to predators
- Strong protection responses when brood is threatened
Feeding and Teaching
Typical in birds and mammals, some fish and invertebrates:
- Bringing food to young, regurgitating food, or providing milk (mammals)
- Guiding young to food sources (e.g., “leading” behavior in some carnivores)
- Teaching foraging techniques, hunting, or tool use (observed in some birds, primates, and other mammals)
Advanced care behaviors enable offspring to learn complex skills and adapt to variable environments.
Protection and Social Support
- Guarding against predators or conspecifics (e.g., infanticidal intruders)
- Shelter and thermoregulation (huddling, nest building)
- Social integration: introducing young into group, mediating conflicts, allowing play
Protective behaviors often involve risk to the parent, which is balanced against expected gains in offspring survival.
Who Cares? Patterns of Parental Care by Sex
Maternal Care
Most common form in mammals and many invertebrates.
Key factors:
- Internal fertilization and gestation can make the female more “committed” from early stages.
- Physiological constraints (e.g., lactation) tie the female to offspring.
- Paternity uncertainty (males lack full certainty they are the genetic father) can reduce male incentive to invest.
Maternal behaviors include nest building, nursing, grooming, carrying, defense, and teaching.
Paternal Care
Less common but widespread in certain taxa (many fish, some birds, some mammals, some invertebrates).
Conditions favoring paternal care:
- External fertilization and male control of territories where eggs are laid (male can guard many eggs from different females).
- High certainty of paternity.
- Offspring survival increases greatly with male care relative to costs.
Behavior examples: nest guarding, mouthbrooding, carrying, feeding fry, incubating eggs.
Biparental Care
Especially frequent in birds, some fish, and a few mammals:
- Offspring require such high investment that success is low unless both parents contribute.
- Monogamous pair bonds often accompany biparental care.
Behavioral elements:
- Division of labor (one guards, one feeds)
- Coordination (alternating incubation, synchronized feeding trips)
- Communication over nest defense and provisioning rate
Cooperative Breeding and Alloparental Care
Alloparental care: care by individuals other than the biological parents (e.g., helpers).
Occurs in:
- Social birds (e.g., some jays, bee‑eaters), mammals (e.g., meerkats, some primates), and insects.
Helpers may be:
- Older siblings
- Other relatives
- Occasionally unrelated individuals
Benefits:
- For offspring: extra food and protection
- For parents: more total offspring raised, reduced workload
- For helpers: experience, possible indirect genetic benefits (helping relatives), or future inheritance of territory
Behaviorally, cooperative breeding requires social coordination, recognition of kin and roles, and often complex communication.
Parent–Offspring Conflict and Sibling Competition
Parent–Offspring Conflict
Parents and offspring share genes but have different genetic interests:
- A parent’s optimum: spread investment across multiple current and future offspring.
- An individual offspring’s optimum: obtain more resources for itself, even at some cost to siblings or future siblings.
Behavioral manifestations:
- Persistent begging for food or attention beyond what parents “prefer”
- Offspring resistance to weaning or independence
- Parental refusal, aggression, or withdrawal of care at some point
Natural selection shapes both offspring behavior (to demand) and parental behavior (to regulate and limit).
Sibling Rivalry and Cannibalism
When resources are limited, siblings may compete:
- Begging competition (louder calls, more conspicuous displays) in bird broods
- Direct aggression or dominance hierarchies among littermates or nestlings
- Facultative siblicide: stronger offspring kill weaker siblings when food is scarce (known in some birds of prey and seabirds)
- Cannibalism of siblings in some fish, amphibians, and insects; can be adaptive if it improves survival of remaining siblings or the cannibal
These behaviors are shaped by the balance between kinship benefits and survival benefits.
Reproductive Strategies: Many Small vs. Few Well‑Cared‑For
While not repeating full life‑history theory, it is useful here to see how parental care relates to reproductive strategies.
“Quantity” Strategy: Many Offspring, Little or No Care
- Typical in many invertebrates, fish, amphibians, and some plants.
- Behavioral traits:
- High fecundity (many eggs/gametes)
- Synchronization with favorable environmental conditions (e.g., mass spawning)
- Little to no post‑spawning behavior directed to offspring
This strategy works when mortality is very high and unpredictable, and each additional unit of care would not greatly increase survival.
“Quality” Strategy: Few Offspring, High Care
- Typical in mammals, many birds, some reptiles, and social insects.
- Behavioral traits:
- Low fecundity per event but repeated breeding with similar or increasing investment
- Intensive care: feeding, protection, teaching
- Long juvenile periods, extended social learning and bonding
Here, additional care substantially boosts offspring survival and future success, so investment pays off.
Many species are intermediate, combining some care with moderate offspring numbers.
Sexual Selection and Parental Care: Linked Behaviors
The distribution of parental care between sexes is often tightly linked to sexual selection patterns:
- Strong sexual selection on males, little male care:
- Polygyny; males compete for many females, invest more in mating effort than care (e.g., many mammals).
- Weak sexual selection, high male care:
- Monogamy or role reversal in mate choice; male investment in care becomes critical (e.g., many birds, some fish).
- Cooperative systems:
- Complex social structures; selection acts on group‑level traits like coordination and communication.
Hence, mating behavior and parental care form an integrated reproductive strategy, not isolated sets of behaviors.
Human Reproductive Behavior and Parental Care (Brief Biological Perspective)
Without going into cultural specifics, humans show:
- Relatively few offspring per female
- Extended period of juvenile dependence and learning
- Strong parental and often alloparental care (grandparents, other relatives, non‑relatives)
- Complex pair bonds and social institutions affecting mating and care
These patterns fit a high‑investment strategy where learned behavior and social transmission are crucial. Biological and cultural evolution interact in shaping human reproductive and parenting behavior.
Summary
- Reproductive behavior includes mate search, attraction, competition, and choice, structured into different mating systems.
- Parental care ranges from none to extreme investment and can be maternal, paternal, biparental, or cooperative.
- Care is shaped by trade‑offs between current and future reproduction; adaptive patterns depend on ecology, life history, and certainty of parentage.
- Conflicts arise between parents and offspring and among siblings over the distribution of limited parental investment.
- Overall, reproductive behavior and parental care are prime examples of how behavior evolves to maximize reproductive success under specific ecological and social conditions.